OUTDOOR USE ONLY!
This is a very simple design; improvements in hull design would be advantageous, but are not strictly necessary.
Less than an hour's build time
1/2 a coconut shell
an old coke can
50cm of 4mm copper pipe
some small lumps of charcoal
2" of garden hose
a marker pen
a way to light the charcoal (a small gas blowtorch works well)
1. Cut the top off the Coke can and fold over its top edge to hide the burrs.
2. Cut a 3/4" slot around halfway down the side of the can and fold the metal strip inwards - it's not needed.
3. Coil the pipe around the marker pen, bare hands... no jig needed - it didn't fold closed for me, but be careful.
4. Cut a 3/4" slot 1" deep into one side of the coconut wall.
5. Feed the ends of the pipe coil through 2" of garden hose, then push the garden hose into the slot in the coconut wall - seal any gaps with dripped candle-wax. Also seal/plug the water's end of the hose with candle-wax in the same way.
6. Tip up the boat, take a mouthful of water and blow/push it into the copper pipe - make sure there's no air in the coil or the pipe, then place the coconut in the water while keeping your fingers over the ends of the pipe. The pipe ends should remain underwater, and the pipe should contain no air.
7. Elsewhere, place the small lumps of charcoal into the Coke can and light them.
8. Using tongs, place the Coke can under the coil in the coconut - pass the coil outlet pipes through the slot in the can.
9. Wait until the water in the coil heats up, and your boat will begin to move forwards.
Working principles and details here...
Video of the boat working here...
This will be E2's next field trip project - SHHH! - they don't know it yet.