A small sheet of aluminium foil floats on the water. One coin goes in. Then another. The boat tilts, a little water slips over the edge, and suddenly the cargo is underwater.
What happened—and how could a different design carry more?
This hands-on activity invites children to become boat designers. Using everyday materials, they’ll explore floating, balance, measurement, and the engineering process of testing and improving an idea.
The challenge: Build a foil boat that carries the greatest number of identical coins before water enters or the boat sinks.
You’ll need:
- Several equally sized squares of aluminium foil—for example, 20 × 20 centimetres
- A shallow basin of water
- Identical coins or other uniform, waterproof weights
- A ruler, pencil, and recording paper
- A towel for spills
An adult should supervise the water activity and keep coins away from younger children who might put them in their mouths.
Build, test, and improve
- Imagine your boat. Before folding the foil, sketch a design. Will it have a wide base, tall sides, or a narrow shape? Predict how many coins it will carry and explain your reasoning.
- Build the first version. Shape one foil square into a boat. Avoid holes, and make sure the sides can keep water out. Place the empty boat gently on the water.
- Add the cargo. Add one coin at a time, distributing the weight carefully. Count the coins until the first water enters or the boat sinks. Record the number it successfully held before that point.
- Investigate what happened. Did the boat tip? Did its sides bend? Did water enter through a low edge? Describe the result before deciding how to change the design.
- Redesign with a fresh foil square. Use the same amount of foil. Change one feature where possible—such as the width of the base—and test again using the same coins and loading method.
- Compare your results. Which design carried more? Can you explain why? Rebuild and repeat your strongest design to see whether the result is consistent.
What’s the science?
Water pushes upward on an object placed in it. This upward force is called buoyancy. A floating boat displaces enough water for that upward force to balance the weight of the boat and its cargo.
Shaping foil into a hollow boat allows it to displace water while keeping its total weight relatively low. Adding coins makes it sit lower in the water. Its shape, stability, and the height of its edges affect how much cargo it can carry before tipping or taking on water.
A wider boat may spread the load more effectively, but width alone doesn’t guarantee success. The design still needs sturdy sides and careful loading.
Keep the conversation going
- Where should the first coin go? Why?
- What changes when you place several coins on one side?
- Did your results match your prediction?
- How could you make the comparison fairer?
- What would you test next?
For an extra challenge, give every team the same foil allowance and ask them to present their design with evidence: “Our boat carried this many coins, and here’s what we changed.”
Celebrate careful observation and thoughtful improvements alongside the strongest boat. A sinking design can still produce an excellent discovery.
