Sophie Duncan investigates steam propulsion

31st October 2003 at 00:00
This steam-propelled boat makes use of a pulsating water engine. It is called a pop pop boat, and once you've made one you will see why. It demonstrates Newton's third law of motion: for every action there is an equal and opposite reaction.

The design is straightforward, but makes use of a night light, so follow appropriate safety measures.

Take a foil tray, and bend the front to shape it like the prow of a boat.

Make two holes in the base of the tray, towards the back. These holes need to be side by side, a few centimetres apart. Take 20cm of narrow copper tubing, with an internal diameter of about 3mm, and wind it round a piece of dowel about 2 cm in diameter, leaving straight lengths of tubing at both ends. This copper coil forms the engine of the boat.

Place the straight ends of the copper pipe through the holes in the base of the foil tray, ensuring about 0.5cm sticks out through the bottom. To avoid an early sinking of the boat, seal the holes with modelling clay. Turn the boat upside down, and using a pipette squirt some water into the engine.

Turn the boat right way up again, put it on the water and place a night light beneath the copper tubing. Light the wick and watch what happens.

The water in the engine turns to steam, which gets forced out of the bottom of the boat. After the steam is forced out there is a partial vacuum in the tube and water is forced into it. The cycle repeats. Can your students create designs that work more quickly?

This experiment can be found in the Little Book of Experiments on

Sophie Duncan is project manager for science at the

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