Page:The New International Encyclopædia 1st ed. v. 18.djvu/129

* SHIPBUILDING. 99 SHIPBUILDING. and therefore its angular velocity, the best means so far devised is the bilge keel (q.v. ), or 'rolling chock.' Horizontal, tliwartship water cluinibers 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 ol)jectionable that they have not been adopted. Vessels are designed to have a certain met.acentric 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, vessels must not only be siillieiently 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 gieater. 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 be capable of being closed in rough seas. Comfort and health require that the sides of the ship, and particularly the bow, 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 hull 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 (c) 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 dilTerence in resist- ance between a smooth and a rough bottom is versr 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-inaking resistance is not usually impor- tant in well-designed ships, and ought not to exceed about 8 per cent, of the frictional re- sistance. Eddies are chiefly to be 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, riidder. 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 cliaracter 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 stem 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 (lircrjiciit 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, and, 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, but if a wave hollow exists there the Avave-making resistance is increased. Some of the variations in power required to drive vessels at different speeds may be due to this cause. A study of the behavior of models and of full- sized ships of different designs and under dif- ferent conditions has shown that for every de- sign there is a certain critical speed below which wave-making resistance increases quite regu- larly and moderately, but beyond which it in- creases with great rapidity. It is further shown that the greater the length of the entrance and the run the higher is this critical limiting speed. It was at one time supposed that of two designs of equal length and displacement that with the least midship section would give the least resistance, but experiment has shown that this is not necessarily the case. If two designs of equal length and displacement are tested, ono having fair lengths nf entrance and run and con- siderable length of parallel middle body, andt the other having no parallel middle body and a much greater beam, but tapering from the mid- ship section to the bow and stern, the latter will have the higher limiting speed. Ships, however.