If you've ever watched Hayao Miyazaki's Ponyo, you will have seen the pop pop boat in action. The children's toy uses the heat from a candle to power an engine that propels the boat forward. They're pretty neat as children's toys, but what is far cooler, of course, is creating pop pop boat so large it can carry an actual human. That noble goal has been successfully achieved by a team, who demonstrated it to YouTuber and science communicator Steve Mould.
First off, how do pop pop boats work? While it might sound simple, the inner workings of the toy are a little more complicated than you'd expect.
You start by filling a tube leading to the "engine" – a small metal container underneath a candle – before placing the boat in water and lighting the candle to begin heating the water. Lo and behold, the boat moves forward. The complicated part is how exactly that works.
Inside the "engine", the water is heated, evaporating some of it and causing pressure inside the engine to increase. This causes water to be forced further down the tube and out of the end. But as it cools, the drop in pressure forces water back into the tube again towards the engine, where it is heated and the process repeats. So if water is constantly being sucked in and pushed out again, how does this generate forward thrust?
One theory was that, as the water is expelled, it is fired out in one direction, pushing the boat forward, but as it comes back into the tube the water comes from all directions rather than the one. The result is a net forward motion. However, according to Mould – who had a see-through pop pop made boat made by the scientific glassblowing team at the University of Southampton to see for himself – this isn't quite right.
In fact, as the boat expels the water, the jet causes the boat to move forward. That is its equal and opposite reaction. However as the water comes back into the tube, it is met by the water and gas further up the tube, and it is this that it pushes against rather than causing the boat to move backward. The result is a net forward motion, with no way to steer.
Mould was able to try out a life-sized version of the pop pop boat, courtesy of the AHHAA Science Centre in Tartu Estonia. It's slow-going, but nevertheless awesome to see.