Page:The American Cyclopædia (1879) Volume XIV.djvu/892

866 SHIP and the patterns are skilfully cut for a smooth and even fit. The edges are bound around with a rope called a bolt rope to take the strain from the canvas, and in each corner an iron ring or thimble is inserted and held fast by a rope called a cringle, which goes round the outer concave surface of the ring, and is spliced each end into the bolt rope. Through these rings are passed the ropes, called earings, by which the sail is stretched or bent to its place. The same contrivance is repeated at one or two places on the edge of the sail, that it may be ithortened in single or double reefing ; and on the line horizontally with these oarings short lengths of cord, called reef points, are secured through the sail and hang loosely on each side, which are used when the sail is reefed to tie around the part which is taken in. Sails may be classed as square sails and as fore-and-aft sails. The former hang by the earings and rope bands from yards, and are drawn out by the lower corners or clows to the ends of the yards below. They are made to swing round with the yards so as to present their surface to a side wind; but the fore-and-aft sails are better designed for sailing on the wind, and the square sails for running with a free wind. Shoulder-of-mutton sails and gaff topsails are triangular fore-and-aft sails, the foot of which may be attached to a boom, or in the latter case to the gaff, and the top, by which they are hoisted, terminates in a point against the mast. Lateen sails, much used in the Mediterranean, are suspended from a very long yard, which is hoisted by the mid- dle from the dock. One end of the yard is brought down by a brace, and the other pro- jects above the top of the mast, and rakes with it well aft. The sail serves very well as a fore-and-aft sail. The great superior- ity in the rig of American fore-and-aft ves- sels, by which they have been able to attain the highest speed of sailing craft, is in the great spread of their sails, their skilful cut, and perfect stretch, which causes them to keep full while their plane is more nearly in a line with the wind than could formerly be practised. It belongs to the naval architect to deter- mine the amount and disposition of sail which his ship is to carry. The former is pro- portioned to the immersed midship section, for every square foot of which a well de- signed ship may carry 35 or even 86 sq. ft. of plain sails, i. ., courses, topsails, topgallant sails, jib, and spanker. Yachts often carry as much as 100 to 1. In regard to the manner of disposing the various sails, it is important that their common centre of effort should be at such a point that the ship when in trim will carry, on a wind, a small weather helm. It has been found that when the pressure of the wind on the sails forward of a perpen- dicular erected on the centre of load water line, is to the pressure on the sails abaft as '78 to 1, the ship will work well, all other con- ditions of a good ship being fulfilled. The Theory of Working Ship. The principle upon which a vessel is made to advance against the wind may be explained as follows : Sustained in a state of equilibrium in the water, she is readily susceptible to any force applied to change her position. This involves a move- ment of the water to admit her passing through it. On the line of the keel this easily takes place from the wedge-like shape of the hull ; but a movement sidewise is resisted by the great body of water pressing against the hull for its full length. Whenever therefore the sails are filled by a breeze blowing against them from behind, even if at a considerable angle with the length of the ship, it is easy to per- ceive that her motion must be forward on the line of the keel. As the wind draws further forward the sails are braced further round, so that they may still receive it upon their after side. The wind of course strikes them to a greater disadvantage the nearer their plane ap- proaches its direction; but so long as it im- pinges even obliquely upon their after surface, a portion of the force is exerted to press out the sails in a forward direction, while the re- mainder passes uselessly along the plain of the sails. The former portion tends to push the ship directly in a course at right angles with this plane; but the shape of the ship being opposed to this movement, this force also is resolved into two, one acting to propel the ship sidewise and the other forward. Thus this last result may prove effective even when the head of the ship is pointed obliquely to- ward the wind, as mentioned of fore-and- afters, at an angle of 40 or 46, and in the case of ordinary sailing frigates at an angle of 60. This may be shown by the annexed figure, where the sail A B, oblique to the line of the keel and to the wind V 0, is im- pelled in the direction C D with a force ex- pressed by the square of the sine of the angle of incidence A V. If C D represent the force of the wind on the sail, as expressed by the square of the sine of incidence A V, we have only to construct G II to see that such a direction is composed of the two effects H and G with respect to the body E F on which it acts. Now the sharper we brace the