A putt putt boat a.k.a. pop pop boat is a toy steam boat which had no moving parts. The toy was popular in the 20th century back when tin toys were considered “cool.” Although I think such a toy is timeless because of its ability to entertain and its simplicity. It is also interesting because it has some interesting science behind it.
The boat moves forward by turning the water in the boiling chamber into steam which forces water out of the ends of the tubes propelling the boat forward. A vacuum is created as the water rushes out of the pipes and so more water is sucked in and then the cycle repeats. The cycle can repeat itself more than 5 times a second giving the boat almost a continues motion.
Some people wonder why the boat can move forward at all, since it first pushes the water out, but then sucks it back in so it would be logically to assume that the boat can’t move forward at all. The expanding steam pushes the boat forward and then more water is sucked back in and the water's momentum is transferred to the boat. This means that the water suck in doesn’t cause the boat to lose or gain its net velocity so the only thing driving it forward is the initial pulse.
A putt putt boat usually consists of only a boiler, a hull, two tubes protruding from the boiler and a candle. The boiler is the component that gives the boat the distinctive putt-putt sound. This is
due to the very thin piece of metal covering the boiling chamber. The piece of metal usually has a convex or concave shape which is briefly inverted as the pressure changes in the chamber creating the putt-putt sound.
These instructions will tell you how to build a putt putt boat, but this boat will not make the putt-putt or pop pop boat sounds; in fact it will be comparatively silent. This is because this design is simpler to make since it cuts out the need to solder anything by making the boiler into
one long copper tube. Even though it does not make a sound it still works in the same way.
You will need:
Start off by cutting the balsa wood into a smooth and streamlined boat. Make sure that your putt putt boat can take the weight of the tube and the candle and that there is enough room for both on board. Once you know it will fit both the candle and the tube. Make it as light and small as you can. This will make the boat more efficient.
Then take the copper tube and twist it around a thin round piece of wood about 1.5cm in diameter making 4 loops in the middle of the copper tube. (If your loops will be bigger remember to increase the length of tube and size of boat.)
Once you have done that, drill two holes side by side in the bottom of the boat going straight down. Then push the two ends of the tube through the holes making sure that the 4 loops are over the candle. Bend the two ends of the copper tube that are protruding from the bottom of the boat so that they point backwards like an exhaust pipe. If there is water leaking through the two holes use two small dabs of hot glue or PVA. Then add some personal touches e.g. a paint
job, a wind shield, fins, etc.
To prepare your boat take the straw or a length of bendy plastic tubing and place it over one the two copper tubes ends and suck until you see water coming up the straw. The boat's engine needs to be full of water to function. Some boats run dry after a certain time due to a build up
of gas pushing out the water; this gas is from the dissolved gases in the water which is released when it turns into steam. This shouldn't happen, but if it does you may have to change something
about your copper tubes position so the gases can escape. Once your engine is full of water you can light the candle and watch it go. The putt-putt boat will keep going until the candle goes out.