I use cardboard/poster-board, cut into 3" wide strips and a hot glue gun. For sharp curves you use a shorter piece and shape it to fit the profile of the boat. Then you take the next piece and glue it to the end of the piece you just shaped. Keep sticking pieces together to form the outline of the wood you need. Once you have the outline glued together, glue in plenty of pieces that run the full width and length of the form to keep your form from expanding or compressing in the middle when you move it. Once you've finished building your cardboard form you lay it over your plywood and trace the outline onto the wood and cut.
If your wood isn't large enough and turning the sheet sideways doesn't solve the problem, you'll have to put a splice somewhere. Find an area where a splice could be fastened down on both edges and go that route. Or if you plan on using something like 3/4" material, you could take 2 sheets of 3/8" and laminate them together, offsetting the joints of the 2 layers, and go that route. Some times you just have to be a little creative.
