How do big ships float?

Drop a little stone in a pond and it sinks straight down. Yet a ship made of heavy steel, as big as a building, floats happily on the sea. How can that be?

Water pushes back up

Whenever something sits in water, the water pushes up on it. We can call this gentle push the upward push. The more water an object shoves out of the way, the harder the water pushes back up.

If that upward push is strong enough to hold the object’s weight, the object floats. If it is too weak, the object sinks.

A wide, hollow shape

Here is the ship’s clever trick: its hull is wide and hollow, like a giant bowl. That big shape pushes a huge amount of water out of the way.

So the water pushes back up with a really big force — big enough to hold the whole heavy ship! Even though steel is heavy, the ship is mostly full of air inside, which makes it light for its size.

Try it yourself

Squeeze a ball of foil tight and it sinks. Open it into a little boat shape and it floats! You just changed how much water it pushes aside — the same trick that holds up the biggest ships.

Wonder fact: The largest cargo ships can carry over 20,000 containers — yet that wide hull keeps them bobbing safely on the waves!