Making homemade toys like a paddle boat is an exciting way to introduce scientific principles to your kids---in this case, propulsion. You can add some fun with a sail or sailor and decorate your boat with paints or felt-tip pens.
Cut the piece of plank to a point at one end, the shape of a boat hull. With the point facing upwards, the boat should be longer than it is wide.
With the point facing away from you, cut a square from the base of the boat hull you have just shaped. Your boat hull should resemble a tall house, and now you are cutting a large door into it at the bottom. Measurements do not need to be specific, but the height of the square should be roughly one third the height of the boat; make sure you leave at least half an inch on each side intact. Don't discard the square you've just cut.
Trim the wood square so it can easily fit back in the square cut from the base of your boat without touching the edges. This will be used as the propeller.
Attach each end of the rubber band to the thin sides left at the base of your boat. Slip the propeller inside the rubber band. Wind the propeller toward you until the rubber band is taut. Place in the water, release the propeller and watch your creation go.
It is possible to use other materials such as cardboard and plastic, but wood is the most effective and durable. Experiment with different sizes of rubber bands and propellers to see how variations affect the propulsion. For a more advanced propeller, cut a second square of equal size. On each square, cut a notch halfway up one side. Make the notch half as long as the square, and as wide as the wood is thick. Slot the two sections together so they form a cross when viewed from the side and glue using wood glue.