MATERIALS:  ALUMINUM ALLOYS

Aluminum Alloys are a fine medium for the construction of all types of marine vessels. Being a metal, almost any vessel that can be built in steel can also be built in aluminum alloys. There are a few glaring exceptions such as a Dory, which depends on a heavy bottom for a large portion of her stability. The reverse is not true for vessels designed in aluminum because the lesser weight of aluminum alloys permits a lesser displacement for the same type of steel hull.

In practice, to achieve the same strength as steel, it is mandatory to increase the scantlings approximately 50%, thus 1/8 steel = 3/16 aluminum; framing is also proportionally greater in sectional area. While the theoretical difference in material weight of aluminum is 1/3 that of steel, in actual practice the difference in structural weight as built gives only about a 30% savings over the steel hull. This must not be translated directly as a reduction to displacement as the engine, fuel, water, all other equipment, and joiner work remain virtually the same. I was at one time the Consulting Naval Architect for Kaiser Aluminum Company and can cite hundreds of reasons for and benefits of choosing aluminum alloys. Unfortunately, it is not a trickle-down effect that has much meaning in small vessels. Instead I prefer to think about some of the benefits as well as the demerits of aluminum construction.

When welding aluminum, I would place cleanliness at the top of the list. One cannot expect to obtain sound welds using the same procedures that are standard practice in steel construction. Indeed, after working in aluminum and then returning to steel one finds the habit of clean joints so ingrained that it is automatically carried over. Weld rejections become a rarity.

Welding aluminum is very similar to welding steel, EXCEPT in cutting and otherwise preparing a plate or shape for welding using power tools, such as a saw, sanders, routers, and grinders, which can impart enough heat to actually melt some or all of the metal in the joint, causing a heavy scale to form. The scale melts at roughly three times the temperature of the parent metal; therefore, when welding, an undue amount of heat has to be induced, and this slag then precipitates into the weld zone causing inclusions and porosity. Unless carbide blades are used for sawing, a lubricant must be used to prevent the blades from gumming up. This is usually a stick wax rather than a cutting fluid. The solution is simple, and that is to wipe all effective areas with toulol and then, with a hand file, clean up all the edges to be welded. This may sound like a monumental task, but for 20 of joint about 2 minutes is all that is required, as the thickness of the scale is not much more than .0001 or 1/000 thick. Weld cracking is common when welding aluminum and most often it can be traced to poor preparation and welding sequence. After the plate or shape is ready to be welded, use a hand stainless wire brush to the joint or seam to be welded and then wipe the whole with toluol. Now weld, using standard procedures and techniques. I do not believe in welding from one side only on shell and deck plating as both are subject to flexing and to impact loads. The exception is when it is impossible to make the weld on the closed side, at which time I use ceramic backing strips which are grooved to form the back bead. The strength of a completed aluminum weld is about 80% of the parent metal strength due to the loss of strength in the HAZ (heat affected zone) which is alleviated to a certain extent by careful location of butt joints and seams. When welding from both sides, it is, as in steel, necessary to get back to sound metal in the root of the first pass. This is best done by power gouging, but this is very noisy. Today we have small grinding wheels made for working aluminum that do not gum up and by using light pressure and rapid movement will not melt the aluminum. The same wheel is not suitable for, say, beveling.

Some of the advantages of aluminum are: it is lighter; welding and sanding require less time; requires no paint except for antifouling purposes; does not rust; can be cut with woodworking tools and plasma arc; and it is easier to machine. Also, the scrap has a high value; it is non-magnetic; and longer and wider sheets and plate can be used because of its weight and stiffness. The savings in the cost of paint and sandblasting are quite significant. For example, sandblasting the exterior hull, decks, and cabin trunk of a 40 steel troller is $1600 to $2000.  If  the inside is also blasted, add $3000 to $4000. The barrier coats and primer, inside and out, plus the finish color coats cost about $2000. This buys a lot of aluminum. The pilot house of wood used on steel vessels costs more than an aluminum pilot house used on aluminum vessels. Special shapes can be extruded at a nominal cost for dies. Unused portions of a plate that would normally be scrap in steel can be cut into flat bars and pieces for other small fixtures. It is well to remember that in cutting, say, with a Skil saw, there is NO distortion. I have found that 20 x 8 plates are quite handy; whereas, in steel, say, 4 x 14 or 5 x 12 would be my choice in that they must be cut with heat.

There are some detractions of using aluminum in boatbuilding. There is a limited number of suitable alloys for marine use; these are in the 5000 and 6000 series; and the numerous remaining alloys should never be used. Except in rare instances, aluminum should not be heated to make several bends, and cannot be forged to make fittings and other parts. It costs about 6 times more per pound than mild steel, thus extra care must be exercised to derive the most out of each plate to minimize the scrap. Extra care must be exercised to isolate any dissimilar metals from the aluminum and, with the exception of laminated wood cabin tops, wood bonded to aluminum should be avoided due to the probability of poltice corrosion. 

 

48' aluminum Chinese Junk, K'UNG FU-TSE, which I designed and built at my yard in Virginia. My family and I  lived aboard and cruised for 16 years, enjoying many memorable experiences. She proved to be a great sea boat and sailer.

 

 

 

 

 

 

 

 

K'UNG FU-TSE in the Caribbean. This is one of many experimental rigs that she had during my ownership.

 

 

 

 

 

 

 

57' aluminum centerboard ketch, LUCAYA, used as a cargo vessel. She is a fine seagoing vessel and extremely fast.

 

 

 

 

 

 

HOME    MATERIALS