Structural 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 a prominent position, and laid down minimum requirements acceptable for merchant vessels. Strength is obtained not only by the use of the best mate rials but by the disposition of the material used in such manner as to best accomplish the de sired results. The methods used in determin ing the strength of ships are very similar to those used in determining the strength of other structures, such as bridges, but there is an es sential difference due to the fact that it is prac tically 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 archi tect, 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 accu rately 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 de velopment of carrying and other desirable qual ities. The natural tendency of such societies, however, is to make sure of adequate strength, necessarily giving to weight and cost merely sec ondary 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 experi ence accumulate, these questions adjust them selves, but with the rapid development in the size of ships, and the variations in types which have been witnessed since the last quarter of the 19th century, 'shipbuilders and the insur ance 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 prob lems which must be solved by the naval archi tect 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 pres ent 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 feature of the problem of stability are com paratively simple, and may be readily under stood by considering a ship floating in still water. Under these conditions, the whole weight of the ship may be regarded as concen trated 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 grav ity must be exactly equal to the upward pull through the metacentre. Evidently, if the meta centre is directly below the centre of gravity, there will be unstable equilibrium, since any slight accidental deflection of the centre of grav ity 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 down ward 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 umetacentre" is supposed to have originated from the Greek word ((meta,) meaning or ((goal) beyond which the centre of gravity cannot pass. Pos sibly it would have been more logical if the metacentre had been called the centre of buoy ancy, thereby causing the nomenclature of the centre of buoyant forces to correspond more exactly to that of the centre of gravity. But in the 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 ob viously pass through the centre of buoyancy, and it therefore follows that the centre of buoy ancy 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 inclina tion 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 in clination at which this occurs is called the cap sizing angle, and the angular range through which the vessel can be inclined without cap sizing is called the grange of stability.) Gener ally 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 metacen tre 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 degrees, and, in the majority of cases, should be much more. It is frequently over 100 degrees 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 metacen tre 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. Sev eral of the large trans-Atlantic liners are pur posely designed with comparatively small initial metacentric height, as such a condition permits the vessel to respond less quickly to wave ac tion and causes easy and slow rolling.