A pontoon is a watertight tube that mounts on the bottom of boat to provide buoyancy. You can make a simple pontoon by welding together two or three 55-gallon drums end to end. For the wood crafter interested in honing old-fashioned boat building skills, a pontoon can also be handmade by creating a ribbed skeleton, and then fastening thin flexible plywood over the ribs as a wood skin. Fibreglass is then applied as the finishing treatment to seal the pontoon.
Design the shape of the pontoon. The cross-section of the pontoon can be as simple as a square, or more complex curved shapes. If using curved surfaces, the plywood will have to flex and bend as it forms to the surface. On the other hand, a curved surface pontoon will not have sharp corners, which will make the pontoon easier to waterproof. For this example, we will use a modified square, with a flat top and bottom with rounded side walls that is 16 inches on diagonal. The top and bottom surface is 10 inches wide. The profile resembles the shape of a fish bowl. The pontoon is 96 inches long.
Cut 5 ribs from the 3/4 inch plywood, each shaped like the profile described above.
Rip two 3/4 inch plywood plates 9 1/2 inches wide by the length of the plywood sheet on the table saw. Fasten these strips to the plywood ribs with the 2-inch screws so that the ribs are 24 inches apart. Apply a heavy bead of construction adhesive between the ribs and the plywood before screwing them together. The plywood is a slightly shorter width than the flat surface of the rib. Center the wood plate on each of the ribs, which will leave 1/4-inch between the edge of the wood plate and the edge of the plywood rib.
Measure the actual length of the curved part of the rib, plus the thickness of the top and bottom plywood plate. This dimension will be approximately 13 1/2 inches. Rip 2 strips of 1/4 inch plywood the width of your measured length by 96 inches long.
Wet the 1/4-inch plywood strips thoroughly, and allow it to soften. Once the plywood is flexible, apply one piece to each side of the pontoon with the 1 1/4-inch screws. Make sure the plywood is flush with the top and bottom edges of the 3/4-inch plywood. It can hang over a bit, but should not be too short. Apply a heavy bead of the construction adhesive to each rib before attaching the 1/4-inch skin.
Measure the circumference of the curved side of the pontoon again and cut two more pieces of 1/4-inch plywood skins. Wet the plywood, and once flexible, apply a moderate coating of waterproof wood glue to the curved surfaces of the pontoon which are attached to the pontoon. Attach the second layer of 1/4-inch plywood over the first layer of curved plywood with 2-inch screws. Make sure the fasteners screw into the ribs, and not just into the 1/4-inch plywood underneath it. Allow the assembly to dry
Cut 2 pieces of 3/4-inch CCA marine plywood which fit each end of the pontoon like a cap. Apply a heavy bead of construction adhesive to the end rib, and then screw the plywood to the end of the pontoon with the 2 inch screws. These final pieces seal the pontoon, but the pontoon isn't watertight yet.
Use the belt sander on all the seams of the pontoon. All joints should be flush, with the edge of one piece of plywood perfectly aligned with the surface of the plywood adjacent to it. Remove any overhanging plywood to correct uneven joints.
Mix the fibreglass resin with the hardener according to the manufacturer's instructions. The product hardens chemically, so only mix the amount that can be used in approximately 15 minutes.
Soak the fibreglass cloth in the resin, and apply the cloth to the pontoon. Start with the seams, and overlap each seam with the fibreglass cloth. Use the plastic smoothing tool included with the fibreglass cloth to remove any air bubbles under the fibreglass. When complete each piece of fibreglass should be completely smooth, adhering to the wood surfaces.
Continue covering the surface of the pontoon with fibreglass cloth until the entire pontoon is covered. The pieces of fibreglass cloth should overlap each other approximately 1 inch. Allow the pontoon to dry in an outdoor location, such as a garage, or pole building. Once the fibreglass dries, the pontoon will be waterproof and buoyant.
As an option, the fibreglass can be sanded smooth with an orbital sander and a second coat of fibreglass applied. A second coat of fibreglass will improve the longevity of the pontoon, and guarantee a watertight seal.