NAVAL ARCHITECTURE.

CHAPTER I.

THE DISPLACEMENT AND BUOYANCY OF SHIPS.

A SHIP floating at rest in still water must displace a volume of water having a weight equal to her own weight. The truth of this fundamental condition may be easil demonstrated. Let Fig. 1 represent the ship (in profile view and athwartship section), WL being the surface of the water. If it is supposed that the water surrounding the ship

becomes solidified, and that the ship is then removed, there will remain a cavity representing in form and volume the water displaced by the ship: this is termed the "volume of displacement" (or, shortly, the "displacement") of the ship, being represented in the diagrams by WKL. If the cavity is then filled up to the level of the surface WL with water of the same density as that in which the ship floated, and afterwards the surrounding water again becomes liquid, there will obviously be no disturbance or change of level in consequence of the

B

substitution of the water for the ship. Therefore the total weight of water poured into the cavity-that is, the total weight of water displaced by the ship-must equal her weight.

This fundamental law of hydrostatics applies to all floating bodies, and is equally true of wholly submerged vessels as of ships (like that in Fig. 1) of ordinary form, having only a portion of their volume immersed.

Ships which are of equal weight may differ greatly in form and dimensions, and consequently the forms of their respective displacements will differ; but when they are floating in water of the same density, the volumes must be equal to one another, because the weights of the ships are equal. On the other hand, when a ship passes from water of one density to water of another density, say from the open sea to a river where the water is comparatively fresh, her volume of displacement must change, because the weight of water displaced must be the same in both cases. Under all circumstances the volume of displacement, multiplied by the weight per unit of volume of the water in which the ship floats, must equal the weight of the ship. It is usual to express the volume in cubic feet, and for sea-water to take 64 lbs. as the weight of a cubic foot: so that the weight of the ship in tons multiplied by thirty-five gives the number of cubic feet in the volume of displacement when she floats in sea-water.

At every point on the bottom of a ship afloat, the water pressure acts perpendicularly to the bottom. This normal pressure at any point depends upon the depth of the point below the water surface; and it may be regarded as made up of three component pressures. First, a vertical pressure; second, a horizontal pressure acting athwartships; third, a horizontal pressure acting longitudinally. Over the whole surface of the bottom a similar decomposition of the normal fluid pressures may be made; but of the three sets of forces so obtained, only those acting vertically are important in a ship at rest. The horizontal components in each set must obviously be exactly balanced amongst themselves, otherwise

the ship would be set in motion, either athwartships or lengthwise. The sum of the vertical components must be balanced by the weight of the ship, which is the only other vertical force; this sum is usually termed the "buoyancy;" it equals the weight of water displaced, and the two terms "buoyancy' and "displacement" are often used interchangeably.

The total weight of a ship may be subdivided into the "weight of the hull," or structure, and the "weight of lading." The latter measures the "carrying power" of the ship, and is therefore frequently termed the "useful displacement." Useful displacement for a certain degree of immersion is simply the difference between the total displacement and the weight of the hull: so that any decrease in the weight of hull leads to an increase in the carrying power. If the ship is a merchantman, savings on the hull enable the owner either to carry more cargo in a vessel of a specified size or else to build a smaller vessel to carry a specified cargo. If the ship is a man-of-war, such savings on the hull render possible increase in the offensive or defensive powers, or in the coal supply, engine power, or speed; or else enable certain specified qualities to be obtained on smaller dimensions than would otherwise be practicable. Hence appears the necessity for careful selection of the best materials and most perfect structural arrangements, in order that the necessary strength may be secured in association with the minimum of weight. It is in this direction that all recent improvements in shipbuilding have tended, and the use of iron hulls instead of wood has greatly facilitated progress. For example, in wooden ships of war it is common to find the weight of hull equal to one-half of the total displacement; whereas in iron ships the hull weighs only 30 or 40 per cent. of the displacement. Iron merchant ships also, even of the largest size, frequently have hulls weighing only one-third of the displacement. Hereafter a description will be given of the principal structural improvements to which the savings in weight of hull are due.

Having given the draught of water to which it is pro

posed to immerse a ship, the volume of her immersed part determines the corresponding displacement, and this displacement can be calculated with exactitude from the drawings of the ship. This is the method adopted by the naval architect; but any details of the method would be out of place here. At the same time an approximate rule by which an estimate of the displacement of the ship may be rapidly made may have some value. Assuming that the length of the ship at the load-line is known (say L), also the breadth extreme (B), and the mean draught (D), the product of these three dimensions will give the volume of a parallelopipedon circumscribing the immersed portion of the ship. This may be written:

Volume of parallelopipedon =V (cubic feet) = L x B x D. The volume of displacement may then be expressed as a percentage of the volume (V) of the parallelopipedon; and for the undermentioned classes of ships, the following rules hold:

Classes of Ships.

1. Fast steamships, such as her Majesty's yachts or the Holyhead packets

2. Swift steam-cruisers of Royal Navy (Inconstant and Volage classes); corvettes and sloops. 3. Gun-vessels of Royal Navy; merchant steamers (common forms).

4. Old classes of unarmoured steam line-of-battle

ships and frigates in Royal Navy

5. Early types of ironclads in Royal Navy (Warrior and Minotaur classes)

6. Modern types of rigged ironclads, with moderate proportions of length to breadth

7. Mastless sea-going ironclads (Devastation class); cargo-carrying steamers of moderate speed.

Displacement equal to Percentage of Volume (V).

43 to 46 per cent.

46 to 52 per cent.

55 to 60 per cent.

50 to 55 per cent.

55 per cent.

60 to 62 per cent.

65 to 70 per cent.

This table may be found serviceable in rapidly approximating to the displacement of a ship for which the principal dimensions are known; although it makes no pretensions to completeness or exactness. Taken in connection with