It has been said that the centre of lateral resistance represents the point through which the resistance of the water to the sideway motion of a vessel acts, and the centre of effort of the sails represent the point through which the force acts which endeavours to impart sideway or broadside motion to the vessel. It is therefore evident, if these two horizontal forces do not act in the same vertical line, that some disturbance must take place in the direction of the vessel's motion. In short, the horizontal distance represented by x q or C E s in Fig. 11 is a coupling lever tending to turn the vessel towards the wind. When such conditions exist, a vessel requires what is known as " weather helm ; " that is, if the vessel's head has a tendency to fly up in the wind, the rudder is twrned to leeward by bringing the helm or tiller to windward. (The distance k, x in Fig. 11 shows the length of the lever upon which the rudder acts to turn the vessel, but this part of the subject will be fully treated in the next chapter.) It is obvious that if C E were directly over sr no such lever would exist, as the force accumulated in C E would have no tendency to turn the vessel, either on or off the wind, and the vessel would " steer herself." If on the other hand were at q, and C E at s, it is clear that the effort of the sails would be striving to turn the vessel's head of the wind, and she would in fact require " lee helm." Thus, two bad faults in a vessel whilst sailing by the wind are dependent on the fact that the centre of effort of the sails does not act in the vertical line in which is the centre of lateral resistance. It would appear to be a very simple matter to so arrange a vessel's sails that the centre of effort came over the centre of lateral resistance, presuming the latter to be determined ; but, owing to the concavity of the sails, and the fact that the pressure varies considerably on them (there being always a plus pressure on the fore part or luff of the sail), the centre of effort cannot be accurately computed. That is, it is always some distance ahead of the calculated centre, as already shown ; but as the centre of lateral resistance is likewise ahead of the calculated centre, it is found in practice both safe and useful to treat the calculated centres as if they had been correctly determined. In " Yacht Designing," in summing up this question we
find the following : " Experience teaches us that, to obtain the largest average of advantages, the calculated centre of effort of the sails should be some distance forward of the calculated centre of lateral resistance, and this distance may vary from •01 to of the length of the load line. With such a ratio as •03 the vessel may carry lee helm' in light winds and be slack in stays, and probably a ratio of -02 will be a safe one to adopt. Thus, if a vessel be 50ft. on the load line, then 50 x .02=lft., which is the distance the calculated centre of effort of the lower sails is to be ahead of the calculated centre of lateral pressure on the immersed surface of the hull." This, it should be understood, refers to racing yachts with much tuckered or very raking keels ; for cruisers it will be better and safer to have the centre of effort and centre of lateral resistance in the same vertical line. In the case of yawls it is generally found that the calculated centre of effort requires (relatively to the centre of lateral resistance) to be a little further aft that in either cutters or schooners, as the mizen is not a very effective sail on a wind.
As the longitudinal component (F a, Fig. 10, page 19,) of the force of the wind acts through the centre of effort of the sails considerably above the centre of lateral resistance a couple is formed (C E, q, Fig. 11) tending to depress the bow ; but as the longitudinal stability of a vessel is so great, but little depression will actually take place. Thus, take the case of Seabelle at a speed of 7 knots, her resistance in the water would be about half a ton and the distance (C E, q) is 50ft., and at such speed the moment would be (50 x 0.5) equal to 25 foot tons, which would only cause Seabelle to be depressed by the head lfin. Of course, as the resistance increased, as it would very rapidly in the case of vessels with full bows driven at high speed, the moment would increase and the bow would be further depressed. In small yachts and boats, which are made to carry sail areas (by aid of trimming ballast up to windward) out of all proportion to their size, the depression of the head is often very considerable, and has to be met by shifting some of the ballast or crew farther aft.