It’s called the pop-pop boat, the put-put boat or putt-putt boat because that’s the sound it makes, as it putters around the bathtub or pond. That is also how you can search for images, plans and videos on the internet. Put-put boats have been around for generations.
Basically, you build a small boat-shaped model. This can be as complex or as simple as you wish. Then, you put a tea light or votive candle in it with the engine. The engine is simply a piece of soft copper 1/8″ tubing with a single loop or two in it. Above is one made from a soft plastic bottle and here is a fancier one
Let some water run into the tubing, float your boat and light your candle, and wait for it to putt-putt about.
How’s it work? Here are the principles.. Gases expand when heated but liquids do not. Gases contract mightily when cooled but liquids do not. Copper conducts heat. Now, when the candle heats the copper loop (with some water in it already) the water turns to steam, the steam pushes out, the boat makes one “putt”. (Newtons’s 3rd law of motion). When the coil is out of steam, the aft part cools, water is sucked back up, where it is heated, and the process repeats.
These toys are cheap but more fun to build yourself. The most important trick is safety around an open flame (fire extinguisher within arm’s reach?), followed by using copper tubing instead of other materials.
I have used these from 7th grade on up (way up!). You can also have regattas or boat races! The trick here is to have them go in a straight line – that can be a contest in itself. I have even put a small wading pool in the classroom for a week’s worth of put-put boats and small sailboats races and experiments. Do NOT use the inflatable kind! Have a small pump to drain the water into the sink drain when you are finished and you are good to go. Mop up your own mess and have chocolate for the janitorial staff to make up for the stress they quietly nourished while watching your pool for the week.
http://sci-toys.com/scitoys/scitoys/thermo/thermo.html#boat How-to, plus he sells boats and parts.
http://www.sticksite.com/putputboat/ Very nice plans for cutting and building these boats using aluminum pop cans. A little more complex though.
http://www.sciencetoymaker.org/boat/index.htm A complete site for putt-putt aficionados, complete with galleries, etc.
http://www.nmia.com/~vrbass/pop-pop/ A pop-pop boat page.