A boat propelled by pedals and gears instead of engines or wind is called an HPB, or Human Powered Boat. Constructing one of these for yourself from scratch would involve a good knowledge of marine engineering just to get the hull crafted correctly. An easier, less time-consuming method utilizes materials already designed for flotation to form the basic structure of the pedal boat you are building. A discarded boat, or pontoons from a float plane, would serve as a good base to start with.
Preparing the Base Craft
Purchase a small fishing boat, about 9 feet long. If the boat includes an engine, working or not, remove it by unscrewing the bolts that secure it to the engine compartment.
Inspect the craft for leaks by floating it on a lake or pond. Pay close attention to the engine compartment area, and look for cracks in the structure of the boat. If you spot a leak or crack that could become a leak, note the location, and remove the craft from the water. Allow time for the boat to dry, and use towels to mop up any of the leaks. Find the holes and cracks, and use water sealant epoxy to seal up these areas.
Secure a piece of 3/4-inch treated plywood over the center of the boat. Drill pilot holes into the craft if necessary, and use galvanized sheet metal screws along the edges to prevent the board from shifting while you are pedaling.
Preparing the Pedals and the Paddles
Bolt two pedals to either side of the largest gear from the 12-speed assembly. The bolt should be located in the center of the gear, and should allow the gear to spin freely when the pedals are used.
Elevate the gear so that the bottom of the gear is at least 6 inches from the plywood support. Mount the gear onto an A-frame pipe with the center sprocket. Bolt the two legs of the "A" to the plywood support. Use the locking pin that held the back tire onto the 12-speed to secure the gear to the A-frame's upper point. The gear should spin freely now, using the pedals.
Cut a 6-foot piece of PVC pipe into foot-long sections, using a hacksaw. Cut each of these sections into two lengthwise strips. These strips will serve as the paddles on your HP Pedal Boat. Sand the edges of each section until they are smooth.
Cut a circle of shatter-resistant acrylic roughly the same diameter as the rear wheel of a 12-speed. This will be the piece to which the paddles will be attached. Make four cuts into the acrylic, the same thickness as the PVC pipe paddles, from the outer edge to about 6 inches inward.
Drill a hole into the center of the acrylic the same size as the pin that held the 12-speed cassette of gears in place on the rear wheel of the bike. Secure the cassette to the left side of the acrylic circle, using the pin or a nut and bolt of the same size.
Insert the PVC paddles into the grooves you just created. Use a generous portion of waterproof epoxy to seal the paddles to the wheel. Let the epoxy dry overnight.
Mount the acrylic wheel at the rear of the boat. The paddles should reach at least 4 inches below the bottom of the craft into the water, to move the boat forward and backward. If the boat you chose housed the engine in a drop-down area, mount the wheel here. If not, you will have to mount it farther back.
Bolt a deep-sea fishing chair to the plywood support, close enough to the A-frame with the pedals so that you can sit comfortably and reach the pedals. Link a bicycle chain from the gear on the A-frame to the smallest gear on the acrylic wheel. Make sure the chain will not interfere in the movement of the paddles.
Install a rudder on the right side of the acrylic wheel. A rudder assembly can be purchased online or at a local sporting-goods shop. Make sure the rudder control is long enough to be used from the chair you installed.