But as the centre of effort is too high, there is no equilibrium until the bows are depressed; and the force exerted in depressing the bows is W (S+8) GF-WS. GV=W(S. FV+s. GF) and the bows must be depressed until F becomes the point velique. Then bGF will be the angle of the ship's square sails with the horizon. Let it =-X. .. the part of the force of the wind which propels the vessel in her course while =W (S+8) cos? X W (S+s) sin X acts vertically downwards, like an additional weight in the ship. But when the centre of effort is at V, the force propelling the ship is W. S. Now the amount of force lost by the malposition of the centre of effort is W (S+s) sin2 X; and unless or W (S+8) cos9X>WS 8. cos2X>WS. sin X then the vessel will actually go slower. Moreover, the area of resistance is considerably increased by the downward pressure of the wind, which has not been taken into account. In the above consideration, we have supposed the plane of the sails, when the vessel is at her proper load-line, to be vertical. If the masts were raked, there would not be the downward pressure of the wind. If the centre of effort were too low, the stern would be depressed, and then the part of the wind resolved vertically, would tend to lift the vessel instead of acting as an additional weight. When the bows are depressed, the ardency is increased, when the stern settles the ardency is diminished. A very broad stern enables the builder to put the centre of effort a little lower than the point velique ; so that the addition of the extraordinary topsails brings it right. For the breadth of the stern makes the depression to be very slight. A broad stern increases also the stability. The commander of a yacht should be provided with a pendulum, that he might immediately be made aware of any alteration in the water-line of his vessel. It may then be rectified by a different disposition of the weights in the vessel. To determine the proper amount of sail for a vessel, is by no means an easy matter. In the first place, it is easily seen how that two vessels may have the same length, breadth, depth, tonnage and stability, and yet, from a difference in form, require the centre of effort at a different height, and be obliged to carry a very different area of canvass. But hereupon no standard can be set up by theory; the average amount of sail proper for a vessel must be determined by experience alone. The America' has about 56 square feet of canvass (exclusive of jib) for every square foot in her midship section; while our yachts usually have only 42 feet. Chapman says, that the area of canvass should vary as the stability to the power of 3. It is clear that it must vary with the stability, and the proper ratio can only be found by experiment. For three famous American schooners, I have calculated the moment of the sails round the middle line of the ship at the height of the load-line (that is, the area of sail multiplied by the height of the centre of effort), and also I have determined the value of the expression flda for each of them. CHAPTER IX. RIGGING AND MANAGEMENT OF YACHTS. EVERY rig has its own peculiar advantages; some suiting best in some places, others being better adapted to other seas. For small craft, a dippinglugsail gets more out of the boat than any other rig; there is little weight aloft, and it is a very lifting sail. A standing lug does better for short tacks, but is not so lifting a sail. It is more handy than the other kind of lug, but is not equally good on both tacks; for, on one tack, the part before the mast becomes a backsail. An improvement in this kind of sail was suggested by my father; it consists in having only the bolt-rope at the luff, in order to make the sail stand; but there is no canvass before the mast, and the weather-leech has hoops round the mast as in a trysail. This would do admirably for vessels larger than mere boats. Small schooners |