Boat project in process

Originally our first boat was too small and was not catching enough air to move it very fast through the water. We changed the sail from a piece of a plastic grocery bag to a Ziploc bag, which was not as efficient as the plastic bag sail.
Our second boat we created was bigger and we changed the sail from the bag to a thin piece of foam which seemed to be the best option. While this worked fine at home testing it, at school in the tank it continuously hit the side. In order to keep it going straight we added popsicle sticks to the bottom in hopes that it would keep the boat going straight instead of curving. This was still unsuccessful.
The third attempt, we cut the second boat down a bit. Covered in Duct tape to keep water out of the boat, pointed more sharply at the front of the boat, and a tin foil sail, with popsicle sticks on the bottom, seems to be the most efficient so far