Why Don't Metal Ships Sink? Physics Explains

Many have wondered how a massive iron ship can stay afloat on water. The answer lies in one of physics' most famous laws – Archimedes' principle. The same principle applies to both cargo ships and submarines.

The question of how a giant metal ship floats on water without sinking has captivated people for centuries. Intuitively, heavy iron should sink into water – yet ships weighing hundreds of thousands of tonnes regularly sail back and forth across the seas.

Archimedes' Principle at Work

The answer lies in Archimedes' principle, which states that a liquid exerts an upward force on an object equal in magnitude to the weight of the liquid displaced by that object. In other words, a ship will not sink as long as it displaces water weighing more than the ship itself. Therefore, the material – iron, steel or aluminium – is not actually the key factor, but rather the shape and volume of the object.

Because a large ship is hollow inside and contains enormous amounts of air, its average density is less than that of water. By contrast, a small solid piece of iron sinks immediately to the bottom because it lacks sufficient volume to displace enough water.

Submarines – Same Rules, Different Application

The same physical laws explain how submarines work. A submarine has ballast tanks that are filled with water to make it sink and purged with compressed air to make it rise. This allows a submarine to change its average density relative to the density of water – increasing it to descend and decreasing it to ascend.

The application of these principles is not limited to maritime affairs. The same logic applies to hot air balloons and airships, where gas is lighter than the surrounding air. The laws of physics are universal, and humanity has cleverly harnessed them both at sea and in the sky.