page_title: Pop pop boat
section_title: Principle of operation
hierarchical_section_title: Pop pop boat / Principle of operation
caption_attribution_description: English: Pop pop boat with accessories
context_page_description: A pop-pop boat is a toy with a very simple steam engine without moving parts, typically powered by a candle or vegetable oil burner. The name comes from the noise made by some versions of the boats. Other names are putt-putt boat, crazy boat, flash-steamer, hot-air-boat, pulsating water engine boat. Around the world they may be called Can-Can-boot, Knatterboot, toc-toc, Puf-Puf boat, Poof Poof craft, Phut-Phut, or Pouet-Pouet.
context_section_description: A pop pop boat is powered by a very simple heat engine. This engine consists of a small boiler, which is connected to an exhaust tube. When heat is applied to the boiler, water in the boiler evaporates, producing steam. The expanding steam is suddenly pushed out of the boiler, making a 'pop' sound, and pushes some of the water out of the exhaust tube, propelling the boat forward. The boiler is now dry, and can therefore not generate any more steam. The momentum of the column of water in the exhaust tube keeps it moving outward, so that the pressure inside the boiler drops below atmospheric pressure. In the case of a diaphragm type engine, the boiler also bulges inward at this point, also making popping sound. The pressure outside the boiler now forces water back into the boiler. This water then boils and the cycle repeats. The popping noise is more pronounced when a diaphragm-type boiler is used: coil-type boilers are much quieter. 
  Any air in the boiler can act as a spring and support the oscillation of the water, but if too much air enters the boiler, the oscillation will stop because all the water has been displaced, and no steam can be generated.  Water contains some dissolved air, which can build up in the engine during operation. Therefore, engines must "burp" out air periodically in order to run for a long time.
  In pop pop boats with two exhaust tubes, the water is expelled from both tubes during the first phase of the cycle, and drawn in from both tubes during the second phase of the cycle. The water does not circulate in through one tube and out through the other.  The internal-combustion analog of the pop pop boat engine is the valveless pulse jet.
  The operation of the pop pop boat may seem surprising, since one might expect that if water is going in and out through the exhaust tube, the boat should merely shake back and forth.  But while the water pushed out carries away with it momentum, which must be balanced (by Newton's third law) by an opposite momentum on the part of the boat, the water sucked in quickly impinges on the boiler tank and transfers its momentum to the boat. The initial reaction force on the boat (which would pull it backwards) is therefore cancelled by the pushing of the water when it hits the inside of the boiler.  The result is that the inflow of water causes no appreciable force on the boat. It works because the velocity or momentum of the water as the exhaust in the pipes are greater than the velocity of the in flowing water. 
  Some authors have argued that the reason why the pop pop boat works is that the water being propelled out the back of the boat forms a narrow jet, while the water being drawn back in on the second half of the cycle is drawn in from all directions. This asymmetry may be seen as well in the way in which one blows out a candle: it is easy to extinguish a candle by blowing on it, since all of the air expelled is moving in a concentrated, directional jet. However, it is difficult to put out the flame by sucking in air, the air being sucked in comes from all directions.  This observation, though correct, may be misleading as an explanation of why the pop pop boat moves forward.  The asymmetry of the shapes of the inflow and the outflow is a consequence of the viscosity of water, whereas the boat would be able to operate in an ideal fluid. Furthermore, as they pass through the exhausts, the inflowing and the outflowing water carry the same momentum (in opposite directions), relative to the boat.  The important difference is that the momentum of the outflow is expelled, whereas the momentum of the inflow is soon transferred to the boat. The sucking/blowing asymmetry does make the boat more efficient, even if it's not the principle on which it operates. 
  The physics of the operation of the pop pop boat is very similar to that of the "Feynman sprinkler," a submerged sprinkler which is seen to turn weakly or not at all as water is sucked in through it.  In both cases, the reaction force on the solid device caused by the sucking in of the fluid is balanced by the fluid imping
caption: Pop pop boat with accessories. (Melted candle in the 'pan' with a lit piece of wick provides the heat source)
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