Foil Boats and Coracle Racing

The Ironbridge during a minor flood with the city of Ironbridge behind

This week we were discussing activities and demonstrations we could offer as part of a collaboration between the Discovery Center of Idaho and Boise Parks and Rec. We wanted activities related to water since the Parks and Rec site will be near Boise’s new whitewater park. There are all sorts of great demos, but one that came to mind was the foil Float-a-Boat challenge.

Before moving to Boise, I lived in Ironbridge, England and worked at Enginuity. Each year, we would collaborate with another local organization, the Green Wood Centre, to help with one of their big events. They would teach people to build coracles, small round boats that look like large bowls. After a weekend of building, the entire community was invited down to the River Severn to try paddling these unusual boats.


Racing across the River Severn. Note that my coracle is rotated about 60° off the direction of travel. Yeah, they spun a lot.

Adults could participate in one of three races which were entertaining to watch. Because of the design, these boats could turn easily, but racing them in a line was nearly impossible for the novice paddlers. The younger ones had their own race which involved spinning in as many circles as possible in three minutes. It was nauseating to watch. I spun around enough while trying to race across the river and back that I never felt the need to just spin.

While the experienced adults were racing, I worked with anyone who wanted to try building their own (much smaller) boat out of foil. It is impressive just how much weight these simple boats can hold. Interestingly, one of the best designs for holding lots of weight looks just like a coracle.

So, how fast can you paddle a coracle? And how much weight can your foil boat hold?


How many marbles can your foil boat hold?


  • Aluminum foil (buying precut squares makes it easier and more fair)
  • Pool or bucket of water
  • Weights (marbles, paper clips or pennies)

What to do

  1. Fold the foil into a boat
  2. Place the boat on water
  3. Slowly add weights until it sinks
  4. Try building another boat to hold more

Inquiry Questions

  • What shape works best?
  • What boats look similar to your best boat?
  • If your boat had to carry half the weight and move quickly, how would your design look different?

What’s happening?

Buoyancy is what keeps boats afloat. It is the force of the water pushing up on the boat to counter gravity. The amount of buoyancy an object has is equal to the weight of the water the object pushes out of the way. If you crumple the foil into a small ball, it takes up little space and will sink because it weights more than the small amount of water it replaces. If you make a canoe shape, the foil still weights the same amount, but now, takes up much more space. As long as that space is filled with air, the aluminum foil canoe will float. As you add weights, it gets pushed down in the water. Once water comes over the top, the boat is no longer “pushing” lots of water out of the way so it is no longer as buoyant and it sinks.

A canoe is designed to carry weight and cut through the water easily. Is that the best design for this challenge? What other types of boats might work better? Remember, the more water the boat displaces or pushes away, the more buoyant the boat will be and the more weight it will be able to hold.

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