Engineering Wonders of the World
Volume I

DESIGNING A SHIP. 353 understood by reference to Figs. 4 and 5. Suppose a vessel to be floating upright at ox ...x water-line wl, the up- Stabihty. 1 ward force due to the dis- placement acts at the centre of buoyancy B1. Now suppose the vessel to be inclined through a small angle ws w1 (Fig. 4), the centre of buoyancy will move to b, and the vertical, through, b, will cut the centre- line at m. If the centre of gravity, g, of the weight of the ship and cargo is below m, as in Fig. 4, a righting moment will be brought into play tending to make the vessel return to the upright—that is, the vessel is stable. Should, however, the point g be above m (Fig. 5), the moment is an upsetting one, tending to heel or incline the ship still further. The point m is termed the “ metacentre,” and the distance g m the “ metacentric height,” the latter being commonly referred to as a measure of the stability possessed by the ship. The metacentric height can be in- creased by adding to the vessel’s breadth, and therefore raising the point m, owing to b moving further out for a given inclination; or by lowering the centre of gravity g, either by reducing the weight of the upper portion of the ship and cargo or by adding weight in the bottom. Care must be taken, in choosing a metacentric height, that a good “range ” of stability—that is, maximum inclination at which the vessel remains stable—is secured. In this respect a good freeboard is very bene- ficial. Battleships require relatively larger metacentric heights in order that they may be stable with different compartments flooded, as might be the case when damaged by the ene- my’s fire, and it is for this reason that such a large breadth, is adopted in this class of vessel. Coming now to the problem of strength, the sizes or scantlings of the Strength. . , various parts or a ship s struc- ture are largely the outcome of practical experience, which has been embodied in the rules of the great registration Societies— (1,408) Lloyd’s, Bureau Veritas, British. Corpora- tion, and other kindred institutions. Wood and iron, as already indicated, have of late years been superseded almost entirely by mild steel as the material for the structural portions. More recently a stronger variety of steel, known as high-tensile steel, of re- duced scantling, has been employed in the con- struction of the upper portions of high-speed vessels, such as torpedo-boat destroyers, where economy in weight is of great importance. Considered in the direction of length, the ship may be compared to a girder for which the supporting forces are not directly under- neath. the loads, especially when the vessel is among waves. Consequently, to secure ade- quate longitudinal strength without unduly increasing the weight of the structure, a suitable depth in relation to the length of the ship must be provided. All continuous longi- tudinal materials—such as the shell plating, deok plating, tank top or inner bottom, stringers, etc.—are considered to form part of the “equivalent girder.” For purposes of com- Fig. 6.—HALF SECTION OF CARGO STEAMER WITH TRANSVERSE FRAMING.’ parison, stresses are calculated assuming the vessel to be (1) supported on the crest of a wave at the middle of her length with her 23