Stability

STABILITY.

It has been said that the weight of water which a yacht or vessel of any kind displaces is equal to her own weight, and consequently the pressure of the water on the immersed vessel is equal to her own weight. This pressure is diffused all over the immersed part of the hull, and the pressure of any individual particle of water on the hull is in a direction at right angles to the point of contact. The concentrated pressure, or the resultant of the pressure, on the immersed portion of the hull, acts vertically through the centre of buoyancy ; and, as before said, this pressure is equal to the weight of the ship. Thus there are two equal forces acting in opposition to each other, and balancing each other—the weight of the displaced water pushing upwards through the centre of buoyancy, and the weight of the ship pushing downwards through its centre of gravity ; and these two forces never act in any other than a vertical direction.

The centre of gravity of a yacht or ship is a determinable point, where the action of all her weights is concentrated; therefore it is sometimes called, in relation to ships, the "centre of gravity of the whole mass." The " whole mass " includes the hull, ballast, spars, sails, fittings, crew, stores, and everything which the ship or yacht contains that is of any weight at all. If on a plank A (Fig. 3) a number of weights, a a a a, &c.

be placed at any irregular or equal intervals, and the plank be made to balance on a pointed stake at E, then E will be the common centre of gravity of the plank and all the weights placed upon it. Thus the exact position of the centre of gravity of a ship depends upon the disposition of her weights—no matter whether these weights be timbers, keel, plank, ballast, spars, rigging, sails, crew, stores, or anything else that is of weight—and it follows in a ship that, if the weights are placed further forward, the centre of gravity will be shifted forward, and the contrary if the weights be shifted further aft. In a like manner, if the weight of the masts, sails, or gear be increased, the centre of gravity, with regard to its vertical position, will be brought higher ; on the other hand, if the ballast be increased in weight, or if it be stowed deeper down in the hull, the centre of gravity, with regard to its vertical position, will be carried lower.

Thus we have two distinct, but balanced, forces—the weight of the water the ship displaces acting upwards through the centre of buoyancy, k (Fig. 4), and the weight of the ship acting downwards through its centre of gravity, o.

A necessary condition for the ship to be in equilibrium is that the resultant of the two forces, represented by the weight of the ship acting through o, and the weight of the water she displaces, acting through k, should have effect in the same vertical line. (See a a, Fig. 4.)

If the direction of action of either be shifted, a struggle will instantly commence to regain a position where they will balance each other again, or act in the same vertical line. For instance, let a portion of a yacht's ballast or other weight be shifted forward until her centre of gravity is shifted from o to 8, Fig. 4, then the yacht will sink down by the head until the two forces are directly over each other again, as 8 le, in the vertical line b b, Fig. 5 (p. 8).

Now if the

centre of buoyancy had been carried to le by the vessel being hove down by the head otherwise than by having a portion of her ballast or weight moved forward, such, for instance, as by a pressure of wind on her sails, she would regain the position depicted in Fig. 4 the instant the force or pressure which had hove her down was removed. This force that brings a vessel back to her original condition of equilibrium is called her righting power, or statical stability ; and for the motion we have described would be termed her longitudinal statical stability. When a vessel is placed among waves, the centre of buoyancy is continuously carried forward or aft, as she is differently water-borne by the passing waves. A constant struggle is thereby maintained between the centre of gravity of the vessel and her centre of buoyancy to keep in the same vertical line a a, and an uneasy violent motion is acquired, whose force, to some extent, is dependent upon the urgency of the righting power. A vessel with her weights or ballast stowed low will have this righting power in a greater degree than one with her weights stowed high; and she will be relatively quick in " recovering herself." So also will be a vessel that is very full on the load water-line, and very much cut away under neath; whereas a vessel with what is known as a long body will be comparatively easy in her motions during similar wave disturbance. The pitching and scending motions of vessels form a very complex problem, and are by no means wholly dependent on the conditions just adverted to. For instance, the momentum acquired during pitching, whilst the bow is left unsupported by the water, or scending, whilst the stern is without support, may be much increased by the distribution of the weights or ballast in a fore-and-aft direction, as the radius of gyration would be thereby lengthened ; but the influence on these motions, which any par ticular condition of a vessel may have, can only be determined in a general kind of way, and are not amenable to any precise calculation.