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Bathtub Buoyancy Challenge 1 Results

Do you remember the challenge, to propel a homemade boat across a bathtub without using your hands to touch it, batteries or electricity? We came up with six ways to do this. Did you come with any that we didn't? Let us know!

1.    Wind or air movement
Probably the most obvious way to propel a boat without touching it is to use air movement or wind. We tried both creating a sail and blowing on it and also using a fan to create air. Both were effective, although somewhat slow and hard to steer.

2.    The classic balloon boat –

You may have seen instructions for these or tried out a kit. I found some available for sale on the Internet (see Resources below) or you can make your own. Check these fun instructions to build a balloon boat from ZOOM, the PBS TV show.

Note:  Please be careful using balloons, and always have adult supervision. Children under 8 years can choke or suffocate on uninflated or broken balloons.

3.    Wind-up propeller
My son took a wind-up propeller from a balsa wood airplane and modified it to propel a flat wooden boat. The stretched rubber band creates the potential energy which is then transferred to kinetic energy. It is fast for short distances.

See the red propeller on the right, rubber band across the center and nail holding it on the left.

4.    Magnet Repulsion
Okay this one was my idea. I remembered moving train cars in a wooden train set by reversing the magnets. It works with an egg carton boat, too. I think this type of boat has the best steering once you get the hang of it.

Here is a short video showing one of our trials. Bear with us, we're still learning video technology 🙂

5.    Detergent Power / Soap propulsion

You may have seen instructions for a soap boat that actually moves. We created one using a top of a Styrofoam egg carton and a squirt of dish detergent. This is not technically a chemical reaction, the movement has to do with the surface tension of the water.

Instructions for a soap-propelled boat from CSIRO.

We simplified the technique a bit by just adding the dish detergent to an egg carton boat and it still seemed to work.

6.    Chemical reactions

My dad told me that as a kid he had a boat that ran on Alka Seltzer tablets. You can also make boats propelled by baking soda and water, or vinegar.

ZOOM has instructions for a Soda Bottle Boat

Good luck and I hope you try some of these. They were fun and we learned a lot too.

Resources for Further Explorations (Affiliate links to Amazon):

Balloon Powered Wooden Boat


Rubber Band Paddle Boat

Let's Try It Out in the Water : Hands-On Early-Learning Science Activities by Seymour Simon, Nicole Fauteux, Doug Cushman (Illustrator)


Teaching Chemistry with TOYS by Jerry Sarquis, Mickey Sarquis , and John Williams


Science in Seconds with Toys: Over 100 Experiments You Can Do in Ten Minutes or Less by Jean Potter


Magnetic Boats in the Tub


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