Boats & Gear
- Category: Boats & Gear
- Created: Tuesday, 08 January 2013 22:45
- Written by Charles Doane
HUMANS HAVE BEEN building boats out of wood for many thousands of years. Many assume therefore it must now be obsolete. Wood certainly does not lend itself to mass production the way fiberglass does, though there were a few builders who manufactured wood boats on something like a production basis not long before the advent of glass. Wood does have some distinct virtues. It is light, even compared to modern building materials, and in terms of tensile strength is stronger per pound than common electrical-grade fiberglass. In terms of stiffness, it is stronger per pound than S glass, E-glass, and Kevlar. In terms of its total structural efficiency, it is better than all of these materials, including carbon fiber.
One big problem with wood, however, is that certain lifeforms like to eat it. Various fungi can infest and consume it, causing what is known as dry rot. Marine borers like the Teredo worm, or boring insects like carpenter ants and termites, can also chew their way through a boat pretty quickly. Wood also rots when it gets too wet, is easily ignited, and is soft, with poor abrasion resistance. Structurally, in one important sense, it is deficient in that it is much less dense than other materials and thus takes up a lot of space. A wood hull must normally be much thicker than an equivalent glass hull, and its interior structural parts must also be larger. Indeed, wood cannot be used at all to make certain small parts that carry great loads (such as bolts, tie-rods, and rigging wire) simply because it is too soft and too fat to fit.
Perhaps the biggest advantage wood has over any other material, especially when it comes to building boats, is that it is inherently romantic. For this reason alone, it is likely someone somewhere will always be building wooden cruising boats, and that other people will always be sailing them.
This is the most traditional method of building a wood boat. The principle is simple, though the details are complex. The fundamental structure of a plank-on-frame vessel is defined by a keel, which is the horizontal backbone of the hull; a more vertical stem, which forms the bow; and a vertical sternpost (plus, in the case of many yachts with long overhangs, a much less vertical horn timber that terminates in the transom), which forms the back of the boat. On deep-keel vessels, especially on sailboats, there is also often what is called deadwood fastened beneath the keel. This forms part of the lateral plane well below the waterline and makes up a good part of what sailors normally call the "keel," particularly its aft section. The forward section is normally inhabited by a solid casting of metal ballast, preferably lead, that is fastened to the bottom of the boat.
A full-keel plank-on-frame sailboat under construction. You can see both the deadwood and the lead ballast down low (Photo courtesy of Rockport Marine)
Fastened to the spine created by these parts is a series of parallel transverse frames that describe the vessel's hull form. To help support the hull, lateral stringers are installed inside the frames. The skin of the hull consists of a series of planks fastened to the outside of the frames. These planks may be laid on the frame with their edges slightly overlapping, which is known as clinker, or lapstrake, construction. This is often done with smaller boats, but hardly ever with larger boats, as the many ridges formed where the planks overlap greatly increases wetted surface area. Alternatively, planks can be laid on the frame edge to edge, creating a fair, smooth surface, which is known as carvel construction. To make the hull watertight, the seams between carvel planks must be caulked with long strands of cotton and/or oakum.
Open seams on a carvel hull awaiting caulking. Note the tufts of cotton hanging out where caulking is underway (Photo courtesy of Rockport Marine)
The deck of the boat, meanwhile, is supported by a series of transverse deck beams, the ends of which are fastened to lateral shelves installed along the inside of the hull at the top of the frames. Traditionally, the deck consists of planking fastened to the deck beams with all seams, again, carefully caulked. Another common way to seal decks, often used on yachts, is to cover the planking with painted canvas. These days, however, many wood decks are simply good-quality marine plywood sealed with epoxy.
Even from this abbreviated description it should be clear this is a labor-intensive way to build a boat. Much skill is also required. Just selecting wood to build with is an art, as there are numerous criteria to meet. The best wood should be cut only in winter to minimize the retention of moisture and microorganisms. It should then be air-dried in a climate-controlled environment for as long as possible--many months at a minimum. The lumber should also be carefully milled to produce planks and pieces with the wood grain properly aligned to carry anticipated loads in the boat. Finally, if you're truly fanatic, all pieces should be hewn to size by hand, rather than ripped with power tools, as traditional hand tools do less damage to the fiber of the wood and make it much less prone to fungal attacks.
The biggest issue is embraced in that single verb "to fasten." A plank-on-frame boat consists of hundreds of pieces of wood, all of which must be carefully shaped and then bound together by thousands of small metal fasteners. Even if you use the best fasteners (silicon bronze screws and bolts are preferred, though Monel is technically superior) what ultimately limits the strength of a plank-on-frame boat is not the wood it is made from, but the fasteners holding it together.
This weakness manifests itself in various ways. First, because they are made from many different pieces, and in particular because so many plank seams are permanently submerged, plank-on-frame boats are apt to leak. Many are continually taking on water when afloat, and normally the only variable is the rate at which water is coming aboard. Invariably this increases when conditions get worse. I once sailed across the North Atlantic aboard a plank-on-frame schooner--one time we almost sank; the other time we did (though, fortunately, this was in a river on the other side). Prior to the voyage, a friend warned me: "A wood boat is nothing but a collection of leaks loosely organized as a hull." Nothing in my experience proved him wrong.
Plank-on-frame boats also often have deck leaks. The problem here is that wood in the deck is constantly swelling and shrinking as it gets wet and dries out. If the deck has open seams, all this expanding and contracting is apt to create gaps somewhere. Even with painted canvas covering the seams, or with a solid plywood deck sealed in epoxy, there are again many fasteners securing hardware, each offering a potential route for water intrusion. Other structures sprouting from the deck--deckhouses, hatches, raised gunwales, etc.--also present seams and cracks where they join the deck that water can eventually seep through.
World-famous small-boat cruiser Larry Pardey waters the deck of his boat, Taleisin, to keep the planks swollen tight. That's his equally famous wife Lin peering out the companionway. Larry is a master boatwright (he built Taleisin himself) and maintains his boats scrupulously
Finally, plank-on-frame boats can be a bear to maintain. All that wood, above the water and below, needs to be either painted or varnished on a regular basis. Leaks must be policed and stanched if possible. Moist areas in the structure must be sought out, constantly monitored for rot, and replaced if the rot gets out of hand. As Bernard Moitessier once put it: "The maintenance of my wooden boats had always confronted me with delicate problems and required real qualifications, for I had to be 'Doctor of Rot,' 'Doctor of Teredos' and 'Doctor of Leaks.' " Some people enjoy this sort of work and anxiety. Most, however, like Moitessier, would much prefer to just go sailing.
Plank-on-frame boats still have a strong cult following and a relatively large number of older wooden yachts are sailed and maintained by devoted owners. Some new plank-on-frame yachts are also built from time to time, and a few boatyards--the most prominent are probably Gannon & Benjamin on Martha's Vineyard and Rockport Marine in Maine--even specialize in this sort of work. But the most exciting wooden boatbuilding these days is done with composite wood-epoxy construction.
The key ingredient is modern epoxy, which is not only a tenacious adhesive, but is also highly elastic and nearly impermeable to water. By sealing and coating every piece of wood in a boat with epoxy, and by using epoxy to help glue these parts together (aided, too, by the judicious use of metal fasteners), it is possible to take full advantage of wood's excellent structural properties while negating its tendencies to rot and swell and contract when exposed to water. Epoxy also protects the wood from hungry creatures that want to eat it.
Furthermore, a wood-epoxy hull forms a one-piece monocoque structure that cannot leak unless punctured. In most cases, to improve abrasion and impact resistance, the hull and deck are also sheathed in one or more layers of fiberglass cloth. The result is a boat with many of the virtues of fiberglass, with the added benefits of built-in insulation, plus all the fuzzy romantic feelings inspired by a genuine wood finish.
There are many ways to construct a wood-epoxy boat. One could, for example, build a wood-epoxy plank-on-frame vessel, but this would be labor intensive and the boat would be needlessly heavy and thick. In practice, there are three basic approaches--strip-plank construction, sheet plywood construction, and so-called cold-molded construction. Each has many variations, and to some extent different techniques can be combined in a single hull.
In a simple strip-plank hull the frame is an important part of the structure, and the strip planks, which are narrow--with a square section shape, are both attached to the frame and edge-nailed to each other. Boats were often built like this in the traditional manner (and are still built) without being encapsulated in epoxy. In more modern variations, there is more reliance on epoxy, fiberglass sheathing, and internal accommodations structures (including bulkheads) to support the hull, with framing reduced to a minimum. Some of these vessels are essentially fiberglass boats with solid wood cores. Strip-planked wood-epoxy hulls are probably the most common type built today, as they are generally the most cost effective.
Sheet plywood construction is the least common type, at least as far as larger sailboats go. Mostly this technique is used for smaller boats like dinghies, skiffs, and daysailers. The one major exception are Wharram catamarans, which are usually built of plywood, and may or may not be coated in epoxy. In a plywood boat of any size, a substantial amount of framing is needed, but construction otherwise is relatively simple and fast, as large sheets of plywood can be set in place more easily and quickly than many narrow planks. Plywood construction does limit design options. Normally plywood hulls are hard-chined, although lapstrake construction--as seen, for example, in some very interesting Dutch Waarschip designs--can also be employed.
The third major variation, cold-molded construction, is more properly described as diagonal-veneer construction. Here the hull is composed of several layers of thin wood veneers that are laid up on a diagonal bias over light framing or a jig. The layers of veneer are oriented at right angles to each other and are glued together and stapled in place until the epoxy sets up. Often there are one or more layers also oriented laterally at a 45-degree angle to the diagonal layers. By laminating thin sheets of unidirectional veneer atop one another like this, a light monocoque structure that is strong in multiple directions can be created. These cold-molded boats are, generally speaking, the lightest of wood boats, but this method of wood construction is also by far the most labor intensive. The technique is shunned by some, but is favored by those for whom weight reduction is critical. It is also sometimes used in conjunction with strip-planking, with layers of diagonal veneer laminated over a planked hull in place of fiberglass sheathing.
This Wharram-designed Islander 65 catamaran is being professionally constructed of diagonal veneers (Photo courtesy of James Wharram)
The hull of this large cold-molded yacht has diagonal veneers being laid over strip planking (Photo courtesy of Hodgdon Yachts)
The term cold-molded is something of an historical anomaly. The first laminated wood hulls were composed of veneers laid up in female molds and glued together with adhesives that could only cure in an oven. The expression "cold-molded" was born later when it became possible to use adhesives that cure at room temperature. The term is still used to describe diagonal-veneer hulls, but not other types. Technically speaking, any wood-epoxy hull laid up at room temperature can be said to have been cold-molded.
Whatever they are called, wood-epoxy vessels in fact make superb cruising boats. The only problem is that wood-epoxy construction does not lend itself to series production. If you want a new wood-epoxy boat, you must commission its creation as a one-off, and many people with money to burn have done just that. Many modern wood-epoxy boats are based on traditional designs but take full advantage of modern design and construction techniques to minimize weight and maximize performance. Others are full-out modern superyachts measuring over 100 feet in length and a few are flat-out race boats. For example, Bruce Schwab's very intriguing Open 60 OceanPlanet was a wood-epoxy boat. So, too, was Holger Danske, an early BOC boat designed by Dave Gerr that had a phenomenally low displacement/length ratio of 40. This is one of the lowest D/L ratios ever achieved in an ocean-racing monohull, which gives some idea of just how cutting-edge wood-epoxy can be.
Dave Gerr's ocean racer Holger Danske, a very light boat built out of wood (Photo courtesy of Gerr Marine)
Gusto, a Chuck Paine design, is a modern wood-epoxy cruising boat with more traditional lines (Photo courtesy of Chuck Paine)
Wood-epoxy boats can also, of course, be found on the used-boat market. Recently built boats are relatively rare and normally command a significant premium, but older boats, some dating back to the early 1970s, are often quite affordable. Be sure to have them carefully surveyed, however, as construction techniques have improved considerably in recent years. As with any older boat, there will likely be problems that need addressing.