Rolling of Ships

The

total propelling efficiency or propulsive coefficient is the ratio of the effective horsepower (RV) to the indicated or—in turbine driven ships—the shaft horse power. Its value is gen erally of the order of 0.5. In some recent ships devices such as the contra-propeller have been fitted. In essence this consists of an addition to the hull in front of, or abaft, the propeller, of blades to guide the water to, or away from, the propeller; thus eliminating the rotary motion in the race. Some gain in efficiency is obtained in certain cases, particularly for single screw vessels. Such gain in efficiency is wholly or partially offset by the in creased resistance due to the fixed blades themselves and the de vice is probably of greater value in the case of propellers of large slip and consequently low efficiency.

Cavitation.

The phenomenon of cavitation arises when there is interference with the natural flow of water to the screw. It is accompanied by excessive slip and reduction in thrust and con sequently very much reduced efficiency. The diminution of pres sure accompanying the acceleration of the water ahead of the screw may in certain circumstances be sufficient to bring about these conditions. Such circumstances arise when the depth of immersion is small and consequently the original pressure low, when tip speed is excessive and when the blade area is relatively small in relation to the thrust. Some experiments were carried out on T.B.D. "Daring" by S. W. Barnaby in 1894 for an account of which and the results obtained see Trans. I.N.A. 1897. Later experiments have shown that the circumstances referred to above all have their effect, the critical pressure causing cavitation vary ing to some extent with the type of ship and with the details of the propeller.

A ship in a seaway is subjected to variable forces, which in duce stresses in the pieces and their connections which make up the hull structure. Most of the forces depend on the action of the sea and cannot be predicted with precision ; and at the same time the hull structure is so complex that a quantitative theoret ical analysis of the stresses in the parts is in general of value only for comparative purposes. Such comparisons with the results of experience give the only real guide as to the adequacy of the strength of new ships and to the proper distribution of the mini mum quantity of material necessary.

Longitudinal Bending.

The greatest straining actions to which vessels of ordinary proportions are subject, are due to in equalities in the longitudinal distribution of the weight and the buoyancy. In the standard calculations the ship is regarded as a

beam—(1) on a trochoidal wave of length equal to that of the ship and height of the length, with the crest amidships this condition is known as "hogging." (2) On a similar wave with the trough amidships, the "sagging" condition. The ship is bal anced on the wave to satisfy the necessary elementary conditions of equilibrium and curves of buoyancy and weight per foot run plotted on the base of length. The resulting curve obtained by plotting the differences of the several ordinates of buoyancy and weight gives the curve of loads. By integrating this curve a curve of shearing force is obtained, which integrated in its turn gives the curve of bending moment. The conditions of equilibrium ensure that the end ordinates of the shearing force and bending moment curves are zero. The maximum bending moment, fre quently expressed as a ratio of the product of the ship's length and displacement, occurs near amidships.

The stresses at a transverse section due to bending are obtained from the usual beam formula P — = ; M being the bending mo y 1 ment, I the moment of inertia about the neutral axis, y the dis tance from the neutral axis and p the intensity of stress. In cal culating I, only the continuous longitudinal portions of the structure are assumed effective and a deduction from the area of material in tension is made for rivet holes. The stresses thus obtained vary considerably with the type and size of ship. It is clear that the actual straining actions on a ship necessarily vary with the type, and the stresses allowable calculated on a uniform basis of applied forces must vary accordingly. With regard to size, the larger the ship there is less likelihood of meeting waves as long as herself, and the proportion of height to length of the largest waves is generally less than that assumed. (For particu lars of waves actually observed reference may be made to a paper by Dr. Vaughan Cornish, Journal of Royal Society of Arts, 1912.) For these reasons greater calculated stresses are allowable in large ships than in small or moderate sized ships. For small ships a limiting stress of 6 tons per sq.in. is frequently adopted, with an increase to 8 tons per sq.in. for portions in tension where high tensile steel is used. For large ships the calculated stresses fre quently exceed 8 tons per sq.in. compressive and io tons per sq.in. tensile.