So long as ships depended upon sails for pro pulsion shipbuilding remained a mechanical art bound by rules, traditions, and dogmas which were the result of centuries of experience. But with the advent of steam came the general sci entific awakening and shipbuilding received its due share of attention. Its theoretical side has been given the name of noral architecture.
For convenience we may divide the subject into three principal parts, viz.: (1) Design as it affects the buoyancy, stability, steadiness, sea worthiness, etc. (2) Design as it affects the efficient propulsion and mameuvring powers. (3) Design as regards the strength, habitability. and general structural The various qualities of a ship here mentioned are more or less interdependent, but it is possible to con sider each separately and examine the effects of variation of form or structure which differ ent requirements entail.
A vessel floating freely in still water dis places a volume of water equal in weight to its own, and the weight is called the vessel's displacement. This weight is supported by the pressure of water which acts at all points per pendicular to the surface of the ship's bot tom: but the sum of the vertical components of the water-pressure at all points must balance the weight of the ship, and this sum is termed the buoyancy. The total weight of a fully loaded ship may be divided into the weight of /tall and weight of lading. The latter repre sents her carrying power or useful displace ment, and it. is of course desirable to make this as large as possible (compared to the weight of the hull), being consistent with other necessary requirements. The reduction in hull weight is the principal cause of the substitution of iron for wood in shipbuilding., and, in turn, the dis placing of iron by steel.
In considering ships of different forms it is useful to know something definite concerning their shapes without exhaustive examination, and this is arrived at by comparing them with the parallelepipedon. which has dimensions equal to the length (L), breadth (B), and mean draught (Al) of the ship. If v = the volume of the ship, and V the volume of the parallel epipedon, we have =C = coefficient of fine ness of the ship. If d and D are the corre
sponding displacements (i.e. weights) in tons since 35 cubic feet of sea-water weigh a ton, d y C= D This formula takes no account of the shape of the midship section of the ship, in which there is considerable difference in vessels of the various types. A bluff vessel might have a high rise of floor, and a fine-ended ship a nearly rectangular midship section, and yet the co efficient of fineness be the same. To obviate this uncertainty the prismatic coefficient is used. In this case the volume of the ship is compared to the Yolunie of a prism, whose length is the length of the ship, but whose base is the mid ship section of the ship. If the area of the mid ship section is A, we have prismatic coetlicient, or coefficient of water-lines as it is commonly d X called = C' — X In modern steamships the midship section closely approaches a rectangle, and the ordinary coefficient of fineness suffices. For steamers of exceptionally fine form (particularly those with no parallel midship body), the coefficient is from 40 to 50 per cent.: in large fast steamers, 45 to 53 per in recent battleships, 55 to 65 per cent.; in low-speed cargo steamers, 65 to 78 per cent. The coetlicient of water-lines is greater and varies from about 55 to 33 per cent. in value.
In referring to the displacement of a ship it is necessary to specify sonic particular condi tion, as, of course, the displacement varies with loading. With amen-of-war the condition commonly used is that of normal, or mean load draught. That is supposed to he the average cruising condition, but is usually somewhat less. The deep load condition for a man-of-war is when her full supply of stores are on board and her coal hunkers are full. For merchant ships, displacement is only beginning to he used, and it is generally given for a light load con dition—when the ship is practically empty—or when she is immersed to her Plimsoll mark (see LOAD-LINE) ; it may also be given for a specific mean draught of water. The tonnage of ships is a measure of capacity for cargo, and is fully treated in the article on the MEASURE MENT OF SHIPS.