Soap powered boat science activity | Fizzics Education


Soap powered boat

Soap powered boat

Follow FizzicsEd 150 Science Experiments:

You will need:

  • Cardboard or balsa wood (laminated paper works well too)
  • Tub of water, scissors and glue
  • Detergent or soap


Soap powered boat science experiment - materials needed
1 Soap powered boat science experiment - glued boat on cardboard

Glue the picture below onto some cardboard, or copy it onto you piece of balsa wood.

2 Soap powered boat science experiment - cutting boat out of the cardboard

Cut out the boat picture, along the black lines, to make your small boat.

3 Soap powered boat science experiment - floating the boat in water

Place the boat into a tub of water.

4 Soap powered boat science experiment - adding detergent to boat

Place a drop of detergent, or soap, onto the ‘target’ of the boat.

Why does the boat travel forwards?

5 Two smiling students watching a blue lava lamp made in a cup with oil, blue food colouring and alka seltzer

Why Does This Happen?

The movement of the soap powered boat can be explained by a combination of three principles. All liquids have surface tension along the surface of a liquid, caused by inter-molecular forces within the liquid pulling liquid molecules together. Because of surface tension, liquid surfaces act like a kind of ‘skin’, able to support small insects and materials on their surface.

Isaac Newton’s third law of motion says for every action there is an equal and opposite reaction.
The movement of the detergent provides a ‘pushing force’ backward into the water surface as the detergent molecules spread out, sending the boat forward… i.e. an equal and opposite motion.

The Marangoni effect is also acting upon the boat. This affect describes how a liquid with a high surface tension pulls more strongly on the surrounding liquid than a liquid with a low surface tension. If you change the surface tension of some parts of the liquid you introduce a difference in surface tension or ‘gradient’. A presence of a surface tension gradient will cause the liquid to flow from areas of low surface tension to high surface tension. Your detergent molecules reduced the water surface tension behind the boat, causing the water (and the boat) to move forwards.

To repeat the experiment you may have to get clean water without detergent so that the surface tension is equal throughout the water.

Variables to test

More on variables here

  • Try different shapes of boats and amounts of detergent, does it make a difference?
  • What happens if you use soap instead of detergent?

Recording from a Facebook Live presentation on the Soap-Powered Boat

Want to join the fun? Join our Facebook site and keep an eye out for the next live science event where you can work with our science presenters who run the experiments with you!

Learn more!


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