What is a Pop Pop Boat?

Pop pop engine notice


Pop Pop Boats  - What are they? Otherwise known as flash-steamers, hot-air-boats, toc-tocs or more accurately  a Pulsating Water Engine (P.W.E.)

The name comes from the noise the boats make. 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, Phut-Phut, or Pouet-Pouet.

The engine of a pop pop boat is a steam boiler. Water in it flashes into steam which pushes the slugs of water out of the exhaust pipes propelling the boat by a pulse. The steam bubble then condenses and a vacuum inside the boiler is created sucking cold water back in. The process is repeated over and over as long as a heat is source is under the boiler - usually a candle, olive oil with a wick or similar.

The key principle is that the water being propelled out the back of the boat is a directional jet, but the water being sucked back in on the second half of the cycle is not directional, instead it is sucked in roughly hemispherically, that is, 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, being concentrated. However, it is difficult to put out the flame by sucking in air, since the air being sucked in comes from all directions, and not just the direction of the candle. 

Different Engines
There are two types of pop-pop engine. One is a copper tube coiled in the middle with both ends protruding from the rear of the boat. The other has two tubes connected to a boiler with a diaphragm that improves thrust and produces a popping sound.

pop pop boat coil engine     pop pop boat engines
Copper Coil Engine.
The coil type boats are powered by a basic boiler comprising a thin coiled copper tube. The coils in the middle of the tube are positioned over a flame, which heats the water in the coils. The end of the tubes protrude from the back of the boat into the water. When the water boils, the expansion of the steam pushes the water out of the tubes at the back of the boat as a jet of steam and the boat moves forward. After expansion, the steam cools to create a partial vacuüm, sucking water back into the tubes to allow the cycle to begin again. It's that simple !!
1/8" Dia Copper Tubing available here
3/16" Copper Tubing available here

  Diaphragm Boiler.  
The diaphragm type engine is a slightly more sophisticated version which substitutes the coil with a shallow chamber with a flexible diaphragm for the coil. The flexing of the diaphragm gives a little more impulse to the escaping steam, as well as making a louder pop. The image above shows a standard diaphragm boiler using a very thin sheet of brass soldered to the base chamber. The brass flexes and expands as the water flashes into steam.
Standard diaphragm engines available here
Large diaphragm engines available here

The image opposite shows the action of a diaphragm boiler - the heat underneath flashes the water into steam propelling it out of the pipes at the back of the boat. A vacuum is created sucking water back into the boiler.

animation of a pop pop engine

The diaphragm boiler is a variant of the coil boiler. This type of boiler is more efficient than the coil. However the flip side of it is the fact that it is much harder to make. This type of boiler was patented by Charles McHugh. Below is a snippet by Vance Bass. It illustrates the function and the make up of the Diaphragm boiler.

"A more efficient boiler is the diaphragm type. This variant replaces the coils with a small metal pan whose top is a slightly concave piece of very thin, springy metal such as brass shim. The diaphragm has two apparent effects when the steam flashes under it: first, it makes a click sound to augment the quiet pup-pup-pup of the coil boiler; second, as it pops up and then back into its original shape, it may impart some more concentrated energy to the exhaust jet. The result is a boat which runs well and makes a vaguely motor-like noise. " 
Vance Bass

An internal combustion version would be a valveless pulse jet which works on the same principle, only the working fluid is air, even though it is not a liquid.
Read about the history of Pop Pop Boats here