SHIPBUILDING. In merchantmen, the primary consideration is, to attain the greatest capacity to carry cargo, com bined, as far as possible, with safe and easy movements, and rapid sailing. In this way American builders have suc ceeded in uniting conflicting desiderata in a degree heretofore deemed impossible. Our packet ships carry enormously, while their extreme speed has reduced, by half, the passage to Europe.
The greatest breadth must always be before the centre, and consequently, the bow be more blunt than the stern. The best builders place this point only one third of the length from the stem. Ex perience proves that it is essential to fa, eilitate the escape of the displaced water along the side of the vessel the for, when once a passage is opened for the ship, the fluid tends to re-unite abaft the point of greatest breadth, where, instead of offer ing resistance, it presses the ship forward in its endeavor to recover its level and fill the vacuum constantly opening be hind her. A log tows infinitely easier by its bigger end ; and we find a concurrent testimony in the forms of the finny tribes which divide the element they move in, by a shape gradually diminishing from head to tail. There is a further advan tage in having the bow full towards the edge, that it may check descending into the waves, not abruptly, but gently pitching being the most dangerous to hull and spars of all movements. Sharpness towards the sternpost is vitally essential to fast sailing. Stability increases as the cubes of the breadth ; hence, by adding one quarter to the breadth, you gain a double stability, and, by consequence, a capacity to bear twice as muoh sail, with but one fourth of increase in the resist ance. The pressure of the water increases in descending from the surface, and, from this cause and the augmented difficulty of displacing it, the resistance offered to a ship, in advancing, is three times as great at the lower as at the upper half of the immersed section. An extreme in breadth, as in length or depth, is also dan gerous, and both extremes are to be avoided.
The builder forms a half model of his proposed ship, making it a quarter of an inch to the foot. Moulds are then form ed of all the different parts. In these United States, where there are abundant supplies, builders confine themselves to live oak, pine, chestnut, locust, and cedar. The tree should be taken in the second era of its growth, when it has attained matu rity, without approaching the period of decay. It should be killed, by removing
a ring of bark, at the beginning of winter, when the sap is down, and left to dry and harden before it be cut down.
In laying down the keel, great care must be taken to preserve its perpendic ularity, for which purpose it is pinned with treenails on either side of the blocks : also in raising andpropping the stem and stem, and every piece of the frame. As the floor timbers are the great connecting principles of the ship, to which they bear the same relation as the ribs to the body, too much care cannot be taken in selecting and securing them. Sometimes the frame Ss made completely solid and calked ; and, in this case, the interior covering of plank is dispensed with, excepting a few strengthening streaks.
The planking does not merely serve to exclude the water, but to protect, connect, and bind harmoniously together, and is quite as essential as the skin to the body. It is one of the nicest arts of the builder so to carry up his planking, as with little waste, to keep his seams always fair with the water-lines. When it is necessary to bend a plank at the bow or stern, It is heated by steam, and then forced into place with screws and levers. All being complete, the carpenter makes room for the calker, who carefully stops all the seams with oakum, and smears them with pitch. The scraper follows the calker. Sheathing with wood is practised with iron-fastened ships, because copper causes the bolt-heads to corrode, if placed against them. It consists in covering the bottom with pine boards, sheets of paper soak ed in hot pitch being placed between. Two varieties of sails are principally in use—the square rig, and the fore-and-aft sail. In the square rig, square sails are attached to yards, whose primitive posi tion is at right angles to the masts and to the plane of the keel ; but which are free to move round the mast. In the fore-and-aft rig, the primitive position of the sail is in the plane of the keel ; and the sail is extended at the top and bot tom, by spars, known respectively as the gall and boom : these are attached by crotches to the mast and are free to slide up and down it. Common to both of these rigs are the head sails or jibs— which are triangular in shape and extend ed from the bowsprit to the foremast head.