Foil Boat Buoyancy Challenge
Engineer a foil boat, load it penny by penny, and find the exact moment Archimedes says 'enough.'
Dry the floor around the tub to prevent slips. No sharp tools needed.
Materials
- Aluminum foil (heavy duty works best)
- A tub, sink, or basin of water
- Pennies or similar coins (40+)
- A ruler
- Paper towels
Procedure
Build boat one: wide and flat
Cut identical foil squares (about 30 cm). Fold up the edges of one to make a simple wide, flat-bottomed boat. Do not poke holes.
Load it to sinking
Float it and add pennies one at a time, spreading them evenly. Count until water spills over the rim and it sinks. Record the number.
Build boat two: narrow and deep
Build a second boat from the same foil size but narrow and deep. Load it the same way and record its limit.
Engineer a champion
Try a third design of your own — a barge with folded double walls, or a catamaran of two hulls. Chase the record.
Weigh the winning load
Weigh the winning penny load on a kitchen scale. That weight equals the weight of the water your hull displaced.
What to observe
Wide flat boats hold far more pennies than narrow deep ones of the same foil size.
The boat rides lower with every added penny.
Water spills over the rim right before sinking — the classic failure mode.
Why it happens
A floating boat is in force balance: the buoyant force equals the weight of the displaced water (Archimedes' principle), so the boat plus its cargo can weigh exactly as much as the water its hull pushes aside. A wide, flat hull displaces more water per centimeter of depth than a narrow one, which is why barges carry so much. The boat sinks the moment the load's weight exceeds the maximum displaceable water — cargo ships call that limit the Plimsoll line.
Try variations
Predict the penny limit before testing a new shape — how close can you get?
Try salt water versus fresh water. Which holds more pennies, and why?