Boats & Gear
- Category: Boats & Gear
- Created: Saturday, 12 January 2013 10:22
- Written by Charles Doane
METAL HAS BEEN USED to build ships for about 160 years, and very large metal yachts were being built as early as the late 19th century. In 1895, for example, Nat Herreshoff designed and constructed a radical 123-foot composite metal sloop, Defender, to defend the America's Cup. She was built of aluminum, bronze, and steel and within six years was so debilitated by galvanic corrosion she had to be broken up.
It wasn't until the 1960s (except for some boats built in Holland, where steel has long been a favored material) that metal was used to build sailboats of more moderate size. Bernard Moitessier, an early pioneer, commissioned the construction of his 40-foot steel ketch Joshua in 1961. By the middle of the decade, aluminum was also being used to build both racing and cruising boats. By the mid-1970s, aluminum was the favored construction material in America's Cup 12-meter boats (the first was Courageous, built in 1974) and remained so until the mid-1980s.
These days racing boats are rarely built of metal, though it is still very popular with certain cruising sailors. For those tired of chasing down deck leaks on wood or fiberglass boats, one big attraction of an all-metal boat is that it is very tight. The hull-deck joint is welded and all hardware such as cleats, genoa tracks, stanchion bases, etc., can be welded in place, with no fasteners penetrating the structure. Instead of leaks, however, one must worry about corrosion.
One big advantage of metal boats is that deck hardware can be welded rather than fastened in place. This cleat may be strong enough to lift the boat it is installed on and will never be the cause of a deck leak!
Pound for pound metal is far stronger than wood or fiberglass. Unlike wood and glass, which have most of their strength oriented along the lay of their grain or fibers, metal is equally strong in all directions. Metal is so tough one needn't worry about wasting strength because of this, and much trouble is saved because the material can be laid down any which way. In a wood or glass boat, by comparison, designers and builders must always take care to ensure that material is aligned along anticipated load paths.
The skin of a metal boat must be kept quite thin or the boat will be much too heavy. The thin skin, in turn, must be completely supported by a frame or it will flex too much. The traditional approach is transverse framing similar to that found in plank-on-frame boats. There is a backbone keel with a large number of transverse frames attached to it, plus thin lateral stringers to hold them together. The alternative is longitudinal framing, where fewer but much larger ring frames are joined together by a large number of stringers.
The advantage of transverse framing is that smaller, more closely spaced frames intrude less on the boat's interior than do bulkier ring frames. A transverse frame, however, takes more work to put together, as there are more frames that need to be cut to shape. Whatever type of framing is used, it is always much easier (and less expensive) to build a hard-chined hull. It certainly is not impossible to create a true round-bilged metal hull, but it does take more effort. A compromise shape is seen in radius-bilge hulls where the corners are knocked off a hard-chine hull and replaced with large radiused angles.
Transverse framing for a metal boat under construction. Note the thin lateral stringers holding the transverse frames together (Photo courtesy of Billy Black)
A metal boat must also be well insulated. Metal conducts both sound and heat very well, and living inside an uninsulated hull would be nothing less than an ordeal. The boat's interior would be much too cold when it's cold out, much too hot when it's warm out, you'd be up to your eyeballs in condensation, and every little sound on deck or in the water around you would be greatly amplified, as though you had your head inside a vast tin drum. The most common types of insulation are foam, cork, and fiberglass, which are available in sheets that can be cut and laid in place between the frames. Alternatively, urethane foam can be sprayed onto a hull's interior surface. Fiberglass is the most fire-resistant insulation; urethane foam is the least fire-resistant and also absorbs odors easily. Whatever type is used, insulation should never be laid down in a metal boat's bilges, as it will inevitably get wet there.
Sprayed-on urethane foam insulation in an unfinished storage area on an aluminum boat. Foam is easy to install, but is not fire-resistant and it also absorbs odors
To avoid the awful fate of Defender different metals on a metal boat must be carefully isolated from each other. This is always the case, of course, but on a metal boat it is particularly important, as one of the metal parts that needs protecting is the hull itself. In theory this is simple; in practice it requires constant vigilance.
Any bronze seacocks, winches, or other hardware--not to mention the vast universe of stainless steel hardware and fasteners found on all modern boats, or the intricate web of copper wiring that comprises an electrical system--must all be isolated from the hull by non-metallic spacers and inserts, insulating grease, plastic sheathing, etc., in order to ensure that no galvanic couples are created. Zinc anodes must be scrupulously maintained. Electrical wiring must be carefully organized--with a galvanic isolator or, better, an isolation transformer on the shore-power side--to protect the boat from stray electrical currents. A strong dose of stray current can chew up a metal boat in just a few days.
The primary advantage of any metal boat is its inherent strength. This steel boat, for example, has a massive dent after suffering through a major collision, but it isn't taking on water. A fiberglass boat would have been holed and likely would have sunk very quickly
A small handful of European builders construct metal boats on a series production basis; otherwise, like wood boats, they are built as one-offs. Metal is usually the cheapest material to work with when building just one boat. This, plus the fact that metal boats are so strong, means there will always be a cult of metal-boat cruisers. As mentioned, most acolytes are found in Europe. There are a few metal boatbuilders in North America (mostly in Canada), and there is always a small selection of metal boats available on the North American brokerage market, but the heart of the market will always lie on the eastern shores of the North Atlantic.
Steel is heavy and strong, but is also hard, and working it requires heavy-duty grinding and cutting tools. Cutting and welding steel is laborious, but the welding is not sophisticated and is a relatively easy skill to acquire. Most of all, steel is subject to rusting. Put it in contact with water and oxygen--two things that are never hard to find in the marine environment--and it starts corroding at an alarming rate. Add a little salt, and things only get worse.
A steel boat under construction. Many steel boats, like this one, begin as backyard projects (Photo courtesy of Bruce Roberts-Goodson)
Paint, paint, and more paint is the only answer. Bernard Moitessier liked to boast that a well-trained monkey could do this, but to maintain a steel boat properly you need to be a busy monkey. Modern paint systems are durable and reliable, and if a steel boat is properly prepped and painted as it is built, and if it is designed so that all parts of its interior are accessible to a paint brush, it is possible to keep the rust at bay indefinitely. But the bottom line is you will always be painting a steel boat, both inside and out, or will be worrying about painting it.
Because steel is so heavy, it is not possible to build a small- to moderate-size steel sailboat that performs very well. To sail fast, relatively speaking, you need a steel boat at least 100 feet long. Any steel vessel shorter than 60 feet should have its deck and superstructure built of another lighter material to keep it from getting too top-heavy. In older steel yachts this was common. The problem, however, is that you then don't have the leakproof deck that makes a metal boat seem so attractive in the first place. And all those deck leaks, of course, only help things inside start rusting more quickly.
Example of a small steel sailboat hull. It is certainly strong, but once rigged it won't be winning any races
Most contemporary steel boats, whatever their size, have steel decks. Because steel is hard and stiff and difficult to work, they often have simple shapes and hard-chine hulls. Such boats are cheap and easy to build, and home-built examples aren't hard to find on the used-boat market. If you don't care about performance, are willing to do a lot of painting, and want an inexpensive boat that is incredibly tough, an all-steel boat is an excellent choice. Otherwise, particularly if you are a coastal cruiser, you'd do well to stay away from these.
Aluminum is weaker than steel by a factor of about 1.5 (i.e., to be as strong as steel, aluminum plate must be 1.5 times thicker), but it is also lighter by a factor of almost 3 (i.e., it weighs 1/3 as much). So the bottom line is simple: an aluminum object that is as strong as an equivalent steel object weighs only half as much. On a purely structural basis, aluminum therefore makes an excellent boatbuilding material.
Aluminum is also much softer than steel, so is easier to work with. It can be cut, drilled, and shaped with common woodworking tools and can be more easily tortured into complex forms. Best of all, it doesn't rust. Put it in contact with water and oxygen and it forms a thin oxide layer on its surface that makes it even stronger and more corrosion-resistant. No painting is required except below the waterline, where antifouling is still needed to keep lifeforms from latching on. (Generally, though, it is also wise to paint an aluminum boat's decks some light color to reflect solar radiation and help keep the interior cool.)
Of course, I'm a little prejudiced in favor of aluminum boats, because I own one. This is our boat Lunacy, a 39-foot Tanton design, at anchor in Maine. The hard-chined hull was built in Canada in 1985 and was finished in Rhode Island
Aluminum is considerably more expensive than steel. Commodity prices of course fluctuate, but on a strict per-pound basis, steel is usually about five times cheaper. Though you need only half as many pounds to make an equivalent aluminum boat, you'll still be spending two-and-a-half times as much on materials. This is offset to a large extent by the fact that it takes about half as many man-hours to weld up an aluminum hull and deck. Still an aluminum boat, all told, costs roughly 5 to 15 percent more to build than a steel one.
Though it takes less time, it also takes more skill to build an aluminum boat. Welding aluminum is a sophisticated process requiring special gas-shielded equipment. Ideally, all work should be performed indoors in a well-protected environment. As a result, you almost never see home-built aluminum boats. Also, appropriate marine-grade aluminum (5054, 5083, and 5086 are the best alloys for a hull) is hard to find outside the United States and Europe, as are qualified welders, so making permanent repairs in remote locations is difficult, if not impossible. A steel boat, by comparison, can be easily repaired almost anywhere in the world.
The biggest drawback to aluminum is that it is highly susceptible to both galvanic and stray-current corrosion. It is low on the galvanic scale and wastes away quickly when placed in contact with salt water and more noble metals like stainless steel, bronze, and copper. Different metals must be carefully isolated, and bilges must be kept clean and dry to prevent the inadvertent creation of galvanic couples, as something like a lost coin or camera battery might literally sink your boat someday. Zincs must also be scrupulously maintained and stringent precautions must be taken to ensure no stray electrical current comes aboard.
Otherwise, a properly designed aluminum boat is fast, strong, easy to maintain, and thus a good choice for cruising. They are, unfortunately, relatively hard to come by in North America. A few used examples can usually be found on the brokerage market, but otherwise you'll need to do your shopping elsewhere. A few French builders--Alubat and Garcia come to mind--do produce series-built aluminum cruising boats, but these are quite expensive compared to mass-produced fiberglass boats. They also tend to be bilge-centerboard designs, which are comfortable and well thought out, but are also quite idiosyncratic by American standards and are apt to appeal only to more open-minded sailors.