Mini Power Boats

For example, baking soda + Elvinger I need something stronger than u do find in any grocery store or Tesco.
It must be under 30cm square.
We can not, I bought it in part and shall float my boat!
Help me PLEASE
Oh, and I hand in my project in 2 days!

You can use the rubber band to store energy and then find a way to turn a proppeller with it ..
You can also use a candle to heat water in a small container in the vessel to produce steam that is forced backk ...
If you do not want such a complicated method to use the ball with a straw attached to the opening ...

Have you thought about simply inflate a balloon? Tape it on the boat and let the air out at the right time.