Pop pop boat

  A pop-pop boat (also known as Can-Can-boot, Knatterboot, Putt-Putt boat, flash-steamer[1], hot-air-boat^{[citation needed]}, toc-toc, Crazy boat, Puf-Puf boat, Phut-Phut, or Pouet-Pouet) is a toy boat with a very simple steam engine without moving parts powered by a candle or oil burner. They were invented in the late 19th century. The name comes from the noise the boats make.

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Principle of operation

  The engine of a pop pop boat is a steam boiler. Water in it flashes into steam, which escapes, propelling the boat by a steam jet effect. The remaining steam then condenses and this sucks water in. The key principle is that the water being propelled out the back of the boat is directional, but the water being sucked back in on the second half of the cycle is not directional, but instead is sucked in from all directions equally. This asymmetry is what propels the boat forward. The same principle can be demonstrated by blowing out a candle. It is easy to extinguish a candle by blowing on it, since all of the air being expelled is moving in one direction. However, it is almost impossible to blow out a candle by sucking in air, since the air being sucked in comes from all directions, and not just the direction of the candle.