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Metal Boats For Blue Water
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Click any image below to enlarge This brief essay is intended to bring to light a few of the issues surrounding the use of metal for boats. The pros and cons expressed here are relevant to the choice of one's hull material. These issues are also central to the actual process of designing and building in metal, whether one chooses in favor of steel, aluminum, copper nickel, stainless, or what have you. Therefore, the following is
not intended simply for potential metal boat owners, but also for boat builders and
designers who may wish to make better use of the medium. One of the primary choices one will face when considering metal is just which metal to use and for which vessel type. To begin, here are a few brief thoughts with regard to steel versus aluminum. If an existing design is being considered, in other words, a vessel which already has a fixed hull shape, then we can observe the following: in terms of sea kindliness, some boats may be better if built in steel. This is due to the extreme lightness of aluminum which in some hulls may result in a more harsh motion. This is the case especially with larger boats or very beamy boats. Provided that they have adequate displacement and stability to carry the added structural weight, these types will have a more gentle motion at sea if built in steel. 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. If we are starting from scratch and beginning with a blank sheet of paper, these same considerations would apply. In this case, 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 adds up very quickly indeed. For smaller vessels, say below around 35 feet, this makes for a very burdensome and heavy hull. In larger sizes, say above 35 feet, one can make excellent use of steel. Above 45 feet the material really 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 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 and 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 decrease below a certain point. Therefore, roughly below around 30 feet the boat will no longer be optimum for the task, due to having an ungraceful shape in order to achieve the relatively large displacement necessary to carry the required number of sandwiches and beer (and fuel…)! For smaller 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. The lighter weight of
aluminum will permit a given hull form to be built to a higher standard of strength than
the same hull in steel. Generally, when built to the same strength standard as a steel
vessel, a bare aluminum hull as fabricated will weigh roughly 1/3 less than an equivalent
steel hull. Thus we can see that if we
were to use heavy enough plating on the aluminum hull, in other words, if we were to be
working within the same weight budget, the aluminum boat could be built to have
a higher strength than the same hull form in steel. One cannot readily make use of less than 10 gauge steel plating, due to fabrication issues. Even 10 gauge can be very difficult to keep fair, since it will have relatively much greater distortion while welding than plate of a greater thickness. Of course with a few essential tricks learned, it is not much trouble to avoid distortion altogether in a 10 gauge hull. For an amateur builder however, working in 10 gauge material without knowledge of those tricks, the result would be uncontrollable distortion often to the point of being quite awful. The natural temptation then is to consider the use of greater plating thickness. If one were to begin a design from scratch of course, displacement may be designed to be adequate to carry the greater steel plating thickness. However if one were to be tempted to simply apply greater thickness of steel plating to a design intended for example, 10 gauge steel, the result would be a grossly overweight vessel which will either float far below her intended waterline or will not be able to carry the required amount of ballast. The upshot of the above is that although one can design and build very fine steel boats down to around 35 feet (give or take a few feet), the smaller vessels will necessarily make use of 10 gauge mild steel plate and would therefore require greater skill in building. If the vessel can be large enough or of heavy enough displacement, then one can make use of 3/16 inch mild steel plating which is far easier to use since it will resist distortion all the better. One twist to these considerations is that for smaller vessels which must use 10 gauge steel for plating, we can make very good use of Corten steel. Corten has close to 50% greater yield strength, so it will resist distortion more or less as well as 3/16" mild steel plate. In my view, this is the primary justification for the use of Corten steel for metal boats. Corten is just as easy to weld and cut, 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. So, what about aluminum? Aluminum is light, strong,
corrosion resistant, non-sparking and conducts electricity and heat well. Aluminum is
clean, easy to work with 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 much faster to
work with than steel. Welding it is a very quick process, so there is quite a labor savings. Since one can make use of
much greater plate thicknesses, not only can strength be greater than with steel but as a
result, distortion levels are relatively easily managed. There are a several additional things to consider when choosing between steel and aluminum:
In aluminum, the welds at best are around 70% of the strength of the plate (in the 5000 series). Usually, one would compensate for the reduction in strength in the heat-affected zone either by providing a backup strip at any plate joint and welding it thoroughly on both sides, or by providing additional longitudinal members to span the butt welds in the plating. Additionally, plating butts should be located in the position of least stress. For most general plating, this is ordinarily at one quarter of the span between frames. With proper engineering and design, aluminum weld zone strength 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 that render them nearly useless. Currently,
the best antifouling paint for aluminum hulls is called "No-Foul ZDF" which is
made by the E-Paint Company. No-Foul ZDF uses a controlled release of hydrogen peroxide to prevent fouling. Practical Sailor magazine did a controlled study 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. Consequently, they also discovered that it performs significantly less well than the other AF paints during the second year. Refreshing the coatings annually will result in a top-performing system, as well as frequent inspection intervals for the hull. Another big savings with aluminum is that it is not necessary to sand blast or paint the inside. Generally, due to its very good conductivity one must insulate an aluminum hull extremely well. While steel must always be painted everywhere both inside and out, in a warmer climate however, steel will not necessarily require insulation, except possibly to assist with sound attenuation. As we have seen above, a point in favor of aluminum is that it can be used to design a much lighter weight boat 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, but it will also be less costly to push through the water. The savings in hull structural weights is augmented then by either being able to achieve greater range under power or by being able to carry less fuel to achieve the same range as a heavier vessel. On the other hand, 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. On the plus side, even though an aluminum boat will cost slightly more to build, it will have a much higher re-sale value than a steel boat. Steel is a bit more rugged, being tougher and more abrasion resistant. The welds in steel are 100% the strength of the surrounding plates, whether mild steel or Corten. Steel is less prone to electrolysis problems than aluminum and one can use regular copper bottom paint on a steel hull. 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 painted with an adhesive waterproof paint like epoxy, or protected with a waterproof adhesive bedding, or both. 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 very active
galvanically and will sacrifice itself to any other metal it contacts either directly or
indirectly. Aluminum is anodic to everything except zinc and magnesium and must be
electrically isolated from other metals. A plastic wafer alone as an isolator is not
enough. Salt water must be prevented from entering the crevice, or crevice corrosion
will result. Paint, bedding, and a non-conductive isolator should all be used
together. 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 slightly 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 of the vessel and in the building of it, then stainless can be a viable hull construction media. 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.
These two characteristics conspire against maintaining fairness during weld-up.
Short arc MIG welding would therefore be imperative. One additional material which should be considered along with steel, stainless, and aluminum is Copper Nickel. One can ignore paint altogether with Copper Nickel (CuNi). It is very easy to fabricate and is both easy to cut and to weld using the MIG process. There are two alloys which are most common: 70/30 Cu Ni, and 90/10 Cu Ni. These numbers represent the relative amounts of Copper and Nickel in the alloy. With its higher Nickel content, 70/30 is the strongest and most expensive of the two. In the US, 90/10 Cu Ni is currently priced a little over $4.20 (USD) per pound and 70/30 Cu Ni at around $5.60 (USD) per pound--both based on a minimum order of 2,000 pounds. Of course, the cost would go down quite a bit for a larger quantity order and a vessel built of Cu Ni would make use of a greater quantity than 2,000 pounds. The main issues with Cu Ni 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 with a 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. Cu Ni is slightly heavier than steel per cubic foot; thus, the Cu Ni hull structure will end up being considerably heavier than an equivalent steel hull structure. With the commonly used hull construction metals, one will typically "design to yield." This means that the ultimate failure strength of a material is more or less ignored and instead, the yield strength is used as the guide for determining scantlings. For example, if we were to desire the same yield strength in a 90/10 Cu Ni structure as there would be with a similar steel structure, then we would be tempted to actually double the scantlings resulting in quite a huge weight penalty. In practice, a Cu Ni
structure will not need to be taken to this extreme. More typically, a 90/10 Cu Ni
structure will have around 20% to 25% greater weight than a similar structure in steel.
One would most commonly make use of the same plate thicknesses as with steel, then
compensate for the lower yield strength by spacing the longitudinals more closely, say
using approximately 75% of the spacing prescribed for the same thickness of mild steel
plating. One would of course need to calculate the required strength of the
longitudinals
and frames, increasing the dimensions of the longitudinals and transverse framing as needed to
accommodate the lower strength of Cu Ni. In my view, there is no reason one could not make use of Cu Ni for the hull skin only, taking full advantage of the benefits: not having to paint Cu Ni anywhere on top or bottom, the ease of cutting and welding Cu Ni, the relatively great heat conductivity and consequent favorable behavior with regard to distortion while welding Cu Ni. For the framing then, why not make use of stainless? Provided that the attributes of stainless are kept in mind, this may be a combination with considerable merit. Stainless can be readily welded; one can easily make a weld between stainless and Cu Ni. Stainless steel is strong and stiff and costs around $1.85 (USD) per pound based on a minimum order of 2,000 pounds, less than half the cost of Cu Ni. One could then very nicely avoid nearly all of the cutting hassles present with stainless by making use of NC cutting for the entire frame of the boat. Of course, costs can be reduced even further by the use of a GRP house structure, or possibly wood. If the budget could tolerate the expense, and the vessel were approximately the dimensions required to carry an all-steel structure, Copper Nickel plating and stainless framing would provide an ideal combination for the entire hull and superstructure. If the deck were built of Cu Ni, one could take advantage of the material again by the use of Cu Ni for all deck fittings such (i.e. stanchions, cleats, bitts, etc). Pipe fittings are readily available in either alloy of Cu Ni. I know this proposal will
sound completely crazy to some, but given the budget and the will to be completely free
from all requirements for painting, this is the "bee's knees". The combined use of
stainless and Cu Ni would obviously be far more costly than would be the case with mild
steel, however the savings realized by not having to paint the entire vessel inside and
out would go quite a long way toward easing the cost differential. If we ignore the hull materials for a moment and consider what may impact costs in other ways, we can observe the following: costs will vary more or less directly with displacement, assuming 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 increase exponentially with size. It often seems that the inherent good sense of a vessel is therefore inversely proportional to her size. With regard to the complexity of a vessel this is also the case, but more or less to the third power. Assuming a given budget, the simpler boat can just plain afford to be done better. Estimating actual construction costs is relatively straightforward. A reliable construction cost estimate will be possible only in relation to a specific design, hull material, degree of finish, complexity, building method, whether the hull is computer cut, etc... Assuming we are considering vessels of equal size and complexity and if painted to the same standard on the exterior, when all is said and done, an aluminum vessel may possibly be around 10% more expensive to build than the same vessel in steel. However, if the aluminum vessel is left unpainted on the exterior except where necessary, many yards can build for less in aluminum than in steel. This may come as a surprise, however this has been verified via quite a number of recent construction estimates. Maintenance will be less
costly on an aluminum boat. Taken as a whole then, we can see that any added 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
boat will be much more costly than one built in steel or aluminum, but in terms of
longevity a boat built with Cu Ni will provide the ultimate as a family heirloom… The materials of construction need not dictate the aesthetics of a vessel. Much can be done to make a metal boat more 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. Many metal boats we encounter seem "industrial" in their appearance. In my view, classic and traditional lines if attended to faithfully, nearly 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 made of any other material. My design work is therefore often 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 bringing forth 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 trying to carry forth a bit of traditional elegance. Everyone's needs are
different though, and when considering a new design one should keep in mind that nearly
anything is possible. The eventual form given to any vessel will be the result of the
accommodations it must contain, the purpose for which it is intended, and the budget which
has been offered for its creation. Regarding the notion of a rounded metal hull, in my view they can be excellent. The trade-offs between a rounded hull and a chine type of hull form for metal boats are 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. Another hull shape I’ve developed is a rounded hull with a canoe stern where the shape of the stern nicely balances the shape of the stern. Having a very easily plated
shape these rounded hulls are economically built. These 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, thus having 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 an economically built structure, as well as a better defined keel for windward performance under sail or better tracking under power. Plating on these rounded
hull types is arranged in strips of about 12" to 18" wide running
lengthwise along the hull. The plating is most easily done if the edges are
"joggled" so that one plate fits nicely over another along the edge. Many vessels have been built
using this method. They benefit from the additional strength provided by the joggled
lap and at times can be designed without longitudinals, simplifying the framing. The
joggled laps are very attractive if lined off correctly (as one would do with a wooden
boats' planking). One method which has become popular is to make use of a "radius chine" where a large radius is used to intersect the flat side and bottom plates. Although the radius chine shape takes fairly good advantage of the flat plate for most of the hull surface, it is not a more economical construction method than the types of easily plated rounded hulls described above. In my
opinion, there is no
benefit whatsoever to employ a radius chine shape rather than an easily plated rounded hull
form. The latter would be much more appealing visually. A radius chine hull will nearly
always be easily recognized as a radius chine shape rather than a true rounded shape. Aesthetics is of course a highly personal thing. As we see with many nicely done designs, a single chine can look quite appealing, especially when used with a more traditional style. When building a boat using sheet material, it makes the most sense to think in terms of sheet material, and how one may optimize a hull design without incurring extra labor. I am attracted to the single chine shape for metal boats mainly because metal is supplied as a flat sheet material and in my view a chine hull form is a more honest use of the material. In this regard I feel traditional types have much to offer, keeping in mind the goal of excellent performance. As with many traditional types, there is no aesthetic penalty for using a single chine. Assuming good design for
each type, the claimed differences between the performance of a rounded hull form versus a
single or multiple chine type of hull are unsubstantiated. Since costs are
significantly less using single chine construction, one can make an excellent case for the
use of the simpler hull form. Labor is the largest factor
in hull construction and as we have observed, complexity pushes the cost of labor up
exponentially. Dollar for dollar, a chine vessel can easily be made to be longer within
the same budget. In terms of the vessel's "performance per dollar" the
chine vessel can be said to offer better performance than the rounded type of hull form. A multiple chine hull form offers practically no advantage whatever. The multiple chine shape will require nearly as much labor as the radius chine shape and the only savings will be the cost of rolling the plates for the radius chine. Multiple chine shapes are very problematic visually, typically being much more difficult to "line off" nicely and much less easy to keep fair during construction. My preference therefore is
to take any extra money available and use it to make a graceful single chine boat longer,
therefore realizing some real speed and comfort benefits…! What ultimately defines a
good boat is not whether she is one type or another, but whether the boat has been well
designed, and whether the vessel has satisfied the wishes of those who had her built. The keel of any vessel, sail or power, will be asked to serve many functions. The keel creates a structural backbone for the hull provides a platform for grounding, and 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 vessel tankage (including a meaningful sump), and can serve as the engine’s coolant tank, 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 quite 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. Spreading the ballast laterally is 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 for high lift / low drag / low stall. With metal,
this is easily accomplished. I always prefer integral
fuel and water tanks on a metal boat. Integral tanks provide a 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. There is potentially misleading and incorrect information in the implied promise of “frameless” metal boats, a notion that is pondered by several offbeat builders. The concept of frameless metal boats is attractive, but flawed. If you apply well proven engineering principles to the problem, you quickly discover that frames are simply a requirement. In order Many metal boats are successfully plated and then welded together without the aid of metal internal framing; in order to provide adequate strength, frames must be added before the hull can be considered finished. This construction sequence is a very big plus when trying to maintain fairness during weld-up. As strong as metal is, there
is most definitely a need to support the plating and to reinforce and stiffen the
structure as a whole using frames. Many so-called “frameless” boats do make extensive
use of longitudinals. Bulkheads or other internal structure for example, will
typically be used to reduce the span of the longitudinal stiffeners. Experienced metal boat
builders and designers will 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 know that to leave the boat without
internal framing is quite irresponsible. The most generally suitable
arrangement for internal structure is a combination of transverse frames and longitudinal
stiffeners. The labor involved in fabricating a metal hull can be reduced by a substantial amount via computer cutting. Builders who are sufficiently experienced with building NC cut hull structures estimate that they can save between 40% and 60% on the hull fabrication labor via computer cutting. As an example, for a fairly simple vessel up to around 45 feet, a "typical" hull of that size may take around 2,500 hours to fabricate by hand (complete with tanks, engine beds, deck fittings, etc). If one can save 40% or more of those hours, the savings are dramatic. The number of design hours
one must spend at the computer may amount to some three to four man-weeks, or about 150
hours. With this kind of savings, the effort expended to develop the NC cut files
would be paid for several times over. In fact, the savings are sufficient enough that NC
cutting has the potential to "earn back" a fair portion of the cost by having
developed a custom design. These days, builders who
ignore the benefits offered by computer modeling and NC cut hull structures simply have
their heads in the sand. Small metal boats, unlike tankers and container ships, 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 had 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 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 anyplace 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 softer than steel, so sand blasting aluminum is relatively fast in comparison. The blast nozzle must be held at a greater distance so the blast covers the area more quickly. On a Copper Nickel vessel,
there is simply no need for paint anywhere. Many schemes are used to insulate metal boats. Blown-in foam is an excellent insulator and offers considerable sound deadening. It does offer additional protection for the interior metal work, but only if it is adhered well to the surface. Sprayed in foam, while popular, does have drawbacks which 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 quite toxic. Blown-in foam should be coated with a flame-retarding paint. An alternative to blown-in foam is a good quality, flexible closed cell cut-sheet foam to fit between the framing. Some sheet foams are fire retarding by composition but if not, should be painted just like the urethane foams. The best choice amongst foams for cut-sheet foam installation 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 both better and more expensive than Neoprene. Styrofoam or any other styrene type of foam should be strictly avoided. Purchase a piece at the lumber yard and throw it onto a camp fire...you will be immediately convinced. Sprayed in polyurethane foam
is the best in terms of insulating value, since it nearly completely prevents condensation
by sealing off the air from the metal hull surface. If the insulating value of the
system is the paramount criteria, then sprayed in poly foam will be the preferred choice.
A fire retardant formulation should always be used. Zincs are essential on any metal hull. 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, zincs must be used to control stray current corrosion, to which we can become victim with a metal boat, even without an electrical system! The quantity of zinc and the surface area are somewhat determined by trial and error. As an example, on a metal hull around 35 feet, the best scheme is to start with two zincs forward, two aft, and one on each side of the rudder. With a larger boat of say 45', an additional pair of zincs amidships would be appropriate. As a vessel gets larger the zincs should become more numerous. Since zincs will be effective for a distance of only around 12 to 15 feet, it is not adequate to use one single large zinc anode. The zincs should 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 can be adjusted as needed. Good electrical connection between the zinc and the hull must be assured. Ideally,
metal boats should ideally
not be bonded. This of course is contrary to the advice of the ABYC. However
one must keep in mind that the ABYC rules are primarily aimed at satisfying the
requirements of GRP vessels. Little by little, we are seeing the ABYC create special
case recommendations for aluminum and steel boats. Aboard a metal vessel it is best to make use of a completely floating ground system. The negative side of DC power is that it's not permitted to be in contact with the hull or any hull fittings, anywhere. 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. 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. Needless to say, for the sake of preventing corrosion there should not be a connection between the AC shore power and the hull. This includes that insidious little green grounding wire. Of course this is also contrary to the ABYC recommendations, which are primarily concerned with prevention of shock, rather than the protection of the hull itself. All AC power coming aboard a metal boat should be passed through a marine quality isolation transformer. Other "black box" devices should be strictly avoided, including things like zinc savers, impressed current systems, etc. I know there are plenty of
people out there who will disagree with the above brief statements about electrical
systems. Whether you agree or disagree, please don't come all unglued over these
matters; instead, for much more complete information on these topics, please see the
following. The Metal Boat Society is a non-profit organization dedicated to the promotion, design and construction of metal boats. The MBS is a non-profit organization begun in 1987 to further popular interest in metal boats. Metal Boat Quarterly is sent automatically if you join the MBS. The focus of the Quarterly is to provide accurate, up-to-date information on designing, building, and maintaining metal boats of all kinds, and in general on metals in the marine environment. MBS members are serious metal boat people. Most are either building a boat, intending to build one (or have one built for them), or already own a metal boat. In any case, a potentially valuable resource! The general focus of the Metal Boat Quarterly has been toward mid-range cruising boats, both power and sail. The MBQ does not indulge in advertising, nor serve any corporate vested interests. They mostly try to expose the resources one will need when venturing into the world of metal boats. The MBQ is professionally presented and the intent is that it be a real resource, such as a journal. Receiving the MBQ is the main benefit of membership in the MBS, as well as keeping informed about the excellent Metal Boat Festival, held in August at Oak Harbor, Washington. Many good seminars are given at the Festival by designers, boat builders, travelers, riggers, sailmakers, etc. For a very complete discussion of the issues surrounding corrosion, zincs and bonding, you may download the Metal Boat Society's 1998 Special Issue called just that, "Corrosion, Zincs & Bonding." It is available in PDF format from one of the Metal Boat Society's web pages at: http://www.kastenmarine.com/MBQindex.htm or you may enter the URL of the file directly as: http://www.kastenmarine.com/mbqCref.pdf and yes, capitalization is important! While there, you may want to
download the 1997 Special Issue, called "Marine Metals Reference" available
directly via http://www.kastenmarine.com/mbqMetRef.pdf.
Please note that there are several links at the top of the MBQ Index page which lead to
other Metal Boat Society web pages, including current information about the annual Metal
Boat Festival. We can see that metal has come to 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 or Copper Nickel. By far, steel wins the ruggedness contest. Aluminum wins the lightness contest. Copper Nickel wins 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 U.S. 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 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, 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 that among the dedicated blue water cruisers in the South Pacific, "50% of the boats are metal; the rest of them are from the United States...." Unfortunately, this statement is very nearly 100% true!! It is my hope that the above essay is of some value when considering the choice of hull materials. If you are intending to make use of metal as a hull material, the Metal Boat Society and the Metal Boat Quarterly will be a great asset to your project. Two additional articles which you may wish to review are my "Aluminum for Boats" which first appeared in Cruising World and my "Aluminum vs. Steel" both available at my web site. Please do check out the Kasten Marine Design website for more information about metal boats, as well as for a taste of the types of vessels I have been imagining over the last several years.
See below for further suggested reading:
Many thanks to Michael Kasten in preparing this article for the readers of Pilothouse Online. By: Michael Kasten Kasten Marine Design, Inc.
All images and text provided by Kasten Marine Design, Inc. Copyright © 2000, All rights reserved Pilothouse Online has authorization to reproduce this article on its web site for commercial use. Any and all of this material shall not be reproduced in any form without written permission from the publisher.
Me List
Price: A marine engineer with 26 years of service in the Canadian navy, Ken Scott is the proud owner of Scot-Free II, a 36-foot steel sloop designed by Bruce Roberts. Scott had the hull custom built and then spent three years fitting it out himself. During that time, he learned firsthand about the advantages of metal as a boatbuilding material, as well as the problems many amateur builders encounter. Metal boats are easy to customize and tough enough to withstand abuse that would destroy hulls made of any other material. Find out what it takes to build and outfit your own steel or aluminum yacht - for much less than the cost of a custom fiberglass or wooden boat.
A Our
Price: $39.50 An authoritative and practical guide for designers and builders of aluminum alloy boats. Includes detailed information on tools, techniques, painting and repairs
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Price: For anyone even considering building, buying, or renovating a metal boat, this must-have resource provides a thorough overview of steel, aluminum, and copper-nickel boat construction. Written by one of the world’s most experienced small-craft designers, Metal Boats explains the advantages of metal; how to buy it; how to cut and weld it; how to build or refurbish hulls, decks, and superstructures with it; and how to finish it. The book also covers engineering for metal boats, corrosion prevention, interior design and construction, electrical systems, appendages, fittings, and a portfolio of designs for metal powerboats and sailboats.
Metal
Corrosion in Boats : The Prevention of Metal Corrosion in Hulls, Engines, Rigging
and Fittings List
Price: Understanding metal corrosion and its prevention is vital for the safety and integrity of a boat. Failure through corrosion or fatigue can result in masts falling down, keels falling off or boats quietly sinking in their moorings. Less dramatic are such problems as pitting or fracture of propeller shafts, erosion of pipe work, rusting of steel hulls and galvanic corrosion of aluminum hulls.
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