Ing Mecatronica

The Pop-Pop Boat


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 aboutit 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 faras 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.
Priming. Two pipes run from the heated chamber (boiler) to exit at the back of the boat. Why two? The system must be primed before the heat is applied, by putting water into an exit tube. It will go in only if air can come out. That thesecond pipe serves no other purpose was shown by plugging it after the system was primed and running. The pulsing went on.
Gravity. As the boat sits in the water, the two pipes have a downward slant Boer turning up to the boiler. While the motor was running, I topped the boat up to 45 degrees, keeping the exits of the pipes under water. Put-putting continued. This showed that, whatever the waterdoes, pressure changes in the boiler must be more important in moving it than gravity is.
Temperature of the pipes. After running a minute the heater tray was removed quickly and the pipes were felt with a finger. They were cool up to where they started to bend upward, then too hot for the finger. That showed that live steam may get into the bend but not further.
A puzzling question was the originof 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 totest. 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. Thetop 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...