CENTRE OF EFFORT AND PROPULSION.
If the wind always blew at right angles to the plane of a sail the effort of that wind would be exerted through the centre of gravity of the sail plane ; that is, if the whole effort of the wind were concentrated on one point of the sail, that point would necessarily be the centre of the plane. This point is usually termed the "centre of effort" of a sail. If a vessel has many sails, such as a cutter yacht, Fig. 11, the total effort of the sails is exerted collectively through a point which represents the centre of gravity of the whole of the sails combined. In Fig. 11 the centre of effort of each sail is marked at abed (d, for the topsail, appearing in cat as a), and the common centre of gravity of the four sails lies at the point C E, and this point is termed the czrrrna or EFFORT of the sails.
The most effective angle for the sails to make with the direction of the wind can be theoretically determined upon the assumption that the sails are real planes, and not such as they are, more or less concave surfaces. In "Yacht Designing" this problem is worked out in detail, together with the speed per hour a vessel of 150 tons ought to make upon different points of sailing, assuming the sail area and force of the wind to be constant, as follows :— (It must be understood that the " sail angle " set forth in the table is purely theoretical, and in practice the sails will be trimmed according to the requirements of the vessel and the judgment of the sailing master.) The " real wind" is the true direction of a wind, such as would be indicated by a vessel's vane if she were at anchor or motionless. When a vessel moves obliquely towards the wind, the direction of that wind, as shown by the vane, apparently alters or draws more ahead in the path of the vessel, or, in other words, the vane blows nearly fore and aft. This apparent wind is the force that had to be considered in solving the problem referred to above ; but in speaking of the course a vessel makes with the wind the real direction of the wind is understood. In beating to windward this real direction can be readily determined by aid of the compass ; thus, assuming a vessel on one tack beads N.W., and when
put about that she heads N.E., then between N.W. and N.E. are eight compass points, and the vessel on either tack evidently had pointed four points from the direction of the real wind, which was at N. ; but the coarse, if judged from the vane or flag, would probably be set down as only three points from the wind. The vessel would be said to " turn in eight points" if there were only eight compass points in the arc she described in being put from one tack to the other.
From the table just given we learn that a yacht's speed increases as the angle of her course with the real wind increases, until she brings that wind a little abaft the beam (the vane would show the wind a little forward of the beam), and we have a difference of tpeed of nearly a knot an hour set down between the courses of 34 and 4 points. This seems a very important difference, and it will be well to inquire if by " squeezing" a vessel so close to the wind the loss of speed is compensated for. In beating to windward the proportion the real distance to be reached bears to the distance traversed is for 34 points as 1 is to 1.3 ; for 4 points, as 1 is to 1.414 ; for 44 points, as 1 is to 1.6 ; and for 5 points, as 1 is to 1.825. Thus, if a vessel had to beat twenty miles dead to windward, the distance sailed, according to the following courses (disregarding leeway) would be: There unquestionably could be a great gain here in economising distance, but the matter for consideration is this : would the gain in distance compensate for the loss in speed ? As we know what the speed per hour is on different points of sailing, this question can be answered by a very simple calculation. Thus : Thus, say, two cutters of 75 tons each, equal in every respect as to speed, stability, and spread and effectiveness of canvas, set out to beat 20 miles to windward, and one lay Sit points from the wind and the other 44 points, there would be no loss or gain; if one of them lay 4 points from the wind she would gain 7 minutes, and if she lay 5 points from the wind she would lose 11 minutes. Thus for a cutter beating to wind ward a course of 4 points from the wind would appear to be the most advantageous.