A ship at sea is subject to rolling and pitching and must be designed to be stable and not capsize. Rolling is the motion of a ship from side to side. Pitching is the alternate rising and falling of bow and stern. In general a ship's motion is a combination of rolling and pitching. The principle of hydrostatics (water pressure) governing the stability is as follows: When a ship is floating at rest its center of gravity and its center of buoyancy are in the same vertical line. If the force of winds or waves causes the vessel to keel over as in Fig. 51, the weight of the ship W acting downward through G, and her buoyancy acting upward through B constitute a couple which tends to pull the ship back again upon an even keel with a turning moment equal to W X GP. If the couple be strong enough the ship will swing back towards an even keel. But since the vessel acquires kinetic energy as it swings, it will not stop on the even keel, but will roll some distance the other way, and will continue to oscillate about its mean position for some time.

* A couple is composed of two equal parallel forces acting on the ends of a bar, for example, in opposite directions; so far as producing forward or backward motion is concerned their resultant is zero. They do, however, tend to cause the bar to rotate.