steel yacht hull thickness

Messing about in boats since 1975.  Online Since 1997.

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METAL BOATS For Blue Water Introduction | Which Metal...? | Aesthetics & Hull Form | Design Features Scantling Calcs & Framing | Advantages of NC Cutting | Corrosion Protection | Conclusion   Introduction This essay is intended to bring to light a few of the issues surrounding the use of metal for boats. You can access any of the specific topics via the links above. While the pros and cons of various metals expressed here are quite relevant to one's choice of hull material, they are also central to the actual process of designing and building in metal, whether one chooses in favor of steel, aluminum, copper nickel, monel, stainless, or what have you... The following is therefore not solely aimed at potential metal boat owners, but also at boat builders and designers who may wish to make better use of metal as a structural material for boats.   Which Metal...? One of the primary choices one will face when considering metal is just which metal to use, where to use it, and what metals are best suited to each vessel type . To begin the discussion, here are a few brief thoughts with regard to steel versus aluminum. If an existing boat design is being considered, in other words a vessel that already has a fixed hull shape, then we can very generally observe the following: In terms of sea kindliness, some boats may be better if built in steel, due mainly to the extreme lightness of aluminum, which in some hulls may result in a more active / harsh motion. This is the case to a greater degree with larger boats or very beamy boats. Provided that the design has adequate displacement and stability to carry the added structural weight, boats in general will have a more gentle motion at sea if built in steel. This is not only due to the additional weight, but also to the distribution of that weight towards the perimeter, resulting in a greater roll moment of inertia. On the other hand, somewhat narrower or lighter displacement boats will often be best if constructed of aluminum. They'll generally have a narrower waterplane, and so less inherent shape stability. Therefore, due to having a relatively narrower waterline, they will react less avidly to the water's surface contours (waves), and will have a relatively easier motion at sea. In order to have sufficient stability, weight must be kept down, favoring an aluminum structure. It is usually a simple matter to adapt a steel vessel design to being built in aluminum, since the resulting vessel will have a lower center of gravity and enhanced stability (less structural weight, more ballast). But a design that has been optimized for aluminum construction will not ordinarily be able to be built in steel, due to the substantially greater weight of structure. The exception is an aluminum vessel that has been designed with relatively heavier displacement than needed. If we were to start from scratch and create a new design, we have the chance to optimize the hull form to take best advantage of the preferred material. With steel , we must design a hull with sufficient displacement to carry the structure. At 490 pounds per cubic foot, the weight of a steel structure adds up very quickly indeed. For smaller vessels, say below around 35 feet, this makes for a fairly heavy displacement. In larger sizes, say above 40 feet, one can make excellent use of steel. Above 45 feet and steel structure begins to come into its own. Above around 50 feet, a steel hull can actually be quite light for her length (by traditional cruising vessel standards). I have somewhat arbitrarily given the lower limit of a good steel vessel as being around 35 feet of length. This is of course not a fixed limit. The boundary of what can be built in steel is less a matter of boat length than it is a matter of shape and displacement. With proper design, one can successfully create a steel boat for coastwise or blue water sailing down to around 28 to 30 feet LOD.  Smaller is actually possible but compromises must be made...! Adequate displacement must be maintained to carry the structure, and thus draft and beam may not be decreased below a certain point. Therefore, roughly below around 30 feet the boat will require rather heavy displacement, likely resulting in a less graceful shape in order to carry the structure. There will be that much less carrying capacity remaining for fuel, water, and the desired number of sandwiches and beer...! For small vessels of say less than around 40 feet, one can make a very convincing argument in favor of aluminum . At 168 pounds per cubic foot, we can easily make use of greater plate thickness without much of a weight penalty, and still have a light weight structure.   When built to the same strength standard as a steel vessel, a bare aluminum hull "as fabricated" will weigh some 30% less than an equivalent steel hull. As an added bonus, the lighter weight of aluminum will permit a given hull form to be built with much greater strength than the same hull in steel. In other words, given the same weight budget an aluminum structure will be able to increase scantlings in order to have a considerably higher strength than the same design in steel. What other materials can be considered...?  Any design optimized for steel construction can be readily adapted to being built in Copper Nickel or Monel without having to make changes to the hull shape. The overall weights will turn out to be within a similar range and the placement of internal framing will usually be identical or extremely similar. We can also say that any design that has been optimized for aluminum construction could be adapted to the use of Titanium for the hull structure without requiring any hull shape changes. A titanium structure having an equivalent strength to a steel structure will be approximately 40% lighter than the steel structure, and roughly 10% lighter than an aluminum structure. Since we know from experience that "form follows budget" the choice of materials for a boat's structure ultimately comes down to a question of cost, which we will consider below.   Steel Mild Steel: Due to fabrication issues, one cannot readily make use of less than 10 gauge mild steel plating (0.134 inch, or 3.5 mm). Even 10 gauge mild steel plating can be very problematic to keep fair. It will have much greater distortion levels while welding than plate of a greater thickness. Even so, with a few essential metal boat building tricks learned, it is not much trouble to avoid distortion altogether in a 10 gauge steel hull.  With a few innovative approaches to the arrangement of structure, even less thickness is possible, down to say 12 gauge mild steel.  For an amateur builder however, working in 10 gauge mild steel without knowledge of a few essential tricks, the result will often be excess distortion. The natural temptation then is to use greater plating thickness, but there must be adequate displacement to carry the greater weight. A design intended for 10 gauge steel will be grossly over-weight if the plating is arbitrarily increased to, say, 3/16 inch, and it will neither float at the intended waterline, nor be able to carry the required amount of ballast, and as a result it will not have the intended stability. It turns out that in the battle against distortion, it is better to use a few more strategically placed longitudinals. Other tricks will also ordinarily be employed to preserve fairness, such as temporary external long's, etc. In general it is possible to design and build very fine steel boats down to around 35 feet (give or take a few feet), these smaller vessels will necessarily make use of 10 gauge mild steel plate and they will therefore necessarily require much greater skill in building. If the vessel can be large enough, say over 45 feet, or of sufficiently heavy displacement, then 3/16 inch mild steel plating can be used to advantage (just under 5 mm) and will be far easier to keep fair. For boats above 60 feet, 1/4 inch plate can be used and the boat will still be lighter than one could achieve with traditional plank on frame wood construction. Corten Steel: For smaller steel vessels that must use 10 gauge steel for plating, one can make a very good case for using Corten steel. Corten has about 40% greater yield strength than mild steel. This means that 10 gauge Corten plate will resist welding distortion and denting more or less the same as 3/16" mild steel plate. The higher yield strength is the primary justification for the use of Corten steel for metal boats, rather than imagining there to be any possible corrosion benefits. Although Corten tends to rust much more slowly than mild steel, whether a boat is built of mild steel or of Corten steel it still must be sandblasted and painted everywhere both inside and out. Corten is just as easy to weld and cut as mild steel, so aside from the slightly greater cost of Corten, it is to be recommended for all steel vessels having a steel plate thickness of less than 3/16 inch. "Cor-Ten A" is also known as ASTM A-242, which is an older specification for the current ASTM A-606 (usually for sheet under 3/16") and ASTM A-588 (usually for plate over 3/16" thickness). ASTM A-588 is also known as "Cor-Ten B" and is the more commonly encountered current spec for Cor-Ten, with a minimum yield strength of 50k psi in plates of greater thickness. An alloy sometimes specified for low temperature applications is "Tri-Ten" also known as ASTM A-441. An alternate (newer) spec for this alloy is A-607 when referring to sheet, or A-572 and A-572-M when referring to plate. "Tri-Ten" alloys contain a small amount of vanadium (A-572), or they may contain both vanadium and manganese (A-572-M). The addition of these alloying elements allows these steels to achieve greater strength by producing a more refined microstructure as compared with plain carbon steel (mild steel). The alloying elements provide a smaller crystalline grain size and a fine dispersion of alloyed carbides, thus providing higher yield strength without sacrificing ductility. High Strength Low Alloy (HSLA) Steel Common Names & Properties HSLA STRUCTURAL STEELS ASTM A572-50 EX-TEN 50 Offers 50k PSI minimum yield. ASTM A441 TRI-TEN Offers 50k PSI minimum yield. Resistance to atmospheric corrosion twice that of carbon steel. ASTM A242 COR-TEN A Resistance to atmospheric corrosion four times that of carbon steel. Excellent paint adhesion. ASTM A588-A COR-TEN B Similar to A242. Modified chemistry offers 50k PSI minimum yield. Resistance to atmospheric corrosion four times that of carbon steel.

In General:  The advantages of steel can be summarized as follows...

• Steel is more rugged than aluminum, being tougher and much more abrasion resistant.
• The various HSLA steels are even more so.
• Welds in steel are 100% the strength of the surrounding plates, whether mild steel or Corten.
•  Steel is more "noble" than aluminum, making steel less prone to electrolysis and allowing a steel hull to use regular copper bottom paint.  

Aluminum is light, strong, corrosion resistant, non sparking, conducts electricity and heat well, and is readily weldable by MIG or TIG processes. In terms of ease of construction, aluminum is excellent. It can be cut with carbide tipped power tools, dressed with a router, filed and shaped easily, and so forth. Aluminum is light, clean, and easy to work with.

Aluminum is therefore faster to fabricate than steel and welding aluminum is a very quick process, both resulting in a labor savings. In terms of thickness, 3/16 inch (around 5 mm) is generally considered the minimum plate thickness for MIG welding. However, if pulsed MIG welding is available then 5/32 inch plating (4 mm) can be used, particularly for deck and house structures.

Pound for pound, the cost of aluminum is much greater than steel. In 2012, aluminum in the 5000 and 6000 series costs between USD $3.00 and $3.50 per pound and pre-primed steel plate costs round USD $0.80 per pound.

Since the weight of an aluminum structure will be some 30% lighter than an equivalent steel structure, considering only the cost of materials an aluminum structure will still be roughly 2.5 times that of the equivalent steel structure. That aluminum is faster to fabricate and weld does help to reduce that ratio after labor costs are factored in.

Since aluminum is much lighter than steel, there is the option to use much greater plate thickness within a given weight budget, which means that not only can the overall strength be greater than with steel, but the distortion levels can be much more easily managed. In so doing, of course the cost will be proportionally greater.

Aluminum alloys for use on boats are generally limited to the 5000 and the 6000 series. These two alloy groups are very corrosion resistant in the marine environment due to the formation of a tough aluminum oxide. These alloys are subject to pitting, but the pitting action slows as the oxide film thickens with age.

Aluminum alloys are subject to crevice corrosion, since they depend on the presence of Oxygen to repair themselves. What this means is that wherever aluminum is in contact with anything, even another piece of aluminum or zinc, it must be cleaned, properly prepared, and painted with an adhesive waterproof paint like epoxy, then ideally also protected with a waterproof adhesive bedding such as Sikaflex or 3M-5200 to prevent water from entering the interface.

Paint preparation is critical. Thorough cleaning, and abrasive grit blasting will provide the best surface for adhesion of paint or bedding. Alternately, a thorough cleaning and then grinding with a coarse 16 grit disk will provide enough tooth for the paint to stay put.

Aluminum is anodic to all other commonly used metals except zinc and magnesium, and must be electrically isolated from other metals. A plastic wafer alone as an isolator is not sufficient. Salt water must be prevented from entering the crevice, which means that properly applied epoxy paint, adhesive bedding, and a non-conductive isolator should all be used together.

In aluminum, welds done in the shop are at best around 70% of the strength of the plate (in the 5000 series). Usually, one will compensate for the reduced strength in the heat affected zone either by providing a backup strip at any plate joint, and welding the plate joint thoroughly on both sides, or by providing additional longitudinal members to span any butt welds in the plating.

Ideally, plating butts will be located in the position of least stress. For most general plating, this is ordinarily at one quarter of the span between frames. In other words, with proper engineering and design, the reduced strength of aluminum in the heat affected zone is a non issue.

Aluminum hulls require special bottom paint. Organo-tin based anti-fouling paints can no longer be used as bottom paint except in such diluted formulations as to be nearly useless. Currently, the best antifouling paint for aluminum hulls is called "No-Foul EP-21" made by the E-Paint Company (800-258-5998). 

No-Foul EP-21 is an update of the original "No-Foul ZDF" both of which make use of a controlled release of hydrogen peroxide to prevent fouling. Practical Sailor Magazine did a controlled study of a large variety of anti-fouling paints over several years, during which they discovered that No-Foul ZDF outperformed ALL other antifouling paints during the first year of immersion in all waters. They also discovered that No-Foul ZDF performs significantly less well than the other AF paints during the second year... The conclusion? Refreshing the No-Foul coatings annually will result in a top performing system, as well as frequent inspection intervals for the hull.

The new formulation for No-Foul EP-21 is considered to be an improvement due to the addition of an environmentally preferred booster biocide that helps control slime and grass. Another improvement is the change from a vinyl binder to an epoxy. This makes the paint harder, and allows it to be applied over a wider variety of existing paints.

Other non-copper based anti-fouling technologies continue to appear, and they all should be considered provided that there are no metals present that are more noble than aluminum.

A big savings with aluminum is that it is ordinarily not necessary to sand blast or paint the inside of the hull. Generally, due to its very good conductivity one must insulate an aluminum hull extremely well. The most common insulation is blown-in polyurethane foam, although our present recommendations have drifted away from those materials.  In combination with a light primer or mastic, one can make an excellent case for the use of cut-sheet foams, such as Ensolite and Neoprene, where it is desirable to lightly blast the aluminum, and provide an epoxy primer or other barrier coating prior to insulating.

Various coatings for the interior of an aluminum boat are available which provide sound deadening and insulation. Two products in particular are Mascoat DTM for insulation, and Mascoat MSC for sound attenuation. Our preference is to use Mascoat MSC at 20 mils thickness throughout, with an additional 60 mils thickness in the engine room for sound attenuation. Then to apply Mascoat DTM at 120 mils thickness throughout over that as insulation. With this system it is not necessary to pre-paint the surfaces, nor to use additional insulation, although for colder waters a cut sheet foam can be added.

On the exterior , except on the bottom or locally where things are mounted onto the hull surface, it is completely unnecessary to paint an aluminum hull. This represents such a large cost savings that if the exterior is left unpainted, building in aluminum will often cost LESS than building the same vessel in steel. More or less, the cost difference amounts to the cost of painting the exterior of the aluminum hull...

We have already seen that a point in favor of aluminum is that a much lighter weight boat can be built than would be possible in steel. This is a performance advantage as well as a cost advantage. Not only will the lighter displacement boat be relatively less costly to build, it will also be much less costly to push through the water. Lighter weight means less horsepower is needed for the same speed, which means less fuel will be used to achieve the same range, both of which augment the overall savings in weight.

One might argue that with a lighter boat there will possibly be less room below, the lighter boat being narrower on the waterline, and possibly less deep. With proper planning, this need not be an issue.

On the plus side, even if an aluminum boat costs slightly more than a steel vessel to build (if painted), an aluminum boat will have a much higher re-sale value than a steel boat.  

Stainless Steel

I am occasionally asked, "What about building a boat in Stainless?"

A structure built in stainless will weigh approximately the same as one built in mild steel, although on occasion one may be able to make use of somewhat lighter scantlings due to the somewhat higher strength of stainless. There are several major drawbacks to the use of stainless, not the least of which is cost. Stainless of the proper alloy will cost nearly six times the price of mild steel!

Even if it were not so costly, stainless has numerous other problems:

• Stainless is quite difficult to cut, except by plasma arc.
• Stainless work hardens when being formed and can become locally tempered such as when being drilled.
• Stainless deforms rather extremely when heated either for cutting or for welding, meaning distortion will be very difficult to control.
• Stainless, even in the low carbon types, is subject to carbide precipitation in the heat affected zone adjacent to the weld, creating an area that is much more susceptible to corrosion as well as to cracking.
• Stainless is subject to crevice corrosion when starved of oxygen. This can be prevented only by sandblasting and painting the surfaces wherever an object is to be mounted onto the stainless surface. The same applies to the back side of any stainless fittings which are applied to hull surfaces.

If the above issues with stainless can be properly accounted for in the design and building of the vessel, then stainless can be a viable hull construction media.

Type 316-L stainless is generally the preferred alloy. Type 316-L is a low carbon alloy, and is used in welded structures to help prevent carbide precipitation in the heat affected zone. When available, the use of type 321 or 347 stainless will be of considerable benefit in preventing carbide precipitation, since there are other alloying elements (tantalum, columbium, or titanium) which help keep the carbides in solution during welding.

In my view, as a builder the main battle one will face is the rather extreme distortion levels when fabricating with stainless. Stainless conducts heat very slowly and has a high expansion rate. Both of these characteristics conspire against maintaining fairness during weld-up. Short arc MIG welding will be an imperative. In fact Pulsed MIG will probably be desired in order to sustain the right arc characteristics while lowering the overall heat input.  

Copper Nickel

Another material which should be considered along with steel, stainless, and aluminum is Copper Nickel. One can ignore paint altogether with CuNi, inside, outside, top and bottom. Copper Nickel acts as its own natural antifouling. In fact, bare Copper Nickel plate performs better than antifouling paint..!  Being a mirror-smooth surface, any minor fouling is very easily removed.  

Besides not having to paint CuNi and its natural resistance to fouling, CuNi is also easy to cut and weld, it has relatively high heat conductivity, it is extremely ductile, and it is therefore very favorable with regard to distortion while welding.

There are two alloys of Copper Nickel which are the most common: 70/30 CuNi, and 90/10 CuNi. The numbers represent the relative amounts of Copper and Nickel in the alloy. Having a greater amount of Nickel, 70/30 CuNi is the stronger of the two and also the more expensive of the two.

In the US as of February 2007, 90/10 CuNi was priced around USD $8.50 per pound, and 70/30 CuNi around USD $13.00 per pound, both based on a minimum order of greater than 15,000 pounds. In other words, roughly ten to fifteen times the cost of the same structure in steel. I have not investigated current (2015) prices for CuNi, but we can be certain they are higher (i.e. the value of the dollar less) thus the ratio of costs vs. steel much higher.

The issues with CuNi are not only those of cost, but also of strength. For example, the ultimate strength of 90/10 Cu Ni is about one third less than that of mild steel, and the yield strength about half that of mild steel. In practice, this means that a hull built of Cu Ni will have to use heavier scantlings. CuNi, being slightly heavier than steel per cubic foot, the CuNi hull structure will end up being slightly heavier than an equivalent steel hull structure.

In most materials, we usually "design to yield." This means that the ultimate failure strength of a material is more or less ignored, and the yield strength is instead used as the guide for determining scantlings. For example, if we were to desire a 90/10 CuNi structure having the same yield strength as there would be with a similar steel structure, we would be tempted to actually double the scantlings. Naturally this would result in quite a huge weight penalty, BUT....

In practice, a CuNi structure need not be taken to this extreme. Using the ABS rules to calculate the scantlings, an all 90/10 Cu Ni structure will have around 25% more weight than a similar structure in steel. It is best to use the same plate thickness as with steel, and compensate for the lower yield strength by spacing the longitudinals more closely.

It is unlikely that one would choose CuNi for the internal framing, primarily because of its cost, its relatively low strength, and the relatively much larger scantlings and weight that would result. In other words, there is no reason not to make use of CuNi for the hull skin in order to take full advantage of its benefits, but it is possible to use a stronger and less expensive material for all the internal framing.

What is the best choice for the internal framing...? Probably type 316-L Stainless . As long as the various attributes of stainless are kept in mind, this is a combination having considerable merit. Here is why...

• Stainless can be readily welded.
• One can easily make a weld between stainless and Cu Ni.
• Scantlings of stainless internal framing would not need to be increased, in fact they would be less than those required for mild steel.
• The weight of stainless internal framing would therefore be roughly 10% less than with mild steel, or approximately equal to the weight of a Corten steel internal structure.
• 316-L Stainless costs (February 2007) around USD $4.50 per pound based on a minimum order of 10,000 pounds. Therefore the cost of stainless is roughly half that of 90/10 Cu Ni, and about one third the cost of 70/30 Cu Ni... Combined with there being much lighter scantlings, the overall cost factor would be reduced considerably.

With this strategy the weight can be kept to roughly the same as an equivalent mild steel structure.

And to further reduce costs, NC plasma cutting or water jet cutting can be used for all plates and internal structure.

Are there still more options to reduce costs...?

Fiberglass...! Compared to the weight and cost of an all CuNi / Stainless structure, both cost and weight can be reduced by using fiberglass for the deck and house structures, or possibly just for the house structures. A cold moulded wooden deck and / or superstructure is also a possibility.

Even with GRP or composite wood for the house structures, it probably would be most advantageous to plate the deck with Cu Ni. In so doing, one could then use CuNi for all the various deck fittings: stanchions, cleats, bitts, etc. Pipe fittings are readily available in either alloy of CuNi, so this would be a natural. The resulting integral strength and lack of maintenance would be an outstanding plus.

While the expense of Copper Nickel may seem completely crazy to some, given a bit of extra room in the budget and the will to be completely free from ALL requirements for painting, this is the bee's knees....! The savings realized by not having to paint the entire vessel inside and out - EVER - will go quite a long way toward easing the cost differential.  

Per existing research on a number of commercial vessels, their operators have shown a very favorable economic benefit over the life of a Copper Nickel vessel. This is due to there being a much longer vessel life; far less cost for dry docking; zero painting costs; no maintenance; no corrosion; few if any repairs; etc. 

Per the Copper Alliance, and organization that has studied the economic benefits of CuNi for boat hulls, the cost saved on a commercial vessel's maintenance routine pays for the added cost of the CuNi structure within 5 to 7 years.  And... if the resale value of a CuNi boat is considered, the ROI is further enhanced.  

Monel 400 is an alloy of around 65% Nickel, around 30% Copper, plus small percentages of Manganese, Iron and Silicon. Monel is extremely ductile, and therefore will take considerable punishment without failure. Monel is easily welded, and Monel has extraordinary resistance to corrosion, even at elevated temperatures.

Monel is much stronger than mild steel, stronger than Corten, and stronger than the usual varieties of stainless. As a result of this greater strength, Monel could be used for the entire structure. As compared to a similar steel structure, Monel will therefore permit lighter scantlings and would allow one to create a lighter overall structure than with steel. Alternately one could use the same scantlings in order to achieve a vessel having greater strength .

To reduce costs even more, one could use the same strategy as with CuNi, i.e. use Monel just for the plating, and then use 316-L Stainless for the internal framing. This is probably the sweet spot, offering light scantlings and extraordinary freedom from on-going maintenance costs.

If cost is not an important factor, an all Monel structure may well be the ultimate boatbuilding material of all time.  

Titanium has been used in the aircraft and aerospace industries for quite a long time. As well, several Russian submarines have been built using Titanium. With very high strength alloys available, extreme nobility on the galvanic scale, virtual immunity to corrosion in sea water and in the atmosphere, and about half the weight of steel, there are only a few considerations that stand in the way of Titanium being the "perfect" hull material, not least of which is cost .

Cost :  Due to the higher cost of titanium as compared to, say stainless or aluminum, the choice in favor of using titanium for a fabricated structure such as a boat must be made on the basis of the resulting structure having lower operating costs, longer life, or reduced maintenance in order to justify its use.  In other words, titanium will only be chosen if it is perceived to have a lower total life cycle cost.

Plastic Range: Among the Commercially Pure (CP) grades of Titanium, and with most Titanium Alloys there is little spread between the yield point (the point at which a material is deformed so far that it will not return to its original shape when released) and the ultimate failure point. Thus most grades and alloys of titanium have a very limited plastic range. 

Elongation :  The percentage of elongation before failure is on par with mild steel, and is roughly twice that of aluminum.  Thus most grades of CP Titanium and most alloys are readily formable, and have a fatigue resistance on par with steel.

Stiffness: Another characteristic is "stiffness" which is expressed by the modulus of elasticity. For steel, it is 29 million psi. For aluminum, it is 10 million psi. For Titanium, it is 15 million psi. This indicates behavior that is somewhat closer to aluminum in terms of material rigidity.  In other words, Titanium will flex about twice as much as steel, but about 50% less than aluminum. Interestingly, Ti has about the same modulus of elasticity (stiffness) as Silicon Bronze, but Ti has less stiffness than copper nickel, which has an elastic modulus of 22 million psi.

Welding: Yet another consideration is the welding of Titanium, which is somewhat of a mixed bag due to several of the material's properties.

The melting point of Titanium (3,042 deg F) is well above that of steel (2,500 deg F) and about three times that of aluminum (1,135 deg F). Titanium forms a very tough oxide immediately on exposure to the air, and is highly reactive with nitrogen, therefore welding must be done only after thorough cleaning of the weld zone, and the welding process must assure a complete inert gas shroud of the weld zone both on the side being welded and on the opposite side. The weld zone must then continue to be shielded until the metal cools below 800 degrees.

These factors may provide considerable difficulty, but they are surmountable by thorough attention to detail, good technique, and aggressive measures to assure post-weld shielding. These factors however dramatically increase fabrication costs over that of other metals.

Among the other material properties that contribute to ease of fabrication of any metal are its heat conductivity, and its thermal expansion rate. Aluminum expands twice as much as steel per degree of temperature change, and is three times as conductive thermally. The thermal conductivity of aluminum is a big help, but the expansion makes trouble in terms of distortion. As a benefit though, an equivalent aluminum structure will have greater thickness and thus locally greater yield strength, so the score is more or less even between steel and aluminum, with aluminum having a slightly greater tendency toward distortion while welding.

With Titanium, this latter consideration will be the overriding factor in determining the minimum practical thickness for plating. Thermal conductivity is given as 4.5 BTU / Sq Ft / Hr/ Deg F / Ft for Titanium. For steel, it is 31, for aluminum it is 90. Thermal expansion is given as .0000039 in / in / deg F for Titanium, about 50% the expansion of steel and about 30% that of aluminum. These figures seem to indicate that the material would be fairly stable while welding, but that welds would take a much longer time to cool as compared to steel and vastly longer compared to aluminum. In other words, the heat would not dissipate - it would remain concentrated in the weld zone.

Industry consensus is that Titanium is slightly more prone to distortion due to welding as compared to steel. Considering these factors along with its much higher strength, as a very rough guess a thickness of around 3/32" may possibly be the minimum practical thickness for a welded structure in Titanium, with 1/8 inch thickness being a more likely lower practical hull thickness limit. As a comparison, the minimum thickness for other materials (mainly due to welding ease and distortion issues) is 10 gauge for mild steel (.1345"), and 5/32" for aluminum, although 3/16 inch thickness is a more practical lower limit for aluminum boat structures.

Corrosion:  Titanium is extremely corrosion resistant due to the immediate formation of a tenacious Titanium Dioxide on exposure to air or oxygenated water.  This means it is practically immune to corrosion in sea water, but there is one catch...  Like aluminum, Ti depends on free access to oxygen, therefore it can be susceptible to crevice corrosion wherever it is deprived of free access to oxygen and cannot form a protective oxide.  Crevice corrosion can be prevented in the same way as is done with aluminum, and some grades of Ti are more resistant to crevice corrosion than others. 

Titanium Grades:  Titanium Grade 2 is the most commonly available Commercially Pure (CP) grade, having 40k psi yield, 50k psi ultimate strength and a 20% elongation before failure.  It is highly formable and weldable, and is available in most shapes, i.e. plate, bar, pipe, etc.  These are highly favorable properties for hull construction.

Titanium Grade 12 includes Mo and Ni for a higher strength alloy having superior resistance to crevice corrosion, with 50k psi yield, 70k psi ultimate strength and an 18% elongation before failure. The 20k psi spread between yield and failure is a highly favorable property.  It is a highly formable grade, readily weldable and is available in a variety of plate sizes, pipes and bar shapes.  All of these are highly favorable properties for hull construction, making Grade 12 one of the best choices to be favored for boat structure.

Titanium Alloys :  An interesting Titanium alloy is the experimental Alloy 5111 (5% Al; 1% SN; 1% Zr; 1% V; 0.8% MO) with 110k psi yield, 125k psi ultimate strength and a 15% elongation before failure.  Described as "a near alpha alloy having excellent weldability, seawater stress corrosion cracking resistance and high dynamic toughness." It has a high elongation before failure, a "medium" overall strength of about twice that of mild steel, and has a slightly greater spread between its yield point and failure point than the "high" strength Titanium alloys. It is favored for submarines, but its high strength is not especially necessary for boats or large yachts.

Another Titanium alloy is the proprietary ATI Alloy 425 being made by Allegheny Technologies Inc. (ATI) who are targeting this alloy at ship structures.  With 132k psi yield, 152k psi ultimate strength and a 13% elongation before failure, its use is likely to be relegated to applications requiring very high strength.  Its low elongation before failure is an indication that it could be prone to cracking, and it is unlikely to be a candidate for typical boat structures (i.e. non-military usages).

Light weight, high strength, immunity from corrosion in sea water... sounds ideal.  Although it is obvious that Titanium would be an outstanding hull material, it requires extreme care during construction, thus labor costs would be high. If those factors can be mitigated or if cost is not an issue, then Titanium may possibly be the "ultimate" boat hull material...! 

Despite its immunity from corrosion in sea water, a titanium hull will still require paint below the WL in order to prevent fouling.  

Relative Cost

If we ignore the cost of the hull materials themselves for a moment and consider what may impact costs in other ways, we can observe the following... Vessel construction costs will vary more or less directly with displacement, assuming a given material, and a given level of finish and complexity in the design. Since displacement varies as the cube of the dimensions, we can see that the costs for a vessel will increase exponentially with size.

With regard to the complexity of a vessel the same can be said. Complexity in whatever form affects cost perhaps to the fourth power...! Assuming a given budget, a simpler boat can just plain afford to be made larger!

Estimating actual construction costs is relatively straightforward but it does require a detailed look at every aspect of the process. A reliable construction cost estimate must consider the hull material, degree of finish, complexity of structure, building method, whether the structure is computer cut, the complexity of systems specified and the degree of high finish for the joinery. This is only possible with a well articulated vessel specification, a complete equipment list, and a detailed set of drawings that show the layout and the structure.

Assuming we are considering vessels of equal size and complexity, when all is said and done, and if painted to the same standard on the exterior, an aluminum vessel may possibly be around 10% more expensive to build than the same vessel in steel. If the aluminum vessel is left unpainted on the exterior except where necessary, many yards can build for less in aluminum than in steel, or might quote the two materials at parity. This has been verified by several yards via actual construction estimates for boats of my design.

As compared to a steel boat, maintenance will be less costly on an aluminum boat and resale value will be higher. Taken as a whole, any increased hull construction costs for an aluminum hull will shrink into insignificance in the context of the entire life of the boat.

Of course a Copper Nickel, Monel, or Titanium vessel will be considerably more costly than one built in steel or aluminum, however in terms of longevity a boat built with any of those metals will provide the ultimate as a family heirloom...

For more information, please review our comprehensive web article on Boat Building Costs .  

The materials of construction need not dictate the aesthetics of a vessel. Much can be done to make a metal boat friendly to the eye. On the interior for example with the use of a full ceiling and well done interior woodwork, there will generally be no hint that you're even aboard a metal boat.

On the exterior, if metal decks are preferred for their incredible strength and complete water tightness, one can make the various areas more inviting by devious means. An example would be the use of removable wood gratings in way of the cockpit. Fitted boat cushions made of a closed cell foam work equally well to cover the metal deck in the cockpit area, and some will prefer to laminate a cork or teak deck over a painted and protected metal deck.

Many metal boats we encounter seem "industrial" in their appearance. In my view, classic and traditional lines, if attended to faithfully, will completely eliminate that industrial look. With a bit of classic gracefulness introduced by the designer, a metal boat will be every bit as beautiful as a boat of any other material.

My design work often tends to be drawn toward fairly traditional aesthetics, which some may regard as being somewhat old fashioned. What I have done in these designs however, is to take maximum advantage of up to date materials and current knowledge of hydrodynamics, while retaining the look and feel of a classic boat. In so doing, my overall preference is to provide a boat that is very simple, functional, and rugged, while carrying forth a bit of traditional elegance.

Everyone's needs are different of course. When considering a new design, nearly anything is possible. The eventual form given to any vessel will always be the result of the wishes of the owner, the accommodations the boat must contain, the purpose for which it is intended, and the budget that is available for its creation.  

Regarding Hull Form

Efficiency and performance are high on the list amongst the myriad considerations that go into shaping a hull. With metal hulls, there is always a question of whether a vessel should be rounded or "chine" shaped.

Assuming two vessels are of equally good design, whether the hull is rounded or single chine will not have much impact on their performance, i.e. they will be more or less equivalent. Here are a few considerations that may be of some benefit when considering the choice between rounded or single chine hull shapes...

• If one were to take a single chine hull form and simply introduce a fairly large radius instead of the chine, the newly rounded vessel's wetted surface would be less; displacement would be less; and initial stability would be less, and the comparison somewhat skewed.

In terms of interior hull space, a chine hull form will often be slightly less wide at sole level and slightly wider at the waterline level, so possibly a bit less room to walk around but larger seats and berths. The single chine hull form will have slightly greater initial stability (greater shape stability), and will therefore have slightly greater sail carrying ability at typical heel angles under sail. The single chine hull form will have greater roll dampening (faster roll decay). The rounded hull form will have a slightly more gentle rolling motion. The chine hull form will have slightly greater wetted surface. This implies that the rounded hull form will have slightly less resistance at slow speeds where wetted surface dominates the total resistance. The chine hull can be designed to equalize or reverse that resistance equation at higher speeds due to wake differences resulting from the chine hull being able to have a slightly flatter run.

Aside from these generalities, relative performance would be difficult to pre-judge. We can however observe the following:

• Given the same sail area, when sailing at slow speeds in light airs, one might see the rounded hull form show a slight advantage due to having slightly less wetted surface.
• When sailing fast , a chine hull form will be more likely to exhibit greater dynamic lift, especially when surfing.
• Especially in heavier air, one might even see a slight advantage to windward with the chine hull.

Given that those observations do not reveal any special deficiency with regard to a single chine hull we can additionally observe the following:

• When creating a new design, wetted surface is one of the determining factors of sail area.
• Having slightly greater wetted surface, a single chine hull should therefore be given slightly more sail area, so its slightly greater wetted surface will become a non-issue .
• If the chine hull is given slightly more sail area, it will therefore be subject to a slightly greater heeling force.
• However the single chine hull form will have inherently greater "shape stability" in order to resist that heeling force.
• One can therefore expect the sail carrying ability to be essentially equalized .
• Therefore with good design, there is no performance hit at low speeds, and there is ordinarily a performance gain at high sailing speeds.

Among the above considerations, the one factor that seems to favor the rounded hull form most definitively is that of having a slightly more gentle rolling motion. In other words, a slower "deceleration" at the end of each roll. On the other hand, rolling motions will decay more quickly with a single chine hull form. Even these factors can be more or less equivocated via correct hull design.  

Rounded Metal Hulls

As we have seen, one cannot claim that a rounded hull form is inherently better in terms of performance without heavily qualifying that claim. The primary trade-offs between a rounded hull and a chine type of hull form for metal boats therefore turn out to be purely a matter of cost and personal preference.

I have designed several rounded hulls for construction in metal. These are true round bottom boats designed with the greatest ease of plating in mind. Some are double ended, some have a transom stern, others have a fantail stern, and still others have a canoe stern where stem nicely balances the shape of the stern.

Having an easily plated shape, any of these rounded hull forms can be economically built. These rounded shapes require plate rolling only in a few places and are elsewhere designed to receive flat sheets without fuss. These are not "radius chine" boats. They are simply easily plated rounded hulls.

With any of these types, the keel is attached as an appendage, there being no need when using metal to create a large rounded garboard area for the sake of strength, as would be the case with a glass or a wooden hull. This achieves both a more economically built structure, as well as a better defined keel for windward performance under sail and better tracking under sail or power.

Plating on these rounded hull types is arranged in strips having a limited width running lengthwise along the hull. Usually the topsides can be one sheet wide, the rounded bilge one sheet, and the bottom one larger sheet width.

Examples of these rounded hull types among my designs are Jasmine , Lucille 42 , Lucille 50 , Benrogin , Greybeard , Fantom and among my prototypes such as Josephine and Caribe . While these might be imagined to have a "radius" chine shape, they are in fact true rounded hull forms. In other words, the turn of the bilge is not a radius but is instead a free form curve between bottom and topsides. Both bottom and topsides have gently rounded sectional contours that blend nicely into the curve at the turn of the bilge. With the exception of the turn of the bilge, all of the plating on these designs is developable and will readily bend into place making these vessels just as easily constructed as any radius chine shape. In other words, 85% to 90% of the vessel is able to be plated using flat metal sheets without any pre-forming.

What's the difference between this and a radius chine...?

In my view the visual difference between radius chine and rounded hull forms is very apparent, strongly favoring the rounded shape, yet the labor required and the consequent cost is the same. Due to the gentle transverse curvature given to the surfaces above and below the turn of the bilge, the appearance is a vast improvement over the relatively crude radius chine shape.  

Radius Chine Metal Hulls

Looking around at typically available metal boat designs we quickly observe that the "radius chine" construction method has become fairly common. Here, a simple radius is used to intersect the "flat" side and bottom plates. Although the radius chine shape takes fairly good advantage of flat plate for most of the hull surface, it is not a more economical construction method than the easily plated rounded hull shapes described above - nor is it nearly as attractive.

One reason for the popularity of the radius chine is that nearly any single chine boat can be converted to a radius chine. This is often done without any re-design of the hull by simply choosing an appropriate radius, and using rolled plate for that part of the hull. Radius chine construction does add quite a few extra hours to the hull fabrication as compared to single chine hull forms.

In my experience there is no benefit whatever to employ a radius chine shape over that of an easily plated rounded hull form. The radius chine hull will always be easily recognized for what it is... a radius chine shape rather than a true rounded hull. By contrast a gently rounded hull form will be vastly more appealing visually.  

Chine Hull Forms

A single chine can be quite appealing, especially when used with a more classic / traditional style. A few single chine examples among my sailing designs are the 36' Grace , the 42' Zephyr , the 44' Redpath , the 56' Shiraz , along with a number of others such as the prototype designs for a 51' Skipjack , or the 55' Wylde Pathaway .

As supplied, metal plate is always flat . When building a boat using flat sheet material, it makes the most sense to think in terms of sheet material and how one may optimize a hull design to suit the materials, without incurring extra labor. I am attracted to the single chine shape for metal boats. In my view the single chine shape represents the most "honest" use of the material.

In this regard I feel traditional styling has much to offer, keeping in mind of course the goals of seakindliness, safety, and of excellent performance. As with many traditional types, there is certainly no aesthetic penalty for using a single chine, as is evidenced by reviewing any of the above mentioned sailing craft.

Assuming that by design each type has been optimized with regard to sail area and hull form, it becomes obvious that the typically pandered differences between the performance of a rounded hull form versus that of a single chine, unless heavily qualified, are simply unsubstantiated.

In fact, since costs are significantly less using single chine construction, one can make an excellent case in terms of better performance via the use of a simpler hull form....!

How is this possible...?

With metal boats, labor is by far the largest factor in hull construction, and as we have observed greater complexity pushes the hours and the cost of labor up exponentially. Therefore dollar for dollar, a single chine vessel can be made longer within the same budget .

This means that in terms of the vessel's "performance per dollar" the single chine vessel can actually offer better performance (i.e. greater speed) than a similar rounded hull form...!

By comparison, a multiple chine hull form offers practically no advantage whatever. A multiple chine hull will require nearly as much labor as a radius chine hull. The only savings will be eliminating the cost of rolling the plates for the actual radius. In my view, multiple-chine shapes are very problematic visually, and they are much more difficult to "line off" nicely. There will be just as much welding as with a radius chine shape, and in general a multiple-chine hull will be considerably less easy to keep fair during construction.

If you look at the designs on this web site, you'll soon discover that there are no examples of multiple-chine vessels among my designs, whether power or sail....

Basically, multiple chine shapes cost more to build, and in my view multiple chine shapes are not as visually appealing. As a result the preference has always been to consider the available budget and to make a graceful single chine boat longer for the same cost, and realize some real speed, comfort and accommodation benefits...!

In the end what ultimately defines a good boat is not whether she is one type or another, but whether the boat satisfies the wishes of the owner.  

Keel Configuration

The keel of any vessel, sail or power, will be asked to serve many functions. The keel creates a structural backbone for the hull, it provides a platform for grounding, and it will contain the ballast.

In a metal boat, the keel is not just "along for the ride." In a metal vessel the keel can contain much of the tankage including a meaningful sea water sump, and the keel can serve as the coolant tank for the engine essentially acting as the "radiator." It is usually convenient to allow at least one generous tank in the keel as a holding tank.

A metal hull can take advantage of twin or bilge keels without any trouble. It is an easy matter to provide the required structural support within the framing. Often, bilge keels can be integrated with the tanks, allowing excellent structural support.

An added advantage with both sail and power boats is that the bilge keels can be used as ballast compartments. Having spread the ballast laterally becomes a big advantage in terms of the vessel's roll radius, providing an inertial dampening to the vessel's roll behavior.

Bilge keels can also be designed to permit a good degree of sailing performance to a power vessel which has been set up with a "get-home" sailing rig. Aboard a power vessel, when faced with the choices involved with having an extra diesel engine as a "get-home" device in the event of failure of the main engine, I would very seriously consider the combination of bilge keels and a modest sailing rig.

Bilge keels will usually make use of a NACA foil section optimized for high lift / low drag / low stall. With metal, this is easily accomplished.  

Integral Tanks

Integral fuel and water tanks are always to be preferred on a metal boat. Integral tanks provide a much more efficient use of space. Integral tanks provide added reinforcement for the hull and ease of access to the inside of the hull. Integral tanks are very simple to arrange for during the design of the vessel. If the tank covers are planned correctly there will be excellent access during construction as well as in the future for maintenance.

The one exception to this generality is that polyethylene tanks may be preferred for black or grey water storage, since they can be readily cleaned. This is especially so in aluminum vessels, due mainly to the extremely corrosive nature of sewage. In steel vessels, when properly painted there will always be an adequate barrier, and integral black and grey water tanks again become viable. For aluminum construction, if integral holding tanks are desired the tanks must be protected on the inside as though they were made of mild steel... and the coatings must not be breached...!

Please see my article on Integral Tanks for more on this question...  

Scantling Calcs

Hull size, materials of construction, and the location of the specific region of the structure in question will each have a bearing on the results of the scantling calcs. The method of calculating the hull structural scantlings is usually processed as follows, assuming first that the vessel data is already given (hull length, beam, depth, freeboard, weight, etc.).

Select plate material according to owner preference, available budget, and desired strength or other material properties Select preferred plate thickness according to availability, suited to vessel size and displacement Calculate local longitudinal spacing to adequately support the plate Select frame spacing to satisfy the locations of interior bulkheads or other layout considerations Calculate scantlings required for longitudinal stringers to satisfy their spacing and the span between frames Calculate scantlings required for transverse frames according to the depth of long'l stringers and the local span of the frames.

Per item 3, when considering an alternate material it is possible that due to a difference in plate yield strength as compared to the original design material (say steel), that the long'ls will be placed slightly more closely (say for the same thickness of plate, but a plate of lesser strength).

Generally, since the long's support the plate, they are the primary variable when plate thickness, or strength, or location is changed. It is no big deal to the structure, to the overall weight, or to ease of the building of the vessel (as compared to say steel) to have a tighter long'l spacing. This is the proper strategy to accommodate plate of different strength or thickness.

Once the plate is adequately supported, then scantlings of items 5 and 6 can be calculated according to their spans and the material strengths for the chosen framing materials.

It becomes obvious from the above that it is an advantage (in terms of weight) to select a relatively lesser thickness of plating, and a relatively more frequent interval for internal framing. On the other hand, it is usually an advantage in terms of building labor to select plate of a slightly greater thickness and a less frequent framing interval (so simpler internal structure).

Please see my article on Using the ABS Rule for a more detailed look at how scantlings are determined.  

Frameless Construction...?

There is quite a lot of misleading and incorrect information associated with the implied promise of "frameless" metal boats, a notion that is pandered by several offbeat designers and builders. The concept of "frameless" metal boats is attractive, but flawed.

If one applies well proven engineering principles to the problem of hull design as detailed above, one quickly discovers that for the sake of stiffness and lightness, frames are simply a requirement. For example, in order to achieve the required strength in a metal vessel without using transverse framing will require an enormous increase in plate thickness. Even with light weight materials such as aluminum alloy this would automatically result in a substantial weight penalty..

With light weight materials such as aluminum, one can certainly gain some advantage by the use of greater plate thickness, primarily in terms of maintaining fairness during fabrication, and in terms of ruggedness in use. Still, as strong as metal is, even with light weight materials there is definitely a need to support the plating and to reinforce and stiffen the structure as a whole using frames and stringers.

In general, the most suitable arrangement for internal structure is a combination of transverse frames and longitudinal stiffeners. Framing may sometimes be provided in the form of devious strategies... For example framing may be in the form of bulkheads or other interior and exterior structural features, placed in order to achieve the required plate reinforcement. Many so-called "frameless" boats do indeed make extensive use of longitudinals in combination with bulkheads or other internal structure to reduce the span of the longitudinal stiffeners.

While it is true that many metal boats are successfully plated , and their plating then welded up without the aid of metal internal framing during weld-up, in order to provide adequate strength in the finished vessel, frames must then be added before the hull can be considered finished. Even on a hull that will eventually have substantial internal framing this construction sequence can provide a big advantage when trying to maintain fairness during weld-up.

Experienced metal boat builders and designers have often come to recognize the potential benefits of building a metal boat over molds which do not hold the boat so rigidly as to make trouble during the weld-up. However, the competent among them also know that to leave the boat without internal framing is quite an irresponsible act.

Please see my articles on Framing and Frames First for more on this subject.  

Framing Systems

Framing systems are several, but can roughly be categorized into

Transverse Frames Only Transverse Frames with Longitudinal Stringers Web Frames with Longitudinal Stringers.

Among those, the Transverse Frames Only system is fairly common in Europe. In the US, the most commonly system used is the second system, where transverses are used in combination with longitudinal stringers.

In terms of scantlings, typically, long'ls will be half the depth, but approximately the same thickness as the transverse frames. It is an ABS requirement that transverse frames be twice the depth of the cut-out for the long'l.

Among some light weight racing yachts, a system of Webs with fairly beefy Long'l Stringers is the preferred approach, or alternately Webs with smaller Intermediate Transverse Frames, in combination with Long'l Stringers..

A somewhat generalized walkthrough of the usual design sequence is as follows:

For any given vessel size, plating will need to be a certain minimum thickness suited to that vessel size. For that given minimum plating thickness (for that particular boat) the long'l stringers will need to be a certain distance apart in order to adequately support the plate. The dimensions of the Long'l Stringers are determined by the vessel size, the spacing of the long's and the span of the long's between transverse frames. Finally, the dimensions of the Transverse Frames are determined according to the vessel size, the frame spacing, the span of the frames between supports, and by the requirement that the frames be no less in height than twice the height of the long's.

In other words, by this engineering approach the transverse frames are considered to be the primary support system for the long'l stringers, and the long'l stringers are considered to be the primary support system for the plating.

When a long'l member becomes the "dominant" member of the structure (usually locally only), it ceases to be referred to as a long'l stringer, and becomes instead a long'l "girder" (an engine girder for example).

If long'l stringers are not used, then the frames are the only means of support for the plating. They must therefore be more closely spaced in order to satisfy the needs of the plating for adequate support. In general though, long'l stringers are to be considered highly desirable, primarily because they contribute considerably to the global longitudinal strength of the yacht.

When calculating the strength of any beam, there is a benefit when the beam gains depth (height). Beams of greater height have a higher section modulus. Just as with beams of greater height, when calculating a vessel's global longitudinal strength it is the height of the vessel that makes the greatest contribution. Small and medium sized power and sailing yachts usually have very adequate height , so long'l strength calculations are less critical. For larger yachts or for yachts which have a low height to beam ratio, there it is necessary to consider long'l strength very closely. Witness the catastrophic failures of several recent America's Cup vessels....!

As a general guide to the boundary of acceptability, the ABS rules consider that a vessel must be no more than twice as wide as it is high (deck edge to rabbet line), and no greater than 15 times its height in overall length. Beyond these limits, a strictly engineering "proof" must be employed rather than the prescriptive ABS Section Modulus and Moment of Inertia requirements for calculating the strength of the global hull "girder."

The ABS Motor Pleasure Yachts Rule, 2000, is a very suitable scantling rule for boats of any material. Originally created for "self propelled vessels up to 200 feet, the scope of the Motor Pleasure Yachts Rule has been subsequently restricted to vessels between 79 and 200 feet. In that size range, the ABS Rules for Steel Vessels Under 200 Feet, and the ABS Rules for Aluminum Vessels may also be applied, in particular to commercially used vessels. For sailing craft of all materials, the ABS Rules for Offshore Racing Yachts is applicable to sailing vessels up to 79 feet.

The most appropriate means of assessing the adequacy of structure is to assure that a vessel's scantlings comply with the applicable ABS rule, or alternately the applicable rule published by Lloyd's Register (England), German Lloyds (Germany), Det Norske Veritas (Norway), Bureau Veritas (France), etc.

As we can see from the above, framing is highly desirable for any metal yacht. Without framing, plate thickness would become extreme, and consequently so would the weight of the structure...  

Computer Cutting

The labor involved in fabricating a metal hull can be reduced by a substantial amount via NC cutting. What is NC...? It simply means "Numerically Controlled." Builders who are sufficiently experienced with building NC cut hull structures estimate that they can save between 35% and 55% on the hull fabrication labor via computer cutting.

As an example, a fairly simple vessel of around 45 feet may take around 2,500 hours to fabricate by hand, complete with tanks, engine beds, deck fittings, etc. ready for painting. If one can save, say 40% of those hours, or some 1000, then at typical shop rates the savings can be dramatic. By comparison, the number of design hours one must spend at the computer to detail the NC cut files for such a vessel may amount to some three to four man-weeks, or perhaps some 160 hours.

With this kind of savings, the labor expended to develop the NC cut files will be paid for many times over. In fact, the savings are sufficient that NC cutting has the potential to "earn back" a fair portion of the cost of having developed a custom boat design...! Where there may be any doubt, please review our web article on how we use CAD effectively to develop our designs for NC Cutting .

Anymore, it is inconceivable to build a commercial vessel of any size without taking advantage of NC cutting. While this technology has been slow to penetrate among yacht builders, these days it is plain that builders and designers who ignore the benefits offered by computer modeling and NC cut hull structures simply have their heads in the sand. A possibly entertaining editorial on this is subject is Are We Still in the Dark Ages ...?  

Paint Systems

Small metal boats are not designed with an appreciable corrosion allowance. They must therefore be prepared and painted in the best way possible in order to assure a long life.

Current technology for protecting steel and aluminum boats is plain and simple: Epoxy paint .

When painting metal, a thorough degreasing is always the first step, to clean off the oils from the milling process, as well as any other contaminants, like the smut from welding, which have been introduced while fabricating.

The next important step is a very thorough abrasive grit blasting on a steel boat, or a somewhat less aggressive "brush blast" on an aluminum boat. The process of sand blasting a metal boat is expensive and can in no way be looked at with pleasure, except in the sense of satisfaction and well being provided by a job well done.

While there is no substitute for grit blasting, there are ways to limit the cost of the operation. When ordering steel, it is very much to a builder's advantage to have it "wheel abraded" and primed. Wheel abrading is a process of throwing very small shot at the surface at high speed to remove the mill scale and clean the surface. Primer is then applied. Having been wheeled and primed, the surfaces will be much easier to blast when the time comes.

In terms of the paint system, aluminum boats are dealt with more easily than steel boats. Aluminum must be painted any place a crevice might be formed where things are mounted, and should also be painted below the waterline, if left in the water year-round. The marine aluminum alloys do not otherwise require painting at all.

On an aluminum boat, any areas which will be painted should receive the same aggressive preparation regimen used on steel: thorough cleaning, sand blasting, and epoxy paint. Aluminum is less hard than steel, so sand blasting aluminum is relatively fast compared to steel. The blast nozzle must be held at a greater distance and the blast covers the area more quickly.

On a Copper Nickel or Monel vessel, there would simply be no need for paint anywhere.  

Many schemes are used to insulate metal boats. Insulation is mentioned here in the context of corrosion prevention mainly to point out that regardless of the type used, insulation is NOT to be considered an effective protection against corrosion. As with anywhere else on a metal boat, epoxy paint is the best barrier against corrosion.

Sprayed-on foam is not to be recommended. While popular, sprayed-on foam has many drawbacks that are often overlooked:

• Urethane foam is not a completely closed cell type of foam. With time, urethane foam will absorb odors which become difficult or impossible to get rid of. This is especially a problem when there are smokers aboard.
• Nearly all urethane foam will burn fiercely, and the fumes are extremely toxic. Blown in foam should therefore be of a fire retarding formulation, and should additionally be coated with a flame retarding intumescent paint.
• Sprayed-on foam makes a total mess, requiring extensive clean-up. The clean-up process actually further compromises the foam due to breaking the foam's surface skin.
• Sprayed-on foam requires that an intumescent paint be applied, both for the sake of fire suppression, and in order to re-introduce the seal broken by the clean-up of the spray job.

A much better insulation system is to use a Mastic type of condensation / vapor barrier such as MASCOAT, which adheres well to painted steel surfaces, as well as unpainted aluminum surfaces. It creates a barrier to water penetration, and an effective condensation prevention system. Applied to recommended thicknesses of around 60 mils, it is effective as insulation. Further, it is quite good at sound deadening, is fire proof, and will not absorb odors. Mascoat DTM is used for insulation, and Mascoat MSC for sound attenuation, very effective on engine room surfaces and above the propeller. Both are effective whether on a steel or an aluminum boat.

These mastic coatings can be painted if desired. In more severe climates the mastic coatings can be augmented by using a good quality flexible closed cell cut-sheet foam to fit between the framing. The best choices among these flexible cut-sheet foams are Ensolite and Neoprene. There are several different varieties of each. The choice of insulation foam should be made on the basis of it being fireproof, mildew proof, easily glued, easy to work with, resilient, and if exposed, friendly to look at. Ensolite satisfies all these criteria. Ensolite is better than Neoprene in most respects, but is slightly more expensive. One brand offering good quality flexible foam solutions for boats is ARMAFLEX.

Styrofoam or any other styrene type of foam should be strictly avoided. Go get a piece at your local lumber yard and throw it onto a camp fire.... You will be immediately convinced. The same applies to any of the typical rigid or sprayed-on urethane foams. They are an extreme fire hazard and cannot be recommended.  

Zincs are essential on any metal hull for galvanic protection of the underwater metals (protection against galvanic attack of a less noble metal by a more noble metal), as well as for protection against stray current corrosion.

In the best of all possible worlds, there would be no stray currents in our harbors, but that is not a reality. Regardless of the bottom paint used or the degree of protection conferred by high build epoxy paint, zincs must be used to control stray current corrosion, to which we can become victim with a metal boat, even without an electrical system, due to the possible presence of an electric field in the water having a sufficiently different potential at one end of your boat, vs the other end...!

The quantity of zinc and the surface area must be determined by trial and error by observing real-world conditions over time. However as a place to start, a few recommendations can be made. As an example, on a metal hull of around 35 feet the best scheme to start with would be to place two zincs forward, two aft, and one on each side of the rudder. With a larger metal boat of say 45' an additional pair of zincs amidships would be appropriate. As a vessel gets larger the zincs will become more numerous and / or larger in surface area.

Zincs will be effective for a distance of only around 12 to 15 feet, so it is not adequate to just use one single large zinc anode. Zincs will ideally be located near the rudder fittings, and near the propeller. The zincs forward are a requirement, even though there may be no nearby hull fitting, in order to prevent the possibility of stray current corrosion, should the paint system be breached.

Using the above scheme, after the first few months the zincs should be inspected. If the zincs appear to be active, but there is plenty left, they are doing their job correctly. If they are seriously wasted, the area of zinc should be increased (rather than the weight of zinc). During each season, and to adjust for different marinas, the sizes of the zincs should be adjusted as needed.

Good electrical connection between the zinc and the hull must be assured.  

Bonding is the practice of tying all of the underwater metals together with wires or bonding strips. It is done in order to 'theoretically' bring all of the underwater metals to the same potential, and aim that collective potential at a single large zinc. It is also done in order that no single metal object will have a different potential than surrounding metal objects for the sake of shock prevention.

However for maximum corrosion protection, metal boats will ideally NOT be bonded. This of course is contrary to the advice of the ABYC. Keep in mind though that the ABYC rules represent the consensus of the US Marine Manufacturers Association, and are therefore primarily aimed at satisfying the requirements aboard GRP vessels, about which the MMA is most familiar. Naturally, aboard a GRP boat the boat's structure is electrically inert and not subject to degradation by corrosion, therefore aboard a GRP boat there is no reason to recommend against bonding - except perhaps the fact that bonding all underwater metals using a copper conductor invites the possibility of stray current corrosion of those underwater metals due to the possible potential differential in the water from one end of the boat to the other.

Little by little though, the ABYC is learning more about the requirements aboard metal and wooden vessels, and recommendations for aluminum and steel boats have begun to appear in the ABYC guidelines. Even so, the corrosion vs shock hazard conundrum aboard metal boats is not 'solved' since the solutions are not as simple as they might at first seem. For an introduction to some of the issues with regard to bonding, please see our " Corrosion, Zincs & Bonding " booklet.  

Electrical System Considerations

Aboard a metal vessel, purely for the sake of preventing corrosion the ideal will be to make use of a completely floating ground system. In other words, the negative side of the DC power will not permitted to be in contact with the hull nor any hull fittings, anywhere. With a floating ground system, a special type of alternator is used which does not make use of its case as the ground, but instead has a dedicated negative terminal.

This is contrary to the way nearly all engines are wired. Typically, engines make use of the engine block as a mutual ground for all engine wiring. Also, the starter will typically be grounded to the engine, as will the alternator. And typically the engine is in some way grounded to the hull, possibly via the coolant water, or possibly via a water lubed shaft tube, or the engine mounts, or even a direct bonding wire, etc.

Needless to say, for the sake of preventing corrosion, there should not be a direct connection between the AC shore power and the hull. This includes that insidious little green grounding wire. This whole issue is avoided if a proper marine grade Isolation Transformer is installed, which has as its duty to totally isolate all direct connections between shore power and the onboard wiring. This is done by 'inducing' a current in the onboard circuits, thus the electrical energy generated has been created entirely within the secondary coils, and is therefore entirely separate from the shore side power.

The purpose of the green grounding wire is to return any leakage current back to ground onshore, rather than to leak away through the hull and its underwater metals into the water, seeking an alternate path to ground. If a leakage current of greater than 10 milliamps exists onboard (not at all uncommon), it presents an EXTREME hazard to swimmers nearby. This is especially dangerous in fresh water where a swimmer's body provides much less electrical resistance than the surrounding water, and the swimmer thereby becomes the preferred path for any stray currents in the water. With a leakage current above 20 milliamps, death can (and has) become the result. Above 100 milliamps, and the heart stops. Serious business.

The shore side green grounding wire must be brought aboard and connected to the primary side of the Isolation Transformer. It creates a 'fail safe' return path for the AC current seeking ground. But on the secondary side of the Isolation Transformer it serves no purpose onboard because the secondary side will have created an entirely independent electrical system, generated onboard , and not tied to shore power.

Separately, there should ideally be a green grounding wire in the onboard electrical system, however it should not be tied to the shore side green grounding wire. Recommendations differ here, and the Isolation Transformer should be chosen on the basis of providing COMPLETE isolation of the onboard electrical system from the shore power system... What this means is that if a particular Isolation Transformer's wiring diagram recommends connecting the shore side green grounding wire to the onboard green grounding wire (effectively defeating its very purpose) that Isolation Transformer should be rejected as a candidate for placement onboard.

Other "black box" devices should be avoided, including "zinc savers" or impressed current systems, etc. On a military vessel, commercial vessel, or large crewed yacht where these systems can be continuously monitored, such "active" protection schemes may have some merit. However on a small yacht, which may spend long periods with no-one aboard but which may still be plugged into shore power, an "active" system will not be attended to with any regularity, and could easily fail and develop a fault that could potentially cause rapid corrosion, resulting in considerable damage.

The ideal electrical system onboard will be entirely 12v or 24v DC, energized via a large battery bank. All installations should have an Isolation Transformer on the shore power connection. Onboard, the secondary side of the transformer can then be connected to marine quality battery chargers. Some battery chargers are available that have a built-in isolation transformer, but they should be screened on the basis described above. Then onboard if the only thing the Isolation Transformer connects to onboard is a large battery charger, then there is no real connection between the onboard DC system and the shore side AC system.

Using such a system, it is possible to have onboard AC power provided by inverters, directly energized by the large battery bank. This provides yet another barrier between the onboard AC electrical system and the shore power system. It also provides other considerable advantages.... For one, some types of isolation transformer can be switched in order to accept either 110v AC or 220v AC, and to output either voltage , depending on what the onboard equipment requires (essentially just the battery charger in this case). Since the isolation transformer and the battery chargers are also frequency agnostic, if all onboard AC is generated by inverters, you then have a truly shore power agnostic system. All onboard equipment will either be DC, or will be AC generated onboard by the inverters at the requisite frequency and voltage required by the onboard equipment.

Where this scheme gets defeated rather quickly is where there must be an air conditioning system, and / or a washer / dryer, all of which are very power hungry. But we can still keep from bringing shore power onboard to directly serve those items by using the above described system (i.e. shore power > isolation transformer > battery charger > battery bank > inverter > onboard AC system) in combination with an onboard AC generator. In this way, all AC current onboard will be generated onboard, either via the inverters for low current draw items, or by the generator when high current draw items are used, and frequency / voltage suddenly become a non-issue...

The whole point is to keep shore power OFF the boat by limiting its excursion only to the Isolation Transformer, where it stops completely. With all onboard power being created entirely onboard, there is no hazard to swimmers posed by stray currents attempting to seek ground onshore, because the onboard "ground" is, in fact, onboard...

I know there are those who will disagree with the above statements about electrical systems. Whether you agree or disagree, please don't come all unglued over these matters and instead, for much more complete information on these topics, please see the resources mentioned below...  

We can see that metal can make considerable sense as a hull building material. On the basis of strength, ruggedness, ease of construction, first cost, and ease of maintenance, there is plenty of justification for building a metal hull, whether steel, aluminum, Copper Nickel, or Monel.

Steel wins the ruggedness contest. Aluminum wins the lightness contest. Copper Nickel and Monel win the longevity and freedom from maintenance contest.

Part of the equation for any vessel is also resale. In this realm, aluminum does very well, albeit in this country not as well as composite construction. This is mainly a matter of market faith here in the US where we are relatively less educated about metal vessels. As for resale, a vessel built of Copper Nickel will fare extremely well. After all, the Copper Nickel or Monel vessel will have essentially been built out of money...!

Metal is an excellent structural material, being both strong and easily fabricated using readily available technology. In terms of impact, metal can be shown via basic engineering principles and real world evidence to be better than any form of composite. If designed well, a metal boat will be beautiful, will perform well, will be very comfortable, and will provide the peace of mind achieved only via the knowledge that you are aboard the safest, strongest, most rugged type of vessel possible.

It is said among dedicated blue water cruisers in the South Pacific that, "50% of the boats are metal; the rest of them are from the United States....!" Although this statement may seem so at times, it is fortunately not 100% true!!

It is my hope that the above essay will be of some value when considering the choice of hull materials. If you are intending to make use of metal as a hull material you may wish to review the article " Aluminum for Boats " that first appeared in Cruising World magazine, and the article " Aluminum vs. Steel " comparing the relative merits of both materials. Also, in defense of steel as a very practical boat building medium check out the article on " Steel Yachts ."

In addition, there are two excellent booklets available on our Articles and Other Links page. The first of them, the " Marine Metals Reference " is a brief guide to the appropriate metals for marine use, where they will be most appropriately used. It also contains welding information and a complete list of the physical properties of marine metals. The second booklet, " Corrosion, Zincs & Bonding " offers a complete discussion of electrical systems, corrosion, zincs, and bonding.  

Other Articles on Boat Structure

Metal Boats for Blue Water | Aluminum vs Steel | Steel Boats | Aluminum for Boats Metal Boat Framing | Metal Boat Building Methods | Metal Boat Welding Sequence | Designing Metal Boat Structure Composites for Boats | The Evolution of a Wooden Sailing Type  

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Should I be concerned...3.2mm steel thickness

• Thread starter tomdmx
• Start date 21 Jun 2012
• 21 Jun 2012

I've managed to get my hands on an ultrasound from one of the chaps in the boatyard (he is a surveyor but works for the boat yard also)...so I sounded below and above waterline... In the keel I got a consistent 4.5-5mm thickness but after scraping off 2-3mm of pain on the sides below water line, I got a consistent 3.1/3.2 mm thickness of steel (and above waterline with paint I got 4.5mm). I tested in approx 20 spots... Where I could access the tested areas on the other side, the steel appeared very solid etc as I got concerned about thickness. I then talked to the surveyor chap and said that 3mm is not much but it may be likely that this yacht has been built in part a different type of steel named kardan which fits the years and the fact that the thickness would not be much beyond 3mm..No other signs of rust (ie no pitting etc) is evident either (some on the paint but removing two coats shows nice clean steel)... This is a sloop built in Harlingen boat yard in Holland, by Van Der Vlis under Van Der Stadt design (well the plate states Cumulant, Harlingen boat yards, Van D. Vlis) in the 70s... So is this thickness a problem?  

Steel Thickness From what I've been told in the last few weeks - no. I certainly hope not! The thicker the better. It would've been 4mm probably but over 3 is acceptable. I was panicking about thickness but if you can find a good welder, the great thing about steel is it can be patched and if it's done right you won't even know. If the welder starts talking about putting a patch OVER the steel run away - it can be done properly. The only hassle is removing flammable stuff on the inside. I'm not worrying about my 3.2mm bits as I'm in the land of the steel boat and they say it's ok.  

We are wanting to cross an ocean or three and going to remote areas but my mate can weld well and we're taking equipment (ie a very good portable welder etc) with us.... My issue was that both sides of the tested area appeared very good..yet thickness wasnt there (which got me thinking on corrosion within the steel...) not sure if that makes sense...  

Well-known member

What was it when new? Is that Corten steel by another name?  

I don't think you have any problem at all. You don't say what size your vessel is but boats around the 30 foot or so length are usually built of about 3mm except for the keel. Anything thicker and the boat would be heavy and slow and wouldn't be able to carry much stores and supplies. My boat, a Dudley Dix Hout Bay 33, has 5mm keel and 3mm everywhere else. Provided the plating is well supported by the internal framing this is plenty. If the ultrasound gives consistent results and you haven't seen any worrying rust inside or out you need have no fear. Remember that the loss of 1mm of steel produces (I think) approx 8mm of rust/scale, so any rusting going on under paintwork is pretty evident. The areas that tend to give problems are interfaces between steel and any timber, such as hatch frames or teak decking. The Dutch are fine steel boatbuilders and are well respected for it, so I think you can be confident your boat will carry you through.  

Active member

No problem with thickness at all. Whilst the standard for UK narrowboats has been the 10,6,4 mm (base, sides, top) my old De groot only ever started with a 5mm hull thickness. When it got below 3 mm at the waterline (where most degradation occurs) I had a 6" wide overplate placed continuously around the waterline welded top and bottom.  

The 10M Tucker design steel boat I built has thickness of 4mm plate for the hull, 3mm for the deck, 6mm for the sides of the twin keels and 10mm for the keel feet and tops. This was what I ordered from the steel supplier. When surveyed it was found that the decks and the hull thicknesses were about 10% thicker than this. Having never had a welder in my hands before building my boat my biggest concern was the boat falling apart in a sea because if my **** workmanship. Ten years later my greatest concern upon taking the boat out of the water annually is any wastage in the hull (no probs so far). Welds are not a concern any more. I am grateful that I have more hull thickness than recommended by a superb designer. However if I checked my hull thickness below the water line to-morrow and found it about 3mm I would be worried. I am no expert but I assume there is a rule of thumb as regards hull thickness for a vessel which you are talking about. I just get a feeling that the thickness you have found are not the original spec of the designer.  

• 22 Jun 2012

I think your statement "I got a consistent 3.1/3.2 mm thickness of steel " should put your mind at rest. If you get corrosion in steel it is usually inconsistent. Also Dutch builders of steel vessels have always been masters at welding thin steel without buckling. Many builders use thicker plate because of this problem.  

If my memory serves me right, I think 3.2mm is what the Americans call 10 gauge. If so it's the ideal material for a 31 foot boat. 4mm is way too heavy. Its only appears a little difference, but works out as quite a lot over the full area of a boats hull plating. Seems odd though that they would use the thicker 4mm plate on the topsides. Anyway, 10 gauge was the standard steel for hulls up to 36 ft.  

• 23 Jun 2012

My understanding is that steel boats less than 40 feet have a steel plate thickness of 3mm. Otherwise the boat is too heavy. 40 feet + and it is likely to be 4mm. Our steel boat is 38 feet and 3mm. I'm surprised you got readings on the keel. Anything behind the steel such as a frame, or the lead ingots in our keel give a dead reading.  

We tend to talk about plate thickness in an abstract form. Just recently I was going to buy some 4mm stainless plate to do some small scale fabrication. I chose 4mm because I thought it was the same as some plate I had inherited. I was quite suprised to find the plate I had was only 2mm. Not suggesting this for a hull but so long as its a professionally built hull with adequate framing;welding etc. 3mm should be no problem Remember Van de Stadt was a pioneer in developing construction methods akin to the mirror dingy stitch and glue process where thinner preshaped hull plate sections were welded up first and frames stringers and bulkheads added afterwards-in this form the hull plates themselves are part the of the overall structure unlike traditional designs where the frames;bulkheads and stringers were the structural form and plates did no more than keep the water out so to speak. Latest design plywood Wharram cats use similar techniques.  

• 24 Jun 2012

I'd suggest that if you want to know what the original thickness should have been, Van Der Stadt would be the people to ask http://www.stadtdesign.com . They may well have a construction schedule that will tell you exactly what thickness of plate was specified. I do note that 1/8" is 3.175mm, and also that nominal 3mm plate could be 3.1 or 3.2mm and still be within manufacturing tolerances. If 3mm or (less likely) 1/8" plate was specified then you're fine. If 4mm is what you should have, then your readings show wastage of 20%+, and that would be a problem, but I don't think that's likely. I'm currently looking to buy an ultrasonic thickness tester that can give accurate measurements through paint that ignore the paint thickness. They're not cheap (the cheapest quote I've had was £1100 plus VAT), but they do make surveying steel boats a lot easier (without one I'd either have to get permission to grind back to bare metal (unlikely to be given), or buy a paint thickness gauge and subtract the paint thickness from all of my readings, and I'd still have to shell out for a fairly expensive thickness tester to get a reading through paint at all).  

>frames stringers and bulkheads added If you are looking for a steel boat and it has stringers walk away, they trap moisture and are not needed for structural integrity. Also the frames should be spot welded to allow any moisture to run round the weld, although this is unusual unless built professionally by a Dutch yard and perhaps other steel boat yards around the world.  

KellysEye said: >frames stringers and bulkheads added If you are looking for a steel boat and it has stringers walk away, they trap moisture and are not needed for structural integrity. Also the frames should be spot welded to allow any moisture to run round the weld, although this is unusual unless built professionally by a Dutch yard and perhaps other steel boat yards around the world. Click to expand...

DanBurrill said: Having the shell plating only spot welded to the frames is unusual, because it means that the shell plating is likely to be inadequately supported. There are also plenty of properly designed steel boats that have stringers. Moisture traps on steel (or indeed pretty much any) boats are avoided by having limber holes in frames, stringers, and anywhere else that could trap moisture, and keeping them clear. Click to expand...

chinita said: I am struggling to recall seeing a steel cruising yacht without longitudinals; Click to expand...

srm said: My previous boat, built in the late 50's or early 60's had angle frames at 16inch spaces, 3mm plating for all the curved hull and 4mm in the keel. Only longitudinals were the keel, deck edge and cabin sides. A James Mcgruer 8 metre cruiser racer, the first owner bought plans for the wood design, and had it built of steel in Belgium. Unfortunately, she rusted away from the inside, and is on her second rebuild. Click to expand...

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> > Please support our sponsors and let them know you heard about their products on Cruisers Forums. 01-06-2020, 19:43   Boat: Volkscruiser yachts built in the 1970's and 1980's. How thin does the plating have to be before the yacht is scrapped? Cheers 01-06-2020, 21:01   Boat: Nantucket Island 33 well that the problems start. rust from the inside out. My experience has been that in the bilges ultimately dooms them to scrapping. It's not because the plating is thin so much, but rather the sheer difficulty in gaining access to and repairing the structure within. Plating itself is easily patched and a common "field method" is to try and poke holes in suspect looking areas with a spike although the technical purists would probably use an ultrasonic thickness tester and with percentage loss of thickness as an indicator. 02-06-2020, 05:14   Boat: Island Packet 40 makes it's way all the way through and even then a hole may not appear until you beat all the rust of. 02-06-2020, 06:43   Boat: Murray 33-Chouette & Pape Steelmaid-44-Safara-both steel cutters . companies want them. A much better gage is a good hammer and a bad attitude. The first time I replaced plate I was in a panic. Probably didn’t do a real good job as a result. Now I’ve done it or had it done a number of times it’s no big deal. The General before and after clean up and painting is a bigger deal. 02-06-2020, 09:14   post on a steel or vessel. Max, SAMS, Mexico 02-06-2020, 09:15   Boat: 75' steel sutton ketch,Warwick 70' aluminum sloop . companies want them. A much better gage is a good hammer and a bad attitude. The first time I replaced plate I was in a panic. Probably didn’t do a real good job as a result. Now I’ve done it or had it done a number of times it’s no big deal. The General before and after clean up and painting is a bigger deal. 02-06-2020, 09:51   Boat: PDQ Altair, 32/34 General thickness. Structure and leaks are two separate issues. Areas where the reverse side is not accessible. Accessing the texture of a side you cannot see (the echo changes if there is corrosion) UT can work better than a hammer if there are thick coatings. Requires experienced operator and A-scan ultrasound meter. Plates thicker than 1/8". Thicker than that UT us generally better. Fuel tanks . You really don't want to put a hammer through the side of a 1,000,000 gallon gasoline tank. By extension, a boat in the water! You can UT test through bilge water . 20/20 eyesight (corrected) Bright light Cleanliness Hammer (hard--will damamge paint) UT a-scan meter (not just thickness) and couplant 02-06-2020, 10:14   Boat: Murray 33-Chouette & Pape Steelmaid-44-Safara-both steel cutters . But back to the question about hull thickness foe a moment. Most smaller yachts are 10gage steel. That’s a tiny bit thicker than 1/8”. Larger tend to be 3/16”. Then you get boats with 1/4” and 3/16 topsides. I have seen a 52’ boat made of 10 gage steel. Been 10 years since I saw it, and it was 25 years old then. Saw it last year so it’s still around. 3/16” gives you more “wasting” time. You can go pretty thin and keep out, but the you protection goes to hell. 02-06-2020, 10:35   Boat: Farrier f27 becomes prohibitive. But to the OP, hull thickness would be dictated and gauged by overall hull design such as frame spacing. well that the problems start. Steel boats rust from the inside out. My experience has been that corrosion in the bilges ultimately dooms them to scrapping. It's not because the plating is thin so much, but rather the sheer difficulty in gaining access to and repairing the structure within. Plating itself is easily patched and a common "field method" is to try and poke holes in suspect looking areas with a spike although the technical purists would probably use an ultrasonic thickness tester and work with percentage loss of thickness as an indicator. 02-06-2020, 13:44   Boat: Custom steel, 41' LOD . She's hard-chined (a Colvin Gazelle hull) and 10 gauge steel down to the top of the box keel. The original box keel is 3/8" steel sides and 1/2" steel on the bottom. In 1989 we extended the box keel down a foot (long story) and moved the 6000 pounds of lead ballast down there. The fabricator of the keel extension didn't have any 1/2" steel kicking around so the bottom is now 3/4" plate (after all, it's ballast). The first time I hit a rock with it (I confess to have hit one or two. After all, I the BC coast!) - and I hit HARD - I scrambled below and tore up the floorboards looking for water coming in. Nothing. The next time I hauled out it took me ten minutes with a file to take the burrs off the edges of the scratch in the keel before repainting. Even now, after nearly 40 years, if I remove the foam to do something, the galvanized steel behind is pristine. After removing some tanks, the inside of the box keel is quite accessible and gets painted every five years or so. I have absolutely no concerns about structural rust anywhere - and if it does develop, as others have pointed out, it's easy enough to replace some plate or weld on a patch. The point is, a well-built, well maintained steel boat can last pretty well indefinitely. Go steel! 02-06-2020, 17:48   . The computer can be damaged from EMF caused by welding on the boat. Best remove it. 02-06-2020, 18:57   Boat: 47' Steel Roberts Cutter 02-06-2020, 19:09   Boat: Woods Vardo 34 Cat 02-06-2020, 19:34   Boat: 55' Romsdal 02-06-2020, 19:43   Boat: Woods Vardo 34 Cat but I would assume 144 1" pieces welded together is not as strong as a 1 square foot piece? Or is the weld 100% if done right? , , Thread Tools Rate This Thread : Posting Rules post new threads post replies post attachments edit your posts is are code is are are are Similar Threads Thread Thread Starter Forum Replies Last Post BlackWind Monohull Sailboats 40 19-05-2016 04:45 xymotic Construction, Maintenance & Refit 4 03-01-2012 06:40 Capt. lulz Our Community 4 22-08-2011 23:32 cilrath Construction, Maintenance & Refit 22 03-08-2011 19:09 Rangiroo Electrical: Batteries, Generators & Solar 9 18-08-2007 23:47 - - - - - - - Privacy Guaranteed - your email is never shared with anyone, opt out any time. Log in or Sign up You are using an out of date browser. It may not display this or other websites correctly. You should upgrade or use an alternative browser . Steel plate Thickness Discussion in ' Sailboats ' started by LowcountryData , Nov 21, 2003 . LowcountryData New Member Hello all, We are currently designing a 65' vessel and are having a hard time getting the recommended thicknesses needed before we can submit out plans to a Marine Surveyor. We have asked the major builders ... particularly Bruce Roberts, but without a purchase of their plans we have been basically turned away. We need thicknesses for the following areas and any others you might think of; Areas & Currently suggested thicknesses are; keel = 3/8" bottom hull = 1/4" side hull = 3/16" ribs = 1/4" topside deck = 3/16" stringers = 1/4" I see many posts in here referring to "mm" does anyone have a conversion table handy???<LOL> These we have aquired from several sources, the surveyor will not do any recommendations due to insurace reasons, so we respect that. But he did recommend this site so here we are. Our vessel most closely resembles a NY65 or a Voyager 65, that was why we wanted to get Bruce Roberts input but alas 5700$ to get the answers we want didn't seem reasonable at all. Any help or advice would be greatly appreciated, Thanks in advance,   8knots A little on the slow side Low Country: Get on amazon.com and order Dave Gerr's book "The elements of boat strength" It will walk you through all the math of developing a scantling # for your hull then figure step by step every componet of your hull. Even a simpelton like me figured it out It is the best $24.00 you will could spend in my opinion! Your numbers seem in line off the top of my head. But you will come to understand that your ribs for eg, will have 4 figures you will need. the rib will resemble a "T" the rib itself will have width and thickness figures then the top or inside of the "T" will have a width and thickness thus 4 figures. all are derived from your SN (scantling number) that he explains in detail how to mathmaticly obtain from a small group of numbers describing your hull. A few evenings and some paper,the book and calculator will give you what you require! Good luck! 8Knots   http://www.amazon.com/exec/obidos/t...=sr_8_1/103-0329688-7523015?v=glance&n=507846 This is the link- Click on the "look inside" and go to the table of contents. You will see a list of all the formulas. He does a good job of describing construction method too! 8Knots   gonzo Senior Member You need more than just thicknesses. The design of gussets, frames, knees, beams, etc. is just as important. Each design requires a particular way of building and reinforncing the structure. Bruce Robert's price is not expensive considering the time and liability. If you have a hard time converting metric to standard, it's going to be hard to understand Gerr's or any other engineering book. $5,700 is a tiny percentage of the boat's cost. Also, any mistakes will make your labor and materials worthless. The plate thickness is the least of the complications of the design. Have you considered what the weld schedule will be?   Stephen Ditmore Senior Member I endorse what's been said. There are several approaches to metal boat design. A boat without much internal framing will require a thicker skin than one with closely spaced frames. A frame spacing of 24" is typical for commercial steel vessels.   Guest Guest i think it is best to get a proper and complete set of clasification rules as the scantlings are dependent on lots of factors. this will also alow you to optimize the desig (plate thickness/ frame spacing etc.). in my experience the ABS has rules that alow reasonable "light" scantlings for sailing vessels, ABS rules ( guide for clasing offshore raicing vessels 1994 (?? not quite sure aboutthe title??). The ABS will give more freedom of choosing a construction methode and you will probably end up with a lighter design. Please be aware that local reinforcement (mast/ stays etc) are not included in the formulas but more a less to "surveyors satisfaction". most often for steel sailing yacht i see platethickness of 4 mm for the hull, as this results in a lighter construction, frame spacing will be approximately 400...450 mm. I hope this will help.   YEP... I AGREE! Gonz0's point of $5700 for plans is childsplay in the big picture of constructing a 65' vessel. You will loose that much in dropped washers, welding rod left out in the rain, miscut plate because you only had 3 cups of coffee rather than 4 and all the other pitfalls that WILL happen in the construction of your boat! Thats why they cost 2 Million from a custom builder. Steve is right on the many methods of construction. I have Bruce's "Metal Boats" It was my first book on metal boats. Worth the money anyday! He does go into construction method as does Gerr. After 24 years of study I will still take my numbers to a real N/A to confirm I am not missing something. In my opinion boat design is a combination of......Math, Gut feeling, and previous experience. Those in the field will have a firm handle on all of them and the fee's they must charge are warented by the many sleepless nights they endure trying to produce the best product they can. A reputable designer will work hard for you because the design is an extension of themself, there name, and the firms reputation. For the record Gerr's book outlines all formula's in both Imperial and metric. Thanks for the soapbox 8Knots   mmd Senior Member As a professional designer, I can't help but agree with 8knots' choice in having numbers checked by another qualified designer, and Gonzo's viewpoint on the cost of design services as a percentage of the overall constructions costs. To risk being unnecessarily dramatic, consider the problem in this light: Will the warm feeling of saving a few grand in design costs comfort you when you are in the middle of an offshore passage with the kiddies bunked down for the night, and as you are checking the latest weatherfax you realize that you will not be able to outrun that storm front on your tail before you reach the nearest safe port. Are you really confident in those structural calculations that you made those many months ago? After careful & serious consideration of that question if you are not comfortable, call a designer with some experience in steel construction. A design check by a pro is good insurance and not really that expensive in the larger scheme of things. Your life may depend on it.   SuperPiper Men With Little Boats . . Because of the discussions on this thread, I have ordered Gerr's book. As 8knots suggested, I read the first 2 chapters on-line. I have a mini-pocket-cruiser. With each glass project I wonder: did I apply enough layers? did I extend the tabbing far enough along the hull or bulkhead? is the core thickness correct? And, IMHO, $5700 is a lot of money.   If $5700 is a lot of money, then building a 65 footer is way out of your budget. I also think that prospective buyers are sometimes too eager to dismiss the amount of work a proper desing takes. Three weeks of my time is worth every penny of $5700. Also, I am liable for mistakes.   rjmac Junior Member Ditto....Ditto..... If you are uncomfortable with $5700, 65ft is out of your range....., you need to be realistic, and not meant to be rude.   Your Own Design If you don't know how to calculate structure you may want to rethink doing your own design. Do you know how to calculate stability? Do you know how to calculate powering? Also note that Gerr's book is really only for understanding strength, not for actually doing it, especially for such a large boat. In actual design practice, the internal structure and its spacing is a trade for plating thickness - more closely spaced frames = thinner plate and vice versa. However, closely spaced frames tend to have more labor, so it is a trade requiring some knowledge. Gerr just suggests a standard thickness and spacing which might not be right for a given project. Also, Gerr's book has no official standing, so a given surveyor/underwriter may not accept it. Only class rules or ISO rules have the necessary buy-in from all stakeholders to establish a real standard.   Gerr's and other books that give scantligs and construction techniques have to be checked against whatever rules apply. It takes knowledge and experience to decide which way to go. For example, a rule may call for more closely spaced frames; this may allow thinner plating. Another rule may specify fireproof resins in the engineroom; they have different characteristics than regular epoxy and are not compatible with Dynel.   I think that most of us around here (at the forum) we are in the idea that we can help each other and find interesting information all of excellent topics related to boats but when it comes to this kind of "i'm building a 65' boat but...i don't know what scantlings use and $5700 for set of drawings is too much!....please, FORGET IT! is not worthy to keep having posts toward this, please leave alone this post and let's help people that really value the work and responsability that is to design a boat, and help the ones that need some advice. Glad to be here in the forum and be able to help.   Advertisement: Take it easy "guest". The forum is open to all questions about boats. Lowcountrydata is trying to get information and is welcome to it.   Steel plate for small steel boats Aluminum vs carbon steel for supporting chainplates / shroud load calculation George Sutton steel schooner Brent Swain 26' Steel Yacht Steel masts Boden "Elizabeth" Steel Ketch Cheap steel yacht multi chine in Bulgaria Liza Jane, steel 20' sailboat Watts steel ketch No, create an account now. Yes, my password is: Forgot your password? MarketPlace Digital Archives Order A Copy Steel for Sail and Power S teel ships are the backbone of world trade, and navies around the world maintain their allegiance to the metal. Like-minded builders of smaller commercial fishing boats, tugs and barges also favor the iron/carbon amalgam. So why do we see so few recreational power and sailing vessels being built from what’s arguably the strongest and one of the least expensive boatbuilding materials? Before attempting to answer, it makes sense to take a close look at what steel has to offer.   Riveted iron was the first step in a ship building renaissance, a trend that gave white oak and spruce trees a bit of a reprieve. Eventually, carbon was added to iron increasing its tensile strength and stiffness. At about the same time, welding expedited the building process and steel plate was cut and shaped using highly directable flame heat from oxyacetylene torches. Today, steel can also be cut with laser, plasma, waterjet, and saw blade technology. Metal workers bend hull plate over steel frames, tack weld the plate in place and eventually carefully fuse all the seams together.   Stick welding became a highly prized craft essential to how frames were tacked in place and plate-to-plate seams were joined. At the heart of the process is the welder’s electrical transformer, a tool that turns AC current into lower voltage higher amperage DC current with the capacity to melt metal. Its lower voltage dissipates the shock hazard. Electrical welding harnesses an intentional short circuit. The positive and negative leads meet at the point where the welding rod touches the grounded plate. A key factor in welding involves smoothly working the rod across the seam allowing the high current to momentarily turn both the rod tip and plate into molten metal. When the steel “weld pool” cools, the resulting joint is as strong or even stronger than the hull plate itself. Pluses and minuses Mild steel, as a material, has a long list of desirable attributes along with a couple of potential showstoppers. On the plus side, resides toughness, malleability, and isotropic strength (equal strength in all directions). The net effect of these attributes includes abrasion resistance and a structure that reacts to point loads by deforming rather than tearing. It’s also the least expensive of modern small craft building materials and is relatively easy to repair. The build process can be expedited using computer aided design (CAD), numerically controlled cutting (NC) and laser, plasma or waterjet cutters that steel suppliers use to provide pre-cut hull plating that fits together like puzzle pieces. Steel boat designer Michael Kasten has found that this service can cut building time of a 45-footer by up to 40%. Rust is the enemy of every steel boat owner. Adding carbon to iron increases the metal’s strength but also ups its tendency to oxidize. As steel begins to corrode a powdery, rufous-colored scale quickly grows into flake-like layers of rust as the material’s strength and stiffness disappear. Steel ships are designed with a specific percentage of added plate thickness to account for corrosion over the vessel’s design lifespan (usually 20 years). Small craft designers can’t afford to add the weight of thicker plate and the design process seldom incorporates such corrosion compensation. Instead, contemporary coatings, meticulous preparation and application techniques will do a very good job of holding rust at bay.   Streamlining steel boat construction defies the round bilge smooth curve status quo. And one of the biggest challenges involves generating hard chine aesthetic appeal and maintaining bilateral symmetry. In short, the challenge is bending flat plate into a functional hydrodynamic shape with enough aesthetic appeal to draw a sailor’s eye. In years gone by, master craftsmen struggled to twist and cajole steel plate over round bilge frames that incorporated compound curves galore. In many cases, several hundred pounds of epoxy filler had to be pasted to the hull, troweled out and sanded smooth with “long boards” to mimic the fairness of a timber or FRP hull. Today, single chine, multi chine and radius chine designs prevail. They are designed to minimize the slab-sided look and are much easier to build than a complete round bilge boat. The FRP production boat industry has helped by following automotive trends, and adding a chine to their racers and cruisers.   Fine tuning stability The design challenge also includes how the significant weight of steel is handled. When it comes to vessels less than 50 feet, weight distribution becomes an even bigger issue. For example, to lessen weight above decks and still minimize deck flex, a designer must use thinner plate, 10- or even thinner 12-gauge steel. This requires shorter spans between transverse and longitudinal support or a switch to stiffer Corten steel. Some builders even switch to aluminum above the sheer, a weight saving alternative that ups costs and adds complexity.   This metal transition requires the installation of an explosion welded bi-metallic strip that’s composed of aluminum bonded to steel. It allows a fabricator to conventionally weld one side of the junction strip to the hull’s sheer and then TIG or MIG weld an aluminum superstructure to the opposite side. Welding aluminum requires an inert gas to shield the arc, and the plate is harder to weld but easier to cut. The surface can be left uncoated, it will form a self-protecting, lightly oxidized layer that abates further oxidation. The steel hull, however, must be blasted, primed and painted inside and out. And as Michael Kasten professes, “clean and grit blast the surface, apply epoxy and avoid using sprayed-in foam insulation.” The all-steel alternative can also be designed as a seaworthy vessel if careful attention is paid to weight distribution and the height of the superstructure. Payload location can also be a vital consideration. During the design process, every effort should be made to place machinery and integral tankage as low in the bilge as possible. Chain, batteries, and heavier equipment should also reside in the dry spaces below the cabin sole. If a power cruiser is to be an offshore passage maker, these vertical center of gravity considerations rule out the double-decker riverboat look and it’s also wise to avoid perching a sizable runabout and lifting crane on the top deck, aft of a heavy flybridge. A fringe benefit found aboard lower air draft power cruisers is that it places cabin space closer to the waterline where there is less effect from pitch and roll and windage is lessened.   Sail and power Over the last 40 years I’ve kept track of a small but hearty 45-foot tug/work boat built by Gladding-Hearn Shipbuilding in Somerset, Mass. Dragon belonged to a friend of mine and played a central role in his marine construction business. And whether he was pushing a small crane barge, towing a load of pilings up and down Long Island Sound or when the vessel was loaded up with a sunfish, whitehall rowboat and provisions for a summer family cruise to Block Island — Dragon fit the bill. For decades Captain Jim and now his oldest son Eric have followed a regular rust abatement routine. Their anti-corrosion strategy included regular inspections of hard-to-get-at confines and never painting over rust. The grinder and wire wheel effectively abraded small spots but grit blasting to “white-metal” status was used when appropriate. Their painting preference revolved around PPG Ameron products. High on their to-do list was changing zincs and meticulously servicing the trusty old Detroit 6-71 diesel.   Steel sailboats and power cruisers still hold justified appeal, but it’s important to understand what ownership entails. This is especially true for those considering a DIY build of a steel cruising boat. A good starting point is a thorough review of both Bruce Roberts and Michael Kasten’s in-depth online commentary. There’s plenty of valid detail about building metal boats, both aluminum and steel. Those with experience in welding and metal fabrication have a very significant head start and finished hulls will reflect those who learn metal work during the project and those who start out with essential fabrication skills.   If you’re considering purchasing a steel cruising boat it’s essential to engage a skilled, metal boat-versed, marine surveyor. But before that develop a clear vision of what you are after. A handy way to compare vessels is through the use of parametric analysis. It’s basically, a straight forward spec comparison among two or more vessels and recognition of how the numbers relate to underway characteristics.   Two hulls compared In this case I’ll compare my own well-seasoned 41-foot Ericson (18,000 pounds displacement (six-foot draft, 10’ 8” beam, 8,200 pound   ballast, 750 square foot   sail area) with a classic round bilge, steel 37-foot Zeeland Yawl, (18,000 pounds displacement 5’8” draft, 10’ beam, 5,700 pounds ballast, 550 square foot sail area). Though the two boats’ displacements are similar, the Zeeland Yawl’s working sail area is a lot less. This is likely due to a lower righting moment (ability to resist heeling). A further indicator of this diminished stability is the lower ballast/weight ratio, even though the displacement numbers are the same. The net result is a bit less ability to recover from a deep knockdown or capsize. The designer saw this and responded with a smaller sail area that induces less of a heeling moment.   In real world terms this means that the E-41 would be far more efficient sailing in light air as well as more likely to avoid a knockdown, even when both boats are deeply reefed. But when it comes to sailing higher latitudes with bergy bits floating by, or fetching up on an uncharted rocky shoal, the Zeeland Yawl’s Corten steel hull wins hands down.   The reason welded steel construction has dominated the maritime industry for decades yet made only a slight ripple in the realm of recreational small craft construction is multifaceted. In part it’s due to the production efficiency of molded FRP boat building, the ongoing concern over corrosion, plus the reality that most recreational craft aren’t put to the same rugged use as commercial vessels and work boats. However, for those who sail or power cruise well off the beaten path, steel hulls are still held in high regard and to rank number one when it comes to abrasion resistance and survivability in groundings, collisions, and other blunt force trauma. n   Ralph Naranjo is a circumnavigator and the author of The Art of Seamanship (International Marine/Ragged Mountain Press).   Find anything, super fast. Destinations Documentaries We don't have any additional photos of this yacht. Do you? Nord Star Specifications Name Nord Star Model Custom Class M-SP, Russian River Register Hull NB O-110-1 Hull Colour - Naval Architect Moscow Shipyard Exterior Designer Moscow Shipyard Interior Designer Moscow Shipyard Length Overall 34.6m Length at Waterline - Draft (min) - Draft (max) 1.6m Gross Tonnage - Cabins Total 4 Hull Configuration - Hull Material Steel Superstructure - Deck Material Teak Fuel Type Diesel Manufacturer Caterpilar Power 385 hp / 283 kW Total Power 385 hp / 283 kW Propulsion - Max Speed - Cruising Speed 11 Kn Fuel Capacity - Water Capacity - Generator - Stabilizers - Thrusters - Amenities - Yacht Builder Timmerman Yachts No profile available Naval Architect Moscow Shipyard No profile available Exterior Designer Moscow Shipyard No profile available Interior Designer Moscow Shipyard No profile available Yacht Specs Other timmerman yachts. Dyna-Ski Boats custom builds outboard powered water ski boats for recreational skiers and show ski clubs. We have customers all over the world including Malaysia, the Caribbean, Moscow, Russia, the Cayman Islands and Canada. This blog is used to keep readers informed about what is going on at Dyna-Ski and answers questions that are frequently asked. You can also visit www.dyna-ski.com for more information about our boats. Contact Dyna-Ski at [email protected] or call 715-854-7501. Wednesday, August 13, 2014 Tracking fins. No comments: Post a comment. 213 IMAGES Ultrasonic Thickness Gauging of Steel or Aluminium Yacht Hulls RADFORD YACHT DESIGN Hull thickness Surveys for steel & aluminium yachts Bering 65 under 24 meter steel trawler yacht Ultrasonic thickness measurements of a steel hull Steely Resolve: Linssen Yachts Proves That the Steel Pleasure Boat is VIDEO 40 m Steel Hull Motorsailer EXTREME walkthrough Yacht For Sale Steel Hull EXPEDITION SUPERYACHT With A 5,000 NM Range! BERING 77 STEEL EXPLORER YACHT Check Out This STEEL Hull TRAWLER YACHT With A 4,000 NM Range! THIS Is Hull 1 €585K STEEL Trawler Yacht With A 5,000 NM Range! Ultrasonic thickness testing the steel hull plates on the trawler COMMENTS Metal Boats For Blue Water Above 45 feet and steel structure begins to come into its own. Above around 50 feet, a steel hull can actually be quite light for her length (by traditional cruising vessel standards). ... with 1/8 inch thickness being a more likely lower practical hull thickness limit. As a comparison, the minimum thickness for other materials (mainly due to ... thinkness of hull steel Re: thinkness of hull steel. As one of the other posters said, steel thickness typically is reduced higher on the hull. On a 60 footer the keel bottom will probably be 3/4", the keel sides will probably be 3/8" to 1/2", 1/4 to 3/8 on the bottom, 3/16 to 1/4 on the sides, usually I'd expect 1/8" on the decks. Should I be concerned...3.2mm steel thickness The 10M Tucker design steel boat I built has thickness of 4mm plate for the hull, 3mm for the deck, 6mm for the sides of the twin keels and 10mm for the keel feet and tops. This was what I ordered from the steel supplier. When surveyed it was found that the decks and the hull thicknesses were about 10% thicker than this. Steel Hull Maintenance Posts: 2,103. Boy, I'm not sure where the idea that 4 mm is sufficient for the hull of any boat over 30 feet or so. Delfin is 1/4", or 6.4 mm, and prior to fairing, showed deflection in the bow plates with the ribs on 18" centers. If the steel was Corten, maybe 4 mm would work, but Corten steel has other issues. Steel Hulls That said, small steel boats are generally stronger than larger ones. Here are some thicknesses of steel hulls I had built in the past; Tom Thumb 24: Hull = 3mm, Deck = 3mm - Frameless design - multi chine. Tom Thumb 26: Hull = 3mm, Deck = 3mm - Frameless design - multi chine. v/d Stadt 34: Hull = 4mm, Deck = 3mm - Frameless design - multi chine. Steel hull how thin? But that's NOT what you get on a standard hull survey. But back to the question about hull thickness foe a moment. Most smaller yachts are 10gage steel. That's a tiny bit thicker than 1/8". Larger boats tend to be 3/16". Then you get boats with 1/4" keel and 3/16 topsides. I have seen a 52' boat made of 10 gage steel. Linssen Yachts Proves That the Steel Pleasure Boat is Alive and Well The process starts with clean, shot-blasted steel plates arriving at the factory gates. The thickness of steel used ranges between 4 and 6 millimeters depending on where it is used and the size of the yacht to which it is destined. Linssen cuts these steel plates to shape using a CNC plasma profiling machine. Aluminum and Steel, one designer's view Indeed, the plate thickness for small-boat steel hulls are somewhat heavier than required for basic strength to allow for corrosion, the corrosion allowance. A good rule of thumb is that a steel hull will loose about 0.004 in. of thickness every year. A well maintained and properly built steel boat will do a bit better than this in most places. Sailing Cruisers: The Ultimate Comparison of Hull Materials GRP is far superior to steel in terms of weight. For boats up to about 40 feet, it is also superior to aluminum, since aluminum has a minimum thickness of about 5mm making smaller boats heavier than their GRP competitors. Above the 40-foot mark, the tide can turn, and aluminum may be lighter than GRP, but it depends on the exact construction. Steel plate Thickness Please be aware that local reinforcement (mast/ stays etc) are not included in the formulas but more a less to "surveyors satisfaction". most often for steel sailing yacht i see platethickness of 4 mm for the hull, as this results in a lighter construction, frame spacing will be approximately 400...450 mm. I hope this will help. Steel for Sail and Power Steel boat designer Michael Kasten has found that this service can cut building time of a 45-footer by up to 40%. Corrosion can be kept at bay by proper maintenance and modern coating technology. Rust is the enemy of every steel boat owner. Adding carbon to iron increases the metal's strength but also ups its tendency to oxidize. Steel yacht building questions The cost of a steel hull is very competitive when compared to other materials and the price of steel has remained constant for several years. The cost of the materials alone to build a 35 to 40ft yacht hull and deck is around £4000.00 ($6400.00 US) (UK prices in 1998). This makes the cost of a steel hull particularly good for the home builder. Ost Power 20 GRP Sport Fisherman or general purpose boat Ost Power 20 sport fisherman or general purpose boat. DUDLEY DIX YACHT DESIGN. Ost Power 20 . Versatile new powerboat ~ Production builder ~ Modern styling ~ Sportfisherman ~ Diveboat ~ Patrolboat Ost Yachts website. MENU ... Hull draft - 0.33m [1' 1"] Displ - 1425kg [3140lb] Motor Yacht VALDAI-1 Motor yachts, super yachts, CAD-CAM for naval architect. We like shipbuilding ... Workshop hull drawings : Valdai-1. Valdai-2 . Basic project /code: PC870 /Valdai-1. Modernization project / yacht's name: ... Material of hull/ superstructure: Steel /ALU . Beautiful photos of the yacht's interiors here: ... Nord Star Superyacht Hull Configuration - Hull Material. Steel Superstructure - Deck Material. Teak Decks NB - Engine(s) Quantity. 2 Fuel Type. Diesel Manufacturer. Caterpilar Model ... Timmerman Yachts No profile available. Naval Architect Moscow Shipyard Tracking Fins The older boats had a cast aluminum tracking fin. They are no longer available. We switched to a composite tracking fin a long time ago for several reasons. The main one being a composite fin will break off and do less damage to the hull than a metal one will. Easier and cheaper to replace a tracking fin than to fix a hole and replace a fin. catamaran gulet motorboat powerboat riverboat sailboat trimaran yacht