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Used with permission from The Physics Teacher vol. 35, March 1997, pp. 176-7. Copyright © 1997 American Association of Physics Teachers.
Column Editor: H. Richard Crane
Department of Physics, University of Michigan, Ann Arbor, MI 48109
Often toys can be found that employ simple physics principles in ingenious ways. Such ones are favorites for demonstration lectures: they get attention as well as teaching the physics. But toys for that purpose get hard to find as more and more of them operate with hidden solid-state chips. For the one for this column, I'm safe from chips, because it's a hundred years old. The toy is a jet-propelled boat, Fig. 1.1 Heat from a small candle inside it makes it eject pulses of water from tubes at the rear just under the water level, at five or more per second. That propels the boat at 10 cm/s or more, and it will go for as long as the candle burns.
The boat was invented and patented by an English engineer in 1897. Since then numerous articles2 have been written about it and it has been manufactured and sold in several different countries, under the names Pop-Pop, Put-Put, Toc-Toc, and Puf-Puf. There even is a society for it.3 I am indebted to William Mundus of Ann Arbor, for the loan of all the literature in Ref. 2. Interesting, but after reading it, I, as Omar Khayyam wrote in the eleventh century, "came out by the same door where in I went". At least so far as finding out how it really worked. So then I started on my own, with "reverse engineering" (Fig. 2).
Inspection of simple tests answered several questions.
A puzzling question was the origin of the put-put sound. It comes from the top cover of the boiler, which acts as a diaphragm. It is very thin (0.002 in. or 0.05 mm) hard copper. It would be able to make a pop only by a very sudden movement. It was suggested in the book by Harley2 that the copper is slightly concave or convex, so that with a little change in pressure it reverses, with a pop. Like the bottom of an oil can. Easy to test. I applied gentle air pressure and suction, by mouth, through the exit pipes. The pops were identical to those heard from the operating boat. Suction proved to be unnecessary: the diaphragm returned itself after being forced out. However, as will come out later, there probably is suction at some part of the cycle.
To go further, it seemed necessary to see what goes on inside the boiler. The top cover of the boat was removed (with some difficulty), and the diaphragm of the boiler was cut away. The top of the boiler was then closed by a piece of 1/8-in. (3 mm) thick Plexiglas, cemented on with epoxy. When the boat was "fired up" it operated normally, except silently. That showed, first, that the movement of the diaphragm played no necessary part in the oscillation of the system. Throughout the operation, the boiler contained a little water, but at least 3/4 of the area was dry -- and presumably hotter than the boiling point of water. That showed, second, that the water expelled in the pulses at the rear of the boat is not supplied from the water put in for priming. It means that between the pulses out, water must be sucked into the pipes to replace that expelled.
We now begin to piece together the oscillation cycle. The following is my speculation. Start when there is reduced pressure in the boiler and water is being sucked up in the pipes. A little goes onto a hot surface in the boiler and flash-vaporizes. Resulting steam pressure forces water down the pipes, giving the boat the propelling pulse. The steam quickly condenses in the cooler parts of the system, mainly the boiler, reducing the pressure so that water is sucked up again, some of which goes onto a hot surface and starts the next cycle. It's a typical relaxation oscillation, of which there are many examples.4 So that's my picture of what happens.
You may question very logically how, if water is expelled and sucked in alternately, there can be a net force forward to make the boat go. In the past I spent time puzzling a similar problem. It started with a question posed by Richard Feynman in 19855 about the familiar whirling lawn sprinkler. His question: if the sprinkler is submerged in water, and water is sucked into it instead of being squirted out, will it rotate in the same or opposite sense? (Evidently he assumed it would rotate.) I did the experiment, and couldn't get rotation either way. Feynman's question started a firestorm of letters and articles that didn't peter out until the early '90's. As I remember, the end was quiet, with no agreement.
While the Put-Put is not exactly like the sprinkler, the answer in simplified form may apply to both. It is that as water is drawn into any closed container, there is an exchange of momentum between container and water, giving a forward force on the container. But as soon as water is inside, it has to stop, with the opposite exchange of momentum. So the container gains or loses no momentum. There may be a small displacement, while the water is in motion, but there is no residual velocity. Not so if the container is squirting water out. It gives momentum to the water, gaining the opposite itself, but the water is gone, to the outside: the water does not make the opposite exchange with the container. We know that the system propels, or we would not have gotten to the moon. Just rocket action. So the Put-Put boat does go forward.
Even when you believe the theory, it is well to be reassured by an experiment. With the squeeze bulb of a battery filler syringe from an auto supply store, I rigged up a demonstration. (It might be interesting in a lecture.) The rubber bulb was held collapsed by a string harness. it was mounted on a wood raft, floated in a sink full of water, with the open end of the tube under water. When quiet, the string was burned, letting the bulb expand and suck up water. There was a very small displacement, as mentioned above, but the raft had no residual velocity.
The above suggests one answer to a problem known to physics classes. If you are marooned on ice that has (hypothetically) zero friction, how do you get off? By the method of the Put-Put, if you would just breathe in and out, facing in one direction, you would propel yourself. And with zero friction, your speed would be cumulative.
References
The Pop-pop Pages are copyright © 1997 Vance Bass. They may not be reproduced without permission.
Last updated: 23 April, 1997
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