SAIL CARRYING POWER AND SPEED The success of a yacht in competitive sailing is no doubt largely dependent upon her power for carrying sail, and this " power " is represented by the statical stability of the vessel. A reviewer of " Yacht Designing," said : " We are glad to find so much importance has been placed upon stability as to lay down as an axiom, in which we (the Engineer) most heartily concur, that nearly all the failures in yacht designing are traceable to the want of exact knowledge on the part of the designer of the laws by which the stability of floating bodies is governed; or that if these laws are understood, the means for quantitatively proving this operation in sailing yachts have not been exercised." * In Chapter I. an attempt was made to explain, as simply as possible, the laws which govern stability ; but the mode of calculating its exact value for any particular model would involve a labour much beyond our present purpose. It was explained by Fig. 6 (page 9) that the righting moment or energy of a vessel to recover her equilibrium, after being heeled to any angle was represented by the length of her righting lever x G (corresponding to the angle of heel) multiplied by the weight of the vessel. It is thus obvious if a vessel has so much energy to regain her equilibrium after being heeled, that an equal energy must have been employed in heeling her from that position of equilibrium. In the case of a sailing yacht that energy is represented by a force of wind blowing upon the sails, and exerted collectively through the centre of effort on a coupling lever, the arm of which is the distance the centre of effort is above the centre of lateral pressure or centre of lateral resistance. In Fig. 11 (page 22) this arm is represented by the distance C E q. To fully explain how the sail moment must equal the righting moment to keep a, vessel at any angle of heel, it will be necessary to employ a few figures, abstracted from " Yacht Designing " concerning the well-known racing cutter Kriemhilda. (We select that vessel because she is one of the few whose exact stability has been proved.) The area of the Kriemhilda's sails (including topsail) is 5622 square feet, and the centre of effort is 46ft., above the centre of lateral resistance. The
moment of these sails will be 5622 x 14•5=261,423; and, presuming the wind pressure to be equal to 11b. per square foot, the heeling moment of the sails would be 261,423 foot-pounds. The righting moment in foot-pounds of Kriemhilda must next be arrived at. Her displacement is 115 tons, which, reduced to pounds, is 257,6001b.; at 16° inclination Kriemhilda's righting lever is lft. long, so that for that angle of heel her righting moment would be 257,6001b. multiplied by 1, which in turn would be 257,600 foot-pounds, or nearly equal to the heeling moment of the sails. As the moment of stability at 16° inclination does not quite equal the moment of the sails, she would be heeled a little beyond 16°.
Generally, in speaking of the " stiffness " of a yacht, the term is only used in a relative sense, and if a vessel is said to be not very stiff, nothing more is meant than that she is not so stiff as some other vessels under a similar area of canvas and wind pressure ; that she is in any danger through not being stiff is a contingency never dreamt of. Now, so far as most English yachts of the deep V type are concerned, it may be said that they are absolutely uncapsizable ; but it is not so with regard to many shallow yachts and boats of this country and America. The relative safety or danger of the two different types of vessel can best be illustrated as a curve of stability as shown by Fig. 16.
The distances a k, t d, p s, represent the length of the righting lever (see ,c; G, Fig. 6) of a vessel, heeled respectively to 10°, 20°, 30°, &c. ; then the curved line B passing through the spots at the termination of the distances (as at a, t, p, &c.) is the "curve of stability." The curve A represents the curve of stability of an English yacht of the deep type, and it will be seen that the righting lever is longest when the vessel is heeled to 50°, where it is aft. long; this would be termed the vessel's maximum stability. The curve B represents the curve of stability of a vessel of equal length, but of greater beam, and much less depth and weight. It will be seen that this vessel has the greatest length of righting lever at 30° inclination, where, like the other, it is nearly 3ft. long ; this is the shallow vessel's maximum stability.