The PopPop program simulates the motor of a pop-pop boat (sometimes called putt-putt).

The simulation models a tube-boiler. I was interested in why pop-pop boilers work and thought a simulation might help me understand.


I have been fascinated by pop pop boats since buying one many years ago in Mexico. I've searched in vain for an adequate explanation but none of those that are available explain all the observations.

The Pop Pop boat was probably invented around 1870 by Desiré Thomas Peel or Piot, a Frenchman living in London. He patented it in Britain in 1891. Charles McHugh patented the diaphragm version in the US in 1916.

The general principle of a diaphragm Pop Pop boat is that a small chamber - the "boiler" - is partly filled with water. Two tubes emerge from the boiler with their other end immersed in water. Two tubes are needed in order to fill the chamber - one lets air out as the water goes in. The boiler should not be completely filled. A candle or oil burner is used to boil the water. After a few seconds, the whole system oscillates at perhaps 3Hz. It produces a "pop-pop" noise and water spurts out of both tubes. One of the tubes can be sealed and the engine will continue to run at the same frequency.

The boiler is flat: around 2cm dia and 4mm high. The top surface acts as a resonator so that the engine produces a loud noise. In some engines, the top is bistable and clicks between bulging inwards and outwards but not all do so and the effect seems to have little influence on the efficiency of the engine.

The second version uses 1½ or 2½ turns (2cm dia) of, say, 2mm dia copper tube. A candle can be used to heat the tube but a larger flame is better: the flame heats a smaller area and the copper conducts away more of the heat. The engine oscillates at around 15Hz. Unlike the the diaphragm version, it is almost silent.

Clearly, the engine alternately sucks-in and expels water. When it sucks water in, the water is taken from a hemishphere around the opening of the tube and so produces only a small reactive force. But all the water is expelled in the same direction so a much larger reactive force is produced and the boat moves forward.

The big question is: why does the engine oscillate? Even after 100 years, no-one seems to know.

The water in the boiler flash-boils and forces the water in the tubes out. But why does the water re-enter the tubes? The usual explanation is that the momentum makes the water in the tubes continue a little further and creates a partial vacuum in the boiler. The vacuum sucks water back in and the whole oscillates as a Helmholtz resonator.

But not all of the water in the boiler boils at once, there is still more boiling which should force more water out. Experiments with 2mm dia glass tube show that viscous drag more than critically damps any oscillation - a sealed manometer with large tubes will oscillate, but not one with small tubes.

A Helmholtz resonator oscillates because there is an exciting impulse which is modulated by the oscillation. That's what happens when you blow across the top of a bottle. The jet from your mouth is alternately deflected to pass over the top of the bottle or to enter it. It is not obvious that there is an equivalent mechanism in a Pop Pop boat.

When the engine is filled with air, its resonant frequency is around 3000Hz. Water is 775 times denser than air so when the pipes are filled with water, the resonant frequency should be 108Hz. Not 3Hz.

My view is that as the steam enters the part of the tubes that were full of water, it cools and condenses. The condensation creates a vacuum which sucks water in. As this extra water is drawn towards the boiler, it cools the pipes closer to the boiler and (by conduction) the boiler itself. So yet more water condenses. The condensation stops when water has cooled the walls of the boiler to below 100°C. The frequency is determined by the rates of condensation, cooling and heating.

If you run a boat in hot water, you might expect the frequency to change but it doesn't. The frequency remains constant until the temperature reaches around 75°C when the engine stops and a steady stream of bubbles appears from one of the tubes.




Questions or comments ? E-mail me at peterbalch@btinternet.com

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