Question: Why do boats float?

Sarah Martin answered on 18 Jun 2012:

Hi again meg123! 🙂
Oh, I love sailing, so this is the question for me! Boats float because they are less dense on average than water. Boats are hollow inside, and air weighs much less than water. The science of boats is fascinating actually! I have sailed in a boat with 3 tonnes of lead in the keel to stop it from tipping over – but even that floats, because 3 tonnes of water take up only 3 metres by 1 metre by 1 metre of space and this boat was much bigger, more like 9 metres by 3 metres by 3 metres, and so it was much less dense than water.

Any light object in water (like you for example in a pool) will sink a little and in doing so move aside as much water as the object weighs – that’s the rule. So for example, a block of wood that weighs 100 grams will displace 100 grams worth of water, so 100 mililitres, which is about half a mug full. If this block of wood is the size of a cooking pot, it’ll only sink in a tiny little bit, because it’ll only move aside half a mug of water underneath it. It’s a great way to weigh things if you don’t have scales, but do have a measuring cup: just fill a jug full of water, to the brim, stand it in a larger pot (to collect the water that spills out) and place what you want to weigh in the jug. Then measure how much water spilled and convert from mililitres to grams – hey presto! However this only works with things that float!