XVI. On the unequal distribution of weight and support in ships, and its effects in still water, in waves, and in exceptional positions on shore

Abstract

The object of the present paper is to bring within the grasp of calculation a much neglected division of ship building scienceand art. Many writers of great ability (French, Spanish, Dutch, Swedish, and English) have studied and explained the forces brought into action upon a ship by her own weight and stability, and by the action of the wind upon her sails and of the waves upon her hull; and the result of their investigations has been to encourage the construction of ships of such forms and such dispositions of weights as conduce to moderate and easy motions in the waves of the sea. The relative positions of the centre of gravity and the metacentre, the excursions of the centres of gravity and buoyancy, the inclinations of the axis of rotation, and many other like questions have been very fully and thoroughly discussed, especially by modern English naval architects; in some cases, I venture to say, with even more elaboration and minuteness of inquiry than their intrinsic importance demands. But while the means of securing ease and moderation of movement of the ship at sea have been thus elaborately studied, in order, mainly, as we have been told, to save the fabric of the ship and its fastenings from excessive strains, comparatively few writers upon naval architecture have pursued the subject to its legitimate and necessary development, by seeking to investigate the actual longitudinal bending- and shearing-strains to which the fabric is in fact exposed in ships of various forms under the various circumstances to which every ship is liable. But more than this: not only has the question of internal strain and strength in the ship been left undeveloped, but a serious fallacy has underlain many of the writings even of men of the greatest eminence upon this subject, viz. the fallacy of considering ease of motion identical with moderation of strain . No doubt ease of motion is very desirable in all ships, and violence of motion tends to distress any given fabric; but at the same time it is quite practicable, as will clearly appear hereafter, to so design and build two ships, that in a sea-way the easier of the two shall be the more distressed even with precisely the same structural arrangements, and therefore it is obviously very desirable to examine the actual strains, both static and dynamic, with which we have to deal. The weakness exhibited by many ships, notwithstanding the greatest care on the part of the designers, has long pointed to the necessity of further investigation in this direction; but two modern events—the introduction of armoured ships, and the use of iron and steel in shipbuilding—have added much to the urgency of the inquiry. A long armoured ship, say, like the ‘Minotaur’ or ‘Agincourt’ (400 feet in length, and with fine tapering extremities burdened with towering masses of armour), when pitching in Atlantic waves, undergoes a succession of stresses of great magnitude, undoubtedly requiring to be brought as much as possible within the grasp of calculation, the more so as these stresses undergo continual changes, sweeping through the fabric, so to speak, with prodigious velocity. The employment of iron and steel, and the improvements which the manufacture of both is undergoing, fortunately facilitate the concentration of the strength of the ship in those parts which are subject to the greatest stresses; and to further this object, a closer knowledge of these stresses than has hitherto been possessed is much needed.