Putt-Putt Boats – Matthew Lee

Description of putt-putt boats: putt-putt-what?

“Putt-putt” boats are toy boats that propel themselves through water through the repeated firing of jets of water. In each cycle, water is expelled out of a pipe at the rear of the boat, pushing the boat forward, and then water is drawn in from the same pipe in preparation for the next cycle. This delicate cycle is powered by a candle flame. In this FOTW, we look at two questions. First, how does the boat manage to produce the series of jets? Second, if the boat both sucks in and expels water from the rear, why does the boat move forward at all?

Fig. 1: A putt-putt boat. Note the thin diaphragm below the blue roof.

At this point I recommend that you drop everything and watch this educational video on the action of a putt-putt boat =)


The putt-putt engine: putt-putt-how?

The expulsion of jets of water is simply due to boiling. As shown in the video, the candle flame is positioned under a metal diaphragm that contains water. The diaphragm is connected via a pipe to the openings at the rear of the boat. The heat from the candle causes the water inside the diaphragm (but not the water in the pipe) to heat up to its boiling point and boil suddenly. The consequent rapid increase in pressure due to boiling forces both the steam in the diaphragm and the cooler water in the pipes out of the opening at the rear of the boat. This causes the jet that propels the boat forward.

After the expulsion of water and steam, the diaphragm and pipes are empty of water and are both completely filled with steam. As the diaphragm is constantly being heated by the flame, the steam there remains hot. But the steam that displaced the water in the pipes cools rapidly as the metal pipes conduct heat away from the steam to the surrounding water that the boat is floating in. This causes the steam in the pipes to condense, thereby drawing water from the surroundings back into the pipes. This explains how the pipes are refilled, but how does water get back into the hot diaphragm? Well, the coolwater that enters the pipes comes into contact with the steam in the diaphragm, causing that steam to condense, too. Thus the whole engine is re-filled with water and the cycle repeats.

This is also why Sosuke and Ponyo have to blow water into the engine before the boat can be started – it can’t suck in water on its own if there’s no steam inside to condense.

The reason the putt-putt engine makes the putt-putt sound is due to the flexing of the diaphragm when the water boils. Why, then, did the designers decide to give the diaphragm that shape (instead of a sphere for example) and that thickness (couldn’t they have made it flex-resistant by using thicker metal walls)? Well, the thin, disc-shaped diaphragm sacrificed the energy loss caused by flexing in order to gain a shorter time for boiling. In any case, we all love the putt-putt sound! Putt! Putt-putt!

The putt-putt engine: putt-putt-why-move-forward?

Having discussed why the engine sucks water in and expels it out, we now turn to the question of why the boat moves forward at all. After all, if the net force exerted on the boat during the sucking-in stage equals the net force exerted on the boat by the expelled jet of water, shouldn’t the boat simply oscillate about a fixed point?

According to The Flying Circus of Physics, the reason that the boat does indeed move forward is that when water is expelled from the engine, the jet is directed straight out of the pipe in a single direction. However, when water is sucked into the engine, it is drawn into the pipe from a hemisphere around the pipe opening. Hence, the net force exerted on the boat during the sucking-in stage is less than the force exerted by the expelled jet of water, and the boat accelerates forward.

Fig. 2: Sosuke worries about what would happen if his boat got too big.

Food for thought: think-putt-putt

As shown in the movie, Ponyo, how would scaling up the dimensions of a putt-putt boat affect the behavior of the engine? Examine the effect of increased scale on parameters such as period, velocity and the putt-putt sound made when the diaphragm flexes.


A cute Japanese video: http://www.youtube.com/watch?v=MMyCPE6IhQk

A good website on putt-putt boats: http://www.nmia.com/~vrbass/pop-pop/

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