Published: 7 August, 2003

Yes, even a concrete boat (plus air) can float!

Why do ships float? Why don't ships sink?

That sounds like the question I asked myself during World War II when I read in the newspaper that they were making ships out of CONCRETE!!! It just didn't make any sense to me. First, it has to do with the air that's inside the ship: I'm sure a ship made of solid concrete would NOT float.

Let's test this out. Fill a sink with water and then get a metal or ceramic soup or mixing bowl. If you put the empty bowl in the water right side up it floats. But if you fill it with water, it sinks. The metal or ceramic of the bowl is heavier than water but having air instead of water inside makes all the difference. Why is that?

Let's think about your bowl. When the bowl is lowered into the water it has to push water out of the way to make room for itself. It's the same as when you fill a bathtub too full of water and it overflows when you get in. The deeper you push the bowl (as long as it stays empty), the more water gets pushed out of the way and the harder you have to push down. What you feel is the water pushing up on the bowl, "trying to get back where it was."

Archimedes was a scientist who lived over 2,000 years ago and is famous for having said "Eureka" when he figured out the following: How hard the water pushes up is just equal to the weight of the water that gets pushed out of the way (the displaced water). Thus the empty bowl will float as long as the weight of the bowl plus the air it contains is less than the weight of the displaced water. So, if a boat is mostly air inside, the hull (the outside of the boat) can be very heavy but the total weight of hull plus air can still be a lot less than the weight of the displaced water and the boat will float.

Remember, if you're ever out in a concrete rowboat, or even an aluminum canoe, don't drill a hole in the bottom and let the water in.