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Aluminum and Steel, one designer's view

Jul 27, 2007
 
Sept/Oct 2002
 

Of the four standard boatbuilding materials, fiberglass, wood, aluminum, and steel, steel is my least favorite. Aluminum has many advantage over steel which we'll examine below. Of course there are steel yachts, often large custom boats or workboat conversions. They can work, but, in most cases, not as well as aluminum, which rivals or surpasses fiberglass. Before steel aficionados condemn me for this view, let me say clearly that steel is still a fine material to build a boat from. I have designed boats in steel and will continue to do so. In fact, we recently designed a 65-foot (19.8 m) steel 150-passenger dinner/cruiser charter boat.

Aluminum's advantages over steel can be listed as follows:

1) Light Aluminum: Aluminum is lighter than steel for the same strength. For example, very roughly speaking, and neglecting corrosion allowance in steel, aluminum plate should be between 1.25 and 1.5 times thicker than steel for the same strength. Aluminum, though, weighs 168 lb./cu. ft. vs steels 490 lb./cu. ft. only 34 percent of steels weight. Even taking the larger thickness multiplier of 1.5 this means that aluminum is about half the weight for the same strength, a huge difference! (1.5 times thicker x 0.34 lighter = 0.51 or 51%) Indeed, aluminum structural weight compares favorably to wood or fiberglass. Steel hulls, in vessel of under 100 feet, are nearly impossible to make both really light and long lasting. Aluminum hulls, in contrast, can even be built extra heavy, resulting in a hull that's literally stronger than steel that still weighs significantly less.

2) More Stable and Faster Aluminum Boats: The light weight obtainable from aluminum construction lowers the center of gravity of a boat, making it more stable and thus more seaworthy. Less weight also means you can go faster with the same power or sail area, or have a higher ballast-ratio in a sailboat for more sail area and improved performance. Alternately, you can use less power for the same speed or get greater range with the same tankage. Of course, you have more cargo capacity or allowable weight for joinerwork and auxiliary machinery with aluminum construction.

3) Aluminum Superstructures on Steel Hulls: It is so difficult to make steel light enough that most vessels roughly under 60 ft. must use wood, FRP, or aluminum superstructures rather than steel. If they don't, its nearly impossible to keep the boats center of gravity low enough for adequate stability. Even larger steel craft benefit from lighter superstructures and frequently use this approach.

4) Light Weight Equals Labor Saving: The lighter weight of the components makes building in aluminum less labor intensive than steel. A 3/16-in., 400 sq. ft. steel plate would weight 3,000 pounds, and would require very careful handling and heavy gear. Roughly equal 1/4-in., 400 sq. ft. aluminum plate would weigh just 1,408 pounds. Again, a vast difference.

5) Easy Working Aluminum: Aluminum is softer and easier to bend, cut, and form. It cuts about three times faster than steel. Aluminum beds so easily, in fact, that round-bilge hulls in aluminum are little problem. It can be cut with ordinary woodworking equipment, and quickly and easily drilled, sanded, and filed to exact dimensions. Steel by comparison, takes heavy grinders, and specialized cutting and bending tools. Larger building yards, naturally have specialized machinery for either aluminum or steel, but for aluminum these are simply to make the work go faster still.

6) No Compromise on Hull Shape: The freedom to build nearly any hull shape in aluminum inexpensively means that the hull form doesn't have to be compromised with developable surfaces, or chines, when they're not wanted. This translates into more efficient hydrodynamics, for better performance and increased seakindliness.

7) Faster Welding: Welding aluminum is roughly three times faster than welding steel. Even allowing for somewhat heavier welds (more passes)for the comparably thicker aluminum plate of the same strength, the total man hours in welding aluminum should be about half that for a similar steel hull.

8) No Rust Lower Maintenance: Aluminum doesn't rust at all. Yes, aluminum can corrode when in contact with dissimilar metals or from stray electric currents, but so can steel. Aluminum is so corrosion resistant and totally rust free that it doesn't even have to be painted above the waterline, or on the inside. (Anti-fouling bottom paint is still a must.) Many workboats are routinely left bare above the waterline, but most vessels chose to paint for appearance. The net savings in maintenance over the life of the boat as a result from freedom from rust and generally reduced corrosion is substantial.

9) No Added Plate Thickness to Allow for Corrosion: Even more important, rusting wastes away the steel so the plates get thinner with age. This is why, with small craft, you cant make effective use of most higher-strength alloys. Indeed, the plate thickness for small-boat steel hulls are somewhat heavier than required for basic strength to allow for corrosion, the corrosion allowance.

A good rule of thumb is that a steel hull will loose about 0.004 in. of thickness every year. A well maintained and properly built steel boat will do a bit better than this in most places. Still, in those hard to reach areas like inside the bilge at the base of bulkheads near stiffeners and other obstructions this rule is close.

Another old saying is that steel boats rust from the inside out. This is because its just these hard-to-reach areas, that are difficult to inspect and maintain, that waste away fastest. All this means that in 25 years 3/16-in. shell plate would be reduced to 0.09 inches thick in several areas, approaching just 1/16 in. left. In order to get adequate life in small craft, steel plates must be made heavier than needed for strength with a corrosion allowance to accommodate this wastage.

10) Aluminum is Non-Sparking and Non-Magnetic: Being non-sparking makes aluminum safer both in the building shop and in operation. Fires cant be ignited by the friction spark of some heavy object falling or scraping against the aluminum structure. The lack of magnetic interference is a great plus for navigation and electronics.

11) Aluminum Deforms to Absorb More Energy: Aluminum deforms or stretches beyond its elastic limit more than does steel before rupturing. This is why dents in aluminum canoes and runabouts seldom split open, and can usually be hammered out again. Such plastic deformation accords aluminum still more energy absorption for its weight than steel has.

12) Aluminum is Less Sensitive to Stress Risers: Aluminum's plastic deformation offers another benefit as well. Steel is particularly sensitive to sharp corners in construction. This is called notch sensitivity and the notches or sharp corners are termed stress risers. Even though its still recommended practice to radius corners and avoid hard spots on aluminum, in fact, aluminum is much more resistant to stress concentrations caused by stress risers than steel is. Before the aluminum can fail at such a hard corner it deforms absorbing some of the energy and reliving the stress.

Steel, on the other hand, develops cracks which further increase the stress concentration which then in turn extends the crack and generates new ones as well, a progressively worsening downward spiral in strength. Large ships have literally broken in half and sunk at sea from a small stress-riser cracks propagated at a sharp-corner hatch opening on deck.

13) Custom Extrusions: Custom dies are possible for aluminum to make specialized shapes. This is generally too costly for small one-off projects; however, production yards will often find it economical and practical to have a few specialized extrusions for standard rubrails and similar details made up. This is not doable in steel.

14) No Attack By Bacteria: Aluminum hulls are not subject to attack by sulfate reducing bacteria. These little-known bacteria can accumulate in bilges, ballast tanks, fuel tanks, etc., usually in commercial vessels, and eat their way through 5/16-in. of steel plate in a year! Colonies of microorganisms, sulfate reducing bacteria speeds up corrosion by digesting sulfates and producing sulfides. They can cause catastrophic corrosion quickly if not detected.

15) Higher Scrap Value: Aluminum has higher scrap value than steel, recouping some of its added initial material cost.

16) Reduced Labor Costs Compensate for Increased Material Costs: The much greater ease or working with and forming aluminum substantially reduces labor costs. Usually, the labor-cost reduction, combined with the lower total weight of metal purchased, is enough to offset the substantially higher cost of the aluminum itself, or even to make an aluminum boat a bit less expensive.

Aluminum Disadvantages

No material is perfect, and in spite of the many pluses listed above, aluminum does have some drawbacks compared to steel.

1) Aluminum is Considerably More Expensive: Just a few days ago I got prices on steel at around 29 per pound, and aluminum at $1.50 per pound. Though the relationship shifts with commodity-market vagaries, aluminum will always be significantly more costly. Still, its not quite what it seems. Remember that an aluminum structure will weigh only half as much as a steel one of comparable strength. So, for proper boat-to-boat comparison you need cut total weight of aluminum purchased to half that of steel, or, for convenience, figure on the same weight of metal for both, but half the price for aluminum. At today's prices, this is 29 vs 75.

2) Less Abrasion Resistant: Aluminum is softer than steel. This is a big plus for working and forming, but aluminum hulls are less resistant to abrasion. Still, aluminum is considerably more abrasion resistant, however, than standard wood or fiberglass construction, so this isn't much of a drawback for most boats. In fact, aluminum runabouts are routinely run up on rough concrete launching ramps on their bottoms. Though not the best practice, aluminum hulls can survive such abuse, which most wood and FRP vessels could not. Still, when it comes to pure abrasion abuse its impossible to beat steel. For tugs, canal boats, barges, dredges, and such steel becomes more attractive.

3) Aluminum Can Melt and Burn in a Fire: For a structural metal, aluminum has a low melting point at around 1,080 F. It can even burn in an exceptionally intense fire. Steel, with a melting point of roughly 2,600 F, is the only truly fireproof boatbuilding material. Once again, aluminum is vastly more fire resistant than wood or FRP. Steel, however, is clearly superior in this regard.

4) Welding Equipment is More Expensive: The gas-shielded welding equipment required for aluminum is more expensive than the stick/electrode welding used for steel. What's more, aluminum really should be welded in an enclosed building. If not, breezes will blow the gas shield away from the arc, and cause defective welds. Builders have had good success setting up temporary windscreens for outdoor construction, but this is second best for aluminum construction compared to having a proper enclosed building shed.

5) Qualified Workers and Equipment are Harder to Find: There are fewer yards and welders qualified for and experienced in welding aluminum than steel. Especially when voyaging to out of the way corners of the globe, this may make it difficult to find places to make repairs. Of course, most damage to aluminum can be temporarily patched by hammering the holed area as smooth as possible, drilling holes in the surrounding shell, then bolting on a patch sealed with bedding compound.

6) Aluminum Alloy is Harder to Locate: Aluminum alloys are frequently tough to find in many regions outside of the North America and Europe. Even where aluminum suppliers and manufactures are common, it can be very difficult to purchase the sizes and quantities of material necessary for a small-boat project.

Larger building yards will not have too much difficulty with this through their regular suppliers, due to their high annual purchase volume. Small yards and home builders; however, often have to expend great amounts of effort searching for adequate supplies at reasonable cost, and they may be compelled to import materials from distant sources. Even with this extra effort, compromises on more readily available sizes and shapes may become unavoidable. Steel, by comparison, is obtainable quickly and inexpensively, in small and large quantities, all over the world.

Evaluating all the factors above, aluminum is a clear choice over steel for most boat applications, and its tougher and more abrasion resistant than fiberglass at the same weight. Only conditions involving truly extreme abrasion or fire hazard make steel more attractive.

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