Page:The American Cyclopædia (1879) Volume XIV.djvu/887

 SHIP Nil of considerations as the building of a ship. A hollow shell is to be constructed in which lightness and stability are the first requisites. If the vessel be a man-of-war, it is a nice point to determine her displacement, or the entire weight of the structure itself with all that she carries of spars, armament, men, supplies, &c., that from this her depth in the water may be known, and the line of her lower ports be fixed so high as not to be washed into in time of action. The form is to be specially suited for easy and rapid progress, and at the same time must be adapted to resist the severest strains, caused not merely by the weight of the struc- ture and of its load, but by the shock of the waves, and their constantly varying figure, the effect of which is to continually change the places of support, and throw large portions of the weight first upon one point and then upon another. It has often been observed that after a vessel has left the stocks upon which she was put together, and lies upon still water, a line that had previously been drawn straight along her top side from stem to stern is deflected several inches by the set- tling of the ends, which is owing to a want of precision and strength in the work to meet the inequality of the weights on the different transverse sections. The effect is to separate to some extent the planks and connecting pieces at the top, and compress those in the bottom of the structure. When the ship en- ters rough water, she is at one moment sup- ported at the two extremities like a bridge, and the great weight bears down the middle, threatening to bend the whole structure and produce the effect called sagging; the next instant her bow and stern hang unsupported over the great wave which bears up the ship across her centre, and the two ends tend to droop ; the latter change of form is called hogging. If the ship was thus affected when first launched, it is obvious that the distortion must increase as she works in a heavy sea, and that her timbers and fastenings must be great- ly weakened by the motion. In various other ways the strength of her framing is severely tried. Driven obliquely across the waves, she is lifted high upon their summits, and at any moment is dashed into the trough against the next coming swell, the force of which she receives upon her bow, side, or quarter, with a shock that quivers through every timber. When following too nearly the line of the waves, she is rolled violently from side to side, and the great weight and long purchase of the heavy yards and masts act with fearful power to strain the sides, to which they are fastened by the shrouds and stays. Again, when mov- ing directly across the waves, each end is in turn elevated and depressed. In all these movements the force of the strain is told by the creaking of the timbers. The structure is put to still severer tests when the ship touches an uneven bottom, and the weight is supported by a few points upon a hard un- yielding surface. Then, beaten by the waves, raised up and dashed down again by them, her frame is most perfect if she is not soon parted and broken up. Indeed, the only vessels ever known to come off from a rocky exposed coast after remaining aground for a considerable time were iron ones, as the Great Britain, which lay a whole winter on the coast,of Ire- land, and the Vanguard, which was for several days on a rocky beach. The strength of ships, like that of roofs and bridges of long span, depends on the skilful arrangement and fitting of the timbers, so that they shall take the strains they are to meet to the best advantage, as well as on the bolts and fastenings by which they are held in their places. The keel is the foundation or backbone upon which the whole structure is built up. It receives the great upright timbers of the stem and stern, and those called floor timbers that support the ribs, which give form to the sides. The deck beams at different stages, securely fastened at their ends to opposite ribs, hold these to- gether against any spread of the sides or lat- eral hogging, and also act as struts to prevent collapsing of the sides. Curvature on the length of the ship is guarded against by the planking on the ribs and that of the decks, the planks being laid longitudinally and strongly bolted down to the timbers. In northern Eu- rope since the middle of the last century a sys- tem of trussing has been introduced for greater security in this respect. Three parallel rows of pillars were set up extending from one end of the ship to the other, one row on the keel- son, and one each side on timbers laid for the purpose and bolted to the ribs. On the top of the pillars of each row and directly under the lower deck was secured a longitudinal tim- ber like an architrave; and diagonal braces extended from the top of one pillar to the foot of the next in the same row. By such arrange- ment the stiffness was materially increased, but at the expense of stowage room, and the trussing was not altogether secure of remain- ing in place in the violent movements of the ship. A much superior method was introduced in 1810 by Sir Kobert Seppings, surveyor of the navy, which is known as the diagonal bra- cing. This was formed of a system of timbers crossing the ribs on the inside of the ship at angles of about 45, and braced by diagonals or struts. This framing started below at the keelson or horizontal timbers at its side, t< which it was strapped down, and terminated above under the horizontal shelf which sup- ported the ends of the cross beams under the lower deck. The shelf was thus braced up and supported ; and in large ships the second hon zontal shelf was likewise sustained by a coi tinuation of the diagonal bracing above t lower deck. These shelves secured to sides of the ship are always provided for support of the deck beams, and serve t selves to stiffen the structure in their acti like internal hoops. In place of this method