EFFORT OF THE WIND.
Tax effort of the wind at right angles to the surface of a sail will require more explanation than at first sight may appear to be necessary. In sailing before the wind very diminished speed is attained to what may be realised with the wind abeam, assuming, of course, the sail area to be the same ; the reason is that the velocity of the wind, or its effective force, is diminished by exactly the speed of the yacht ; or say the real wind is travelling at the rate of 14 miles an hour, and the yacht at the rate of 8 miles an hour, then the apparent wind, which is impelling the yacht, will only have the speed of 6 knots past her. The effect of the apparent wind on a yacht can be thus briefly explained : a yacht with 1000 square feet of immersed surface would meet with a gross resistance in the water of about 4001b. at a speed of 8 knots, and would have about 2000 square feet area of canvas. If the wind moved at a velocity of 14 knots, it would exert a pressure on a fixed sail of about 11b. per square foot; but as the sail is not fixed, the pressure is diminished until it balances a resistance met with by the yacht in the water at a certain speed, say 8 knots. At this speed the resistance of the yacht would probably be increasing as the cube of the speed ; and as the pressure on the sail would be diminishing as the square of the speed, it is quite evident that a limit to an increase of speed before the wind is very suddenly reached. Thus, say it were desired to increase the speed of the yacht to 10 knots, the resistance she would meet at that speed would be about 8001b., and as the wind pressure on the sail would be only that due to a velocity of 4 knots (14 —10 = 4), the sail area would have to be in creased to about 8000 square feet to maintain the 10-knot speed of the yacht. Beyond this we can imagine that the sail area could be increased until the yacht attained a velocity equal to the velocity of the wind, when no farther increase of speed could be attained.
If a vessel were always sailing before the wind, or nearly so, no Mus e 2 tration would be necessary to show how the impulse of the wind acts in other ways as a propelling power; but, as a vessel can be made to move ahead nearly " in the wind's eye," or, in other words, almost in direct opposition to the direction of the applied force, it is evident that a curious problem has to be solved in accounting for close-hauled sailing.
In Fig. 10, D E is a vessel moving ahead in the direction of the arrow and the line D E. The letters F G represent the boom and projection of a sail. The arrow W marks the direction the wind is blowing towards the vessel, i.e., four compass points, or 45°, from her course. The line F H shows the direction of the wind (which is the same as the arrow W) * relative to the plane of the sail, F G. The length of the line F H repre sents the force or strength of the impulse of the wind on the sail.
At the first view it would seem that the force of wind coming from such a direction as W upon a sail trimmed at such an angle as F G could Fmk. 10. only drive the vessel astern in an oblique direction ; but the force F H as it strikes the sail is decomposed or resolved into other forces, so that in reality the force is not exerted in the direction that it is apparently applied to the sail. The line F H will be regarded as the diagonal of a parallelogram or four-sided figure, such as F G H I. The wind F H by the construction of the figure is resolved into other forces—the first, F G, along the plane of the sail or direction of the boom, and has little or no effect ; the other force is exerted at right angles to the plane of the sail, and in Fig. 10 is represented by the line F I.
The force F I (which represents the whole horizontal exertion of the wind on the sail) is apparently employed in driving the vessel side ways, although at the same time a little ahead; but F I (the force exerted on the sail) is decomposable into three other forces, two acting horizontally and one vertically, thus : let F I be the diagonal of a parallelogram F alb; then the line F b represents the magnitude of the force that is driving the vessel to leeward in a direction at right angles to her keel ; and the line F a represents the magnitude of the force that is driving her ahead in the direction of her keel.