the vessel efficient as a fighting machine. The foregoing statement is quite as true of the humble 10-knot collier, which will carry in coal twice the weight of her hull, machinery, and outfit, as of the trans-oceanic passenger liner which carries a comparatively small amount of cargo but a large quantity of bunker coal to enable it to make a high speed, and provides luxurious_accommodations for numerous passengers. It is true also of the man-of-war, which carries a proportionately large crew and a fair amount of cargo in the shape of consumable stores, coal and ammunition, besides a heavy weight of armor, armament and ammunition, and the necessary military adjuncts required by the special service upon which employed. Considering, then, all ships as bearers of burdens, there are two essential characteristics which they must show: They must be able to go from point to point at an appropriate speed and with all reasonable safety for ship, cargo and crew. Ability to keep the sea under all the usual conditions of its intended service is indispensable for every ship, and we will now consider briefly the detailed factors entering into the problem.

Buoyancy. When a ship is entirely waterborne, the weight of water which it displaces is exactly equal to the weight of the ship itself and everything contained in it. To float at all, the volume of the enveloping surface of the ship must be greater than the volume of water which equals in weight the displacement of the ship. Clearly, for safety, there must be a margin, or reserve of buoyancy, in the ship over and above the buoyancy equal to its weight. The percentage of reserve buoyancy varies widely according to the type of vessel, passing from approximately zero in the case of diving, or submarine, boats (when in condition to dive), to as much as 100 per cent or more in the case of passenger vessels with large deck areas and high sides. In certain types of men-of-war, notably the large cruiser class, the percentage of reserve buoyancy is also very high. In the case of men-of-war, the reserve buoyancy is practically fixed by the design; but, in the case of merchant vessels, and particularly cargo carriers, which are subject to overloading, the reserve buoyancy is now practically determined by the marine insurance companies. The business of insuring ships and their cargoes is a large and important one, but is carried on by a comparatively small number of very powerful companies or associations, and these companies, for their own protection, have a well-equipped, scientific and technical staff and have prescribed conditions affecting the safety, or seaworthiness, of ships, which must be complied with in order to obtain insurance at a reasonable rate. In England, the Board of Trade, which is the department of the government charged with authority over matters relating to shipping, has acted in conjunction with the large insurance companies in laying down requirements for reserve buoyancy, these authorities being still further assisted by representatives of the national associations of naval architects. The actual amount of reserve buoyancy required varies somewhat with the size and type of vessel, and likewise with the character of the service, the maximum amount being required for vessels engaged in winter service in the North

Atlantic Ocean. Speaking broadly, the objects aimed at in the load-line requirements are to obtain the greatest possible carrying capacity, compatible with safety of the vessel under all conditions of weather, after making provision for minor casualties which might still further reduce the reserve of buoyancy.

Structural Strength.-Buoyancy, however, is not the only requirement for seaworthiness of a ship. It is essential that the ship should be strong enough to withstand the stresses due to the action of the waves. In this direction, also, the insurance companies have taken prominent position, and laid down minimum requirements acceptable for merchant vessels. Strength is obtained not only by the use of the best materials but by the disposition of the material used in such manner as to best accomplish the desired results. The methods used in determining the strength of ships are very similar to those used in determining the strength of other structures, such as bridges, but there is an essential difference due to the fact that it is practically impossible to determine the maximum load to which a ship may be subjected. It is comparatively easy to calculate the strength of the hull structure, knowing the disposition of the weights and buoyancy of the vessel. The stresses upon each part of the vessel's structure, when floating in still water, may be determined with accuracy; but the maximum stress will occur, not in still water, but among waves, and while we know that, whatever the motion of the sea, the buoyancy of the submerged portions of a ship must equal the total weight of the vessel and its contents (subject to minor corrections, due to the dynamic effect of the motions of the ship itself), it is obviously impossible to foresee the possible combinations and contours of waves to whose action the ship may be subjected and the consequent distribution of buoyant forces and structural stresses. Hence, the naval architect, in determining the structural requirements of a ship so far as they affect its strength, must be guided largely by experience. If he provides strength equal or superior to that of ships of the same type and not very dissimilar in size, which have shown no weakness in service, he has reasonable assurance that he is safe. But, when dealing with vessels of a novel type or size beyond precedent, the skill and judgment of the designer are taxed to the utmost to accurately estimate in advance and provide against the maximum stresses that may occur in service. In this respect, also, the accumulated experience of the marine insurance societies has been of great value in determining adequate strength associated with weight of structure which is not excessive, and which will permit a maximum development of carrying and other desirable qualities. The natural tendency of such societies, however, is to make sure of adequate strength, necessarily giving to weight and cost merely secondary consideration; so that whenever a novel type of ship is put forward, there is apt to be a period of discussion and unsettlement. The builders and owners naturally desire to provide only the minimum strength necessary, in their judgment, for complete seaworthiness, while, from the point of view of the insurer, if any error is made it should be positively on the side of excessive strength. As the results of experience accumulate, these questions adjust them

8 ft. 6 in.

Beam, extreme, 73 ft 6 in.

Depth, bottom

Length over all, about 630 ft. of keel to upper deck at center, 56 ft. 1 in. Upper deck to promenade deck, Promenade deck to bridge deck, 8 ft. Bridge deck to boat deck, 8 ft. Boat deck to captain's bridge, 7 ft. 8 in. Bottom of keel to upper navigating bridge, 88 ft. 3 in. Total water ballast, 5,072 tons.

Total coal capacity with reserve bunker, 6,195 tons. Total cargo capacity. excluding reserve bunker, about 30,000 tons. Total stores, 250 tons. Firstclass passengers, 253. Intermediate class passengers, 68. Steerage pas

sengers, 1,300 to 2.400.

Crew, 250.

selves, but with the rapid development in the size of ships, and the variations in types which have been witnessed during the last quarter of the 19th century, shipbuilders and the insurance societies have not been entirely in accord on such questions.

Stability. In addition to the elements of adequate buoyancy and strength, there must be considered an equally important factor in the seaworthiness of the vessel,- namely, stability. This quality is all-important in rendering the vessel safe and enabling it to resist the capsizing effect of wind and waves. The stability problems which must be solved by the naval architect are practically peculiar to his profession. Sailing vessels, to avoid capsizing, must be handled with skill, and sail must be reduced in ample time to avoid the disastrous consequences of undue wind pressure. Steamers of the present day carry practically no sail, and are liable to be capsized by the sea only. But with them, too, there is room for skill in handling with reference to the direction of the waves, etc. It is the duty of the naval architect to provide a vessel, in either case, which, when handled with ordinary skill, will be stable under all probable conditions of wind and weather. The general features of the problem of stability are comparatively simple, and may be readily understood by considering a ship floating in still water. Under these conditions, the whole weight of the ship may be regarded as concentrated and acting downward through its centre of gravity. The upward forces of buoyancy may also be regarded as concentrated into a single upward pull through an imaginary point called the metacentre. With the ship at rest, the downward pull through the centre of gravity must be exactly equal to the upward pull through the metacentre. Evidently, if the metacentre is directly below the centre of gravity, there will be unstable equilibrium, since any slight accidental deflection of the centre of gravity would result in further deflection, the forces of buoyancy and gravity acting as an upsetting couple; if, however, the centre of gravity is below the metacentre, the resultant of the downward pull through the centre of gravity and the upward pull through the metacentre produces a righting moment tending to return the ship to the upright position. The name "metacentre" is supposed to have originated from the Greek word "meta," meaning "limit" or "goal" beyond which the centre of gravity cannot pass. Possibly it would have been more logical if the metacentre had been called the centre of buoyancy, thereby causing the nomenclature of the centre of buoyant forces to correspond more exactly to that of the centre of gravity. But in naval architecture, the centre of buoyancy is the name given to the centre of gravity of the volume of water displaced by the immersed portion of the ship. The line of action of the resultant upward forces of buoyancy must obviously pass through the centre of buoyancy, and it therefore follows that the centre of buoyancy and the metacentre are always found in one vertical line. The metacentre is not a fixed point but rises and falls as the ship inclines, owing to the varying shape and proportions of the immersed portion of the hull, and in every ship there is finally found an angle of inclination at which the metacentre is found directly below the centre of gravity. For

inclinations greater than this, there is a tendency for the ship to capsize instead of right itself. The inclination at which this occurs is called the capsizing angle, and the angular range through which the vessel can be inclined without capsizing is called the "range of stability." Generally speaking, the less the freeboard (or height of side above water), the smaller the range of stability. In practice, the range of stability necessary for safety is affected somewhat by the initial metacentric height, or the distance between the centre of gravity and the metacentre when the ship is upright. The greater this distance, the greater the effort required to heel the ship, and hence the range of stability may be made less with safety. In practice, with any type of ship the range of stability can seldom be safely made less than 50°, and, in the majority of cases, should be much more. It is frequently over 100° for vessels of high freeboard. For safety alone, it is not always necessary that, initially, when in the upright position, the metacentre should be above the centre of gravity. If the metacentre is slightly below the centre of gravity, the vessel will heel over a few degrees to one side or the other until it reaches an inclination at which the metacentre rises above the centre of gravity and the vessel becomes stable again; if the freeboard of the vessel is high and the range of stability is great, such a vessel may be perfectly safe. Several of the large trans-Atlantic liners are purposely designed with comparatively small initial metacentric height, as such a condition permits the vessel to respond less quickly to wave action, and causes easy and slow rolling.

Rolling. Closely associated with the question 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 location of the centre of buoyancy so that the metacentre shifts to one side or the other of a vertical line through the centre of gravity, causing a tendency to heel or roll the vessel until the metacentre 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 equilibrium. An analysis of the theory of the rolling 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 the centre of the cylinder. 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 pivots at the height of the metacentre. In such an imaginary 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 oscillation, and the further the metacentre from the centre of gravity, the shorter the period of oscillation. In actual ships floating in water, however, the question is complicated by the varying position of the metacentre and the resistance of the water, which, in the absence of new disturbing 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 compared with the vessel, the vessel of great metacentric height tends to float like a board, keeping its deck fairly parallel to the surface of the water; while the vessel of small metacentric height will at times be found rolling toward the wave crest instead of away from it, a very undesirable condition with low free-board vessels. In practice, vessels vary widely in their periods of oscillation. For a large vessel, perhaps 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 22 to 3 seconds, while a large vessel of small metacentric height may take as much as 20 seconds to the single roll. While rolling through small angles, say under 10°, 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° or more. If there did not exist a retardation of roll in heavy rolling there would be grave danger of vessels, otherwise perfectly safe and seaworthy, being capsized by an accumulation of roll, every passing wave adding a little to the amplitude of roll,well illustrated by the fact that with properlytimed 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 produce complete lack of synchronism between the period of the ship and that of the waves. The naval architect, however, in the original design of the vessel utilizes the resistance of the water and provides "bilge" or "rolling keels," which aid materially in preventing heavy rolling. Bilge keels are projections at the bilge of the ship, approximately 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 conditions, to less than half what it would be without them.

Speed and Resistance of Ships.-It has already been pointed out that an essential characteristic of all ships is mobility. The speed of a ship is a simple, concrete fact, readily appreciated by anyone and comparable with the speed of other ships; therefore, in many cases, it is considered the most conspicuous and important 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 comparatively recent development and largely due to the late William Froude, who, through an elaborate series of experiments, established the truth of the funda

mental laws upon which are based the present theories of the resistance of ships. The resistance of a given ship, moving 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 exposed and the speed of the ship. It varies slightly with variation of form, due to this variation affecting the velocity of the water over the hull, but this variation is too slight to be taken account of in practice. The second element of resistance is what is called "wave-making resistance," due to the fact that a ship in moving through water produces waves and the force required to produce these waves proportionately reduces the power available for propulsion and thus, in effect, increases the resistance to the motion of the ship. The third element is what is called "eddy 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 accuracy from the results of experiments upon the friction of plane surfaces drawn through water at known speeds. Mr. Froude demonstrated that the remaining resistances (wave and eddy making) of a fullsized ship could be estimated with great accuracy from a careful determination 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 4 knots. By model experiments, also, it is comparatively 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. Froude in passing from one fullsized 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 countries. 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 electrically-actuated carriage which spans the basin. When erected in 1899, this was the largest experimental basin in the world. Later experimental 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 establish, from actual trials, the relationship between this effective horse-power and the indicated horsepower which the ship's engines must exert.