Swimming, floating, sinking

There are two forces acting on every body which is dipped partially or fully into a liquid substance: the weight force which acts vertically downwards and the counteracting lifting force (upwards).

Swimming

If the density of a body is smaller than the density of the liquid substance in which the body is dipped, it will swim on the surface of the liquid substance. Just part of the whole volume of the body dips into the liquid substance.

Ships, however, are made of iron and the density of iron is much greater than the density of water. They are able to swim anyway, because the have a great volume and therefore can displace a lot of water.

While swimming, the lifting force equals the weight force of the swimming body and it also equals the weight force of the displaced water.