When looking for a voyaging boat, an important decision is that of hull material. While the majority of voyaging yachts are constructed of fiberglass, metal boats are also popular. Which metal to use, however? Steel and aluminum each have their advantages and their drawbacks. Let’s take a look at some of these factors for each material.
Weight, strength and displacement
Steel, at some 491 lbs/cubic ft is considerably heavier than aluminum at 166 lbs/cubic ft, which, if that were the whole story, would put steel at a significant disadvantage. However, of greater significance than weight alone is the strength-to-weight ratio of the metal. Considering the mass of metal necessary in the structure to achieve a desired degree of strength, steel comes out ahead. This would indicate that boats made of very thin steel sheet are, theoretically, quite feasible. The problem comes about in the welding of thin — say 10 gauge — metal sheet. It is difficult to accomplish without severe and unacceptable distortion of the panels. Because of this, the use of steel in boats less than roughly 30-feet (plus or minus a few feet) is questionable. This is not to say that it cannot be done — indeed, it has been done — but rather that the difficulties in doing so make it of questionable economic viability.
One option is the use of Corten steel instead of common mild steel, not for its unique resistance to corrosion (which is of doubtful value in this application), but for its strength, which is nearly 50 percent greater than that of mild steel. However, Corten is significantly more costly and thus offsets the cost savings of a mild steel hull. Also, there are tricks of the welding trade that can be employed, but these are labor intensive and time consuming — again, nullifying any cost savings in using the common mild steel grades.
While the greater weight of steel does bring with it the advantage of a more comfortable motion at sea, it does so at the cost of diminished performance as well as the requirement for greater power both in the sailing rig and in the auxiliary engine. It is at the larger hull sizes, greater displacements, and more demanding service conditions that steel becomes more and more advantageous.
As far as strength is concerned, steel is unquestionably the stronger metal. Steel has a tensile strength of about 50,000 psi. Tensile strength is the ability to withstand static stress loads before tearing. Steel is also very ductile, having typical elongations of some 36 percent (elongation is a measure of steel’s ability to stretch or deform without rupturing). It also has high resistance to abrasion. All of this means that steel hulls are better able to survive collisions with floating debris, cargo containers, etc., and going ashore on the rocks without taking water or sinking — very desirable features for a serious offshore voyaging boat. A couple of ugly dents and a scratched-up paint job are certainly a more attractive outcome than a ruptured hull and possible sinking. Other measures of strength are impact resistance and stiffness, and steel is tops in both of these.
Series 5000 aluminum alloys are the grade commonly used for hull structures because of their superior corrosion resistance. Typically, they have tensile strengths on the order of 40,000 psi, depending on temper. Not as high as that of steel but still pretty fair. At the higher tempers, tensile strengths can be even higher, but at the cost of ductility. Typical elongations of 22 percent show that aluminum, too, is quite ductile; again, not as ductile as steel, but appreciable.
Aluminum, at only one-third the weight of steel, offers significant savings in weight, which can be realized in terms of greater carrying capacity for stores or crew, reduced fuel usage, or greater range for the same fuel capacity. The lighter displacement hull can also mean an easier boat to drive — a smaller rig and auxiliary engine — for the same size boat.
Also, in this size range, there is another option: Use steel only where its strength and toughness properties are required, in the hull, and use aluminum for on-deck and above-deck structures such as decks, cockpit, cabin house, etc. Foam core materials can be used in the interior for additional weight savings, all of which also lowers the vessel’s center of gravity. Care must be taken to insulate those areas where the aluminum structures are joined to the steel hull to avoid galvanic corrosion.
So, for smaller boats, say 35-feet and under, aluminum clearly has the advantage. Hull plating of greater thicknesses can be used, resulting in a hull of perhaps even greater strength than if the same boat were built in steel.
Between 35- and 50-feet, either metal is viable, and the choice must be made on the basis of other considerations, such as maintenance and repair or cost. Larger than 50-feet, steel becomes very attractive; a steel hull can be very light for its length, stronger and tougher than aluminum, and cost-effective. However, maintenance and repair are still important considerations.
Ease of fabrication
In terms of ease of fabrication, aluminum is much easier and faster to work with than steel. Aluminum can be cut with any tool or power tool that, preferably fitted with carbide blades or cutting tips for the heavier gauges, is suitable for cutting wood. Holes and shapes can be drilled or cut with common portable drills or routers. Surface finishing is done with common power sanders — as opposed to grinders or grit blast blasting equipment needed for steel — and proceeds more rapidly. Material handling is considerably easier, faster and safer because of the relative weight of the materials, all of which produce significant savings in labor hours and obviate the need for heavy duty and specialized lifting and handling machinery.
However, greater care must be taken in the production process. There are eight principal groups, referred to as series, of aluminum types. Within each series there are a number of grades, referred to as alloys. To the naked eye they all look pretty much the same, but only the 5000 series and 6000 series are suitable for marine applications. The 5000 series is used for hull plating and the 6000 series is primarily for extruded parts such as spars, rails and tubing. It is critical that aluminum alloys of other series not be used.
All aluminum alloys are highly active — that is, anodic galvanically. Only zinc and magnesium are more anodic than aluminum. Any metal that is less anodic — that’s just about all of them — in contact with the aluminum in a wet environment like the bilge will cause severe corrosion of the aluminum. This is true even of small particles embedded in the surface from wire brushes, hand files, and the like previously used on other metals, or pennies or tools dropped into the bilge.
As with steel, welding is an important part of the fabrication process. But unlike with steel, special welding techniques and conditions are essential. Aluminum must be welded by one of the gas-shielded welding methods such as MIG (metal arc inert gas shield) or TIG (tungsten inert gas shield). It cannot be done out of doors except in an enclosed area where wind or drafts cannot blow away the protective gas shielding the weld. Preparation of the weld area also is critical; impurities trapped in the weld will make it susceptible to cracking, corrosion, etc. So the work area must be kept clean. That’s the downside. The upside is that with skilled welders in proper facilities, aluminum welding is a very fast process and results in significant labor savings.
Maintenance and repair
The commonly accepted opinion is that a steel hull requires constant chipping and painting to just hold its own against the ravages of rust. There was a time when that was close to true, but since then, our dedicated chemist friends have been busily concocting new and more effective coating systems with some success. If initially the hull is grit blasted down to clean metal and modern steel coating systems are properly applied under the right environmental conditions, the subsequent maintenance program need not be at all onerous. Chips and scratches in the top coat, lightly sanded, primed and painted promptly throughout the season, and an annual light sanding and a cosmetic finish coat are about it for the topsides.
The same is true below the waterline. Putting on a proper coating system is not a trivial task, but it is very important. Typically, the process consists of grit blasting, chemical cleaning, primer coat, two or more barrier coats, and finally the antifouling coat. However, given a proper coating system, replacement of any damaged barrier coat and the application of a fresh coat of antifoulant once a year are all that is necessary. Problems arise when the barrier coat is breached. In the interior, annual repairs to any damage to the barrier coat in the bilges should do the trick for a number of years. The rest is pretty much up to your particular aesthetic standards.
Aluminum hulls may be left unpainted above the waterline and frequently are on commercial and workboats. Unlike steel, the oxide that forms naturally on the surface of the aluminum offers sufficient protection, is quite hard, and scratches in the protective oxide are self-healing. Most non-commercial owners prefer to paint them, not just for aesthetic reasons, but also to reduce glare and heating effect (hot surfaces and bare feet). Below the waterline, the procedure is much the same as with steel hulls. However, most of the common antifouling paints are metallically based — copper, mercury — and, as mentioned above, will cause severe corrosion unless they are isolated from the aluminum by thick barrier coats and are carefully monitored to detect damage to the barrier coat and subsequent corrosive damage to the hull.
Antifouling paints containing tributyltin oxide (TBTO), once used extensively on aluminum hulls, have been prohibited from sale to the public except in 16-oz. spray cans for outboard and outdrive lower units. Tin-based coatings can still be used on aluminum boats, but they can be purchased and applied only by EPA- and state-licensed operators, who must pay a fee and carry liability insurance. That means, for most of us, this is a job for the yard.
Another disadvantage of aluminum is the scarcity of yards with the skills and facilities necessary for making repairs to aluminum hulls, especially in foreign countries. There is also the problem of finding compatible alloys. These problems are especially difficult in the more remote locations that many serious bluewater voyagers favor.
All is not lost, however, as one of aluminum’s more attractive features — the fact that it is so easily cut and drilled by common hand tools — makes it relatively easy to make temporary repairs using a wood or aluminum patch, properly bedded and screwed or bolted over the damaged area. Something to keep in mind for your emergency repairs kit.
Per pound, steel is, by far, the less costly of the two metals, but the per-pound cost of the hull material is not a sufficient basis on which to judge the cost of a boat; there are too many variables to be considered. Labor and cost of fitting out the boat are much more significant factors, even in boats of the same design and suitable for either material. First of all, the weight of material used in the aluminum boat is far less than that in the steel boat. Also, if labor rates are high in the area where the boat is built, the savings in labor with aluminum may very well offset its higher cost to the point where aluminum construction may be no more costly — or even less costly — than steel construction. The bottom line is that, in smaller sizes, aluminum is significantly more expensive than steel, but this cost difference decreases as size increases. Designer Michael Kasten sums it up pretty well. Given vessels of the same size and complexity, painted to the same standard on the exterior, an aluminum boat may cost some 10 percent more to build than the same boat in steel. However, if the aluminum boat is left unpainted on the exterior, except where necessary, many yards, proficient in the material, could build the aluminum boat for less than they could a steel boat. Also, given that over the life of the boat maintenance costs will be less, the cost differences may not be that significant. As a bonus, the aluminum boat will have a much higher resale value.
Ev Collier is a freelance writer, author of The Boatowner’s Guide to Corrosion, and is a current or former member of the Materials Research Society, the Institute of Electrical and Electronics Engineers, the American Boat and Yacht Council and the Society of Naval Architects and Marine Engineers.