Wednesday, November 5, 2008

How to Build Plywood Boats

A guide for the beginning boat

builder - these basic techniques are used by the pros

FIBERGLASS may be the number one choice for production-line boats, but when it comes to the amateur, plywood re­mains the overwhelming favorite.

And well it should be. It's cheap, strong, easy to handle, and it provides a handsome boat-whether outboard skiff or an ocean­cruising sailboat.

Although only average carpentry skill is needed, building a boat is not much like building a kitchen corner cabinet. Here are standard procedures that apply to all ply­wood boat construction, whether you are

starting with a set of plans, or building from a kit with pre-cut parts.

First, a word about materials is in order. Of course, the plywood must be marine or exterior grade. Any other type will deterio­rate rapidly when exposed to a marine en­vironment. In addition to the plywood it­self, everything else that goes into or onto the boat must be suitable for a marine en­vironment. This includes adhesives, fasten­ings, and paint. Your local marine dealer is the best source for each of these. Do not use common brass fastenings in the hull, if it will be kept in salt water. If such fastenings trap moisture, they corrode rapidly.

Plans and patterns. Measurements for many framing members can be taken from the plans and marked directly on the wood stock. Where curves must be cut, full size patterns should be used. The heavy paper available from building supply firms is ex­cellent for patterns.

Plans usually show a scaled-down "pat­tern" for each curved member, with grid

superimposed over it. The grid may repre­sent one- or two-inch squares. Mark off full-size squares on the paper, and draw in the outline for each item, using the plans as your guide.

To transfer the pattern to the stock, tape the pattern in place, and tap a nail through the drawing at two- or three-inch intervals to leave a line of indentations in the wood. Remove the pattern, connect the indenta­tions with a heavy pencil line, and you're ready to cut. Naturally, you cut a little out­side of the line, and use a plane or rasp to achieve the finished shape.

Building forms or jigs. For most boats, it's necessary to set up a building form or jig (two words for the same thing) to support frames, transom, and stem during construction. The jig ensures that frames are at the proper intervals, and at right angles to-and centered on-the keel's centerline. It is also used to set up the frames at the proper height in relation to each other.

Often, the plans will give complete in­structions for building the form. Because each type of boat has its own requirements, no special instructions can be given here, other than to make sure it is set up to do its job properly, and that it rests on a firm foundation that will not shift or settle dur­ing construction.

Keels, stems, transom, knees. These members make uu a boat's backbone. Use

the best grade wood you can get-wood that is clear, uniform in grain, and free of knots. While solid timbers may be called for in your plans, it is possible to laminate several layers of thin wood to achieve the required thickness, and wind up with a stronger unit than solid wood would pro­vide.

Transom knees and stem are often made up in sections that fit together like pieces of a jigsaw puzzle, with glue and bolts at the joints. Sometimes it is desirable to add plywood side pieces (gussets).

Transoms and frames. These are the first units to be mounted on the building jig in most cases. As they determine the final shape of the boat, every care must be taken to ensure accuracy in their construc­tion.

The transom itself is usually heavy ply­wood, 3/4 -inch thick, backed on the inside with 3/4 -inch framing members. The total 11/2-inch thickness along all stress areas provides plenty of strength for outboard motor installations. ,

To make up the transom, cut the ply­wood to shape, allowing a little excess for trimming. Cut the framing members to shape and size, and fasten them to the ply­wood. Coat mating surfaces with water­proof glue, and insert screws through the plywood into the framing members. Al­ways drill pilot holes for the screws, and countersink the heads.

frame and the stem, trim the panel to pro­vide a "transition joint." Aft of this joint, the bottom panel will lap over the edge of the side panel; forward of this joint, the edge of the bottom panel will butt against the edge of the side panel.

Where two or more sections of plywood are needed to make up each side panel, in­stall butt blocks as outlined earlier.

Bottom panels are installed in the same manner as the side panels. Use a plane, wood rasp, or "Surform" type tool to trim them for a good butt joint along the center­line and at the stem. Trim them flush with the side panels aft of the transition joints.

Fiberglassing. If you plan to fiberglass the seams, the bottom, or the entire hull, this is the time to do it. Fiberglass tape over seams adds strength, and prevents moisture from entering the end grain of the ply­wood. As fiberglassing the entire bottom

adds to its strength, you can use thinner, easier-to-bend plywood bottom panels and still have the required strength. Fiberglass­ing the entire hull permits use of thinner side panels as well, and you can add color pigment to your final resin coat to elim­inate the need for a paint job.

Before fiberglassing-or painting­cover all screw heads with wood putty. Sand the putty flush with the plywood after it has dried. If you do not use fiberglass, coat all exposed plywood edges with wood sealer to help prevent deterioration.

Turn the hull right side up. Block it carefully so that it has adequate support, and won't shift around as you work on the interior and topsides. Trim the side panels flush with the sheer clamps.

Now you can paint the interior of the hull. There will be areas that are impos­sible to see, and difficult to reach.


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