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This article first appeared in Cruising World magazine, September 1997, entitled "To Thine Own Chines Be True."
It has been updated in places since then, as below.Aluminum For Boats
Copyright 1997 - 2013 Michael Kasten
Aluminum is a sheet material with virtues aplenty. To honor them best, I advocate simplicity, ample framing... and yes, even single chine hull shapes!
Why Build An Aluminum Boat?
For things that go bump in the night. For ease of construction. For longevity. For good resale value. For the benefit of being able to create a custom design and build it economically, without the huge penalty of having to build a mold first, as with fiberglass. For freedom from the stench of fiberglass, and from the dread fiberglass boat pox. For repair-ability. For lightness and strength. For the competitive edge in performance. And most importantly, for the security of safe cruising.
It takes over 60,000 pounds per square inch (psi) to tear apart a chunk of mild steel, and 30,000 psi to deform the same piece; to make it yield. With aluminum, around 45,000 psi will tear it apart, and around 35,000 psi will deform it. Yes, you read that correctly: size for size, aluminum has a higher yield strength. In these facts lie the extreme benefits of metal for hull construction: The "plastic range" of either metal is quite high, so the material can take a terrific beating without failure.
Aluminum is light, strong, corrosion-resistant, non-sparking and weldable. Because aluminum is not abrasion-resistant, it can be cut with carbide tools. Aluminum is subject to electrolysis, pitting and crevice corrosion, but these liabilities can be managed as long as the installation of dissimilar metals and electrical items are correctly done. After that, it is a matter of attending to these matters during the life of the boat.
In terms of seakindliness, some boat shapes may be better if built in steel. Aluminum's extreme lightness can introduce a faster pitching and rolling motion in some hulls. For example, very beamy boats will exhibit a gentler roll if built in steel. Fairly narrow or light-displacement boats, which tend to have a narrower waterplane and less inherent form stability, will benefit most from aluminum construction. These are of course generalizations. Given a blank sheet to begin a design, the roll behavior will be considered along with the choice of materials.
In terms of size, one can successfully build a steel boat for coastwise cruising and serious blue-water sailing down to around 30 feet LOD. Below that, the steel vessel will either have to be built with excessive displacement or with quite thin plate that will be more difficult to build due to distortion while welding.
An aluminum bare hull, built to the same strength standard, will weigh roughly 25% to 35% less than the same hull in steel. As a result, if high strength is of the highest priority, the aluminum boat can be built to the same structural weight as the steel vessel, and then be considerably stronger. If low weight is the highest priority, then the weight saved will allow us to create aluminum cruising boats down to, say, 20 feet.
One can build good aluminum boats in sizes even smaller than that, as is evidenced among my small Pram designs.
On Cost
The aluminum to build a bare hull costs just under twice as much as the mild steel to build the same design. But aluminum is faster to work with, so the savings in labor helps even the score. The labor saved can be substantial since aluminum can be cut with common carpentry tools and is welded much faster than steel.
Another significant advantage with aluminum is that there is no need to sandblast or paint the interior. You do have to insulate an aluminum hull, but that won't ordinarily require sandblasting. Painting the exterior of an aluminum boat is unnecessary, representing another big savings.
After you've factored in the added costs of painting steel, the margin for building an aluminum hull drops to being a very minor amount when compared to building in steel. As a percentage of the entire construction project, the additional cost of the aluminum becomes very slight indeed. Once built, maintenance on an aluminum boat is less expensive, and resale value higher. These factors more or less even the score between the two materials.
New construction methods have trimmed metal hull building costs substantially. The most dramatic savings can be effected by computer lofting, and then computer cutting the actual parts for the hull. Essentially, the builder receives a "boat kit" ready for assembly.
A parallel method, also ideally done by computer, involves cutting and fitting the plate only, by itself, without a prior support structure.
One last note about cost: When comparing like for like, boat costs tends to vary more or less directly with displacement (not length), assuming a given level of complexity in the design. Displacement, and therefore cost, varies as the cube of the overall dimensions.
On Hull Shape
Aesthetics are a personal thing. For my own part, I am attracted to the single-chine shape for metal boats because metal is a flat-sheet material. When building a boat using sheet material, it makes the most sense to think in terms of that material's characteristics and how one may optimize a hull design without incurring extra labor.
In metal, a single-chine hull is easier and less costly to build than one with radius or multiple chines or one that is fully rounded. Further, with a good design there is no performance penalty with a single-chine hull. The slight gain in wetted surface, if any, can be offset by slightly greater sail area, made possible by slightly greater ability to carry sail due to the form stability provided by the chine.
This line drawing and the ones that follow demonstrate degrees of design complexity for sheet materials, from single chine to a fully rounded hull. I prefer the simplicity and economy of a single-chine metal hull, as shown here. In my view, it is a more honest shape for a metal boat. To the left is the hull shape given to my Highland Lass, Zephyr, and Grace sailing designs. Nearly all of our designs are single chine metal boats... and for good reason. An example that provides a visual comparison between single chine and rounded versions of the same design is our cutter Awahnee III.
Further, the reputed seakindliness of a radius-chine or round-bottom hull can be approached in a single-chine hull by giving it a slightly more "slack" shape. A big advantage of the single-chine shape is economy; the cost to build a rounded or radius-chine hull is considerably higher due to the work involved with the added shaping and welding.
A single chine can look quite appealing, especially when used with a more traditional style. In my view, it makes the most sense to take any extra money available and use this to make a graceful single-chine boat longer rather than radius chine or multiple chine, thereby netting some real speed and comfort benefits… in other words, a bigger boat for the same money, with inherently greater speed potential due to the increased length.
Multi-chine designs allow building with flat plate, without requiring that any plates be rolled. Although considerably more time consuming than a simple single chine, these shapes remain within the construction realm of the amateur or one-off builder. I have not made use of the multiple chine shape for any of my designs, primarily because it involves considerably more work than single chine, without conferring any real benefits. See below..!
Radius-chine hulls employ flat panels everywhere except for a narrow 'rounded' plate that joins topside to bottom, rendering a quasi-rounded hull without requiring that every metal sheet be rolled; only those at the radius. However... it will always be recognized as a radius chine vessel, and not a true rounded hull shape. Therefore if a radius chine is being considered, there is very little reason not to simply take the next step and go to a true rounded shape, as follows...
A fully-rounded metal hull is beautiful to behold. They need not be expensive to build if correctly designed, where only the minimum amount of plating needs to be rolled. These are not "radius chine" boats. They are instead just easily plated, rounded hulls with no reverse curvature, so these hulls can be built economically. As soon as there is a request to vary from a simple single chine shape, a rounded shape is preferred. This is the shape given to my Benrogin 40, Jasmine 48, Lucille 42, Lucille 50, Fantom 36, Greybeard 38 and Mermaid 61 sailing designs.
Radius-chine and multi-chine boats cost about the same amount to build, and a true rounded hull - provided it's designed correctly - need not be any more time consuming nor any more expensive to build than a multi-chine or radius-chine shape. And as a very big bonus... it will look vastly better!
It is generally our first choice to make use of a single chine hull shape for metal boats. If for some reason a chine shape is not desired, we nearly always find that a well-designed rounded hull is the next best choice. It will share the same ease of construction as a multi-chine or radius chine vessel, but with a little bit of transverse curvature in the topsides and bottom.
Designing true rounded metal hulls for ease of plating is not at all difficult. Our goal with a rounded metal hull is that the topsides and bottom will not require any pre-forming at all, there being just enough curvature to sweeten the appearance, but not so much as to require rolling. In other words, 90% or more of the vessel is still able to be plated using flat sheets, and without any fuss at all.
One excellent technique when building a completely rounded metal hull involves using "joggled" plate seams, akin to "lap strake" planking in wood. According to this method, an offset is pressed in along one edge of the plate. The offset is just enough to take the thickness of the plate below it. Each plate is a strip about 12 to 18 inches wide. Bernard Moitessier's steel boat Joshua was built that way, and it certainly withstood the ultimate test...!
Alternately, the plating can be lapped by instead jogging the frames to match the plate contour. Just above the lap, the frame jogs out to meet the plate above, etc. These lapped plate methods provide a much easier fit-up, and a much more easily achieved weld seam. If "lined off" nicely, as one would do with wooden planking, they can also look very good. The plate overlap creates its own longitudinal stringer and reinforcement.
With any of these types including the fully rounded metal hull, as can be seen in each of the examples above, it is most economical in terms of labor if the keel is attached as an appendage. In other words given the strength of metal, there is no particular need to create a large reverse-curved garboard area merely for the sake of strength, as would indeed be the case with a glass or wooden hull. This saves an enormous amount of construction time, and is therefore the most practical approach.
On "Frameless" Construction
With the notion of metal's extreme strength, we have come to a point of faith which has at times created a misconception: There is potentially misleading and incorrect information pandered by some in the implied promise of "frameless" metal boats. The concept of frameless metal boats is attractive, but flawed.
The definition of "frameless" must be clarified…
Achieving the required strength in a metal vessel without using framing imposes an enormous weight penalty due to the required increase in plate thickness. If one applies well-proven engineering principles to the problem, one quickly discovers that frames are simply a requirement. Designers may employ devious strategies, such as using bulkheads, interior furniture or other features to achieve the required reinforcement, but responsibly designed and built metal boats, whether of steel or aluminum, definitely do use framing.
Despite recent talk about "frameless" construction, responsibly designed and built metal boats definitely do use framing. The added plate thickness required to forego framing completely would render a heavy hull indeed. Here, just three out of a total of 17 transverse frames for this design are illustrated. For more information, see our articles on structure, below.
Without the aid of metal internal framing, many metal boats are successfully plated, and the plating then is welded together prior to the addition of the frames. This construction technique renders a high degree of fairness.
Other methods use a "folded plate" strategy, with perhaps one large plate per side, to make the plating much faster to erect. To give the vessel adequate strength in the final product, though, frames must be added before the hull can be considered finished.
Many so-called "frameless" boats make extensive use of longitudinals, which, in "folded-plate" construction, are often pre-welded to the plate. Bulkheads or other internal transverse structures are used to reduce the span of these longitudinals. Strictly speaking, then, these boats do have framing, and with good design, the framing will be adequate to the task.
Classification societies, such as the American Bureau of Shipping, Lloyds, and Det Norske Veritas are somewhat conservative in their approach, but working through their formulae demonstrates the benefit of framing, primarily to bring the weight of the vessel within a reasonable range while maintaining the required hull rigidity.
Studies of failures in aluminum crew boats and offshore supply vessels show the need for being very conservative in terms of the allowable areas of unsupported plating, in terms of scantlings for framing, and in terms of the welding between frames and plating.
Most often, the best framing style makes use of a series of strong transverse frames combined with longitudinals which provide the primary support for the plating. The longs, then, are held by the frames.
In my view, the frames in a metal boat should always be located where required by the interior bulkheads. Bulkheads can then be bolted directly to these web frames, and all is as it should be, simple and strong.
With a few tricks of the trade up your sleeve, an absolutely fair hull is the result.
Some boats are built "Frames First" while others are built by applying the "Plating First" as described above. For further reading about the various advantages of each, please see our online article: Metal Boat Building Methods.
On Protection And Coatings
Aluminum alloys for use on boats are generally limited to the 5000 and 6000 series. These two alloy groups are both corrosion resistant in the marine environment due to the formation of a tough aluminum oxide.
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 such as epoxy, or it must be protected with a waterproof adhesive bedding, or both. A plastic wafer alone as an isolator is not enough. Salt water must be prevented from entering the crevice; otherwise corrosion will result.
Anodizing, a process of electrically causing the formation of a tough oxide film on the surface of aluminum, slows pitting, but anodizing will not prevent pitting or crevice corrosion.
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. In this case, paint, bedding, and a non-conductive plastic or rubber isolator should all be used together. Unlike tankers, small metal boats are not designed with an appreciable corrosion allowance.
In terms of the paint system, aluminum boats are dealt with more easily than steel boats. Aluminum must be painted wherever things are mounted to the aluminum surface, and below the waterline if left in the water year-round. Otherwise, marine aluminum alloys do not require painting at all.
Present technology for protecting metal boats is plain and simple: epoxy paint. Once the metal is protected with a 12- to 16-mil dry-film thickness of epoxy, it can then be top coated with whatever is appropriate to the situation.
The top coats can be, for example, foam, enamel, linear polyurethane, or bottom paint. More durable top-coats better protect the investment in epoxy. To assure that this "secondary system" sticks to the epoxy barrier, use a standard extended-recoat-time epoxy primer, which makes an excellent tie-coat.
For anti-fouling paint, the excellent offerings from the E-Paint Company should be of interest to metal-boat owners. Called "No-Foul," these paints release hydrogen peroxide to prevent marine growth, eliminating the inherent problems that accompany copper-based paints on aluminum hulls.
Whether on steel or on aluminum surfaces, paint preparation is critical. Thorough cleaning and sandblasting provide the best surface for adhesion of paint or bedding. Alternately for aluminum, cleaning and then grinding with a coarse 16-grit disk will provide enough tooth for the paint to stay put. If the surface finish must be extra fine, as on an aluminum spar, then a thorough sanding, cleaning and etching with a product like Alodine before painting will give good results.
The interior of an aluminum boat does not require painting. It would be the ultimate, though, to epoxy prime the interior if a blown-in urethane foam will be used. A chromated vinyl-acid "wash primer" would be a very acceptable second choice inside, in order to provide the best surface for adhesion for the foam.
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! With a scratch at the bow, and another at the stern, the boat itself becomes the preferred path for any ambient currents in the water. In the best of all possible worlds, there would be no stray currents in our harbors, but that is not reality.
Zinc anodes should always be used on an aluminum boat, and generally in the same quantities as with a steel boat, in order to prevent stray-current corrosion. The quantity and placement of zincs are discovered by experiment over time, and will differ from one marina to the next.
As an example, on a 40-foot metal hull, the best scheme is to start with two zincs forward, two aft, and one on each side of the rudder. With a larger boat, say over 45 feet, an additional pair of zincs amidships would be appropriate. Surface area, not zinc volume, is the important factor.
After the first few months, inspect the zincs. If they appear active but plenty of material remains, the zincs are doing their job. If they are seriously wasted, the area as well as the weight of zinc should be increased.
Of course, welding zincs on is best, but for an aluminum boat, the zincs will instead usually be bolted to studs welded onto the hull plate, or bolted using stainless bolts into a heavy bolting plate welded to the hull. Good electrical connection between the zinc and the hull is imperative.
The Bottom Line
Can aluminum compete with fiberglass as a production hull material?
Jimmy Cornell's Ocean Cruising Survey, a valuable indicator of trends among world-voyaging cruisers, shows that metal boats are on the increase. A metal hull was the number-one wish of those with other hull materials. "My next boat will be metal..." was heard over and over, particularly by those who were already cruising aboard a metal boat.
It is said among dedicated blue water cruisers in the South Pacific, "50% of the boats are metal; the rest of them are from the United States...." Although it may seem so at times, this statement is fortunately not 100% true!!
In terms of cost, we usually observe that displacement is more important than length. Aluminum is the ideal material for building a lightweight boat.
The second cost determinant is complexity. This reaches into all aspects of the design, including hull shape. The simpler the design, the lower the cost. For example, a well-designed single-chine hull will perform extremely well, and the savings will allow a slightly longer boat. Dollar for dollar, this translates into a *real* performance advantage.
With correctly applied protective coatings where needed, adequate zincs, a proper electrical system, and good care over time, an aluminum boat will last indefinitely.
Further Considerations...?
We believe in metal as the ultimate boat structure, and as a result we have created quite a number of metal boat designs. To review them, please see our Sail Boats Gallery and our Power Boats Gallery. We have also created quite a number of Prototype Designs, most of which are also intended for metal structure. Sail or power - mono or multi-hull - if the structure is well-designed and well built, the resulting boat will be excellent.
We are often asked about one metal vs. another - most commonly steel vs. aluminum. Despite the excellent case we have made for aluminum above, we do not have a distinct preference. There are so many varying factors that will contribute to making that decision for each boat, and for each owner. Some boats are designed for one material only, other boats can make use of either.
In general, any of our designs that have been developed for steel can very quickly be re-specified for construction in aluminum. The design conversion from steel to aluminum is done for a minimal extra cost. Where NC cutting files exist for a steel boat, they will need to be re-done in order to work for aluminum structure, and there will be a cost incurred for that conversion.
Designs originally developed for aluminum structure are not as readily converted, since they will have been designed specifically to save weight. To convert an aluminum design to steel will ordinarily require a re-work of the hull shape in order to support the extra weight of steel.
For more information, or if one of our designs is of interest, please inquire.
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
Direct Quote from an aluminum boat owner...
As an owner since 5 years of an aluminum boat I could not agree more with your preference for this material. She is a great sailboat and requires very little in the way of maintenance. I do a lot more reef snorkeling than the paint, polish, varnish and wax guys!--Peter Kminek
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