Explainer #17 · Flight & Motion
Why do boats float?
A boat floats when the water it pushes aside weighs as much as the boat itself. A cubic metre of water weighs 1,000 kg. Squeeze 1,000 kg of steel into a solid lump and it only takes up about 0.13 cubic metres, so it pushes aside just 130 kg of water: 130 kg holding it up, 1,000 kg pulling it down, so it sinks. Shape the same 1,000 kg into a wide, dry hull full of air and it sinks in only until it has pushed aside 1,000 kg of water, and there it floats. Let water flood in and the boat gets heavier without getting bigger, so it sinks.
The principle
A boat floats when the water it pushes aside weighs as much as the boat. A huge steel ship can weigh tens of thousands of tons, yet it sits on top of the sea. The secret isn’t the steel. It’s how much water the ship shoves out of the way.
Step 1: Water pushes up. Push a block under water and you can feel the water pushing back. It’s just like pushing a beach ball down in a pool: the deeper it goes, the harder it shoves back at your hands. Let go, and it pops straight back up.
Step 2: Weight = water pushed aside. When something goes into water, it has to shove some water out of the way, just like the water level rises when you climb into a bath. A cubic metre of water weighs 1,000 kg. A 300 kg block sinks in until it has pushed aside 300 kg of water, and then it stops: 300 kg down, 300 kg pushing up.
Step 3: A 1,000 kg lump of steel sinks. Steel is about eight times heavier than the same amount of water, so 1,000 kg of steel is a small lump of about 0.13 cubic metres. It can only push aside about 130 kg of water. That’s 130 kg pushing up against 1,000 kg pulling down, so it sinks.
Step 4: Shape it like a boat. Take the same 1,000 kg of steel and make it a wide hull with dry air inside. Now it sinks in only until it has pushed aside 1,000 kg of water, and there it floats. If you pushed the whole hull under, it would push aside about 3,000 kg, so it has lots of spare lift. Load 800 kg of cargo and it sits lower, until it pushes aside 1,800 kg. That’s why a loaded ship sits deeper than an empty one.
Step 5: Let water in and it sinks. A hole lets water flood into the hull. The water inside adds weight, but the boat doesn’t get any bigger, so it can never push aside more than about 3,000 kg. Once boat, cargo and water together weigh more than that, it sinks. It’s like a cup floating in the sink: fill it up and down it goes.
Common myth
Heavy things always sink
It’s not how heavy something is that decides whether it floats. It floats if the water it pushes aside weighs as much as it does. The same 1,000 kg of steel sinks as a lump (130 kg of water pushed aside) but floats as a boat (up to 3,000 kg). A giant ship is wide and full of air, so it pushes aside a huge amount of water; a small steel nut is solid, so it pushes aside almost none.How this video was made: Rendered 3D animation, simplified and drawn as a side view of a water tank. The green arrows show the water's upward push and the red arrows show the weight; the kilogram numbers are rounded example values and arrow lengths are not to scale.
