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December 30, 2009 9 min read

Sheet Plywood

Plywood is a very versatile material widely used in wood boat construction. It is dimensionally stable and needs only to be epoxy coated to protect it from moisture to become an almost ideal boatbuilding material. Several construction techniques are used to fabricate boats from plywood.

It is not necessary to use marine grade plywood in boatbuilding. Marine plywood is basically exterior plywood with a lower void content. Several years ago the plywood association that sets specifications started degrading marine plywood by allowing a higher void content. We believe that the extra cost for fir marine plywood over A-B exterior is not justified today.

Fir plywood boats that see continuous outdoor exposure should be sheathed on the exterior surfaces. Almost all fir plywood is rotary cut and exposed exterior surfaces will eventually check if unreinforced epoxy coatings are used. Sliced veneer plywood has almost no tendency to check. However, we know of no source of sliced fir plywood so plan to put a light layer of cloth on all exterior fir plywood surfaces. Sliced veneer mahogany plywood is readily available in many coastal areas. This plywood is considerably more expensive than fir. Builders using this for small boats rarely fiberglass the surface. Large boats are commonly fiberglassed if only because repair is far more costly if the epoxy coating is breached in impact. Better to build in the protection during production.

Frame, Stringer, Planking Construction

Plywood was probably first used in this type of construction which came along even before epoxy and polyester resins were developed. This is a simple and straightforward method of boatbuilding.

The builder makes the frames from a set of drawings or table of offsets. Some of the frames may be temporary while others stay as bulkheads in the finished boat. The frames are mounted bottom up on a horizontal ladder-like structure called a strongback. Here the rungs are attached to the rails precisely at ninety degrees to the centerline. Strongbacks are precisely leveled so that plumb bobs may be used to aid in attaching the frames to the rungs. Usually an overhead taut wire will represent the centerline in space. The frames are mounted on the rungs of the strongback so that centerlines and reference lines align precisely. Temporary bracing is often used until the stringers are added.

After this notches may be cut into the frames to mount the stringers, which are epoxy glued into place. Once the epoxy has cured the temporary bracing is removed as the frames and stringers are quite rigid. Plywood is fitted and glued to the frame/stringer structure. After this the strongback is superfluous.

The plywood is epoxy coated, faired, and fiberglassed. All the unit operations of epoxy use have been employed so far. There are a number of tricks that can make the epoxy work go easier. For example, the plywood frames could have been coated and sanded even prior to cutting from 4x8 sheets. Plywood planking should have been coated and sanded on the inside before the planking is attached. Some designs use molded chines rather than chine logs. Here, biaxial tape would be ideal to attach adjacent planking.

It is important to remember that there is no one right way to do most anything (except for correct mix ratios!) when boatbuilding. Build it in your head first and you'll think of a lot of short cuts that work.

Stitch & Glue Construction

In a way this uses plywood in hull construction in almost the reverse order of the frame, stringer, planking building method. Here the panels are cut to the correct shape and wired together with soft copper wire through small holes cut in adjoining planking edges. Sometimes a frame or two are used to facilitate in the bending and joining of the panels.

Once wired together the joints are glued by applying large epoxy fillets to the inside edges. The wire is removed and the outside edges are taped with glass cloth and epoxy. Several bulkheads are added and stringers may be glued to the interior panels to increase stiffness.

This construction method, also called sewn seam construction, lends itself very well to small boat construction where the internal structure of frames and stringers are not necessary. Lately several large production boats (56 feet) have been built on the West Coast using this method. The commercial builder of one of these made his own plywood on a 10 by 60 foot vacuum table. Interestingly, many huge fiberglass boats are partially constructed using this method (bows and sterns are still made in a standard fiberglass mold).

Some designing skill is required to take a three dimensional concept and reduce it to a two dimensional drawing that can be laid out on plywood so that edges meet when the panels are cut and sewn together. The first time builder should purchase plans rather than suffer the frustration involved in ruining several sheets of plywood "trying to get it right." The classified sections of the boatbuilding magazines carry ads for these plans. Often, there are also ads for kits where the panels are already cut.

Epoxy coating and perhaps even fiberglassing the panels prior to assembly will save lots of time in this construction method. Learning to make the "no sand" interior fillets described is a must. Removing the copper wire can be tricky. Some builders fillet right over it and then cut it off flush on the outside.

Here it becomes part of the boat. We heard of one builder who hooked the wire up to a car battery for a second or two and then pulled the red-hot wire out with a pair of pliers. We like the idea of making small fillets using Quick Cure 5 and wood flour, or alternately using SilverTip EZ-Fillet wood flour putty every six inches along the inside seam. After a few minutes the wires can easily be removed and the regular fillet applied. Talk to your plans designer and see what he recommends.

Glued Lapstrake Construction

Many people believe that traditional lapstrake construction makes the most beautiful boat of all. They long for a more modern method that produces a light, strong boat without the maintenance problems of the traditional lapstrake boat. Several builder/designers have developed techniques for doing this out of plywood.

Basically, plywood is cut to the proper shape and one edge of each plank is beveled. These planks are attached to temporary mold frames and permanently attached to the stem and transom. The boat is usually built upside down with the very bottom planks applied first. Successive planks are laid on these with about a half inch overlap. The laps are epoxy glued. Planking continues in this fashion to the gunnels.

The result is a lightweight, stiff boat. The stiffness comes from the doubling at the laps combined with the bent planks. So far, only small boats have been built using this method. A chief aesthetic advantage of this style is that it allows for more roundness in the finished hull than other plywood methods. A possible disadvantage is that it is more tedious than sewn seam construction.

We can't conceive of building a boat in this method without epoxy coating and fiberglassing the plywood panels prior to cutting the planks. The boat would be impossible to fiberglass once assembled. Doing it first means that except for the cut plywood edges the boat is essentially "epoxied" before it is even assembled. We'd seal these after assembly and use fillets to round them into the adjacent planks.

There are several books available on glued lap construction. Check in the various boating magazines to find them. Buy a good design. As much as in stitch and glue construction, cutting and beveling the planks is critical to achieving good results.

Strip Plank Construction

Strip planking has been used to construct boats with lengths from eight feet (prams) to huge power and sailboats. The most common strip planked boats built today are canoes in the fifteen to eighteen foot range. Strip planking readily allows the builder to make compound curved hulls even in shorter lengths.

Strip planking is simply the edge gluing of long strips over temporary mold frames. Most often these are male mold frames although some advantage is possible using female mold frames. The strips are cut somewhat longer than the boat so that they are long enough to be bent around the curve of the mold. Larger boats use scarfed strips to achieve the proper length. Western red cedar is often used to build canoes and smaller boats. It is lightweight and attractive lending itself well to bright finished hulls. Redwood, Alaskan yellow cedar, Sitka spruce, and fir are also used.

Several companies are now marketing veneer faced end grain balsa strips. This material shows promise for boats over twenty feet. It is lighter than solid material for a given thickness. It is also weaker and much more expensive. Because it is weaker more exotic fiberglass fabrics must be used to build strength. This further adds to the cost of using this material and adds enough additional weight to largely offset its lower density when used for smaller boats. In larger boats where lightweight is the ultimate goal and the money is there to buy it, balsa strip is worth considering.

In a way the smaller strip planked boats may be thought of as wood cored fiberglass composite boats, as the sheathing is structural. Here the strips are thin and edge contact area is low enough that the strips need the structural reinforcement of cloth on the inside as well as the outside. As boats get larger strip thickness and glue surface area increase. The overall strength coming from the glued edges begins to predominate and the structural importance of the sheathing becomes less important. Still, most builders of large strip plank boats use a healthy layer of reinforcing material on the outside of the hull. Some large strip planked boats use a double layer of diagonal veneer planking (see section on cold molding). Except for scarfing and edge gluing not much epoxy is used in strip planking until the hull is completed. One neat trick when canoe building is to use Quick Cure to glue the butt ends to the side strips when closing the "football". The outside of a strip-planked boat is faired and fiberglassed before taking the hull off a male mold. In female molding the inside may be fiberglassed and ribs molded in place prior to demolding the hull. Since the sheathing is an important structural part of strip plank boats the designer's recommendations should be followed when selecting sheathing materials.

Cold Molding

The term "Cold Molding" was coined to differentiate the process from hot molding. In hot molding layers of veneer are glued together under heat and pressure. Plywood is hot molded. In cold molding some pressure is used at least to keep the veneer in contact but heat is not generally used. Boat hulls have been built using the cold molding process long before the advent of epoxy resin adhesives. The development of modern epoxy resins has made cold molding into a viable building technique for the professional one-off builder as well as the serious amateur.

Cold molding is the process where strips of veneer are laid diagonally to the hull's centerline over a male mold or plug. Three or more layers are used with each layer at ninety degrees to the one below it. The net result is a large piece of plywood in the shape of a boat hull.

The length of the strips is sufficient to reach from the keel to the sheer with the thickness and width largely determined by the size of the boat. Many materials have been used for cold molding including veneer, plywood, and door skins. Western red cedar, fir, spruce, Alaskan yellow cedar, mahogany, and redwood have all been used to build successful boat hulls.

Epoxy resin is the preferred adhesive because of its great gap filling properties. Veneer is often stapled either permanently or with removable staples. Because the "clamping pressure" varies over the surface small gaps result in areas away from the staple. Thickened epoxy like SilverTip GelMagic fills these gaps to make a solid structure. Some builders prefer to use vacuum bagging techniques to clamp the veneer until the epoxy cures.

Vacuum bagging is a very simple process that uses atmospheric pressure to achieve clamping forces. Essentially, a "bag" is created by using the part to be clamped as one side with a polyethylene film as the other. The two sides are joined with some type of mastic sealant and the air in the bag is removed with a vacuum pump. In order to work properly the veneer must be molded on some type of mold that is impervious to air. The bag must totally cover the veneer (which may be stapled at the ends to hold it into place until the bag is evacuated) and be affixed to the mold surface. Obviously, a more elaborate mold must be constructed for vacuum bagging a cold molded boat.

In the past several years many techniques have been developed that allow hull panels to be cold molded using vacuum bag techniques. The hull panels are cut to shape, butted together and joined along the keel line using sewn seam methods. Two of these techniques, Constant Camber and Cylinder Molding, are especially suited for making long narrow hulls of the types used for catamarans and trimarans.

Cold molded boats are epoxy faired and fiberglassed before removing them from the mold. Once removed they are epoxy coated on the inside, and frames and bulkheads are added.

Just as some strip planked boats have several layers of veneer cold molded to the outside, some cold molded boats are built by permanently attaching the veneer to a latticework of frames and stringers. Some carvel planked and caulked boat hulls have been preserved using cold molding techniques. The careful reader will have noted that it is possible to combine elements of several building techniques to produce a strong hull.

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