It is a well-known fact that a solid block of iron which may be as small as a needle readily sinks in water but something as big as a ship floats on water. It is interesting to know why it so happen?
According to Archimedes principle, the apparent loss in weight of a body totally or partially immersed in a liquid is equal to the weight of the liquid displaced. The centre of gravity of the body and that of the displaced liquid (centre of buoyancy) must lie in the same vertical line which is called the centre line of the body. In the case of a solid iron block, the weight of the liquid displaced is less than the weight of the block — so it sinks.
The case is different in case of an iron ship due to its special construction. Its body is shaped in such a manner that it displaces a large quantity of water, and therefore, experiences an upward thrust greater than its weight. Therefore, when it floats on water, its weight is equal to the weight of water displaced by its immersed portion. In other words, when the ship enters the water the volume of the water displaced is much greater than the volume of actual iron immersed, and as a solid it cannot displace more than its own weight of a liquid. The ship sinks only to the extent the weight of the displaced water is equal to the weight of the ship. Thus the remaining portion of the ship stays out of water.