Page:The New International Encyclopædia 1st ed. v. 18.djvu/127

* SHIPBUILDING. 97 SHIPBUILDING. general. In the reign of Henry VII. ship con- struction was much improved and ships began to take on iiuuh of the form which they have preserved to the present day. During the next four centuries improvements of design and con- struction were continuously made until the wooden sailing ship reached its culminating point in the clipper ship of the nineteenth cen- tury. 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 naral 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 ellieient propulsion and manwuvring powers. (3) Design as regards the strength, habitability, and general structural arrangement. 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 com])onents of the water-pressure at all points must balance the weight of the ship, and this sum is termed the ttiioynneii. The total weight of a fully loaded ship may be divided into the ireight of hull and ireight of lading. The latter re])re- 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 exliaustive 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 (M) of the ship. i v ^ the volume of the ship, and V the volume of the parallel- V epipedon, we have ^ =: C = coefficient of fine- i ness of the ship. If d and D are the corre- I sponding displacements (i.e. weights) in V" tons since 35 cubic feet of sea-water weigh a ton, L C ,-v 1 rfX35 D L X B X M' 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 volume of a prism, whose lengtli 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 coefficient, or coefficient of water-lines as it is commonly called = C = . ., t ■ AX L In modern steamships the midship section closely approaches a rectangle, and the ordinary coefficient of fineness suffices. For steamers of exceptionally fine form (particularly tho.se with no parallel midship body), the coefficient is from 40 to 50 per cent. ; in large fast steamers, 45 to 55 per cent.; in recent battleships, 55 to 65 per cent. ; in low-speed cargo steamers, 65 to 78 per cent. The coefficient of water-lines is greater and varies from about 55 to 83 per cent, in value. In referring to the displacement of a ship it is necessary to specify some particular condi- tion, as, of course, the displacement varies with the loading. With men-of-war the condition commonly used is tlat of normal, or mean load draught. That is supposed to be the average cruising condition, but is usuallj' somewhat less. 'The deep load condition for a man-of-war is when her full supply of stores are on board and her coal bunkers are full. For merchant ships, displacement is only lieginning to be 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 Me.sube- MENT OF Ships. After considering the volume of displacement of ships, the next point to be examined is the shape of the volume as regards stability and steadiness. These two expressions are linked together in the minds of many people as if they were convertible terms. Instead of being so they are in a measure antithetical, as we shall presently see. When a vessel is at rest in still water it is evident that her centre of gravity and the centre of gravity of the volume of water she displaces (which is called the centre of buoyancy) must — lie in the same ver- tical line, for only in that condition will the forces of Bi Fig. 1. Bi Flo. 2. weight and buoyancy act in exactly opposite directions and ])roduce equilibrium. The rela- tive positions of these points are shown in the accompanying diagrams, in each of wdiich G is tile centre of gravity of the ship and B the centre of buovancv.