Curiosities: Why don’t heavy ships sink?

By Eddy Montilla

You drop a stone or an iron bar into the water and they sink quickly. A ship is thousands of times heavier and, however, it can float. How can this be possible?

     To start, things don’t float because of the material they are made of, but because of their density and relation between their weight and the weight of water they displace. Whenever you put something in water, two things happen: It experiments an upward buoyant force and it also displaces water that (believe it or not) weighs. The upward force mentioned before is equal to the weight of the fluid displaced by the object (Archimedes’ principle). A ship is very heavy, but somehow water is heavy too. So, if the weight of water displaced by the ship is bigger than the weight of the ship, then the ship floats. They sink into the water a little bit, but they are designed in such a way that the weight of the ship is displaced before it is completely underwater. Remember also that a ship is not solid steel throughout. They are designed with hollow areas that are filled with air. The combination of the steel and the air is light compared to the average density of water.

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