It may be observed that the keel of a ship is not horizontal, or parallel to the plane of floatation when the ship is in still water, but is made lower towards the stern than it is forwards, in order to allow greater length to the rudder, and thus increase the power of the latter in giving a direction to the ship's motion.
The keelson, e fig. 3, is placed longitudinally above the floor timbers, and immediately over the keel, and it is united to the latter by bolts which go through both, and through the floor-timbers : its transverse section is represented at e, fig. 4. In large ships two addi tional keelsons, t, t, fig. 4, about thirty feet long, are bolted to the floor-timbers sufficiently near one another that the step (foot) of the mainmast may rest upon them ; they serve to relieve the bottom of the ship from the pressure of that mast, and strengthen it against the upward action of the water. The timbers E and F, fig. 3, called the sternson and stemson, are also attached interiorly to the stern-post and stem-post, in order to increase the strength of the fabric.
The whole assemblage of rib-timbers is covered on the outside, and either wholly or partly on the inside of the ship, with planks of oak from three to six inches thick ; and in order to make the latter bend so as to lie close to the curve surface of the ribs, they are, previously to being applied, moistened by steam : the exterior planking appears in the section, fig. 4. The planks are fastened to the ribs both by bolts and trenails (plugs of oak from one to two inches diameter), which pass quite through the ship's side, and are tightened by wedges driven into them at each extremity.
When the ribs do not join closely side by side, it is recommended that, before the planking is applied, the intervals both on the exterior and interior sides of the ship should be filled up with pieces of wood, as long as the curvature of the ribs will permit; the lines of junc tion with the ribs being well caulked. Sir Robert Seppings, however, proposed that for ships of war, where there are intervals between the principal rib-timbers, there should be introduced in those intervals other ribs extending from the keel up to the orlop or lower deck, since by this construction the lower part of the ship will be one compact mass of timber. He observed that the filling up of the spaces between the ribs not only adds to the strength of the ship by causing its bottom to have a solid thickness, but it tends to preserve the health of the crew, since those openings become receptacles for dirt, by which the air within the ship is vitiated. Channels, or water-courses, may be
cut down the ribs at their interior lines of junction, and covered by planks or battens ; by these channels the water is able to descend to the limber passage along the keel, and pass to the pump-well. For merchants' ships Sir Robert Scppings recommended strakes, or courses, of thick planks to extend longitudinally through the ship along the in terior sides of all the ribs, and to cover the abuttinga of the futtock pieces in each alternate rib : such strakes appear at n, o, p, in fig. 4. lie considered that no other interior planking would be required, but he recommended that battens should be fastened over the junctions of the ribs in the vertical planes.
A ship being a vast fabric consisting of comparatively short pieces of timber connected together by scarfing, and the principal parts of the frame-work, the ribs, the longitudinal timbers, and the planks being nearly at right angles to one another, it must oinecessity happen that when the ship is not supported in its whole length and breadth, it will bend by its own weight. This will take place not only in the event of being lifted up by a wave under some place in its length, or pressed unequally by the force of a wave acting obliquely upon one bow or quarter, but even while floating in still water, from an excess of the weight in one transverse section over that in another. In this last case it has occurred, a horizontal line having been traced from head to stern by means of a spirit-level while the ship was on the stocks, that immediately upon the ship being launched the two ex tremities were observed to sink as much as three or four inches; the ends of the planks separating in the upper part of the structure, while the timbers below were in a state of compression, and the whole body of the ship becoming curved in a vertical direction. M. Dupin has shown (' Phil. Trans., 1817) that the strain is greatest at that transverse section of the ship which divides the whole length into two parts, in each of which the weight of the displaced fluid is equal to that of the corresponding part of the ship and its loading. To counteract the tendency to arch or bend was the object of Sir Robert. Seppings in the application of diagonal braces to the interior side of a ship.