Rolling.— Closely associated with the ques tion of stability is the question of rolling in a seaway. When a vessel is floating in disturbed water the effect is to change the relative loca tion of the centre of buoyancy so that the meta centre shifts to one side or the other of a verti cal line through the centre of gravity, causing a tendency to heel or roll the vessel until the mctacentre again becomes immediately above the centre of gravity. Moreover, by this time the vessel has acquired certain angular velocity so that it swings beyond the position of equi librium. An analysis of the theory.of the roll ing of ships at sea would be too complicated to be instructive in an article of this character, but, as in the case of stability, there are certain broad, underlying principles. These would be comparatively simple if in a floating body of ship-shape form the metacentre were fixed. In the case of a floating circular cylinder, such a condition does exist, the metacentre being fixed and remaining always at hte centre of the cyl inder. In such a case, the motion of a ship rolling is very closely analogous to what It would be if the vessel were suspended on *ma at the height of the metacentre. In such an im aginary case, in conformity with the well-known principles' covering the motion of compound pendulums, the closer the metacentre is to the centre of gravity the longer the period of (met latino, and the further the mesacentre from the centre of gravity the shorter the period of oscil lation. In actual ships floating in water; how ever, the question is complicated by the vary ing position of the metaoentre and the resistance of the water, which, in the absence of new dis turbing causes, rapidly' brings rolling ships to rest. But the fact remains that vessels of large metacentric height are inclined to roll very quickly, while those of small metacentric height are sluggish in their rolling motion. When floating among waves which are large as com pared with the vessel, the vessel of great meta centric height tends to float like a board, keeping its deck fairly parallel to the surface of the water; while the vessel of small metacentrtc height wilt at times be found milling toward the wave crest instead of awap from it,— a very undesirable condition with low free-board ves sels. In practice, vessels vary widely in their periods of oscillation. For a large vessel, per haps the shortest period met with in practice would be that of a low-freeboard monitor, which, on account of its large metacentric height, may make a single roll from extreme inclination in one direction to the extreme in the other in from two and one-half to three seconds, while a large vessel of small metaceu tric height may take as much as 20 seconds to the single roll. While rolling through small angles, say under 10 degrees, the motion of a vessel is practically isochronous, that is to say, the period or time of completing a roll varies but little with the angle. This ceases to be true when vessels reach large angles of roll, say 30 degrees or more. If there did not exist a re tardation of roll in heavy rolling there would be grave danger of vessels, otherwise perfectly safe and seaworthy, being capsized by an ac cumulation of roll, every passing wave adding a little to the amplitude of roll,— well illustrated by the fact that with properly-timed impulses comparatively small forces will give large oscillations to a swinging weight. In actual practice, the skilled seaman can do much to limit excessive rolling by shaping the course of the vessel so as to pr-'duce complete lack of synchronism between the period of the ship and that of the waves. The naval architect, how Fver, in the original design of the vessel util izes the resistance of the water and provides "bilge" or trolling keels," which aid materially in preventing heavy rolling. Bilge keels are projections at the bilge of the ship, approxi mately from one foot to three feet in depth, and extending usually for about half the length of the vessel and so situated when practicable as to offer maximum resistance to rolling. When properly fitted, bilge keels will often reduce the maximum angle of rolling, under adverse con ditions, to less than half what it would be with out them.
Speed and Resistance of Ships.— It has already been pointed out that an essential char acteristic of all ships is mobility. The speed of a ship is a simple, concrete fact, readily appreciated by comparable with the speed of other ships; therefore,.in many cases, it is considered the most conspicuous and irn. portant quality of a ship, whether man-of-war or passenger steamer. The keen interest taken
by the general public in the speed records of passenger steamers engaged in trans-oceanic service fully illustrates this fact. The present accepted methods of determining the power necessary to drive a given ship at a given speed, and conversely, the form of ship best adapted to be driven by a given power, are of rpm paratively recent development and largely due to the late William Fronde, who, through an of experiments, established the truth of the fundamental laws upon which are based the present theories of the resistance of ships. The resistance of a given ship, mov ing at a given speed, is made up of three main factors: first, the skin friction of the water on the surface of the ship. This is dependent only upon the surface ex and the speed of the ship. It varies fitly with variation of form, due to this variation affecting the velocity of the water over the hull, but this variation is top slight to be taken account of in practice. The second element of resistance is what is called uwave-making resistance," due to the fact that a ship in moving through water pro duces waves and the force required to produce these waves proportionately reduces the power available for propulsion and thus, in effect, in creases the resistance to the motion of the ship. The third clement is what is called teddy making?' due to eddies of the water behind square corners of the hull and attachments, such as stern-post, propeller strut, etc. The eddy making resistance is, however, comparatively small. The skin frictional resistance of a ship can be readily calculated with sufficient accu racy from the results of experiments upon the friction of plane surfaces drawn through water at known speeds. Mr. Fronde demonstrated that the remaining resistances (wave and eddy making) of a full-sized ship could be estimated with great accuracy from a careful determina tion of similar resistances experienced by a small model of a ship when towed at a speed corresponding to the desired speed of the ship, the corresponding speeds of model and ship being in the ratio of the square roots of their linear dimensions. For a ship 500 feet in length, and a small model 20 feet long. the ratio of linear dimensions is 25; so that the actual speed of the model corresponding to 20 knots for the ship, would be 20 ÷V25, or four knots. By model experiments, also, it is com paratively easy to investigate the general effect of changes in shape and dimensions of vessels without having recourse to experiments with full-sized ships. The principles applied in passing from models to full-sized ships were also applied by Mr. Fronde in passing from one full-sized ship to another,—being quite applicable if the two ships are similar, and applicable with fair approximation if the two ships are reasonably similar in proportions and shape.
Model Basins.— Experimental model basins are now found in nearly all shipbuilding cc ratries. That of the United States is located at Washington. It is about 500 feet long, and, at its maximum section, the water is about 42 feet wide and 14 feet deep. Wooden models 20 feet long, made by special machinery, are used in this experimental work, the model being towed back and forth through the water by an elec trically-actuated carriage which spans the basin. When erected in 1899, this was the largest ex perimental basin in the world. Later experi mental basins built in Germany, however, are somewhat longer but not so deep or wide. From data obtained with models towed in the experimental- basin, the effective horse power, as it is called, necessary to tow the full-sized ship without engines, is determined with great accuracy. It is therefore necessary to estab lish, from actual trials, the be tween this effective horse power and the Indi cated horse power which the ship's engines must exert. This ratio depends upon the fric tion of machinery, efficiency of propellers and to some extent upon the shape of the stern of the ship and other minor factors; it is found, in practice, that it ranges from .50 to .60, although there is seldom reason why it should not be made as great as .55,, a lower value being usually due to mistakes in design of hull, unsuitable propellers or some such cause. The tabulated data obtained from ex periments with models in the experimental basin, supplemented by progressive trial data taken tinder actual seagoing conditions, from the full-sized ships, have in recent years greatly aided the naval architect and the engineer in their design work.