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Pop the question

When I was a child in the 1950s I had a toy pop-pop boat, so called because of the popping noise it made as it moved across the water. I have recently found them in toy shops again, and I want to know how they work. The boat is about 12 centimetres long and contains a flat, horizontal water boiler of a thin pliable material that looks like shim brass. It is about 30 × 35 millimetres on top and 3 millimetres deep. The boiler has two pipes that connect underneath it at the front, run along the bottom of the hull, and exit at the stern below the waterline. They are plain pipes with no valves or other mechanisms. You fill the pipes and boiler completely with water and float the boat on a pond. Then you place a small candle under the boiler and soon the boiler emits a popping sound as the top membrane starts to vibrate up and down two or three times per second. This causes water to pulse in and out through the pipes and propels the boat forward. The boat does not use up the water in the boiler and operates as long as the candle stays alight. What is the physics behind this?

• I have been fascinated by pop-pop boats since buying one years ago in Mexico (see photo). I have searched in vain for an adequate explanation but found none that could account for all the observations, which prompted me to try a few experiments.

The pop-pop boat was probably invented around 1870 by Desiré Thomas Peel or Piot, a Frenchman living in London. He patented it in the UK 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 the other ends immersed in the water in which the boat is floating. A candle or oil burner is used to boil the water. After a few seconds, the whole system oscillates at perhaps 3 hertz. It produces a pop-pop noise as 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 in boats with a diaphragm, and the top surface acts as a resonator so that the engine produces a loud noise. The other version does not have a boiler and just uses one or two 2-centimetre-diameter turns of 2-millimetre-diameter 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 15 hertz but, unlike the diaphragm version, it is almost silent.

The engine alternately sucks in and expels water. The water being sucked in is taken from a wide hemisphere around the opening of the tube, and so produces only a small reactive force. But all the water is expelled in one direction, so a larger reactive force is produced, and the boat moves forward.

The big question is why does the engine oscillate? The water in the boiler flash boils and forces the water in the tubes to squirt out. But why does the water then re-enter the tubes? The usual explanation is that the momentum of the water leaving the tubes creates a partial vacuum behind it in the boiler. This sucks water back in and the whole system oscillates as a Helmholtz resonator.

My experiments show this is unlikely. Rather it seems that as the steam enters the part of the tubes that were full of water, it cools and condenses. This creates a vacuum which sucks water in. As this extra water is drawn towards the boiler, it cools the pipes 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.

Peter Balch

Edinburgh, UK

• This is an example of a flash-steam pulse-jet engine. Steam filling the pipe condenses at the cold end, contracting and drawing in water. The first drop of water meeting hot metal is flash-vaporised and expels the water again, filling the pipe with steam, and this cycle continues. The water needn’t be very cold; as a boy in 1947 my first boat operated well in a hot bath.

Toy boats like this appeared on the market in the 1940s under the name Jet-Pop. Actually a working steam engine, in the post-war austerity period it made a cheap and simple toy. As an impecunious schoolboy crazy about model boats, at a time when a midget electric motor cost a year’s pocket money, I was attracted by the engine’s simplicity. I have since owned and built several boats. My engines consisted of a simple loop of copper tubing making a 540-degree turn over the flame. Soldering a metal plate to the coil improves the heat uptake.

The recommended fuel was metaldehyde, a white, crumbly substance bought from ironmongers in block form. A pea-sized piece burns with a hot smokeless flame for several minutes. If metaldehyde is unobtainable try a few large roasted peanuts.

The boiler with a diaphragm in your reader’s example no doubt slows the cycle by buffering the pulse. Your reader reports a frequency of 3 hertz, whereas I recall my engines purring at at least twice this rate, evoking another notorious example of a pulse-jet engine – the V1 doodlebug, or buzzbomb.

Ian Clark

Whitby, North Yorkshire, UK