This is a rubber band race boat I designed for the Maker Faire 2015 Moat Boat Paddle Battle Challenge. See it in action on a test run here: https://www.youtube.com/watch?v=8XWNFfDlJNM
The propeller is a store bought model boat propeller because consumer 3D printers are not capable of printing high strength, high resolution models that propellers require. A large pitch propeller is preferred.
I used a 2" T-pin as the propeller axle. I cut a slot in the tail of the propeller to secure it on the T-pin's head.
I use Autodesk Fusion 360 to estimate the weight and volume and calculate whether the model has enough buoyancy.
To fine tune the boat's balance, I used a little stack of rare earth magnets. To hold the propellers after winding, I added a little hole to fit a drill bit. A small loop on the deck is used for dragging the boat back after it runs away. To reduce drag, I coated it with epoxy.
Printing requires a bit of support as shown in after print picture here. No issues if you print it length wise.