How can boats float, no matter how heavy they are?

Answer

Dear Paul,

Two forces act on an object that is on or in water: the weight of the object (pulling downwards) and the buoyant force of the water on the object (directed upwards). If the weight is greater than the buoyant force, the object will sink; if the buoyancy force is greater than the weight it will float.

The buoyant force also depends on the shape of the object. If you crushed all the metal from a ship’s hull into a lump and threw it into the water, the metal would immediately sink.

Archimedes’ law says that the buoyant force on an object in water is equal to the weight of the displaced water (i.e. proportional to the part of the volume of the object that is under water). The ship’s hull is relatively thin and the interior is largely filled with air. As a result, the ship takes up a relatively large volume and there is therefore a large buoyant force, greater than the weight of the loaded ship. This keeps the ship afloat and a lump of equal weight but with a smaller volume does not.

Many people find Archimedes’ law confusing, so I’ll give another way to explain the same thing:

metal has a greater density than water and so you expect metal to sink in water. However, the average density of the metal that makes up a ship along with the air contained within the ship is less than the density of the surrounding water. That’s why a ship floats.

Regards,
Sylvia

PS: Recently I was asked if there are liquids that can float on a gas. You may also find the answer to that question interesting.