So far the subject of ship stability has been discussed considering that the sea is calm and the ship is stationary, not moving in any mode at all. It is evident that this assumption is wrong and the motion will have to be taken into consideration somehow. It was in mid-1850s when the idea of dynamical stability was introduced in an attempt to relate the stability of a ship to its rolling motion. It is of course clear that most ships which capsize do so after rolling excessively and indeed the capsizing itself is a kind of rolling motion. Therefore, it is quite right that we should relate the rolling motion to stability, although other modes of motion may be ignored for the time being while we consider vessels in their intact condition.