Drop a solid block of steel into water and it sinks like a stone, because steel is nearly eight times heavier than water. So how can a giant steel ship float? The trick is the shape. You don't make the steel lighter — you spread it out into a hull wrapped around a huge pocket of air. When you weigh the steel and the air it encloses all together, the whole thing turns out lighter than water, so it floats. Slide the steel from a solid block into a wide hull in the simulator and watch it rise up and bob. Then load it with cargo until it sinks.
Most people think floating depends on what something is made of. In fact it depends on average density: a solid steel block sinks at about 7.9, but shaped into a hull around a pocket of air the steel and air together average below water’s 1.0, so it floats.
What's actually happening
It looks like a contradiction. Steel is one of the densest things in everyday life — a block of it sinks instantly, dropping to the bottom of any pool. Yet the heaviest objects humans have ever moved, supertankers carrying hundreds of thousands of tonnes of steel and oil, sit calmly on the surface of the sea. The puzzle dissolves the moment you stop asking 'is steel heavier than water?' and start asking 'is this whole shape heavier than the water it takes up?'
That whole-shape question is what Archimedes worked out over two thousand years ago: an object floats if it can push aside a weight of water equal to its own weight. A solid steel block can't — it's so dense that even fully underwater it only displaces a small, light volume of water, far less than its own heft, so down it goes. But take that same steel and beat it into a thin hull wrapped around a large hollow full of air, and everything changes. Now the object as a whole is mostly air, and its average density, the steel and the air counted together, drops below that of water. Lowered into the sea, the hull only has to sink until the water it has shoved out of the way weighs exactly as much as the ship. Then it stops, and floats.
This is why a ship's waterline matters so much, and why there are painted marks on every hull showing how deep it is allowed to sit. Loading cargo adds weight without adding much air, so the average density creeps upward and the ship settles lower, displacing more water to carry the extra load. Keep loading and eventually the average density passes 1.0 — the ship can no longer displace its own weight before the water pours over the top, and it sinks, exactly like that solid block. The same steel can float or sink; the only thing that changed was how much air it was wrapped around.
The same steel can float or sink; all that changes is how much air it wraps around, so the average density crosses water’s 1.0.
- 1Tear off two equal squares of kitchen foil. Scrunch one into a tight, dense ball and drop it into a bowl of water — it sinks.
- 2Fold the other into a little open boat shape with raised sides and set it gently on the water — it floats, because it now encloses air and its average density is below water’s.
- 3Drop coins into the floating boat one at a time. It rides lower and lower, then suddenly takes on water and sinks — your hull just passed the average density of water, exactly like an overloaded ship.
Common questions
A solid block is so dense it displaces only a small, light volume of water, far less than its own weight. Beaten into a hull around a large hollow of air, the same steel becomes mostly air, dropping its average density below water's.
Loading cargo adds weight without much air, so the average density creeps upward and the ship settles lower. Once it passes 1.0, the hull can no longer displace its own weight before water pours over the top, and it goes under.
Yes. During both world wars ships were built from ferrocement. Concrete is denser than water, yet a hull-shaped one floats fine, because the shape, not the material, sets the average density.