Take a few sheets of marine plywood, some 2 by 4s, add a bit of imagination and a few available hours on a weekend, and you can build a plywood boat. A well-built plywood boat, with a reinforced transom on which to mount an outboard motor, can become a viable source of recreation for anyone who wants to spend some time on the water.
Build the ribs using the 2 by 4 lumber. Cut the gussets to strengthen the rib joints from 1/4-inch marine plywood. Similar to the ribs on the human body, the ribs secure the rest of the boat's structure. The transom---which supports the outboard motor---is a reinforced rib and is the stern-most rib structure.
Build the keel by cutting a 2 by 4 to length of boat. Attach the stern post, also cut from a 2 by 4, to the "aft" end of the keel with #12 6-inch wood screws. Again, as in the human body, the ribs need a backbone to link them together; the keel serves as the backbone of the boat. The stern post is an extension of the keel and provides extra stability and reinforcement to the transom.
Turn the ribs upside down and attach the keel and stern post to the ribs, beginning with the transom and working forward, equally spacing the ribs and attaching with #12 3-inch wood screws. The last set of ribs should be attached at the forward end of the boat to form the bow. Attach the keelson to the inside of the ribs with #12 6-inch screws, screwed through the ribs and into the keel. The keelson provides extra stability and rigidity to the keel and ribs and transfers of longitudinal forces to and from the keel and the hull.
Attach the stringers to the inside of the ribs on the bottom, then to the outside of the ribs on the sides, using #12 3-inch wood screws. The stringers provide even more lateral strength to the hull, helping to prevent longitudinal flexing that crack the keel---or even the boat---in half.
Cut the marine plywood to fit. The plywood forms the "skin" of the hull--the hull plating. Use the #12 3-inch screws to attach the marine plywood to the ribs and stringers at every point where they touch. Fill the seams with caulk, both inside and outside the boat.
Cut the 1 by 10-inch planking to fit between the uprights of the ribs, 2 inches below the gunwales, to form the seats. Cut gussets to act as seat supports and attach them to the ribs with #12 2-1/2-inch screws, with the top of the seat support 3-1/4-inch below the to top of the gunwales. Attach the seats to the seat support with #12 2-1/2-inch screws.
Tie the boat to the shore to test float the boat with no occupants. Watch for two or three hours to see if water leaks into the boat. Mark the leaks with a china marker (grease pencil) as they are noted. Haul the boat out of the water; allow the hull to dry for a day. Re-caulk the entire boat with special attention to the marked leaks. Allow the caulk to cure according to the directions on the caulk label. Test float the boat again to ensure the leaks are closed. Haul the boat out and allow to dry for a day. Prime and paint the boat. Allow the paint to dry according to the manufacturer's directions.
- Carpentry work has inherent hazards; proper safety precautions, including gloves and safety glasses should be used as appropriate. The illustrations shown are not finished hull designs; finished and dimensioned plans for a boat hull are available from most boating magazines and boating websites.
- Boats are much easier to build upside down, with the exception of attaching the keelson and completing the finishing work, such as adding seats.