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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 slightly more 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 an increase in the 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.

In general, it is easy 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). However, 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 a vessel that has relatively heavy displacement already.

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 hull. 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 cruising or serious blue water sailing possibly down to around 28 to 30 feet LOD.

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 sleek and graceful hull shape.

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 greater strength than the same hull in steel. In other words, given the same weight budget the aluminum structure will be able to have a higher strength than the same design in steel.

Further, 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 hull structure without hull shape changes. A titanium structure having an equivalent strength to a steel structure will be approximately 40% lighter than the steel structure, and thus even lighter than an aluminum structure.

Choice of materials for a boat's structure usually comes down to a question of cost, which will be considered 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. For an amateur builder however, working in 10 gauge mild steel without knowledge of those 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 a 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. In the battle against distortion, it is better to use a few more longitudinals, strategically placed. Other tricks can be employed 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.

Corten Steel: For smaller steel vessels that must use 10 gauge steel for plating, we 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 for 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: Steel is more rugged than aluminum, being tougher and much more abrasion resistant. The various HSLA steels, 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

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. Pre-primed steel plate will cost 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 work with 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. This means not only can the overall strength be greater than with steel, but the distortion levels can be relatively more easily managed. In so doing, of course cost would be proportionally greater as well.

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 very 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.

Another 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 one can make an excellent case for the use of cut-sheet foams, such as Ensolite and Neoprene. In the latter case, it is desirable to lightly blast the aluminum, and provide an epoxy primer coating prior to insulating.

However there are other coatings for the interior of an aluminum boat which provide sound deadening and insulation. Two products in particular are Mascoat DTM for insulation, and Mascoat MSC for sound attenuation. Our usual specification 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.

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 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 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, and also 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:

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. Copper Nickel is very easy to fabricate, being both easy to cut and to weld using the MIG process.

Besides not having to paint CuNi and its natural resistance to fouling, CuNi is also easy to cut and weld, it has relatively great 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 (2012) 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 main 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 need to make use of 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 will 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, then we might 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, say using approximately half the stringer spacing prescribed for the same thickness of mild steel plating.

It is unlikely that one would choose CuNi for the internal framing, primarily because of 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 only in order to take full advantage of its benefits, but 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...

With this strategy the weight can be kept to roughly the same as would be the case for a 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.
 

Monel

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

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, 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.

Plastic Range: Among the higher strength Titanium alloys there is little spread between the yield point and the failure point. Thus titanium has a very limited plastic range. However, elongation before failure is fairly high compared to, say steel.

Stiffness: Another characteristic is "stiffness" expressed through 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, before it is made to yield (the point at which a material is deformed so far that it will not return to its original shape when released), 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, 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.

An interesting Titanium alloy is the experimental alloy 5111 (5% Al; 1% SN; 1% Zr; 1% V; 0.8% MO), 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.

Another Titanium alloy is the proprietary "ATI 425 Alloy" being made by Allegheny Technologies Inc. (ATI) who are targeting this alloy at ship structures.

Although I believe Titanium would be an outstanding hull material, it requires extreme care during construction, thus labor costs would be very high. If those factors can be mitigated or if cost is not an issue, then Titanium may possibly be the "ultimate" in terms of heirloom boat hull materials...!

However, 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.
 

Aesthetics

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...

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

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

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....

Why...?

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.).

  1. Select plate material according to owner preference, available budget, and desired strength or other material properties
  2. Select preferred plate thickness according to availability, suited to vessel size and displacement
  3. Calculate local longitudinal spacing to adequately support the plate
  4. Select frame spacing to satisfy the locations of interior bulkheads or other layout considerations
  5. Calculate scantlings required for longitudinal stringers to satisfy their spacing and the span between frames
  6. 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

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:

  1. For any given vessel size, plating will need to be a certain minimum thickness suited to that vessel size.
  2. 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.
  3. 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.
  4. 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.
 

Insulation

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:

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

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

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...
 

Conclusion

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