(HDPE), and is very easy to cut using an Xacto knife or other sharp utility knife.
Near the bow of the boat is a small candle, such as those used to keep food warm on the table. I chose a candle that comes with a small aluminum can, and is less than an inch high, so it would sit low in the boat and not drip wax all over. The aluminum cup keeps the melted wax in place, so the boat can go for hours. I used a small amount of sticky tape to keep it in place.
The engine is made of 1/8 inch soft copper tubing. This can be found at the better hardware stores, and is normally used in refrigerators. You can use the easier to find 1/4 inch soft copper tubing, but the 1/8 works best with the candle's small heat source, and weighs less, so it is less likely to tip the boat.
The best way to cut the tubing is with a tubing cutter (get it at the hardware store when you get the tubing). You can use a hacksaw, but you will have to clean the debris out of the cut with a knife or sandpaper.
Gently bend the tubing around a large pen or pencil (I used the handle of a wooden spoon) to form the coil in the center. Poke two holes in the back of the boat with a nail, and force the copper tubes through the holes. The holes in the soft plastic will close around the tubing, forming a water-tight fit. It is also possible to simply lay the tubing on the top of the back part of the boat, and then bend it down so the ends are under water, and pointing to the rear.
Gently bend the tubing so the coil is just above the top of where the candle flame will be. The boat is now finished, and ready to launch.
The copper tubes must be full of water, and both open ends must be under water. The easy way to fill the tubes is to hold one end under water, and suck on the other end.
When the tubes are full of water, and the boat is resting in the water with both ends of the tubing under the water, light the candle.
When the coil of copper tubing is hot enough to boil the water inside, the boat will jerk ahead suddenly, then start moving evenly forward. If you put your fingers in the water just behind the tubes, you can feel little pulses of water, about 5 or 10 pulses per second. These pulses are pushing the boat along.