Sailboats use the wind to push them along, and motor boats have a propeller. This little boat is powered by something else – soap!

You will need
- Good pair of scissors
- Thin, stiff plastic such as a disposable plastic plate or a berry punnet
- Liquid soap or dishwashing detergent
- Sink with a plug
Safety
You need to cut plastic in this activity. Younger scientists should ask an adult to help.
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What to do
Cut out a rectangle of plastic. Ours was about 3 centimetres wide and 6 centimetres long.
Cut the two corners off one end to make a pointy front to your boat.
Cut a little notch into the back of the boat.
Plug the sink and fill it to a depth of about 1 centimetre.
Put the boat on the water.
Carefully put a drop of soap or detergent on the notch in the back of the boat.
Watch as the boat sails forward!
When you’re finished, wash your hands so you don’t accidentally get soap in your eyes!
What’s happening?
You might not realise it, but water is super sticky! When you have a bath or wash your hands, the water sticks so hard you need a towel to rub it off. And then, that’s just the water sticking to something else instead!
When you put your boat in the water, the sticky water pulls on it. The water at the front of the boat pulls it forward, while at the back the water pulls backwards. There’s lots of pulling involved, but it’s all perfectly balanced so the boat doesn’t go anywhere.
Then, you add some soap. Soap contains surfactants, special chemicals that make water less sticky. When the soap mixes with the water behind the boat, it weakens the pull at the back of the boat. But the pull at the front is still strong! It overpowers the pull at the back, and the boat moves forward.
As more and more soap ends up in the water, the boat will stop working as well. But you can always run another sink!
Why surfactants?
So why are these surfactant chemicals in soaps and detergents?
Water doesn’t stick to everything very well. Famously, water doesn’t like sticking to oil and fat. And that’s no good if you need to wash some oil or fat off your hands and dishes.
Surfactants make water less sticky to water, but they actually make water more sticky to oils and fats. So those greasy stains on your plates get pulled off by the surfactants and out into your washing up water!
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