How Does a Putt-Putt Boat Work?

For more on collimated vs. diffuse fluid flow, see “Smartphone ‘Fan’ App Blows Out Candles—But How?” posted here on 2021-12-14, but there’s more to the putt-putt boat than that.


This is related to the Feynman sprinkler problem, in which Feynman asked which way a rotating sprinkler would turn if immersed in water with reversed flow. I posed this problem to my colleagues back in 2002:
The winner got lunch at a Chinese restaurant.
The solution is explained in this video, which includes an experimental demonstration.


Beautiful, thank you.

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How big a boat can move using this, I wonder? Be interesting if this will work on full-size boats,

I doubt if it would scale up to a full-size boat. It works because the heat source can cause the water at the closed end of the tube to flash to steam and eject the water from the tube. This is only a practical means of propulsion if the mass of the water in the tube is comparable to that of the boat and can overcome the friction moving through the water. For a full scale boat, you’d need a massive tube which would in turn require an extremely intense heat source.

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That is why I phrased my question as “how big a boat?” It is the kernel of an idea for a possible SF story. A low tech way to do steam propulsion in a world under a technology ban (like L Sprague de Camp’s Krishna). And it doesn’t necessarily have to work on a full-size boat. Just one large enough to squeeze a research grant from a ruler eager for steam propulsion.