racing sailboat keel design

Sailboat Keel Types: Illustrated Guide (Bilge, Fin, Full)

The keel type is one of the most important features of your boat. But the different designs can be confusing, so I've set out to create a very clear guide that will help you understand sailboat keels once and for all.

What are the most common sailboat keel types? The most common sailboat keel types are full-length keels, fin keels, bulb keels, wing keels, bilge keels, and lifting keels. Full keels are popular among cruisers, while fin keels are generally used for racing. Bilge keels and lifting keels are typically used in tidal waters, on small fishing boats for example.

In this article, we'll explore the most common keel types together. I'll use diagrams to really hit home the differences of all these keel types, and we'll discuss what keel types are best for liveaboard, ocean cruising, and lake weekend trips. After reading this article, you'll know what to choose - and why.

Sailboat Keels Explained

On this page:

Overview of sailboat keel types, keel types: fundamentals, modified full keel, centerboard.

If you just want a quick overview, here's a list with the most common keel types and a short description. More detail will follow below.

The most common keel types

• Full keels run from front to aft and are the most stable keel type, making them the most popular cruising keel.
• Fin keels offer the best performance but are less comfortable. This makes them popular for racing. Fin keels are bolted on to the hull and generally run deep and thin.
• Bulb and wing keels are both variants on the fin keel.
• Bulb keels carry additional ballast in the tip, making them more stable.
• Wing keels have two tips at the end of the keel, which reduces crossflow, improving directional stability.
• Bilge keels are double fin or double full kees, which allows the boat to be beached, making them the most popular keel for tidal waters.
• Lifting keels are moveable keels that can be lowered and raised, allowing the boat to enter shallow waters as well.
• Centerboard keels are a pivoting lifting keel, allowing to sail both coastal and inland waters.
• Leeboards are fins on the sides of flat-bottomed hulls boats, making a keel unnecessary.

Properties of each keel type

What does a keel do?

What does the keel do? A keel is a vertical blade running down from the hull. It is weighted and acts as a ballast, countering the boat's tendency to heel and preventing it from tipping over. The wetted surface under the waterline reduces slippage to leeward by creating a track, which counters the sideway force of the wind on the sails.

The reason sailboats don't tip over is that the weight of the keel counters the buoyancy of the hull, which means it will pull the boat downward. This downward force reduces heel and prevents the boat from rolling.

A canoe doesn't have a keel. Try stepping into that: it will want to roll.

It counters the horizontal force the wind puts on the sails. Whenever the force on the sails increases, the resistance of the water on the keel increases proportionally.

The heavier the keel, the less heel you'll get.

A keel reduces slippage to leeward. Slippage is simply the amount you fall off course because of the direction of the wind and current. Leeward is the side of the boat behind the wind.

So if you don't have a keel, you will fall off course quite a lot because the wind will push you over the water surface.

You will also heel quite a lot since there is nothing beneath the water surface to counter the force of the wind high up in your sails.

A keel fixes both of these issues and makes sailboats one of the most reliable boats in heavy winds and storms.

You can read on about how keels work here.

Keels can be classified by multiple dimensions. You can look at them from the side or the front. You can also classify them based on properties.

Before I dive into each keel type in-depth and show examples, let's make sure we have the same starting point.

There are essentially two sorts of keels:

Fixed keels

Movable keels.

Fixed keels are keels that are integrated into the hull or bolted on. They can't be moved or lifted.

When looking at fixed keels, you can divide them up further based on the side view. There are three main categories:

Bilge keels

Full keels are more comfortable, provide better stability and protection, but are also slower than fin keels.

Fin keels are less comfortable, provide less stability, are more vulnerable, but they're also a lot faster than full keels.

Bilge keels are double keels: one on each side of the hull. This allows them to be beached, which comes in handy in tidal waters. They are generally a lot slower and less maneuverable compared to fin keels.

Movable keels can be lifted from the water, creating a shoal (shallow) draft, allowing the boat to enter both shallow waters and coastal waters. This makes it a very versatile keel type. There are two main designs:

Lifting keels

Lifting keels can be lowered and raised through a slit in the hull. Examples of lifting keels are the daggerboard and centerboard.

Leeboards are wooden swords attached to the side of the hull and prevent slippage to leeward, but they don't stabilize the boat, nor counter heel by adding ballast.

With fin keels, there are different tip designs available. The most common two tip designs are:

These are both variants of the fin keel. Generally, these keel designs are mentioned in one breath with full keels and fin keels, creating confusion on what kind of keel they are. But it's important to understand that they are a sub-category of fin keels.

Rudder design

As with the tip of the fin, there are different rudder designs that may apply to both fin and full keels. The two most common rudder designs are:

Skeg rudder

Spade rudder.

A skeg is a structural part of the keel in front of the rudder that protects the rudder. The keel encompasses the rudder, preventing any rogue ropes, weeds, or rocks from damaging the rudder.

A spade rudder is an unprotected rudder: it doesn't have any structural protection from the keel design. It is simply attached to the hull. This design is very common.

Alright, we understand the big picture. Let's dive into more detail for each keel type and discuss the pros and cons.

Fixed keel Good for cruising and liveaboards Comfortable

What is a full keel? A full keel runs from front to aft for at least 50% of the hull and is fully integrated into the hull. It has the largest wetted surface of any keel type, and it is also the heaviest. This results in directional stability and reduced heeling, providing the most comfortable ride, but also the slowest.

The wetted surface simply means the amount of water contact area. With such a large wetted surface, it decreases slippage to leeward the most of all keel types, while it counters heeling the most as well.

The full keel is the most comfortable and stable keel type available. However, comfort comes at a price. It delivers the worst performance due to this large wetted area. It is the slowest of the keel types, and it has the worst windward performance.

This makes full keels particularly great for longtime cruisers or liveaboards who prefer comfort over speed, but less ideal for daysailers who need to navigate in and out of slips regularly.

Since it runs for at least 50% of the hull, it doesn't need to run as deep as a fin keel, resulting in a more shoal draft.

Heavier keels result in increased displacement, so a full keel boat will need a larger sail area to compensate for its weight.

For a more detailed discussion on full keel advantages, I recommend reading William's excellent article 5 Surprising Advantages of a Full Keel Sailboat here.

Example sailboats with a full keel:

• Nicholson 22
• Island Packet 380
• Beneteau Oceanis 411 Clipper
• Beneteau First 50
• Jeanneau Sun Shine 38
• Dufour 455 Grand Large

There are a lot of great cruising boats with full keel designs , some of them considered classics.

Full Keel with skeg rudder

Full keels with a skeg rudder design have a protected rudder, thanks to putting a structural part of the keel directly in front of the rudder. This helps with fending off any hazards to the rudder, like floating pieces of rope, rocks, or garbage, and protects it in case of running aground. The skeg design ensures the rudder is nearly impossible to break off.

Fixed keel Good for cruising and liveaboards Faster than a regular full keel

What is a modified full keel? A modified full keel is a full keel with a cutout at the front, reducing the wetted surface slightly, which increases performance without sacrificing too much comfort and stability. After the full keel, it has the best directional stability and the least amount of heel.

The modified full keel is popular among (bluewater) cruisers, thanks to its increased handling and performance. Most modified full keels have a skeg rudder, ensuring it is well-protected.

The slightly reduced weight and wetted surface improve windward performance quite a lot, but it is still one of the most stable keel designs out there.

Example sailboats with a modified full keel:

• Hallberg-Rassy HR 40
• Dufour Arpege 30
• Beneteau Oceanis Clipper 281
• Jeanneau Sun Odyssey 37.2

Fixed keel Good for racing Fast

What is a fin keel? A fin keel is a long, weighted blade attached to the bottom of the hull. It is lighter, faster, and more maneuverable than a full keel, but also more vulnerable. The increased distance between ballast and sails provides a lever, reducing the need for a large wetted surface or additional ballast.

Fin keels are generally bolted onto the hull and run deeper and thinner than a full keel. They are also lighter. This helps increasing performance (a lot), making fin keels a lot faster in all situations.

There are some major disadvantages to fin keels, however. Fin keels are a lot less comfortable than full keels and allow for more heel and a less solid track, so less directional stability. Fin keels are also a lot more vulnerable than full keels. They can break off when running aground, or get damaged.

They are very popular among racers and perform better when maneuvering in tight spots, like getting in and out of slips.

Example sailboats with a fin keel:

• Catalina 30
• Jeanneau Sun Odyssey 36.2

Fin keel with skeg rudder

Fin keels with a skeg rudder use a small structural part in front of the rudder to protect it. This design is mostly integrated into the hull, making it less vulnerable, and a great compromise between speed and safety.

Fin keel with spade rudder

Fin keels with a spade rudder have a completely exposed rudder, and typically a fin that is simply bolted on. The keel isn't integrated into the hull, making it more vulnerable and less comfortable.

Fin keel variant Good for cruising Less crossflow

What is a wing keel? A wing keel is a fin keel with a horizontal foil at the tip, which is wing-shaped and generally weighted. Its shape reduces crossflow, improving directional stability, and its ballast decreases heel, resulting in a more comfortable ride. The addition of a wingtip allows for a shorter fin, reducing draft.

Wing keels are good for cruising since this design improves directional stability compared to a regular fin keel or a bulb keel.

We'll discuss the wing keel's advantages and disadvantages in more detail in this article.

Fin keel variant Good for cruising Stability

What is a bulb keel? A bulb keel is a high-aspect-ratio fin keel with additional ballast at the end, which generally has a bulb or teardrop shape. This ballast improves stability and utilizes the distance between force and counterforce as a lever. This design reduces the need for a deep fin, resulting in a shoal draft.

By placing the weight at the largest possible distance from the force on the sails, you need relatively little extra weight for the same reduction in heel, making bulb keels very effective for cruising.

This design reduces the wetted area while increasing the weight of the keel just slightly, which increases sailing comfort big time.

Example sailboats with a bulb keel:

• Bavaria B/One
• Beneteau First 24

Fixed keel Good for racing Can be beached

What is a bilge keel? A bilge keel is a twin keel which uses double fins, allowing the boat to be beached and rest on its keel upright. Bilge keels have double the wetted surface, which increases comfort and directional stability while decreasing heel. Modern bilge keels often provide decent windward performance, thanks to better design.

The bilge keel does sacrifice speed compared to the fin keel but doesn't necessarily offer worse performance overall. Older designs performed considerably worse than other keels and were especially slow.

Bilge keels have some major advantages over full keels and fin keels. The most important is that the boat can be beached, making it a popular design in tidal waters. Bilge keels are especially common along the British coastline, where fishermen keep their boats in tidal harbors.

Another major advantage is that the boat can be stored resting on its keels, making dry storage and maintenance a lot easier.

Of course, there are many more pros and cons to the bilge keel , which we go into here.

Example sailboats with a bilge keel:

• Dufour Dynamique 62
• Hunter Duette
• Patagonia Patago 39
• Macwester 27

Lifting keel Good for daysailers Versatile

What is a centerboard? A centerboard is a type of retractable keel that rests on a hinge and can be lowered through a slot in the hull. It folds out like a pocket knife and allows you to increase or reduce the draft of the boat. Centerboards are mostly used on small fishing boats.

The centerboard is a very versatile keel type, allowing you to have both a very shoal draft for inland waters, as well as steadying the boat and reducing heel for larger bodies of water, or even oceans.

I've sailed a Cornish Crabber with a centerboard for a week, and while we stayed inland, having the option to increase the keel depth really came in handy when crossing the IJsselmeer (a former sea in The Netherlands).

There's more to the center

Olaf Roethele

https://www.theyachtmarket.com/en/new-boats/cornish-crabbers/adventure-17/218/

My name is Olaf and I am the owner of a Cornish Crabber 17 Adventure boat.

I would like to ask you if you can imagine to install on this boat a Torqeedo 2.0 Pod motor? Therefore i guess a modification of the keel/skeg is necessary ?!

Best regards from Uruguay,

You completely missed the hybrid planing/water-ballast keel of the Macgregor range

Thanks a lot for this explanation

Roger Bannon

Very well written article which provides an excellent guide for us small wooden boat builders. Thanks.

Hulda Whiddon

Great job site admin! You have made it look so easy talking about that topic, providing your readers some vital information. I would love to see more helpful articles like this, so please keep posting! I also have great posts about Airport Transfer, check out my weblog at <a href=” https://rosuchebnik.ru/bitrix/redirect.php?goto=http://uy5.de/flyingstar/flughafentransfer-frankfurt-rech&quot;&gt;UY5&lt;/a&gt ;

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Practical Boat Owner

• Digital edition

Keel types and how they affect performance

• Peter Poland
• June 19, 2023

Peter Poland looks at the history of keel design and how the different types affect performance

The Twister is a well-proven example of a generation of production yachts with ‘cutaway’ full keels and keel-hung rudders. Credit: Graham Snook/Yachting Monthly

Having been a boatbuilder for around 30 years until the very early ‘noughties’, I’ve already witnessed – and even taken part in – a lot of changes in the world of yacht design and building.

Yacht design originally evolved as traditional workboats developed into leisure craft.

In his History of Yachting , Douglas Phillips-Birt writes that the Dutch, who gave the name ‘yacht’ to the world, were probably the first to use commercial craft for pleasure in the 16th century.

They created the first yacht harbour in Amsterdam in the 17th century.

When the schooner America visited the UK in 1851 and raced around the Isle of Wight, this led to the America’s Cup and the resulting merry-go-round of race-yacht design that continues to this day.

The Jeanneau Sun Odyssey 35 offers three different fin keel configurations with different draughts plus a lifting keel version with a centreplate housed in a shallow winged keel stub. Credit: David Harding

The creation of what is now the Royal Yachting Association ( RYA ) in 1875 led to the introduction of handicap rules, establishing the sport in Britain.

These rating rules – and their numerous successors down the ages – have helped determine the evolution of yacht design and keel shapes.

Many early yachts were closely based on workboats, commercial cargo carriers or even privateers and naval vessels.

Initially, the ballast was carried in a long keel and the bilges .

New racing rules of the day taught designers to seek and tweak performance-enhancing features.

Maybe racing did not always improve the breed, but it certainly kept it moving ahead.

Artwork inspired by Ted Brewer’s illustration of keel types (excluding centreplate or lifting keels)

The late, great designer David Thomas believed that fishing boats, pilot cutters and oyster smacks had a large influence on the sport of sailing.

Each type of workboat was built to fulfil a specific purpose. And many had to be sailed short-handed while carrying heavy cargoes.

So they needed to combine form and function, sail well and be able to cope with heavy weather.

Proof of the versatility of working boat designs was provided by Peter Pye and his wife, Anne.

They bought a 30ft Polperro gaff-rigged fishing boat (built by Ferris of Looe in 1896) for £25 in the 1930s.

Having converted her to a sea-going cutter, and renamed her Moonraker of Fowey , they sailed the world for 20 years.

It proves how the simplest working boat design can cross oceans and fulfil dreams.

Racing influence on keel types and design

Most early yacht designs were schooners, but during the latter half of the 19th century the gaff cutter rig started to dominate the scene.

Many notable yachts were built at that time and the most important racing design was probably the yawl Jullanar (1875).

Designed and built by the agricultural engineer EH Bentall, she had, in his own words, “the longest waterline, the smallest frictional surface, and the shortest keel”.

She proved to be extremely fast and in her first season won every race she entered. Jullanar became the forerunner of such famous designs as GL Watson’s Thistle (1887), Britannia (1893), and Valkyrie II and Valkyrie III , both of which challenged for the America’s Cup during the 1890s.

Compare the She 36’s graceful overhangs with the vertical stems and sterns of most modern cruiser/racers

In the USA, Nat Herreshoff experimented with hull forms for racing yachts and produced the ground-breaking Gloriana in 1890.

She was a small boat for the times, with a waterline length of 46ft. Her hull form was very different to anything yet seen in the USA.

With long overhangs at bow and stern, her forefoot was so cut away that the entry at the bow produced a near-straight line from the stem to the keel.

It was a revolutionary design, and nothing at the time could touch her on the racecourse.

Many French models, such as this Beneteau, have opted for substantial pivoting keels. Credit: Peter Poland

Herreshoff wrote: “Above the waterline everything on Gloriana was pared down in size and weight… and every ounce of this saving in weight was put into the outside lead.”

Early English rating rules produced the ‘plank-on-edge’ yacht, where the beam became narrower and the draught got deeper.

New rating rules were then adopted to discourage this extreme type and eventually the Universal Rule was introduced in the USA and the International Rule – which produced the International Metre Classes – took over in Europe.

Yet again, racing rules proved to be a major influence on design development.

By the start of the 20th century the big, long-keeled racing yachts like the J Class attracted a lot of public attention, but after World War II everything changed. Yachts built to the Universal Rule fell from favour.

The age of the racing dinghy arrived and the ocean racer became the performance yacht of the future.

To new extremes

A 300-mile race from New York to Marblehead saw the start of offshore racing and the first Bermuda race was run in 1906.

The British were slower to compete offshore, but in 1925 seven yachts took up the challenge to race round the Fastnet Rock, starting from the Isle of Wight and finishing at Plymouth.

EG Martin’s French gaff-rigged pilot cutter Jolie Brise won the race and the Ocean Racing Club was formed.

In 1931 this became the Royal Ocean Racing Club (RORC), which remains the governing body of offshore racing in Britain.

The ‘cutaway’ modified full keel was famously used by Olin Stevens on his mighty Dorade. Credit: Christopher Ison/Alamy

The early competitors in RORC races were long-keeled cruising boats, many of them gaff rigged and designed for comfort and speed.

But everything changed in 1931 when the young American Olin Stephens designed and then sailed his family’s 52ft yawl Dorade across the Atlantic to compete in that year’s Fastnet race.

She won with ease. Then she did it again in 1933, having first won the Transatlantic ‘feeder’ race.

At 52ft LOA, with sharp ends and 10ft 3in beam, some said Dorade looked like an overgrown yawl rigged 6-metre. But her triple-spreader main mast was revolutionary. As were her cutaway forefoot, lightweight construction, deep ballast and 7ft 7in draught.

Dorade took the long keel format to new extremes.

In the USA, the Cruising Club of America (CCA), founded in 1922, played much the same role as the RORC did in Britain.

It introduced its own rating rule which influenced the evolution of yacht design in the USA.

The Elan 333. Both the deep (1.9m) and shallow (1.5m) draught models feature an elegantly faired bulb keel and spade rudder. Credit: Peter Poland

Beam was treated more leniently under the CCA rule, so wider American designs later offered more space for accommodation and a bit more inherent form stability than RORC-rule inspired yachts.

Many famous designers of long-keel racing yachts at this time developed their skills at the yachtbuilding firms they ran, such as William Fife II (1821–1902), his son William III (1857–1944), Charles E Nicholson (1868–1954) of Camper & Nicholsons and Nat Herreshoff of Bristol, Rhode Island.

Around the same time several British yacht designers made their names, including George L Watson (1851–1904) who set up one of the earliest Design Offices and Alfred Mylne (1872–1951), who designed several successful International Metre Class yachts.

Norwegian designers Colin Archer (1832–1921) and Johan Anker (1871–1940) also joined the party.

Continues below…

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In 1873 Archer designed the first long keel Norwegian yacht, but his real interest was work boats – pilot boats, fishing craft, and sailing lifeboats – some of which were later converted into cruising yachts.

Erling Tambs’s Teddy was a classic Colin Archer long keel canoe-stern design in which he wandered the globe with his young wife and family.

He proved the seaworthiness of Archer’s yachts, as well as their speed, by winning the 1932 Trans-Tasman yacht race.

Fellow Norwegian Johan Anker – a one-time pupil of Nat Herreshoff – became equally famous, thanks to his Dragon-class design that still races today.

As a new generation of designers arrived on the scene in the 1930s, hull tank testing became more sophisticated.

Long keel designs became as much a science as an art.

The leader of this new wave of designers, Olin J Stephens, had been a junior assistant to Starling Burgess who designed race-winning J Class yachts, including the iconic Ranger .

Tank testing was then in its infancy but the USA was ahead of the game and Stephens stored away everything that he learned. He enjoyed a head start over his contemporaries.

Keel types: Fin keels

Between the 1930s and the 1980s more fin keel designs began to arrive on the scene and his firm Sparkman & Stephens produced many of the world’s top ocean racers.

He also designed America’s Cup 12-Metres that defended the cup up to 1983 until Ben Lexcen’s winged keel shook the sailing world.

Many S&S fin keel and skeg production boats – such as the Swan 36 (1967), 37, 40, 43, 48, 53 and 65, She 31 (1969) and 36 and S&S 34 (1968) – still win yacht races and are much sought after as classics.

The S&S 34 has several circumnavigations to its name. Stephens, of course, had his rivals.

Among these was the Englishman Jack Laurent Giles, whose light displacement race-winner Myth of Malham had one of the shortest ‘long keels’ of all time.

(L-R) A Sigma 38 designed by David Thomas and Gulvain (1949) by Jack Giles as a development of his Fastnet-winning Myth of Malham have very different keel types. Credit: Peter Poland

The Dutchman EG Van de Stadt designed the Pioneer 9 (1959) which was one of the first GRP fin keel and spade rudder racers.

Towards the end of his career, Olin Stephens also came up against Dick Carter, Doug Peterson, German Frers and the Kiwis Ron Holland and Bruce Farr.

The development of new shaped keels went hand in hand with this rapid evolution in yacht design.

The full keel, as still found on motor-sailers such as the Fisher range, gave way to the ‘cutaway’ modified full keel as famously used by Olin Stephens on his mighty Dorade , designed back in the late 1920s.

She still wins ‘classic’ yacht races in the USA. American designer Ted Brewer wrote in ‘ GoodOldBoat ’ that Dorade’s offshore racing successes proved that the full keel is not essential for seaworthiness.

The Nicholson 32’s modified ‘cutaway’ long keel results in excellent performance and handling. Credit: Genevieve Leaper

As a result of its improved performance and handling, the modified ‘cutaway’ long keel caught on quickly and became the standard for around 35 years.

This keel type is found on numerous popular designs such as the Nicholson 32 , 26 and 36, Twister 28 and many Nordic Folkboat derivations.

The modified full keel format had a cutaway profile, giving good handling and directional stability while having less wetted surface than the full keel designs.

These yachts can perform well in all conditions and have a comfortable motion.

Even though they are generally of heavier displacement than fin keelers, they are not much slower in light airs , despite their added wetted surface area.

Their main drawback is a wide turning circle ahead and reluctance to steer astern when under motor.

Keel types: Increased stability

The modified full keel was subsequently cut away more and more for bluewater and inshore racers in an attempt to reduce wetted area until, finally, some designers took it to extremes.

As a result, much-reduced directional stability produced craft that were difficult to steer in breezy conditions, broaching regularly.

Whereupon the fin keel and skeg-hung rudder took over, reinstating increased directional stability, improving windward ability, reducing drag and restoring – when under power – control astern and on slow turns.

This fin and skeg format was later followed by the NACA sectioned fin keel with a separate spade rudder .

Soon, many performance cruisers followed this race-boat trend.

The Hanse 430 has a spade rudder and bulbed keel (draught 2.16m or 1.79m shoal draught. Credit: Peter Poland

Many builders now also offer shoal draught fin keel options and shallower twin rudders.

Some, such as Hanse, incorporate L- or even T-shaped bulbs on some Hanses and Dehlers at the base of finely shaped cast iron fins.

A new international competition had encouraged the initial development of modern fin keel yacht designs.

The revamped One Ton Cup was launched in 1965 for yachts on fixed handicap ratings (typically around 37ft long).

This spawned later fixed-rating championships for Quarter Tonners (around 24ft), Half Tonners (around 30 ft), Three-Quarter Tonners (around 33ft), and finally Mini-Tonners (around 21ft).

All these yachts were eventually handicapped under the International Offshore Rule (IOR) that replaced the old RORC and CCA rules.

The revamped One Ton Cup helped encourage the developed of modern fin keel designs. Credit: Getty

Countless production fin keel cruisers designed and built in the 1970’s to 1990’s boom years were loosely based on successful IOR racers that shone in the ‘Ton Cup’ classes.

The IOR handicap system’s major drawback was its Centre of Gravity Factor (CGF) that discouraged stiff yachts.

Once the international IRC rule replaced the IOR, more thought was given to increasing stability by putting extra weight in a bulb at the base of the keel.

GRP production boats followed suit. The keel foil’s chord needed to be wide enough to give good lateral resistance (to stop leeway), yet not be so wide as to add unnecessary drag.

Exaggeratedly thin foils are not suited to cruising yachts because they can be tricky upwind.

Tracking is not their forte and they can stall out. A bonus was an easier ride downwind thanks to wider sterns.

Keel Types: Lead or iron?

And then there is lead. Almost every production cruiser has a cast iron keel for one simple reason; it is much cheaper than lead. But it’s not as good.

Not only does it rust; it is ‘bigger’ for the same given weight. A cubic metre of iron weighs around 7,000kg, while the same cubic metre of lead weighs around 11,300kg.

An iron keel displaces far more water (so has more drag) than the same lead weight. We had always put iron keels under our Hunters – as did our competitors.

But when we came to build the Van de Stadt HB31 cruiser-racer, designer Cees van Tongeren said “No. We use lead.” “Why?” I asked. Cees replied: “If we use iron, the keel displaces more, so the boat sails worse.”

Rustler 36 long keel’s cutaway forefoot delivers responsiveness and manoeuvrability – a reason the design is so popular in the Golden Globe Race. Credit: Beniot Stichelbaut/GGR/PPL

Which explains why top-flight race boats have lead keels – or at the very least composite keels with a lead bulb or base bolted to an iron upper foil, thus lowering the centre of gravity (CG).

Some modern production cruiser-racers offer high-performance lead or lead/iron composite keels – but at a price.

Many Danish X-Yacht and Elan race-boat models, for example, have a lead bulb on the base of an iron NACA section fin.

Rob Humphreys, current designer of the popular Elan and Oyster ranges, said: “The T-keel is good if you have sufficient draught available. If not, the fin element has too short a span to do its job. This is because the T-bulb doesn’t contribute as usefully to side force as a ‘filleted L-bulb.’

“I developed and tested this shape (a blended-in projection off the back of the main fin) for the maxi race boat Rothmans in 1988/9, and have since used it on the Oysters and Elan Impressions. The ‘filleted’ keel we tested for Rothmans had slightly more drag dead downwind (more wetted area) but was significantly better when any side-force occurred; and side-force goes hand-in-hand with heel angle – which is most of the time! When the model spec allows for reasonable draught, the keel option with the lowest centre of gravity will invariably be a T-keel, with a longer bulb giving the greatest scope for a slender ballast package. An L-keel is a compromise and doesn’t suffer from the risk of snagging lines, mooring warps, and nets. [many modern production cruisers have 100% cast iron L- or T-shaped keels]. A lead bulb is preferable to a cast iron keel in terms of volume and density, but it costs more. However, a lead T-keel in a production environment will almost certainly use a cast iron or SG Iron fin, which may rust.”

The Mystery 35, designed by Stephen Jones and built by Cornish Crabbers, has a lead fin keel. Photo: Michael Austen/Alamy

Rustler Yachts also uses lead instead of iron for their keels.

The Rustler 36 long keel (designed by Holman and Pye and winner of the 2018 Golden Globe Race) has a cutaway forefoot to improve responsiveness and manoeuvrability.

The long keel creates more drag but, as with the Rustler 24, the cutaway forefoot makes the 36 more nimble than a full long keel boat, which are more difficult to manoeuvre in reverse under power.

The rest of Rustler’s offshore range – the Rustler 37, 42, 44 and 57 – designed by Stephen Jones – have lead fin keels.

As does his Mystery 35 built by Cornish Crabbers.

These offer an excellent combination of directional stability, performance and lateral stability. The yachts track well, are comfortable in choppy seas, and have good manoeuvrability, all without the flightiness of shorter chord fin keels found on many production family cruisers.

A digital future

Influential designer David Thomas said: “When I started designing, I integrated sharp leading edges to the keel; until someone told me a radius was better. Then we were all taught that an elliptical shape was better still. With the advent of computers, designers could better visualise the end-product; and clever ‘faring programs’ speeded this up.”

So where next? A combination of lighter and stronger materials, rapidly developing computer programs, a desire for maximum interior volume and low costs has led us to today’s production yacht.

Twin rudders improve the handling of broad-sterned yachts when heeled.

The IRC rating rule permits low CG keels, wider beam and near-vertical bows and sterns.

And designers now have an array of new computer tools at their disposal. But maybe there’s still that element of black magic?

As David Thomas so succinctly said: “You can design a yacht 95% right, but the last 5% can be down to luck.”

Keel types : the pros and cons

Full length keel

The Fisher 31 and many motor-sailers have long keels. Credit: Peter Poland

Pros: Directional stability. Heavy displacement leading to comfort at sea.

Cons: Poor windward performance. Large wetted surface leads to drag. When under power at low speeds, the turning circle is wide unless fitted with thrusters. The same applies to manoeuvring astern.

Cutaway modified long keel form with keel-hung rudder

Pros: Reduced wetted surface area leading to increased boat speed. Better windward performance and handling than full length keel. Rudder on the aft end of the keel improves self-steering ability on some designs.

Cons: Under engine, this keel form has a large turning circle ahead and poor control astern. Since the rudder is not ‘balanced’, the helm on some designs can feel quite heavy.

Fin keel with skeg-hung rudder

The skeg gives protection to the rudder. Credit: Graham Snook/Yachting Monthly

Pros: The further reduction in wetted surface area leads to more boat speed. Directional stability and close-windedness are also improved. If full depth, the skeg can protect the rudder against collision damage.

Cons: When combined with a narrow stern, this keel format can induce rolling when sailing dead downwind in heavy winds.

Fin keel with separate spade rudder

Fin keel with spade: Low wetted surface and aerofoil shapes enhance performance. Credit: Graham Snook/Yachting Monthly

Pros: The fin and spade rudder mix reduces wetted surface and gives a more sensitive helm – especially if the blade has ‘balance’ incorporated in its leading edge. Handling under power in astern is precise and the turning circle is small.

Cons: The rudder is fully exposed to collisions. There are no fittings connecting the rudder to a keel or skeg, so the rudder stock and bearings need to be very robust.

Shallow stub keel with internal centreplate.

Pros: When lowered, the plate gives good windward performance. The plate can act as an echo sounder in protected shallow water. There is normally no internal centreplate box to disrupt accommodation. With the plate raised, off-wind performance is good.

Cons: The plate lifting wire needs regular inspection and occasional replacement. Windward performance with the plate raised is poor.

Lifting or swing keel

Boats with lifting keels tend to surf earlier downwind. Credit: Graham Snook/Yachting Monthly

Pros: Shallowest draught so more cruising options; can also be moored on cheaper moorings. Surfs early downwind. Small wetted surface so can be fast.

Cons: Reduced living space due to internal keel box. With a raised keel, poor directional control. Susceptible to hull damage if grounding on hard material.

Twin or bilge keel

Bilge- or twin-keelers can take the ground on the level. Credit: Graham Snook/Yachting Monthly

Pros: Can take the ground in a level position. Modern twin-keel designs with around 15º splay, around 2º toe-in and bulbed bases perform well upwind. Good directional stability due to the fins. Modern twin keels with bulbed bases lower the centre of gravity.

Cons: Older designs do not point upwind well. Slapping sound under windward keel when at a steep angle of heel on older designs. Antifouling between the keels can be tricky. Can be more expensive than fin keels.

Wing keel: A low centre of gravity gives a good righting moment. Credit: Graham Snook/Yachting Monthly

Pros: Low centre of gravity means good righting moment. Shallow draught. Sharper windward performance.

Cons: Larger surface area means it is more likely to pick up fishing gear, like lobster pots. Difficult to move once it is grounded. And difficult to scrub keel base when dried out alongside a wall.

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Basics of Sailboat Hull Design – EXPLAINED For Owners

There are a lot of different sailboats in the world. In fact, they’ve been making sailboats for thousands of years. And over that time, mankind and naval architects (okay, mostly the naval architects!) have learned a thing or two.

If you’re wondering what makes one sailboat different from another, consider this article a primer. It certainly doesn’t contain everything you’d need to know to build a sailboat, but it gives the novice boater some ideas of what goes on behind the curtain. It will also provide some tips to help you compare different boats on the water, and hopefully, it will guide you towards the sort of boat you could call home one day.

Table of Contents

Displacement hulls, semi displacement hulls, planing hulls, history of sailboat hull design, greater waterline length, distinctive hull shape and fin keel designs, ratios in hull design, the hull truth and nothing but the truth, sail boat hull design faqs.

Basics of Hull Design

When you think about a sailboat hull and how it is built, you might start thinking about the shape of a keel. This has certainly spurred a lot of different designs over the years, but the hull of a sailboat today is designed almost independently of the keel. 

In fact, if you look at a particular make and model of sailboat, you’ll notice that the makers often offer it with a variety of keel options. For example, this new Jeanneau Sun Odyssey comes with either a full fin bulb keel, shallow draft bulb fin, or very shallow draft swing keel. Where older long keel designs had the keel included in the hull mold, today’s bolt-on fin keel designs allow the manufacturers more leeway in customizing a yacht to your specifications.

What you’re left with is a hull, and boat hulls take three basic forms.

• Displacement hull
• Semi-displacement hulls
• Planing hulls

Most times, the hull of a sailboat will be a displacement hull. To float, a boat must displace a volume of water equal in weight to that of the yacht. This is Archimedes Principle , and it’s how displacement hulled boats get their name.

The displacement hull sailboat has dominated the Maritimes for thousands of years. It has only been in the last century that other designs have caught on, thanks to advances in engine technologies. In short, sailboats and sail-powered ships are nearly always displacement cruisers because they lack the power to do anything else.

A displacement hull rides low in the water and continuously displaces its weight in water. That means that all of that water must be pushed out of the vessel’s way, and this creates some operating limitations. As it pushes the water, water is built up ahead of the boat in a bow wave. This wave creates a trough along the side of the boat, and the wave goes up again at the stern. The distance between the two waves is a limiting factor because the wave trough between them creates a suction. 

This suction pulls the boat down and creates drag as the vessel moves through the water. So in effect, no matter how much power is applied to a displacement hulled vessel, it cannot go faster than a certain speed. That speed is referred to as the hull speed, and it’s a factor of a boat’s length and width. 

For an average 38 foot sailboat, the hull speed is around 8.3 knots. This is why shipping companies competed to have the fastest ship for many years by building larger and larger ships.

While they might sound old-school and boring, displacement hulls are very efficient because they require very little power—and therefore very little fuel—to get them up to hull speed. This is one reason enormous container ships operate so efficiently. 

Of course, living in the 21st century, you undoubtedly have seen boats go faster than their hull speed. Going faster is simply a matter of defeating the bow wave in one way or another.

One way is to build the boat so that it can step up onto and ride the bow wave like a surfer. This is basically what a semi-displacement hull does. With enough power, this type of boat can surf its bow wave, break the suction it creates and beat its displacement hull speed.

With even more power, a boat can leave its bow wave in the dust and zoom past it. This requires the boat’s bottom to channel water away and sit on the surface. Once it is out of the water, any speed is achievable with enough power. 

But it takes enormous amounts of power to get a boat on plane, so planing hulls are hardly efficient. But they are fast. Speedboats are planing hulls, so if you require speed, go ahead and research the cost of a speedboat . 

The most stable and forgiving planing hull designs have a deep v hull. A very shallow draft, flat bottomed boat can plane too, but it provides an unforgiving and rough ride in any sort of chop.

If you compare the shapes of the sailboats of today with the cruising boat designs of the 1960s and 70s, you’ll notice that quite a lot has changed in the last 50-plus years. Of course, the old designs are still popular among sailors, but it’s not easy to find a boat like that being built today.

Today’s boats are sleeker. They have wide transoms and flat bottoms. They’re more likely to support fin keels and spade rudders. Rigs have also changed, with the fractional sloop being the preferred setup for most modern production boats.

Why have boats changed so much? And why did boats look so different back then?

One reason was the racing standards of the day. Boats in the 1960s were built to the IOR (International Offshore Rule). Since many owners raced their boats, the IOR handicaps standardized things to make fair play between different makes and models on the racecourse.

The IOR rule book was dense and complicated. But as manufacturers started building yachts, or as they looked at the competition and tried to do better, they all took a basic form. The IOR rule wasn’t the only one around . There were also the Universal Rule, International Rule, Yacht Racing Association Rul, Bermuda Rule, and a slew of others. 

Part of this similarity was the rule, and part of it was simply the collective knowledge and tradition of yacht building. But at that time, there was much less distance between the yachts you could buy from the manufacturers and those setting off on long-distance races.

Today, those wishing to compete in serious racing a building boat’s purpose-built for the task. As a result, one-design racing is now more popular. And similarly, pleasure boats designed for leisurely coastal and offshore hops are likewise built for the task at hand. No longer are the lines blurred between the two, and no longer are one set of sailors “making do” with the requirements set by the other set. 

Modern Features of Sailboat Hull Design

So, what exactly sets today’s cruising and liveaboard boats apart from those built-in decades past? 

Today’s designs usually feature plumb bows and the maximum beam carried to the aft end. The broad transom allows for a walk-through swim platform and sometimes even storage for the dinghy in a “garage.”

The other significant advantage of this layout is that it maximizes waterline length, which makes a faster boat. Unfortunately, while the boats of yesteryear might have had lovely graceful overhangs, their waterline lengths are generally no match for newer boats. 

The wide beam carried aft also provides an enormous amount of living space. The surface area of modern cockpits is nothing short of astounding when it comes to living and entertaining.

If you look at the hull lines or can catch a glimpse of these boats out of the water, you’ll notice their underwater profiles are radically different too. It’s hard to find a full keel design boat today. Instead, fin keels dominate, along with high aspect ratio spade rudders. 

The flat bottom boats of today mean a more stable boat that rides flatter. These boats can really move without heeling over like past designs. Additionally, their designs make it possible in some cases for these boats to surf their bow waves, meaning that with enough power, they can easily achieve and sometimes exceed—at least for short bursts—their hull speeds. Many of these features have been found on race boats for decades.

There are downsides to these designs, of course. The flat bottom boats often tend to pound when sailing upwind , but most sailors like the extra speed when heading downwind.

How Do You Make a Stable Hull

Ultimately, the job of a sailboat hull is to keep the boat afloat and create stability. These are the fundamentals of a seaworthy vessel. 

There are two types of stability that a design addresses . The first is the initial stability, which is how resistant to heeling the design is. For example, compare a classic, narrow-beamed monohull and a wide catamaran for a moment. The monohull has very little initial stability because it heels over in even light winds. That doesn’t mean it tips over, but it is relatively easy to make heel. 

A catamaran, on the other hand, has very high initial stability. It resists the heel and remains level. Designers call this type of stability form stability.

There is also secondary stability, or ultimate stability. This is how resistant the boat is to a total capsize. Monohull sailboats have an immense amount of ballast low in their keels, which means they have very high ultimate stability. A narrow monohull has low form stability but very high ultimate stability. A sailor would likely describe this boat as “tender,” but they would never doubt its ability to right itself after a knock-down or capsize.

On the other hand, the catamaran has extremely high form stability, but once the boat heels, it has little ultimate stability. In other words, beyond a certain point, there is nothing to prevent it from capsizing. 

Both catamarans and modern monohulls’ hull shapes use their beams to reduce the amount of ballast and weight . A lighter boat can sail fast, but to make it more stable, naval architects increase the beam to increase the form stability.

If you’d like to know more about how stable a hull is, you’ll want to learn about the Gz Curve , which is the mathematical calculation you can make based on a hull’s form and ultimate stabilities. 

How does a lowly sailor make heads or tails out of this? You don’t have to be a naval architect when comparing different designs to understand the basics. Two ratios can help you predict how stable a design will be .

The first is the displacement to length ratio . The formula to calculate it is D / (0.01L)^3 , where D is displacement in tons and L is waterline length in feet. But most sailboat specifications, like those found on  sailboatdata.com , list the D/L Ratio.

This ratio helps understand how heavy a boat is for its length. Heavier boats must move more water to make way, so a heavy boat is more likely to be slower. But, for the ocean-going cruiser, a heavy boat means a stable boat that requires much force to jostle or toss about. A light displacement boat might pound in a seaway, and a heavy one is likely to provide a softer ride.

The second ratio of interest is the sail area to displacement ratio. To calculate, take SA / (D)^0.67 , where SA is the sail area in square feet and D is displacement in cubic feet. Again, many online sites provide the ratio calculated for specific makes and models.

This ratio tells you how much power a boat has. A lower ratio means that the boat doesn’t have much power to move its weight, while a bigger number means it has more “get up and go.” Of course, if you really want to sail fast, you’d want the boat to have a low displacement/length and a high sail area/displacement. 

Multihull Sailboat Hulls

Multihull sailboats are more popular than ever before. While many people quote catamaran speed as their primary interest, the fact is that multihulls have a lot to offer cruising and traveling boaters. These vessels are not limited to coastal cruising, as was once believed. Most sizable cats and trimarans are ocean certified.

Both catamarans and trimaran hull designs allow for fast sailing. Their wide beam allows them to sail flat while having extreme form stability. 

Catamarans have two hulls connected by a large bridge deck. The best part for cruisers is that their big surface area is full of living space. The bridge deck usually features large, open cockpits with connecting salons. Wrap around windows let in tons of light and fresh air.

Trimarans are basically monohulls with an outrigger hull on each side. Their designs are generally less spacious than catamarans, but they sail even faster. In addition, the outer hulls eliminate the need for heavy ballast, significantly reducing the wetted area of the hulls. 

Boaters and cruising sailors don’t need to be experts in yacht design, but having a rough understanding of the basics can help you pick the right boat. Boat design is a series of compromises, and knowing the ones that designers and builders take will help you understand what the boat is for and how it should be used. 

What is the most efficient boat hull design?

The most efficient hull design is the displacement hull. This type of boat sits low in the water and pushes the water out of its way. It is limited to its designed hull speed, a factor of its length. But cruising at hull speed or less requires very little energy and can be done very efficiently. 

By way of example, most sailboats have very small engines. A typical 40-foot sailboat has a 50 horsepower motor that burns around one gallon of diesel every hour. In contrast, a 40-foot planing speedboat may have 1,000 horsepower (or more). Its multiple motors would likely be consuming more than 100 gallons per hour (or more). Using these rough numbers, the sailboat achieves about 8 miles per gallon, while the speedboat gets around 2 mpg.

What are sail boat hulls made of?

Nearly all modern sailboats are made of fiberglass. 

Traditionally, boats were made of wood, and many traditional vessels still are today. There are also metal boats made of steel or aluminum, but these designs are less common. Metal boats are more common in expedition yachts or those used in high-latitude sailing.

Matt has been boating around Florida for over 25 years in everything from small powerboats to large cruising catamarans. He currently lives aboard a 38-foot Cabo Rico sailboat with his wife Lucy and adventure dog Chelsea. Together, they cruise between winters in The Bahamas and summers in the Chesapeake Bay.

• Mastering the Depths: The Sailboat Keel Guide

The keel of a sailboat is more than just an architectural feature; it's the foundational element that ensures stability, performance, and the directional integrity of the vessel. Acting as the backbone of the boat, it runs longitudinally along the bottom, playing a crucial role in preventing the boat from being blown sideways by the wind. But the keel is not just about stability; it's intricately linked to how a sailboat interacts with the water and wind, affecting everything from speed to maneuverability. Diving deeper into the specifics, the variety in keel designs is vast, each tailored to suit different sailing needs and environments. From the slender and deep fin keel, known for its speed and agility, to the bulb keel with its characteristic bulb at the bottom for enhanced stability, the design choices are many. There's also the wing keel, designed for shallow waters, reducing the draft without losing stability, and the full keel, which is excellent for long-distance cruising due to its exceptional directional stability. Not to forget the twin keel, which allows a sailboat to stand upright on low tides and offers remarkable stability in heavy weather conditions.

The Balancing Act: Keel Types and Sailboat Performance

The keel's influence on a sailboat's performance cannot be overstated. It's a balancing act, quite literally, where the keel's design directly impacts the boat's stability, speed, and maneuverability. The choice between a deep keel and a shallow keel, for instance, is a decision that touches upon the very nature of one's sailing. Deep keels excel in open waters, offering unmatched upwind performance, while shallow keels are better suited for navigating coastal areas and shallower bodies of water.

But how does one choose the right keel? It's a combination of understanding the specific sailing environment, the performance expectations, and personal sailing style. Whether it’s for competitive racing or leisurely cruising, the keel must align with the sailor's aspirations and the challenges of the waters they intend to navigate.

Navigating Choices: Selecting the Right Keel and Ensuring Its Care

Selecting the right keel for your sailboat involves a detailed analysis of various factors, including the type of sailing you're interested in and the conditions you expect to face. Each keel type brings its own set of advantages and challenges. The maintenance and care for sailboat keels are equally important. Regular checks, cleaning, and upkeep are essential practices that extend the longevity and preserve the performance of the keel, preventing long-term issues that could hamper your sailing experience.

Read our top notch articles on topics such as sailing, sailing tips and destinations in our Magazine .

Check out our latest sailing content:

The voyage of evolution: from simple structures to sophisticated designs.

The history of sailboat keels is a fascinating journey of evolution, showcasing human ingenuity and the relentless pursuit of better, more efficient designs. This evolution is not just about improving performance but also about adapting to the changing environmental conditions and the growing demands of the sailing community. Innovations in sailboat keel designs continue to emerge, driven by a desire to enhance efficiency, reduce environmental impact, and push the boundaries of what's possible on the water.

Charting the Future: Innovations and Trends in Sailboat Keel Design

The future of sailboat keel design is as promising as it is exciting, with new advancements focusing on optimizing performance, eco-friendliness, and adaptability to diverse sailing conditions. These innovations are not just about pushing the limits of speed and agility but also about ensuring that sailing remains a sustainable and accessible sport for generations to come.

The keel is a testament to the intricate dance between form and function, a crucial component that defines a sailboat's relationship with the sea. Understanding the various types of keels, their functions, and how they impact sailboat performance is essential for every sailor. As we look to the horizon, the continuous innovation in keel design promises a future where sailing is not only about embracing the wind and waves but doing so with an eye towards efficiency, sustainability, and harmony with the marine environment.

So what are you waiting for? Take a look at our range of charter boats and head to some of our favourite  sailing destinations.

I am ready to help you with booking a boat for your dream vacation. Contact me.

Denisa Kliner Nguyenová

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• Sailboat Reviews

A Look at Sailboat Design: Fin Keels vs. Full Keels

Details like keel design count when considering cruising sailboats..

Photos by Ralph Naranjo

When a keel tears away from a sailboats hull, it makes the loss of a rig or rudder seem like a minor inconvenience. History shows that its an uncommon occurrence, but because we now annually hear of such incidents, weve decided to take a closer look at keels and see what keeps the ballast where it belongs.

The International Sailing Federation (ISAF) Offshore Special Regulations devotes pages to helping sailors prevent and respond to a crew overboard incident. There is nothing about how to handle the loss of a keel or ballast bulb. Some might say this is because such occurrences are so infrequent, while others note that, if youre still upright once the ballast breaks off theres not much you can do other than blow the sheets, douse the sails as quickly as possible and attempt to stop any leaks.

When solo sailor Mike Plants Open 60 Coyote lost her lead bulb in 1992, Mike was lost at sea. Other adventure-sailors have survived near instantaneous capsize precipitated by keel loss. In 2003, round-the-world racer Tim Kent and his crew capsized when Everest Horizontal lost its ballast on the way back from Bermuda. US Sailing Safety at Sea Committee Chairman Chuck Hawley was aboard the racing sloop Charlie, on the way back from Hawaii, when a loud groaning sound led to a deep heel as the lead peeled away from the keel bolts and ballast headed straight to the bottom. This encounter at least had a happy ending thanks to the crews quick actions to douse sail. Apparently the keel had been cast with too little antimony (an additive that causes lead to become a harder alloy). The point here is that keeping the keel attached is as important as keeping the crew safely on board. And for the offshore monohull sailor, preventing a keel loss, like preventing crew overboard, requires some informed forethought.

A ballast keel on a sailboat is a classic example of potential energy poised in a balancing act. The buoyancy of the hull itself offsets the effect of thousands of pounds of lead or iron. At rest, gravitys attraction for the dense material strains against the buoyancy of the hull, and the adjacent garboard region is continuously in tension. Few sailors spend much time contemplating how keel bolts corrode and what cycle-loading does to the resin matrix comprising the garboard region just above the ballast. What is apparent, is that the attachment material, whether it be wood, metal or fiber reinforced plastic (FRP), must be able to support a mass of metal weighing as much as a small truck-and do so day in and day out for decades.

Underway, every tack causes the rig and sailplan to try to lever this ballast package free from the hull. And when the helmsman starts daydreaming about lobster for dinner and wanders off course onto a granite ledge Down East, the keel designed to handle sailing loads takes it on the chin. Its easy to see why experienced designers and builders lose sleep over their decisions about keel shape, structure, and what kind of safety factor should be built into the structure.

Its surprising to discover that with better materials and computer-aided design, we still hear about incidents such as the Rambler capsize in the 2011 Fastnet Race (PS, May 2012). Just as significant is a spate of smaller race boat keel-ectomies that have caused ISAF to send out a cautionary note to sailors around the world, and introduce new structural standards for race boats. Keeping the ballast attached to the boat involves an awareness of a chain-like set of failure points. And one of the most difficult decisions each designer must make is how to marry foil efficiency with a structural safety margin that covers the boats intended usage and the unintended use of the keel as a depth sounding device.

For decades, engineers and naval architects have had to contend with some racing sailors Icarus-like quest-a trend that prioritizes shedding weight and making the keel foil a long, thin appendage with a high-aspect ratio. Though not quite a flight toward the sun with wings made of wax and feathers, some race-boat scan’tlings walk a fine line between lightweight and structural failure. The challenge lies in attaching a lead bulb on a high-tensile steel foil to a lightweight, high-modulus, FRP hull. Interconnecting the dense metallic ballast to the lower-density foam/fiberglass hull structure is a true engineering puzzle. Part of the challenge lies in the dissipation of point loads (confined to a relatively small area) and how to handle the resulting stress risers. A stress riser is the point at which theres an abrupt change in a materials flexibility, such as where a stiff, fin keel meets the more elastic hull bottom. In FRP composites like those found in a balsa-cored hull, stress risers are a likely place for delamination to occur. Over time, these can result in the failure of the FRP composite.

The see-saw effect of the keel counteracting a vessels righting moment is a mathematically predictable energy transfer. Even the effect of groundings such as those that turn hull speed into a dead stop can be quantified. But its the cumulative effect of fatigue (localized structural damage caused by cyclical loading) and corrosion that are harder to pin down.

The term allision refers to hitting a fixed object such as a granite ledge or coral reef. Naval architects analyze the energy transfer and evaluate the stress and strain characteristics that occur. The recognition that the keel-to-hull connection must endure even more punishment than is doled out in heavy-weather sailing episodes is at the heart of how structural specs are devised.

Designers also must consider the jack-hammer-like pounding of a keel on a reef in surf, and realize that there are limits to the abuse a keel and hull can endure. With this in mind, its reasonable to assume that sailboat keels should be built to handle sailing induced loads for decades. It is the extra safety factor built into the boat that defines what happens when the sandbar is a rock pile.

What is harder to anticipate are the unusual encounters that can inflict serious damage to the keel connection. Take, for example, what happens when a sailboats deep fin keel is wedged in a rocky cleft and a good Samaritan with a big powerboat attempts to pivot the sailboat using a line attached to the bow. The distance from the keels vertical centerline to the stem may be 20 feet or more, and with a couple of thousand pounds of bollard pull, the 20-foot lever arm creates a rotary force that can spike to 40,000 foot-pounds or more. This level of torque goes well beyond what most designers and builders model as sailing loads, and its likely to seriously damage the boat.

In plain low-tech talk, extreme fin keels provide a valuable performance edge, but they come with their own set of downsides that every owner needs to be aware of. In essence, the more radical the keel shape, the better the crew must navigate.

A couple of decades ago, PS Technical Editor Ralph Naranjo ran a boatyard and had a client who liked to cut the corners during Block Island Race Week. His first spinnaker reach into a granite boulder stopped the boat and shoved the companionway ladder upward six inches. This underscored how an allision that causes the keel to stop abruptly transfers a shock wave through the entire hull. The resulting compression cracked several transverse members in the New York 40 and damaged the core in the canoe body near the garboard.

The FRP repairs had to be tapered and all delamination problems resolved. The moderate-aspect-ratio lead fin keel absorbed a good deal of the blunt trauma. Judging from the cannonball-size dent on the leading edge of the lead keel, it was clear that the impact was significant. The dent offered grim proof of the advantage of having soft lead instead of steel as keel ballast. New floor frames were added, the broken transverse members were replaced, and the boat was off and sailing.

The next season, the boat had another Block Island encounter, and only because the Petersen-designed New York 40 was a pretty ruggedly built boat was a second repair even considered. This time, an equally violent keel-to-hull trauma came from an on-the-wind encounter with a different rock. The extent of the delamination was greater than it had been in the first go round, and more extensive core removal and repair was required. The keel was dropped in order to check the bolts and the garboard. With the bilge fully opened for the FRP repair work, the repair crew made a pattern of the canoe body dead rise and fore and aft contour. As the glass work was being completed, they fabricated a stainless-steel grid that would spread keel loads fore and aft as well as athwartship. The new grid reinforced the keel attachment and returned the sloop to the race course.

Afterward, Naranjo and the owner discussed the details of the repair, including the possibility of hidden, widespread damage from the two groundings. These included the dynamic loads imposed upon the chainplates and rigging, the likelihood of hidden resin-cracking, and potential for more delamination and core shear linked to the torque induced by the accident. In short, any serious allision causes overt and hard-to-detect damage far from the actual impact zone, and these can lead to more problems down the road. When buying a used boat, look for a good pedigree, but also look for signs of previous blunt-force trauma. A good surveyor will be skilled in such structural forensics, and he or she will do more than comment on the gelcoat shine.

In the early days of wooden ships and iron men, a lack of dense metal ballast put less point-loading in the garboard region of the hull. Bilges free of cargo were filled with rocks or tighter-fitting granite blocks cut for more compact stacking. The principal of ballasting a vessel was to lower her center of gravity (CG) and create both an increase in the righting arm and a greater righting moment to offset the heeling moment created by the rig and sail plan. The keel also helped lessen leeway and would evolve into an appendage that added lift.

Movable ballast had a few downsides, not the least of which was its propensity to move in the wrong direction at the very worst moment. Even small boat sailors have found out what can happen to unsecured pigs of lead ballast when the boat heels far enough over for gravity to overcome friction. Whether stones, lead, movable water ballast, or a can’ting keel are used to augment the boats righting moment, a sailor must anticipate the worst-case scenario. This is when the weight ends up on the leeward side of the boat and a bad situation can turn into a real catastrophe. Fixing or locking ballast in place, controlling the volume of water put in ballast tanks, and limiting the can’ting keels range are sensible compromises.

Internal ballast, the ballast inside a keel envelope thats contiguous with the hull, is still seen in many new boats. Island Packet is an example of a builder has stuck with this traditional approach of securing ballast without using keel bolts. Its a sensible design for shoal-draft cruisers, and the upsides are numerous. These high-volume, long-range cruisers arent encumbered by the demands prioritized by light displacement, performance-oriented sailors. Instead, Island Packets combine a rugged laminate and a long-footed, shallow-draft keel. This may not place the lead or iron ballast as deep as the tip of a fin keel, but it does keep the all-important CG low enough to deliver a powerful righting moment along with shoal draft.

In order to deliver the high angle of vanishing stability (AVS) also known as limit of positive stability (LPS), designer Bob Johnson puts what amounts to an internal bulb in the very lowest point in the boat. This long slug of iron or lead (depending on the model) is then covered by Portland cement, locking it in the Island Packets monocoque structure. The result is a contiguous FRP structure spreading keel loads efficiently over a considerable amount of hull skin. Keel bolts and the infamous garboard seam are completely eliminated. This approach to sailboat keel design dates back to the Rhodes Bounty II and other prototypes in the production world of sailboats. Now over 50 years old, many of these boats continue to have a tenacious grasp on the lead or iron that they hold.

Encapsulated iron ballast is much less desirable than encapsulated lead, and its sad to see builders skimp on this. Iron, or even worse steel, has been used in many Far Eastern encapsulated keels. It works as long as water and the resulting oxidation havent caused expansion and cracking of the seal. Lead is also denser than ferrous metal, and therefore, the same amount of ballast will have a smaller volume and create less drag.

Encapsulated ballast starts to be less appealing as keels become more fin-like and high-aspect ratio. The reason for this is that the geometry of the support changes, focusing more load on less area of the hull. As hull shapes evolved into canoe underbodies with hard turns in the bilge, and fin-like keels became thinner, deeper, and with shorter chord measurements (thickness), the concept of encapsulated keel became impractical. The Cal 40, Ericson 39, Pearson 365, and a long list of similar genre boats signified the end of an era when performance racer/cruisers would be built with encapsulated ballast.

External Ballast

Performance-oriented sailors and race-boat designers quickly latched on to hull shapes marked by deep-draft, foil-shaped, high-aspect ratio fin keels. From the late 60s to whats currently glowing on CAD screens in designer offices around the world, keels have grown deeper and shorter in chord length, and bulb or anvil-like tips have grown more and more common.

The design development was sound, lift was enhanced, and deeper-not longer-became the answer to getting to windward faster. The challenge was not only in designing an efficient shape, it lay in creating an attachment means that minimized foil flex and twist, retained the low drag coefficient, and still had the ability to withstand an occasional, albeit modest, grounding.

During this same period, marine surveyors and boatyard techs began to see moderate groundings result in major structural problems. The classic example was the allision that produced a moderate dent in the lead at the leading edge of the keel tip. In many cases, further inspection revealed cracks radiating outward from a knot meter or depth sounder mistakenly placed just ahead of the keel. An even closer look often revealed grid damage or a cracked bulkhead just aft of the last keel bolt. Like the New York 40 mentioned earlier, this was a result of a shock wave radiating through the hull structure. As we learned in Mrs. McCrearys science class, Bodies in motion tend to stay in motion, unless acted on by an equal and opposite force. Fin keel sailboats encountering abrupt energy transfers,tend to endure more damage than their long-keel counterparts.

A forensic look at the Achilles heel of external ballast highlights a few pitfalls. First the good news: Lead absorbs impact well, consuming much of the imparted energy through deformation. However, the translation of the remaining energy from the metal keel foil and keelbolts into an FRP hull is where we often find stress risers, and point loading linked to material and hull shape changes. The near right-angle interface between a modern sailboats canoe body and its deep fin keel is a classic load-path hotspot. In the old days, fiberglass techs spoke of oil-canning or the dimpling of a large section of the garboard as tacks were swapped.

Today Naval Architects use Finite Element Analysis (FEA) to better engineer hull structure. Colorized graphics pinpoint load concentration, glowing bright red in the region where the keel joins the hull, the epicenter of the oil-canning. A common solution to coping with this high-load focal point, is to eliminate core in the region and to gradually increase the unit schedule (layers of FRP), or to add an internal FRP grid. Maximum thickness of a keel stub is located where the keelbolts penetrate the stub. In this region, the solid glass thickness is often equal to the dimension of the keel bolt diameter or even greater.

Laminate thickness at the keel bolts is only part of the equation. Just as important is how the transition to the general hull laminate transpires. A bullet-proof keel stub that immediately transitions into a core hull comprising two units of laminate on each side of the panel creates whats equivalent to a tear-on-the-dotted-line weakness. Transitions that involve sharp angles and marked differences in panel strength require a well-reinforced taper that spreads loads gradually rather than abruptly.

Occasionally, we see massive metal frameworks used in the bilge as support for keel bolts; these structures need to be carefully engineered to not create the same hard spot fracture points. When carefully tapered in order to gradually introduce more flex, the problem is abated, as it was in the repair of the New York 40 mentioned earlier. The stainless-steel grid built to support the keel loads incorporated a gradual decrease in stiffness to the framework. The keel was carefully mated to the underside of this grid to ensure full contact (See Keel Bolt Repair Options, online). As a result, the crew relieved the hard spots at the end points and made the transition to the more flexible FRP hull less dramatic.

For cruisers, the take-away lesson is that extra reinforcement, a long garboard keel-to-hull interface, and internal transverse and longitudinal reinforcement really do pay off. Keep in mind that the extra weight this entails is all below the center of gravity and contributes to the secondary righting moment as well as keeping the water out.

This is a big departure from the way many modern production boats are built. They carry a skimpy ballast ratio of 30 percent or less, have less structure to support the keel and are not designed to handle unintended cruising consequences. There are exceptions, and its worth looking at the keel design and structure of the Navy 44 Mark II and the USCG Leadership 44 (see PS, August 2012). These boats utilize external ballast and are examples of rugged keel attachment. They have a relatively long keel-to-stub garboard junction, the laminate scan’tling meets American Bureau of Shipping recommendations, and both utilize an overabundance of 316 stainless-steel keel bolts and an FRP grid to keep the keel where it belongs.

There are many reasons why were seeing more keel problems today. On one hand, light, fast, race-boat design pushes the envelope, and thats probably OK. But when mainstream racer/cruisers start to suffer from lead loss, too much of one good thing (high-aspect ratio) and too little of another good thing (reinforcement) can begin creeping into design and construction.

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Keel design: What’s best?

Posted by Ted Brewer | Boat Reviews

Ted Brewer reviews the ins and outs and ups and downs of keel design

The purpose of a keel, fin, or centerboard is to provide resistance to making leeway; in effect, to keep the yacht from sliding sideways through the water due to wind pressure on the sails. Various shapes of underwater plane have been in and out of style over the past 150 years.

The highly stylized shark fin has extreme rake and a sloping tip chord.

The basic full-keel shape had the longest run, as it was the standard for bluewater sailing craft from pre-Roman times to the earliest days of yachting. The deep, full keel was supplemented in the mid-1800s, for the shoalwater areas of Britain and North America, by centerboard craft. These cover such working types as the sharpies, Cape Cod catboats , and Chesapeake Bay oyster skiffs, to mention a few.

The first truly modern keel yacht, with a cutaway forefoot and highly raked rudder post, was designed by Capt. Nathanael Herreshoff with his Gloriana design of 1891. But it did not catch on for bluewater sailing. Until the late 1920s, the typical offshore yacht, whether cruiser or ocean racer, resembled a sailing fishing craft in the shape of its lateral plane: a long, full keel with deep forefoot and fairly vertical sternpost. Such a shape has the benefits of good directional stability, ease of steering, and the ability to heave to in heavy weather, all desirable traits for a boat. However, its faults may include slowness in stays, excess wetted surface making it slower in all types of air and an inefficient lateral plane shape that has excess leeway, considering its relatively large area. Typical small yachts of this type are seen today in the Colin Archer types and the Tahiti ketch and its copies, while replicas of traditional sailing craft such as Bristol Channel Cutters, Friendship sloops, fishing and pilot schooners, and similar lovely vessels still appear in our waters. Fortunately, many of these workboat types have been developed to the point where the ills of the true full keel have been greatly reduced. Then the result is a handsome cruiser that sails quite well and attracts a great deal of attention wherever she drops her hook.

Successful Sailboat Keel Types

The cutaway keel was revived for ocean racing by Olin Stephens in the late 1920s, with his lovely yawl, Dorade, still sailing and winning classic yacht races more than 70 years after her launching. Her offshore racing successes finally proved that the full keel was not essential to seaworthiness, and it definitely detracted from speed and weatherliness. As a result of its improved performance and handiness, the “modified full keel” form caught on quickly once Dorade showed the way and became the standard for the next 35 years. This type of lateral plane is still sailing in many popular older designs such as the Albergs, the Folkboat, the Luders 33, the Whitby 42, and even some newer yachts.

The modified full-keel form features generally good handling and directional stability plus reduced wetted surface, compared to her true full-keel sister. The yachts can perform well in all conditions and, as they are generally of heavier displacement than contemporary ballasted-fin boats, they do not give away much in light air, despite the added wetted area. A yacht with a modified full keel can sail right up with the best of them if she is given sail area commensurate with her typically heavier displacement.

In my own work, I developed a modified full keel, with the rudder set aft and vertically in the contemporary fashion, in order to improve directional stability and handiness. Then, to reduce wetted area, the lateral plane is substantially cut away ahead of the rudder in what some have termed “the Brewer bite.” The Cabot 36 and Quickstep 24 of my design were early examples of this form. The size of the cutout depends to a large degree on how insistent my client is on having a “full keel,” and I try to make the cutout as large as I can decently get away with. I don’t claim to have originated the shape, though, as the late L. Francis Herreshoff used a not dissimilar profile many years earlier in the design of the lovely 57-foot ketch, Bounty.

Taken to Extremes

Like all good things, the modified full keel was cut away more and more for bluewater and inshore racers in an attempt to reduce wetted area until, finally, some designers took it to extremes. This reduced directional stability and produced craft that were almost impossible to steer in breezy conditions, broaching with monotonous regularity. I can recall working on the design of many short-keel 5.5-Meter yachts in the 1960s, and we always said they were three-man boats with six-man spinnakers! It’s hard to believe none of them were knocked down and sunk, as they were extremely difficult to control on a reach or run, and the hulls were pure leadmines, with 3,500 pounds of ballast in their very short keel and only 1,000 pounds of wood and rig above it!

Olin Stephen’s genius began another fad in the mid 1950s, the keel-centerboard design. After Finisterre showed the way, keel-centerboard yawls were built in sizes from 24-foot midget ocean racers, to the largest offshore yachts, in order to take advantage of favorable ratings under the CCA rule and emulate Finisterre’s record of wins. The keel-centerboard hull has gone out of fashion now, but the type still has merit where a stable, beamy, shoal-draft yacht is desired with little sacrifice of weatherliness or seaworthiness. Indeed, the Bill Tripp-designed Block Island 40 and Bermuda 40 are keel-centerboard ocean racers from the old school and have been in production for more than 30 years now. These classic yachts have made many long ocean voyages, including several world circumnavigations and are first-class bluewater cruisers in every respect.

Keel Types Here to Stay

A rather squared-off fin, not unlike the Cal 40 keel.

The fin shape is not new either, as ballasted fin yachts were pioneered by Herreshoff at the turn of the century for inshore racing. Then, due to excesses and bad design, the shape died out, except for a few one-design classes, until Bill Lapworth dropped a bomb on the ocean-racing scene in the mid-1960s with his Cal 40 design. The Cal 40s made believers out of many yachtsmen who could not believe that a ballasted-fin/spade-rudder yacht was a serious bluewater ocean racer. After wins in the Trans-Pac, many East Coast races, and the 1966 Bermuda Race, it became evident that the fin was here to stay for ocean-going and coastal cruising yachts. Please note that I do not use the term “fin keel” anymore, as I feel it is a misnomer. The keel is the structural backbone of the vessel, and the fin hangs from it. Fish have both backbones and fins; so do yachts.

A less extreme fin keel, with a more parallel tip.

A well-designed fin, in conjunction with a skeg-hung rudder, can provide excellent directional stability, handiness, reduced wetted area and improved weatherliness. The fin/spade rudder combination reduces wetted surface even more. It may have a little (or a lot) more sensitive helm than a fin/skeg rudder yacht, but it has one big advantage over it and all other forms of lateral plane: it can be steered in reverse under power. This can make life a great deal easier in today’s crowded marinas, as many have discovered.

These are some of the reasons that we see fins on the great majority of our new yachts today; they are not simply a fad. There are good fins and bad fins, of course, and it is not always easy to tell them apart. The shape of fins over the years has been limited only by the designer’s imagination. Fins have been set at every angle from the vertical to highly raked aft. They have been deep and narrow, shoal and long, resembling a shark’s fin or whale’s tail, or boxy fins similar to the original Cal 40 design.

A contemporary bulb fin with winglets.

Major Problem

A very deep, narrow fin can be a problem to haul on a marine railway, so the cruising skipper should consider haulout ease when boat shopping. A crane or travel lift is the best method for hauling yachts with extreme fins, but may not always be available in out-of-the-way areas. There is also the danger of damage to the shaft or strut if slings are improperly positioned. Still, the major problem of the high-aspect-ratio fin is structural strength, as it can impose extreme loads at the point of attachment to the keel. Indeed, some years ago I was an “expert witness” in a court case concerning three men who drowned when their yacht sank as a result of its fin tearing off when the vessel ran aground.

The cruising skipper would do well to avoid yachts with extreme fins, both for considerations of haulout ease and structural strength. Fortunately, the heavier, deeper hull and generally shoaler draft of the typical cruising yacht mean there is less height available between the bottom of the hull and the point of maximum draft. So, a longer, lower-aspect-ratio fin is the only solution. On the other hand, the racing sailor will want a fin with an aspect ratio as high as the draft rule will allow. Such a fin is more efficient per square foot, so the area can be smaller and the wetted surface reduced. In Aero-Hydrodynamics of Sailing, C.A. Marchaj recommends about 4 percent of the sail area as a good guide for fin area, and I feel the cruiser should err on the high side, as a small increase in resistance is preferable to increased leeway. On the other hand, I have used as low as 1.75 percent area with good results on an extreme racer with a fin of 2.75 aspect ratio.

Sailboat Keel Aspect Ratios

This “aspect ratio” is the ratio of the span (depth) squared to the fin area; that is, my extreme fin had an 11-foot span and 44 square feet of area, so its aspect ratio was 121/44, or 2.75. If it had a 4-foot span with 44 square feet of area, not uncommon proportions for a cruising yacht, its aspect ratio would be 16/44, or a low 0.3636.

The aspect ratio can also be described as the span divided by the mean chord, the average fore-and-aft length of the fin, and this gives the same result.

A large part of the resistance of a keel is created by the vortices, similar to miniature whirlpools that form when the water flows across the bottom of the keel from the high-pressure (leeward) side to the low-pressure (windward) side. It requires energy to form those vortices and that energy is then not available to propel the boat forward. Obviously, the shorter the keel or fin tip, the smaller and weaker those vortices will be, and that translates to reduced resistance. This is one reason that racing yachts usually feature high-aspect-ratio fins with short tip chords.

However, the formation of vortices can be greatly reduced by using end plates, or wings, to change the flow direction and eliminate crossflow. My own preference, for a fin of average span, is for an end plate that is but a few inches wider than the maximum width of the fin bottom. We tested an actual yacht with such an end plate on one side only and noted a substantial improvement in performance when she was heeled so that the end plate was on the leeward side. Where the draft is shoal and the fin span is on the small side, then a wider end plate, or even a wing, might prove beneficial. However, a wide wing can be a structural weakness, particularly if the boat goes hard aground and has to be towed off, or pounds on the rocks for any length of time.

Sweepback Angles

In the 1970s, I saw more than one very-high-aspect-ratio fin with tremendous sweepback angle. This certainly gives an impression of speed but, as Marchaj pointed out, tank tests have shown that the sweepback angle can be related to the aspect ratio: the higher the aspect ratio, the more vertical the fin should be. Indeed, the very-high-aspect-ratio fin on my BOC racer was set absolutely plumb until a hard grounding set the tip back a quarter inch or so, the result of taking a yacht with a 13-foot draft through a channel dredged to 11 feet! Most cruising-yacht fins are of low aspect ratio, of course, so should have substantial sweepback, up to 57 degrees, with an aspect ratio of 0.5, according to Marchaj. Although most designers try, it is unfortunate that obtaining the perfect sweepback angle is secondary to locating the fin to balance the sailplan, as well as fitting the ballast at the correct spot for proper fore and aft trim. The taper ratio (tip chord length/root chord length) also deserves consideration. Tests on one series of fins showed that a fin with 0.32 taper ratio was 1 percent more efficient than an untapered fin and had very slightly less resistance. This is a small difference but cannot be ignored by the racing skipper. Again, the reduction in drag may be due to reduced vortices from the shorter tip chord. Marchaj also states that the taper ratio should be reduced as the sweepback angle increases. However, the very-low-taper-ratio fins may not be the best solution for a cruising yacht. The tip chord should be long enough so the vessel can be hauled on a marine railway with no major problems. Too, on a moderate-draft cruising yacht, a short tip chord forces the ballast higher, so stability can suffer.

Lower Ballast

Another consideration in the fin profile is whether the tip chord is sloped down aft or parallel to the waterline. The parallel tip chord makes good sense. It allows the ballast to be lower for added stability, it eases blocking up the boat when hauling and, fortunately, tests have shown that it is also superior to the sloped tip chord in other ways. Having the aft edge of the tip chord deeper than the leading edge has no practical effect on aspect ratio, and such a fin develops less lift and more drag than one with a parallel tip.

The National Advisory Committee for Aeronautics (NACA) tested a large variety of streamlined shapes for lift and resistance and the information on these is available in a book, Theory of Wing Sections, by Abbot and Von Doenhoff. These are the shapes that designers refer to when they say their new magic fin has an NACA section. Generally, the shape selected will be similar to NACA 0010-34 or 0010-64 series. The leading edge will be elliptical, as a blunted nose increases resistance while a pointed leading edge promotes stalling. The maximum width will be about 40 to 50 percent aft, and the shape will be streamlined to a fairly sharp (but not razor-sharp) trailing edge. The thickness ratio will be 0.8 to 0.12 of the chord length, although this may be increased to 0.15 to 0.16 at the tip chord. There are advantages to having an increase in thickness ratio at the tip chord, including being able to fit the ballast lower. This need not mean that the fin is bulbed, though. For example, a fin that is 8 feet long at the root and 5 feet long at the tip may have a 0.10 thickness (0.8 feet) at the root and 0.15 thickness (0.75 feet) at the tip. The fin is still slightly thinner at the bottom than at the top, but the thickness ratio has increased.

Increased Resistance

It is not uncommon to see fins wider than 10 to 12 percent of their length, as the designer may need to fatten the fin in order to locate the ballast in the correct spot for proper trim. Very shoal-draft boats may require fatter keels or fins in order to get the ballast as low as possible for stability. Still, extra width does increase resistance so there is a tradeoff; added stability increases performance while a thicker fin reduces performance. Thirty-five years ago, when I worked for Bill Luders, we tank-tested dozens of 5.5-Meter models. These very short-keeled 30-foot sloops had a minimum keel width of 4 inches under the rule, and whenever we tried a model with a wider keel in order to get the ballast lower, we found that overall performance suffered.

We also tested a number of bulb keels on the 5.5 models but they never proved out in the tank, either, although several different shapes were tried. Then, in the late 1970s, I tank-tested the model of the new Morgan 38 at Stevens Institute, first with a fairly fat NACA fin in order to maintain the desired 5-foot draft, and then with a patented bulb fin that we let its designer draw up, with no stipulation on draft. The bulb saved only 2 inches of draft but showed so poorly against the NACA fin that the 38 was put into production with the more conventional shape.

The tip shape, viewed from ahead, may be flat, round, elliptical, or bulbed. Tests show that the flat, squared-off tip develops a bit more lift to windward and that the round or elliptical tip has less drag on a run. The differences are slight but, today, I favor the squared-off tip with an end plate for yachts of average draft. A vee tip was tried in the 1960s on a few yachts, but never became popular. Bulbs and wings, often in combination, are fairly common on contemporary production boats. Usually, they are an attempt to produce a very shoal-draft yacht for use in waters where the bottom is close to the top and, in those cases, they may make sense.

There is a never-ending variety of fin shapes and, to be honest, I’m not sure which is best. Generally, I prefer a fin similar to the old Cal 40, a little shorter perhaps, and fitted with an end plate. Such a fin provides a desirable combination of good performance, ease of haulout, and structural strength, all very important factors for the cruising skipper.

Article first appeared Good Old Boat magazine: Volume 3, Number 4, July/August 2000 .

About The Author

Ted Brewer is one of North America's best-known yacht designers, having worked on the America's Cup boats, American Eagle and Weatherly, as well as boats that won the Olympics, the Gold Cup, and dozens of celebrated ocean races. He also is the man who designed scores of good old boats, the ones still sailing after all these years.

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The Ultimate Guide to Sail Boat Designs: Exploring Sail Shape, Masts and Keel Types in 2023

• June 4, 2023

When it comes to sail boat designs, there is a wide array of options available, each with its own unique characteristics and advantages. From the shape of the sails to the number of masts and the type of keel, every aspect plays a crucial role in determining a sailboat’s performance, stability, and manoeuvrability. In this comprehensive guide, we will delve into the fascinating world of sail boat designs, exploring the various elements and their significance.

Table of Contents

The sail shape is a fundamental aspect of sail boat design, directly impacting its speed, windward performance, and maneuverability. There are several types of sail shapes, including:

1. Bermuda Rig:

The Bermuda rig is a widely used sail shape known for its versatility and performance. It features a triangular mainsail and a jib, offering excellent maneuverability and the ability to sail close to the wind. The Bermuda rig’s design allows for efficient use of wind energy, enabling sailboats to achieve higher speeds. The tall, triangular mainsail provides a larger surface area for capturing the wind, while the jib helps to balance the sail plan and optimize performance. This rig is commonly found in modern recreational sailboats and racing yachts. Its sleek and streamlined appearance adds to its aesthetic appeal, making it a popular choice among sailors of all levels of experience.

2. Gaff Rig:

The Gaff rig is a classic sail shape that exudes elegance and nostalgia. It features a four-sided mainsail with a gaff and a topsail, distinguishing it from other sail designs. The gaff, a horizontal spar, extends diagonally from the mast, providing additional area for the mainsail. This configuration allows for a taller and more powerful sail, making the Gaff rig particularly suited for downwind sailing. The Gaff rig offers a traditional aesthetic and is often found in vintage and classic sailboats, evoking a sense of nostalgia for a bygone era of maritime exploration. The distinctive shape of the Gaff rig, with its graceful curves and intricate rigging, adds a touch of timeless charm to any sailboat that dons this rig.

3. Lateen Rig:

The Lateen rig is a unique and versatile sail design that has been used for centuries in various parts of the world. It features a triangular sail that is rigged on a long yard, extending diagonally from the mast. This configuration allows for easy adjustment of the sail’s angle to catch the wind efficiently, making the Lateen rig suitable for a wide range of wind conditions. The Lateen rig is known for its ability to provide both power and maneuverability, making it ideal for small to medium-sized sailboats and traditional vessels like dhow boats. Its versatility allows sailors to navigate narrow waterways and make tight turns with ease. The distinctive silhouette of a sailboat with a Lateen rig, with its sleek triangular sail and graceful curves, evokes a sense of adventure and a connection to seafaring traditions from around the world.

Number of Masts

The number of masts in a sail boat design affects its stability, sail area, and overall performance. Let’s explore a few common configurations:

1. Sloop Rig:

The sloop rig is one of the most popular and versatile sail boat designs, favoured by sailors around the world. It consists of a single mast and two sails—a mainsail and a jib. The sloop rig offers simplicity, ease of handling, and excellent performance across various wind conditions. The mainsail, situated behind the mast, provides the primary driving force, while the jib helps to balance the sail plan and improve manoeuvrability. This configuration allows for efficient upwind sailing, as the sails can be trimmed independently to optimize performance. The sloop rig is commonly found in modern recreational sailboats due to its versatility, enabling sailors to enjoy cruising, racing, or day sailing with ease. Its streamlined design and sleek appearance on the water make it both aesthetically pleasing and efficient, capturing the essence of the sailing experience.

2. Cutter Rig:

The cutter rig is a versatile and robust sail boat design that offers excellent performance, especially in challenging weather conditions. It features a single mast and multiple headsails, typically including a larger headsail forward of the mast, known as the cutter rig’s distinguishing feature. This configuration provides a wide range of sail combinations, enabling sailors to adjust the sail plan to suit varying wind strengths and directions. The larger headsail enhances the boat’s downwind performance, while the smaller headsails offer increased flexibility and improved balance. The cutter rig excels in heavy weather, as it allows for easy reefing and depowering by simply reducing or eliminating the headsails. This design is commonly found in offshore cruising sailboats and has a strong reputation for its reliability and seaworthiness. The cutter rig combines versatility, stability, and the ability to handle adverse conditions, making it a preferred choice for sailors seeking both performance and safety on their voyages.

3. Ketch Rig:

The Ketch rig is a sail boat design characterized by the presence of two masts, with the main mast being taller than the mizzen mast. This configuration offers a divided sail plan, providing sailors with increased flexibility, balance, and versatility. The main advantage of the Ketch rig is the ability to distribute the sail area across multiple sails, allowing for easier handling and reduced stress on each individual sail. The mizzen mast, positioned aft of the main mast, helps to improve the sailboat’s balance, especially in strong winds or when sailing downwind. The Ketch rig is often favoured by cruisers and long-distance sailors as it provides a range of sail combinations suitable for various wind conditions. With its distinctive double-mast appearance, the Ketch rig exudes a classic charm and is well-regarded for its stability, comfort, and suitability for extended journeys on the open seas.

The keel is the part of the sail boat that provides stability and prevents drifting sideways due to the force of the wind. Here are some common keel types:

1. Fin Keel:

The fin keel is a popular keel type in sail boat design known for its excellent upwind performance and stability. It is a long, narrow keel that extends vertically from the sailboat’s hull, providing a substantial amount of ballast to counterbalance the force of the wind. The fin keel’s streamlined shape minimizes drag and enables the sailboat to cut through the water with efficiency. This design enhances the sailboat’s ability to sail close to the wind, making it ideal for racing and performance-oriented sailboats. The fin keel also reduces leeway, which refers to the sideways movement of the boat caused by the wind. This improves the sailboat’s ability to maintain a straight course and enhances overall manoeuvrability. Sailboats with fin keels are commonly found in coastal and offshore racing as well as cruising vessels, where stability and responsiveness are valued. The fin keel’s combination of performance, stability, and reduced leeway makes it a preferred choice for sailors seeking speed and agility on the water.

2. Full Keel:

The full keel is a design known for its exceptional stability and seaworthiness. It extends along the entire length of the sailboat, providing a continuous surface that adds substantial weight and ballast. This configuration offers significant advantages in terms of tracking and resistance to drifting sideways. The full keel’s deep draft helps to prevent leeway and allows the sailboat to maintain a steady course even in adverse conditions. Its robust construction enhances the sailboat’s ability to handle heavy seas and provides a comfortable ride for sailors on extended journeys. While full keel sailboats may sacrifice some manoeuvrability, their stability and predictable handling make them a popular choice for offshore cruising and long-distance voyages. The full keel design has stood the test of time and is often associated with classic and traditional sailboat aesthetics, appealing to sailors seeking reliability, comfort, and the ability to tackle challenging ocean passages with confidence.

3. Wing Keel:

The wing keel is a unique keel design that offers a combination of reduced draft and improved stability. It features a bulbous extension or wings on the bottom of the keel, which effectively increases the keel’s surface area. This design allows sailboats to navigate in shallower waters without sacrificing stability and performance. The wings create additional lift and prevent excessive leeway, enhancing the sailboat’s upwind capabilities. The reduced draft of the wing keel enables sailors to explore coastal areas and anchor in shallower anchorages that would be inaccessible to sailboats with deeper keels. The wing keel is particularly well-suited for sailboats in areas with variable water depths or tidal ranges. This keel design offers the advantages of increased manoeuvrability and improved performance while maintaining stability, making it a popular choice for sailors seeking versatility in a range of sailing environments.

In the vast world of sail boat designs, sail shape, number of masts, and keel types play pivotal roles in determining a boat’s performance and handling characteristics. Whether you’re a recreational sailor, a racer, or a cruiser, understanding these design elements can help you make informed choices when selecting a sailboat.

Remember to consider your specific needs, preferences, and intended use of the boat when choosing a sail boat design. Each design has its strengths and weaknesses, and finding the perfect combination will greatly enhance your sailing experience.

By gaining a deeper understanding of sail boat designs, you can embark on your next sailing adventure with confidence and make the most of the wind’s power.

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Welcome to the International Star Class, home of the premier one-design racing keel boat in the world.  For 100 years Star sailors have led the way in advancing the sport of competitive sailing, and have been honored by sailing in the Olympic Games since 1932.  With over 2,200 members and fleets in 38 countries, Star Class championship events are hosted at venues around the globe. At a Star Class regatta you will find Olympic and World Champions competing alongside sailors of all ages and skill levels eager to sail at the highest level of the sport. Join us as we lead the sport of sailing into our next century.

"To be Star world champion I think is the biggest achievement that a sailor can have. To win two times is to be a part of the story of Star class, it means a lot. The Star class is very special. It’s the class of all the biggest names."  

Bruno Prada (BRA) after winning the 2011 World Championship with Robert Scheidt

Star Class History

Dozens, probably hundreds, of classes of racing sailboats have come into existence, lasted a few years, and then dropped out of the yacht racing picture. How does it happen that one class, the International Star, designed in the early 1900s is thriving and is raced today on all levels and in dozens of countries of the world? There is no single answer to this question. Many factors have contributed and still contribute to the phenomenal success and long life of the Star.

If you were to ask a top flight racing skipper why he sails Stars he might tell you that he likes the "high performance" qualities of the boat as a superb racing machine. Or he might point to the calibre of the competition, not only on the World Championship and Olympic level but in many lesser events. The skipper and crew who enjoy a varied program of sailing in different localities would emphasize the quantity and quality of intersectional Star events available all over the world. The Star sailor who never travels, on the other hand, would tell you what fine racing he has at home year after year. The major Class officers, those dedicated souls who have to do with running the organization, meeting and solving the multitude of problems that arise in a class of this magnitude, would tell you that the I.S.C.Y.R.A., the Class Association, is the force that holds the Class together and also the power plant that drives it. And every one of these people, all of whom are members of the Star Class, would emphasize the bond of fellowship that exists world-wide among Star owners and sailors.

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America’s Cup boats: 8 facts about the AC75 and why they’re unique

• Toby Heppell
• August 20, 2024

The America's Cup boats to be used on the 2024 edition of the event are immensely complicated high tech bits of kit. They might be officially sailing craft but they behave in some remarkable ways

The AC75 is the class of boat that takes part in the America’s Cup and are arguably the most radical boats the compeition has ever seen.  This type of America’s Cup boat was first used in the 2021 America’s Cup so this is the second event in which these boats have been used.  

The America’s Cup is, fundamentally, a design competition, and successive America’s Cups have featured the most extreme yachts yet – for their time – ever since the first race in 1851.

However, the foiling boats we have seen in the last four editions of America’s Cup racing (the AC72 and AC50 catamarans, and now the AC75 monohulls) do represent a new direction for the highest level of sailing.

There are plenty who argue that this technology is so far beyond the bounds of what most people consider sailing as to be an entirely different sport. Equally, there are those who believe this is simply a continuation of the development that the America’s Cup has always pushed to the fore, from Bermudan rigs, to composite materials, winged keels, and everything in between.

Good arguments can be made either way and foiling in the world’s oldest sporting trophy will always be a subjective and controversial topic. But one thing is certain: the current America’s Cup boats, the AC75s, are unlike anything seen before and are showcasing to the world just what is possible under sail power alone.

Photo: Ian Roman / America’s Cup

1 Unimaginable speed

Topping the 50-knot barrier used to be the preserve of extreme speed record craft and kiteboarders. A World Speed Sailing Record was set in 2009 of 51.36 knots by Alain Thebault in his early foiling trimaran, Hydroptere , and was bested in 2010 by kite boarder, Alexandre Caizergues who managed 54.10 knots.

Only one craft has ever topped 60-knots, the asymmetric Vestas Sail Rocket 2 , which was designed for straight line speed only and could no more get around an America’s Cup course than cross an ocean. Such records are set by sailing an average speed over the course of 500m, usually over a perfectly straight, flat course in optimum conditions.

America’s Cup class yachts, designed to sail windward/leeward courses around marks, are now hitting speeds that just over a decade ago were the preserve of specialist record attempts, while mid-race. American Magic has been recorded doing 53.31 knots on their first version of the AC75 class, Patriot.

Perhaps even more impressive, in the right conditions when racing we have seen some boats managing 40 knots of boatspeed upwind in around 17 knots of wind. That is simply unheard of in performance terms and almost unimaginable just three or so years ago.

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2 A storm onboard the AC75

Related to the speeds the boats are sailing through the water, particularly upwind, is the wind speeds the sailors will feel on deck.

When sailing, the forward motion affects the wind we experience onboard, known as apparent wind. The oft’ trotted out explanation of how apparent wind works is to imagine driving your car at 50mph. Roll down the window and stick your hand out of it and there will be 50mph of wind hitting your hand from the direction your car is travelling.

So when an AC75 is sailing upwind in 18 knots of breeze at a boatspeed of 40 knots, the crew on deck will be experiencing 40 knots of wind over the decks plus a percentage of the true wind speed – depending on their angle to the wind.

The AC75 crews might be sailing in only 18 knots of breeze – what would feel like a decent summer breeze on any other boat – but they experience winds of around 50 knots.

To put that into context, that is a storm force 10 on the Beaufort scale!

3 Righting moment changes

The single most radical development of the AC75 is to take a 75ft ‘keelboat’, but put no keel on it whatsoever.

When the then America’s Cup Defender and the Challenger of Record, Emirates Team New Zealand and Luna Rossa Prada Pirelli respectively, announced that the 36th America’s Cup (to be held in 2021) would be sailed in 75ft monohulls, conventional wisdom had it that the boats would look something like a TP52 or a Maxi72 – both impressively high performance keelboats.

By doing away with the keel entirely, the design is now like nothing we have ever seen, particularly when it comes to how dynamic the power transition is between foiling and not foiling.

The boats are designed to foil on the leeward foil, with the windward one raised to help increase righting moment: to help balance the boat. This means that when the AC75 is not foiling they are extremely tippy – much more so than most other boats of the same size.

Essentially, when the wind catches the sails, the boat wants to fall over as there is too much sail area for the amount of weight underneath the boat – something a lead keel usually counters on a yacht or keelboat.

Once the boat is up and on the foils, however, that all changes, as everything to windward of the single foil in the water balances the sails. That means, the hull, the crew weight, the sail and rig weight, and the windward foil, all work to counter the sails.

What all this means is that the boats go from being extremely tippy, to hugely powerful in just the few seconds it takes to get up on the foil. “The [AC75s] are really very tippy pre-foiling and then they go through the transition where they will need to build significant power. Then immediately [once they lift off] you have more stability than, well, take your pick, but certainly more righting moment than something like a Volvo 70 with a big canting keel.

“That change all happens in a very short space of time,” explained Burns Fallow of North Sails, who was one of the team who developed the soft wing concept back when the concept was revealed.

Photo: Ricardo Pinto / America’s Cup

4 ‘Cyclors’ return to power America’s Cup boats

Bak in 2017 Emirates Team New Zealand stormed to America’s Cup victory in an AC50 foiling catamaran which was, by some margin, quicker than any of the other teams.

The most glaring difference was their use of pedal grinders to produce power rather than traditional pedestal arm grinders. ETNZ’s sci-fi style term for their grinders was ‘cyclors’, cyclist sailors.

The idea had actually been tried before in the America’s Cup; Pelle Petterson used pedal grinders on the 12-metre Sverige in 1977. But ETNZ’s set-up now was very different: here it was part of a linked chain of innovations, the most obvious emblem of a radical approach.

One obvious benefit was the greater power output from using legs to pedal, but beyond this it left cyclists’ hands free and allowed the team to use a highly sophisticated system of fingertip control systems, and thus to use faster, less stable foils, and then to divide up crew roles so ETNZ could be sailed in a different way.

When the AC75 was first introduced in 2021, Cyclors were specifically banned by the class rule. However, with a reduction of crew numbers from 11 to 8 in the second AC75 class rule – in use for the 2024 America’s Cup – cyclors are now allowed once again and all teams look set to be using pedal power onboard.

5 America’s Cup boats may not be heading where they point

With the AC75 sailing on its foil, drag is dramatically reduced, vast amounts of power can be generated and so speeds rapidly increase. But the foils can serve another purpose too.

In order to be able to lift each foil out of the water, the foil arms must be able to be raised and lowered. Hence the foil wings, which sit at the bottom of the foil arms (and are usually a T or Y shape), do not always sit perpendicular to the water surface and the AC75s often sail with them canted over to something nearer 45º to the surface.

The further out the leeward foil arm is canted – essentially more raised – the closer the AC75 flies to surface and, crucially, the more righting moment is generated as the hull and rest of the boat gets further from the lifting surface of the foil.

There is another positive to this: as the lifting foil is angled, it produces lift to windward, which can force the boat more towards the wind than the angle it is sailing.

Due to this negative leeway (as it is known when a foil creates lift to windward) the boat can be pointing at a compass heading of say 180º but in fact will be sailing at eg 177º as the foil pushes the boat sideways and to weather, essentially sailing to windward somewhat diagonally.

6 The foils are heavy. Very heavy.

As the foils work to provide stability to the boat (when it is stationary both foils are dropped all the way down to stop it tipping over) and to provide massive amounts of righting moment, they are incredibly heavy.

A pair of foil wings and flaps (excluding the one-design foil arm which attaches them to the boat and lifts them up and down) weigh 1842kg. To put that into perspective, the entire boat itself with all equipment (but without the crew) weighs between 6200kg and 6160kg. So the foil wings at the base of the foil arms are nearly ⅓ of the total weight of the boat.

It is partly due to this that you will see some teams with bulbs on their foils. If you decide to go for a skinny foil wing (which would be low drag and so faster) then there will not be enough volume to cram sufficient material in to make the foil weigh enough. So some teams have decided to add a bulb in order to make it weigh enough but to also keep a less draggy, slimmer foil shape.

7 Sails can invert at the head

As with everything on the AC75, the mainsail was a relatively new concept when the boat was first announced. It consists of two mainsails which are attached to both corners of a D-shaped mast tube. This has the effect of creating a profile similar to a wing.

It is well established that solid wing sails are more efficient at generating power than a soft sail and for this reason solid wings were used in both the America’s Cup in 2013 and 2017. But there are drawbacks with a wing: they cannot be lowered if something goes wrong and require a significant amount of manpower and a crane to put it on or take it off a boat.

One reason a wing makes for such a powerful sail is that the shape can be manipulated from top to bottom fairly easily with the right controls. With the AC75 the designers wanted a sail that could have some of this manipulation, produce similar power but could also be dropped while out on the water. The twin skin, ‘soft wing’ is what they came up with for this class of America’s Cup boat.

In addition to the usual sail controls, within the rules, the teams are allowed to develop systems for controlling the top few metres of the mainsail and the bottom few metres.

What this means is that the teams are able to manipulate their mainsail in a number of different ways to develop power and control where that power is produced in the sail. But it also means that they have the ability to invert the head of the sail.

Doing this effectively means ‘tacking’ the top of the sail while the rest of the sail is in its usual shape. The advantage here is that instead of trying to tip the boat to leeward, the very top of the sail will be trying to push the boat upright and so creating even more righting moment. The disadvantage is that it would come at the cost of increased aerodynamic drag.

We know that a number of America’s Cup teams are able to do this, though whether it is effective is another question and it is very hard to spot this technique being used while the boats are racing at lightning speeds.

8 America’s Cup meets F1

A new America’s Cup boat is a vastly complex bit of kit. Each team has incredibly powerful Computational Fluid Dynamics (CFD) software packages and simulators in order to try to understand the various gains and losses.

To make these simulators and computer projections as accurate as possible each team has been getting as much data as they can over their three year development cycle.

In the case of this America’s Cup it does seem the development process is genuinely getting closer to Formula 1 (albeit with smaller budgets than a modern F1 team has behind them).

INEOS Britannia have been work alongside the all powerful Mercedes F1 team (both of who are backed by INEOS) and have been open about how much this has helped their development process and after a relatively small amount of collaboration in 2021 the British team and Mercedes have created a much tighter relationship for the 2024 America’s Cup .

But the British team is not alone. When two-time America’s Cup winner, Alinghi announced they would be coming back to the event after some years on the sidelines, they also announced their own tie-in with current F1 World Champions, Red Bull Racing, to for Alinghi Red Bull Racing .

“It’s really similar to F1,” explains Mercedes Applied Science Principal Engineer Thomas Batch who has 11 F1 titles to his name and is was with INEOS in Auckland 2021. “Certainly in this campaign the technology is close to what we have in F1.

“In terms of raw sensors on the boat you are probably talking in the 100s but then we take that and we make that into mass channels and additional analysis with computational versions of those channels that we then analyse and get into in more detail. So you are looking at 1000s of plots that we can delve into [per race or training session].

“That level of data analysis and then feedback with the sailors is very similar to working with an [F1] driver.”

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Types of Sailboat Hulls

Sailboats come in numerous hull shapes. These include single-hull monohulls, along with double and triple-hull multihulls.

There are two main categories of sailboat hulls: monohulls and multihulls. Common monohull types include flat-bottom vessels, fin-keel racers, bulb and bilge keel cruisers, heavy semi-displacement sailboats, and dense full-keel displacement cruisers. Multihull designs include catamarans and trimarans.

In this article, we'll cover the most common types of sailboat hulls along with their best uses. We'll explain the difference between monohulls and multihulls, along with how keel shape influences sailboat performance.

We sourced the information for this article from sailing experts, hull shape guides, and the written wisdom of famous sailboat designers. Additionally, we researched sailboat sales figures to determine the most popular vessel configurations available today.

Table of contents

‍ Importance of Sailboat Hull Design

A sailboat is defined by its rig and hull shape. Sailboat hull shape is one of the deciding factors on how it will handle. Additionally, the shape (and displacement) of a sailboat hull can be used to determine its strengths and weaknesses. Learning about sailboat hull shape can help you understand what kind of boat you need and what your vessel is capable of.

You can easily categorize sailboats based on their hull shape. For example, a heavy deep-draft displacement hull is likely a slow, steady, and comfortable cruiser. In contrast, a sleek flat-bottomed sailboat or catamaran is likely built for speed and could easily outpace even the most nimble displacement cruisers.

The most common kind of sailboat is the monohull. When you think of a sailboat, probably think of a monohull. The term simply means that the vessel has one single hull and nothing more. This is in contrast to multihulls such as catamarans, which are easy to spot and differentiate from traditional designs.

Monohulls are popular because they work. They're easy to build and narrow enough to fit in most marina dock spaces. Monohulls are also generally easy to handle in a variety of conditions, both fair and foul.

One drawback of monohull designs is that they are not quite as stable as most multihulls, though monohulls can recover more easily from a serious roll or capsize. They also cost a lot less, as the vast majority of production sailboats ever constructed were of the same basic single-hull configuration.

Centerboards and Swing Keels

The windward performance of sailboats is greatly improved by the use of a long keel or centerboard. The centerboard is the most simple type of stabilizing device used on sailboats. Usually, the centerboard is simply a long fin that protrudes from the bottom of the hull.

The centerboard keeps the boat on track when the wind is not moving in the boat's direction of travel. This is why sailboats can sail at different angles to the wind without being pushed to the side. A key characteristic of centerboards is that they can be raised and lowered, which is convenient on small boats that need to be trailered or beached.

Swing keels are similar to centerboards in that they can be raised and lowered, though they pivot on a hinge instead of sliding up and down in a truck. Swing keels are either recessed into the hull or held in a housing just below it, which usually also contains much of the boat's ballast. Swing keel designs free up cabin space that would normally be occupied by a bulky centerboard trunk.

Centerboards and most swing keels are an alternative to a permanently affixed keel. They're generally not considered to be as seaworthy as other hull designs, so their use is confined primarily to inland and coastal cruising.

Monohull Sailboat Hull Shapes

When in the water, it's difficult to distinguish between the different types of monohull shapes. In most cases, you have to pull the boat out of the water to figure out what hull shape you're dealing with. Next, we'll go over the most common monohull sailboat shapes and their uses.

Flat-Bottom Sailboats

Flat bottom sailboats are the easiest to build and often the fastest. These vessels have a very shallow draft and are often lightweight, so they slide easily and quickly across the water. Flat bottom sailboats make excellent racing boats and 'gunkholers,' which are primarily used for camping and hopping between shallow Islands.

Flat bottom sailboats usually have centerboards or swing keels, which makes them great for shallow water, beaching, and towing on a trailer. The use of flat bottom sailboats is confined primarily to inland and coastal waters, as a flat bottom does not handle well in swells and rough weather. Flat bottom sailboats pound hard on chop, and they lack the low center of gravity that's necessary for good stability.

Fin Keel Sailboat Hulls

The fin keel is a popular alternative to centerboards, and vessels utilizing this low-profile hull shape have proven to be quite seaworthy. Fin keels are popular on fast racing boats and lightweight cruisers. A fin keel resembles a centerboard, but it usually extends much further from the base of the hull.

The majority of a sailboat's draft comes from the fin keel, as the hulls of these sailboats tend to be rounded and shallow. They resemble flat-bottom designs, but slight rounding significantly increases comfort. Fin keel sailboats are ideal for racing and coastal cruising, and some models can be used for extended offshore passages.

Bulb Keel Sailboat Hulls

A bulb keel sailboat hull usually resembles most fin keel varieties. The hulls of these vessels tend to be shallow and rounded, with a long and thin fin extending from the base of the hull. A bulb keel is essentially just a thin blade with a bulb on the bottom.

Bulb keels are different from fin keels as they usually contain additional ballast weight for stability. The hydrodynamic properties of bulb keels are proven to be efficient. As a result, these boats can also be quite fast. In a direct comparison, a vessel with a bulb keel will likely be more seaworthy than the same sailboat with only a fin keel or a centerboard.

Bilge Keel Sailboat Hulls

The hull shape of a bilge keel sailboat usually resembles that of a bulb or fin keel sailboat, with one major distinction. Instead of one long and thin keel descending from the center of the hull, a bilge keel sailboat has two lengthier fins offset on the port and starboard side.

The idea behind the bilge keel design is that when the vessel heels to one side, one of the two keels will be straightened out. This, in theory, provides better tracking and improves stability. It also distributes ballast evenly on both sides. Bilge keels can also improve motion comfort, and they can reduce the vessel's draft by a small margin.

Bilge keel sailboats offer a balance between seaworthiness and speed. These vessels can be used as bluewater cruisers and coastal cruisers. They can also hold their own in any yacht club regatta.

While a bilge keel sailboat may not be ideal for cruising the North Atlantic during the winter, it can certainly make a safe and comfortable passage maker that can gain a knot or two of speed above its heavier counterparts.

Semi-Displacement Sailboat Hulls

Now, we'll look at some true bluewater cruising designs. The semi-displacement hull features a long and deep keel that runs from about the center of the hull all the way back to the rudder. Semi-displacement hulls get deeper the further back you go, reaching their longest point at the very aft end of the boat.

The offshore benefits of a long and deep keel are numerous, as this hull shape provides an enormous amount of stability and a very low center of gravity. The design itself it's quite old, and it's featured on many classic cruising sailboats and workboats.

Though less common in the modern era than more contemporary fin keel designs, a traditional semi-displacement sailboat offers easy handling and enhanced motion comfort. Semi-displacement hulls tend to have a deep draft and therefore are not ideal for shallow water. They handle confidently in all conditions, though they usually aren't as fast as newer designs.

Displacement Sailboat Hulls

Displacement hulls, also known as full keel hulls, are the bulldozers of the sailboat world. These traditional vessels are deep, heavy, relatively slow, and capable of plowing through the roughest weather conditions.

Displacement hulls have a long keel that begins at the bow and extends all the way after the rudder. Like semi-displacement hulls, full keel sailboats offer excellent motion comfort and confident handling.

Displacement hulls have the best directional stability and downwind maneuvering abilities. Their handling is more forgiving, and they're less jumpy at the helm. Many of these boats heel gently and give the crew more time to respond to changing conditions.

The primary downside to displacement hulls is their high cost and sheer mass. Displacement boats are large and take up a lot of space. They're usually too tall and heavy for trailering, so they tend to remain in the water most of the time.

Displacement hulls aren't made to just sit at the dock or jump around the lake; they're designed for real-deal offshore sailing. They also have the roomiest cabins, as the hull extends further down and longer than any other hull shape.

Now, let's examine multihull sailboat designs and why you may want to consider one. Some of the earliest seagoing vessels had multiple hulls, usually featuring one long hull (occupied by the crew) and a small stabilizing hull off to one side.

Multihulls have only recently become popular, and they make up a decent portion of the modern production boat market. This is because of their numerous design benefits and spacious cabins. Multihulls are almost guaranteed to be more expensive than monohulls (both new and used), and the used market is still saturated with expensive luxury cruising sailboats.

Modern multihull sailboats feature a large pilothouse in the center and plenty of cabin space in each full-size hull. They offer excellent motion comfort and achieve very high speeds. Due to their wide beam, they provide spacious living spaces and excellent stability. Here are the two main types of multihull sailboats.

From above, a catamaran looks like two thin monohull sailboats lashed together and spaced apart. Fundamentally, that's exactly what they are. Except catamarans have a very shallow draft and the capability to reach very high speeds.

Catamarans have two hulls instead of one, and each hull is typically a mirror of the other. They achieve their space using width rather than length, so a 30-foot catamaran has significantly more interior room than a 30-foot monohull.

Their primary drawback is that, due to their width, catamarans usually require two standard dock spaces instead of one. But at sea, they don't heel over dramatically like monohulls, which makes them much more comfortable to eat, sleep, and cook inside of.

Trimarans follow the same basic design principles as catamarans, except they have a third hull in the center. From above, a trimaran looks like a monohull with two smaller hulls lashed to the sides. Unlike a catamaran, the primary living space of a trimaran is in the large center hull. Trimarans are essentially just monohulls with stabilizers on the side, resembling ancient sailing canoes.

Trimarans have the same spatial and stability benefits as catamarans, though they can achieve higher speeds and better sea keeping. This is because of the additional stability provided by the center hall. Trimarans tend to be costlier than catamarans, though many sailors believe that the benefits outweigh the cost.

Best Sailboat Hull Shape for Speed

If we take wave height and weather conditions out of the equation, the fastest sailboats are usually the longest. Sailboats are limited by hull speed and sail plan size regardless of their hull shape. That said, the fastest sailboats tend to be flat bottom monohulls, fin keel monohulls, and trimarans.

Best Sailboat Hull Shape for Motion Comfort

The best sailboat for motion comfort is the catamaran. These wide and seaworthy vessels 'stance up' and minimize rolling. They also come close to completely eliminating heeling.

Wide and stable multihulls are popular because they alleviate some of the most common complaints of sailors. Trimarans are also an excellent choice for comfort, as their stabilizers minimize the effect of rolling in heavy seas.

Most Seaworthy Sailboat Hull Shape

Today, many people consider multihulls to be the most seaworthy design on the market. However, seaworthiness is more than just average stability in rough weather. Many Sailors argue that traditional displacement sailboat hull designs are the most seaworthy.

Displacement hulls have a low center of gravity which improves their knockdown survivability. In other words, in the (rare) event of a displacement boat knockdown, the weight of the keel is more likely to swing the boat back up and out of trouble. Multihulls cannot recover from a knockdown, as they like the pendulum-like recoil ability.

Most Spacious Sailboat Hull Type

The most spacious hull sailboat type is the catamaran. Catamarans have two nearly full-size hulls (one on each side) plus a large central pilothouse that resembles the main cabin of a large powerboat.

Many typical catamarans fit an entire kitchen into the Pilot House along with four private births and two full-sized heads in its hulls. Some mid-size catamarans even come with a bathtub, which is essentially unheard of on equivalent monohulls.

Spaciousness varies on small monohulls. Larger cabins are usually found on bulb and bilge keel designs, as swing keel and centerboard boats need somewhere to hide their skegs. Centerboard boats are the least spacious, as the centerboard trunk must occupy the middle of the cabin space.

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Daniel Wade

I've personally had thousands of questions about sailing and sailboats over the years. As I learn and experience sailing, and the community, I share the answers that work and make sense to me, here on Life of Sailing.

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