The Secrets Of The Pop Pop Boat

Many kids get an early introduction to mechanics with tin pop-pop boats. If you haven’t played with one – you’re missing out! Pop Pop boats are fun toys – but how they work is often misunderstood. To clear this up, [Steve Mould] takes a deep dive into the theory of operation of the pop pop boat.

Most people think these toys operate like a simple steam engine, with water being flashed into steam inside a tiny tin boiler. Turns out that’s not the case. To explain the physics, [Steve] commissioned a glass version of the boat.

The glass boat shows that during normal operation, there isn’t any water at all in the “boiler” at all. The water is only in the boat’s small exhaust tubes. The air inside the tank is heated by a candle. The air expands and pushes the water out of the tubes. This allows the air to cool, and return to the tank. The water then rushes back up the tubes, and the process repeats.

One of the more interesting facts of the video is that the glass boat doesn’t pop. The popping sound associated with the boat is actually made by the tin diaphragm on top of the “boiler”.

[Steve] has gotten pretty good at explaining complex topics using clear cutaway models. If this tickles your fancy, check out his water computer.

28 thoughts on “The Secrets Of The Pop Pop Boat

    1. The way a solar cell works is that any charges activated by light absorption get swept in one direction by the electric field that is generated by the dissimilar charge distributions in the device (the manufacturing doping concentrations). The light-activated charges might jitter back and forth during their travel, but saying that they get “rectified” is a sloppy and fundamentally incorrect way of looking at it.

      1. heh i’m happy to admit my ignorance but you’re going to have to do better than “swept in one direction by the electric field that is generated by the dissimilar charge distributions in the device” if you want me to believe “not a rectifeir” :)

  1. I do wonder about an electric version of this.
    Inductively [ or just with a coil ] heating the pipe. Perhaps inserting rods through the boiler box like a steam locomotive, to increase the efficiency and tuning it’s resonant frequency for better performance. That would allow you to better insulate the hot part. Perhaps the tube could be split with a thin insulating section to increase the abruptness of the thermal gradient. Maybe the tube could be backfilled with a different gas mixture, with better thermal characteristics. Maybe the wrking section could even disconnected entirely from the drive medium and coupled via a wide diaphragm. It would be interesting to see how much electricity could be produced by the design, if it were heated with solar power. Perhaps two connected back to back, made to oscillate in opposite phases of their cycle, using the air spring to recuperate some of the energy from each stroke, or maybe even just pumping mercury through a magnetic field.

    1. Since this is working by way of the Stirling cycle, there are of course ways of optimizing it for efficiency, and these should be similar to what works for more typical Stirling engines. However, there is always a loss when you transfer energy from one medium to another, so I doubt that the two-fluids approach is the way to go. At one point, railroad engineers (er – design engineers, not train drivers) tried using alcohol instead of water in their boilers, because of the lower latent heat of vaporization, which would make it boil easier. This of course didn’t work, because that also meant there was less energy in the vapor!

        1. What on earth are you talking about? Seriously, you really need to explain your claim, since the “‘we were testing it in the boiler’ lie” is not a thing.

          1. He’s joking, by suggesting that the engineers were drinking alcohol, not using it in the boiler.
            I’m surprised you didn’t think of that, Jim.
            (Or did you, and I’m missing the subtleness of your response?)

  2. So it is essentially a Stirling engine, with the water in the tube being the displacer, and the working fluid being a mixture of air and water vapor. Cool.
    His explanation of why the usual explanation of why the boat moves forward is questionable, though. It DOES matter which directiion the water is coming from when it’s sucked into the tube, because it is the change in momentum that causes a force, and on the intake cycle, that momentum is from many directions to one direction, so much of that change cancels out.

      1. I could use one of those when it rains, and the lawn gets too wet. Then I could just suck up the excess and store it for the dry season!

    1. I was thinking the front end of the boat is pointed, and the rear (transom?) of the boat is flat. So, water squirting out of the back of the boat faced less resistance of the boat’s motion than the water being sucked back in.

    1. Sure, as long as the tubes stay the same diameter. Just have lots of them in parallel. If the tubes were also scaled up, then the water wouldn’t work like a piston/displacer in this mechanism

  3. I never had played with (studied?) one of these pop-pop boats.
    But, I wondered why they didn’t have a forward facing intake tube, and a ball valve to force “the steam” out the back.
    Such as the “valve” in the WWII Buzzbombs.
    So, now I know “how” they work, instead of how I “thought” they worked.

  4. Another reason why the boat moves forward and not backward is simple hydrodynamics. The bow offers much less resistance to forward motion than the stern does to backward motion. Therefore, it glides forward (as stated in the video) easily when the water is pushed out, but strongly resists backward motion when the water is drawn back in.

  5. Thank you for the detail analysis of the pop-pop boat
    You have shown that this is a Fluidyne engine.
    Air, expanding and contracting pushes liquid pistons.
    Colin West, way back when, studied these.
    And the Thermofluidics company have, courtesy of Tom Smith, developed the Fluidyne concept by using a gas other than air and exploiting phase change of that gas.
    Thanks for the detailed analysis

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