Ah, the old pop-pop boat! Known around the world by many names including putt-putt, Knatterboot and toc-toc these were a staple “boy’s toy” in the 1940s and 50s. The huge number of localised names for this little steam-powered* boat is testimony to its worldwide popularity and it’s ease of manufacture.
* Kinda, but not in the conventional sense.
Originally conceived by Tom Piot in 1891, the design was refined by American Charles J. McHugh several times between 1915 and 1926 taking it from a coil type boiler to a diaphragm design to allow it to be manufactured more easily.
How does it work?
The toy is simplicity in itself. A small diaphragm – or coil – boiler has two exhausts* extending towards the back of the boat. These are completely filled with water to prime the “engine”. The boat is put in water such as a sink or bath and a small candle or drop of oil is lit under the boiler chamber. With such a small boiler chamber this quickly heats the water to steam. The steam in the engine then forces water out of the exhaust tube and this moves the pop-pop boat forward. As it does this the steam begins to cool and contract, in turn sucking water up the exhaust and the process repeats until the fuel is spent. It is the constant flash boiling of the steam and consequential condensing that gives the boat it’s name due to the sound it produces.
* Earlier models had just one but a second was added simply to make it easier to fill. Both act as inlet/outlets; the water doesn’t flow in one and out the other.
Interested in the physics behind it? Wikipedia tells us:
The operation of the pop pop boat may seem surprising, since one might expect that if water is going in and out through the exhaust tube, the boat should merely shake back and forth. But while the water pushed out carries away with it momentum, which must be balanced (by Newton’s third law) by an opposite momentum on the part of the boat, the water sucked in quickly impinges on the boiler tank and transfers its momentum to the boat. The initial reaction force on the boat (which would pull it backwards) is therefore cancelled by the pushing of the water when it hits the inside of the boiler. The result is that the inflow of water causes no appreciable force on the boat.
However it works it is a fantastic little tin toy and, though I never owned one myself, I remember the whole class playing with one at school during an end-of-term physics lesson. Popularity declined with the increase in use of plastic for kids toys but they are still manufactured and sold albeit in much smaller quantities. Buy one here or make one yourself using the instructions here (diaphragm model) or here (coil type model).