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.

• Select the body to estimate in the browser
• Right click and choose "Properties" and a new dialogue box shows the volume, weight, etc. You might want to change the material to get the right estimate.
• measure volumes before and after hallowing out the hull with Shelling command, this gives me the volume and estimated weight
• Calculate Buoyancy: 1 liter can support 1 Kg. I used an online calculator.

• Make the weigh of the boat about half of the buoyancy.

• I then used Autodesk MeshMixer to check the center of gravity for the model.

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.