Why does the same clay float once it becomes a boat?
Start with a fair comparison: put the 180 grams of clay in the water unchanged. Watch the gold fingerprint—the whole time it is the same lump, from the scale into the tank. It sinks. Water does push up on it, but a solid ball can only displace about 100 cubic centimetres, and that water weighs just 100 grams—not enough for 180.
Now shape the same clay into a little boat. The mass has not changed; the scale still reads 180 grams. What changed is the outer shape: the hull walls enclose dry air, letting the clay occupy much more space. As the boat settles, it displaces more and more water—until it displaces 180 grams. At that point buoyancy equals weight, and the boat rests.
What decides floating is not “how heavy” but how much space the same mass occupies: if the average density (total mass over the outer envelope) is below the water’s, it floats. The clay’s material density never changed at all.
Load the boat and find its limit
- Add cargo one piece at a time. In 20-gram steps, watch the waterline climb: each new weight demands more displaced water.
- Find where the waterline nears the rim. The safety margin shrinks until the surface touches the rim—that is this boat’s boundary.
- Add one more. Water pours over the rim into the cavity; the water that was kept out “returns”, the boat loses that space, and it goes down quickly. Real ships rely on watertight compartments and reserve buoyancy; this toy has neither.
- Switch to brine. With the same cargo, denser water supports the boat with a smaller displaced volume, so the draft shallows.
Note: the boat “dips, then settles” each time. That is normal—reaching the new balance takes a moment of extra displacement.
Pressure, Archimedes and average density
Open: where does buoyancy come from?
Liquid pressure grows with depth: p = ρ g h. The hull bottom sits deeper than the rim, so the upward push exceeds the downward push, while sideways pushes cancel. The result is one net upward force—buoyancy. Shape the submerged part any way you like and the conclusion holds:
That is Archimedes’ principle: buoyancy equals the weight of displaced liquid. It depends on the displaced shape, not the material.
Open: average density is not material density
The clay’s material density is about 1,800 kg/m³ and never becomes lower because of shaping. What drops is the average density:
180 grams of clay plus a cavity of air, divided by the whole outer volume, can fall below 1,000 kg/m³. A hollow boat is really a composite of “clay + a large body of air”.
Limits of the model
Only vertical floating and sinking are modelled—no listing, capsizing, waves, surface tension or true ship stability; the hull is rigid and watertight. When water crosses the rim, the model enters a one-way flooded state: the cavity no longer counts toward closed displacement. Real ships add watertight compartments, reserve buoyancy, hull form and load distribution.