Since the righting moment is the force which makes a ship roll, it is evident that if this force - is powerful the ship will roll quickly and per ' haps deeply, neither of which is desirable. To reduce the time of rolling (i.e. the still-water ' period) the metacentric height may be reduced ; as much as is consistent with safety. or the weights in the ship may he moved away from ; the midship plane if practicable, at the same time preserving the same height of centre of gravity. To reduce the amplitude of the roll, If the ship is made to roll, the position of the centre of buoyancy will be displaced, as shown in Fig. 3. We have then a force acting vertically upward at B', and a force working vertically downward at G', producing a couple and therefore its angular velocity, the best means so far devised is the bilge keel (q.v.), or 'rolling chock.' Horizontal. thwartship water chambers with a central dam, or several dams, and partly filled with water, are useful to re duce small angles of roll, but the noise and shock of the moving water is so objectionable that they have not been Vessels are designed to have a. certain inetacentrie height under various conditions of loading; and the stowage of cargo should, as far as possible, be so arranged as to give proper value to the righting moment. Vessels with double bottoms may, within small limits. vary their righting moments by filling or emptying double-bottom compartments.
To secure seaworthiness, vessel, must not only be sullieiently stable at all moderate angles of roll, but they must be stable at all possible angles. The range of stability is independent of the force of the righting moment and varies in different classes of ships. In battleships and large vessels it usually reaches 70 degrees of inclination on each side of the vertical; for small vessels, such as torpedo boats, the range is usually greater. Seaworthiness further re quires a reserve of buoyancy—that is, only part of the hull below the upper deck must be sub merged, and the openings in the hull must he capable of being closed in rough seas. Comfort and health require that the sides of the ship, and particularly the how, should be high above the water; without high sides a vessel can be kept at sea for a short period only.
The second part of the subject relates to ef ficient propulsion and manoeuvring power. In this we must consider the shape and smoothness of the 1m11 as regards resistance to its move ment through the water. The total resistance is made up of three parts: (a) Frictional re sistance: (b) eddy-making resistance; and (e) wave-making. resistance.
Frictional resistance is due to friction be tween the water and the hull of the ship. It does not depend upon the shape of the hull to any appreciable extent. but upon its smoothness. the area of the wetted surface, the length of the ship. and the speed. It forms the greater part of the total resistance of a ship moving at low speeds and an important part of it at all speeds, particularly if the bottom is rough or foul. For any given ship it varies about as the square of the speed. The difference in resist ance between a smooth and a rough bottom is very great. A smoothly painted bottom has only half that of one of the roughness of fine sandpaper. and only about a third of that of
coarse sandpaper. The difference in the power required to drive a ship when her bottom is foul and when her bottom is clean is then very easily appreciated.
Eddy-making resistanee is not. usually impor tant in well-designed ships, and ought not to exceed about S per cent. of the frictional re sistance. Eddies are chiefly to he found at the stern, where the water rushes in behind the ship.
If the run is long and fine, the speed moderate, and the propeller struts. rudder, etc., well de signed, they are scarcely noticeable: but a ship with too short a. run, badly designed rudder, propeller struts. etc., leaves at full speed a boil ing, troubled, eddying wake behind her. Wave-making is in many respects the most im portant part of the resistance of ships, for it. is the one over which we have the most control, and which is the greatest impediment to high speed. The laws which govern it are not yet fully understood, but the character of the waves and the losses entailed by them have been very carefully examined. A ship moving through un disturbed water puts certain particles of it in motion, carrying some along with her by fric tion and giving motion to others in such a way as to cause them to rise in waves. All the en ergy taken up by the water must come from the propelling machinery. and if it is not returned to the ship in pushing her ahead it is wasted.
The 'entrance' of a ship is the tapered fore body which extends from the stein to the point where her hull has obtained the full dimensions of the midship (or maximum) section; and the run. is the corresponding tapered portion of the after body. These two parts of a vessel are termed the wave-making features, because the movements of the particles of water forming waves depend upon their lengths and shapes. A vessel passing through undisturbed water forms a double series of waves at the bow and at the stern. The former are most readily seen, largely because the action of the screw tends to degrade and confuse those at the stern. One set of waves are called dircrgcnt because their crests make an angle of 40 to 50 degrees with the keel: the other waves are called transverse because their crests are perpendicular to the keel line of the ship. Both sets increase in height with the speed, and this height is a measure of the energy absorbed by them, and indicates the speed with which they are travel ing. The divergent waves are thrown off, leaving the ship, no longer influence it: but the transverse waves move at the same speed as the ship and keep their crests and hollows at about the same points on her sides so long as the speed is constant. Furthermore, the length be tween crests is the same as between the crests of ocean waves moving at the same rate of speed. It is found that if a wave crest is main tained at about the middle of the run the wave making is decreased, hut if a wave hollow exists there the wave-making resistance is increased. Some of the variations in power required to drive vessels at different speeds may be due to this cause.