How to use the surface tension of water to power a paper boat

Surface tension is the force that holds water together

Water molecules are more attracted to each other than to air. This creates the phenomenon we call surface tension. The water molecules on the surface are more attracted to the other molecules on either side of them than to the air molecules. This is why there is a semi-solid surface or skin on water which very light animals like water skippers and spiders can use to walk on.

This skin can be broken by adding soap or methylated spirits to the surface of the water. This is because the surface tension of soap or the methylated spirits is different than the surface tension of water. Once the connections are broken, the water molecules swirl around. The result is motion which can be used to power a little paper boat.


Step 1

Make a paper boat

The boat must be small to work, about one-two inches long and a half to one inch wide. The front of the boat should be pointed and the stern of the boat needs to have a ‘keyhole’ shape cut into it in which to drop the methylated spirits.

Use a pencil to draw the shape on a piece of paper. Use small scissors to cut out the boat.

Step 2

Float the boat on a container of water

Fill an oblong or rectangular container such as a baking dish with water. The water doesn’t have to be deep, a few inches will do. Carefully float the paper boat on the surface.

Step 3

Put a drop of methylated spirits in the keyhole

Get some methylated spirits in an eyedropper or pipette. Gently squeeze one drop so that it falls in the boat’s keyhole. The surface tension of the water will be broken, there will be a swirling motion and Newton’s law of action and reaction will kick in. The boat will shoot forward with a force equal and opposite to the force of the moving water.

Step 4

Keep the boat moving

By continuing to put one drop of fuel at a time into the keyhole, the boat will continue to move across the water. However, if too much fuel is added at a time, the boat will stop. One drop at a time is the key to continuous motion.

Step 5

Make more boats

Make several boats and have races. Try different shapes and sizes to see which is the optimal for forward motion. Try other shapes to see which makes the boat spin the best. Have one person use liquid detergent as the fuel and another use methylated spirits to see which works the best. Imagination is the only limit here!

Boats and Soap — Surface Tension

Boats and Soap. Surface Tension at work. Prepare to be amazed… and stuff.

Things Needed
• paper
• scissors
• rectangular container to hold water, such as a shallow baking dish
• methylated spirits or dishwashing detergent
• pipette or eye dropper

Tips & Warnings
• Methylated spirits is poisonous so make sure that children do not drink it or squirt it at one another.
• Use dishwashing detergent with younger children but still make sure they don’t squirt each other with it, especially in the face!
• Only put on drop at a time of fuel in the keyhole. Too much fuel will ruin the effect.
• This is a possible Science Fair experiment.


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