The displacement of these two vessels is practically the same, but the sail area of Seabelle is less than two-thirds that of Columbia, or, in other words, the sail area of Columbia is 50 per cent. the greater. We have been witness that the Columbia has made 12.5 miles per hour in a lower-sail breeze blowing a little abaft the beam, the wind pressure being equal to 21b. per square foot. Seabelle under similar conditions has made 1P4 knots per hour. At a speed of 124 knots the resistance met with by Columbia would probably be increasing as the fourth power of the speed, and if her sail area were reduced to an equality with Seabelle's, her speed would be reduced in the ratio of .:/ i or about 1 • r us P or to about equality with Seabelle's speed.
If these two yachts set out to sail a course of 50 miles with a strong leading wind, the Columbia would accomplish the distance in four hours, and the Seabelle in four hours twenty-three minutes. Here are two vessels of exactly the same displacement; but one, by adding 35 per cent. to her beam, and only 8 per cent. to her length, is enabled to carry nearly 50 per cent. more canvas ; and it is quite fair to assume that the advantage gained by the Columbia is mainly due to her greater area of canvas, and not to any superiority of model. That is to say, if two designers of yachts are given a displacement of 150 tons to work upon, and one produces a vessel that will carry 50 per cent. more canvas than the other, it is quite evident that the vessel with the larger area of canvas should be very much the faster, as it is unlikely that her designer would allow her to suffer from defective modelling of entrance and run. [As a matter of curiosity, we might here state that the measurement for competitive sailing under the rules of the New York Yacht Club was until two years ago one of displacement,* and if Seabelle and Columbia set out to compete under these rules, they would have been rated upon equal terms, or rather Seabelle would have given a few seconds time. We need not stop to inquire whether this would be an equitable arrange ment, but it can be assumed that English yachtsmen consider the advantages which are due to length and beam are fairly taxable, as both these quantities are terms in the Y.R.A. measurement formula. By this formula Columbia would be rated as of 264 tons, and Seabelle 140, and the time allowance would be 16 minutes for a course of 50 miles.] The Seabelle and Columbia are representatives of extreme types, but among vessels so little varying in form as those belonging to this country, very varying stability characteristics are to be found. These varying characteristics can best be illustrated by curves of stability, which exhibit the righting power at successive angles of inclination, and a suitable contrast will be found in the curves for Jullanar and Florinda for the purpose of illustration.
The Jullanar affords an example of the kind of curve of stability a vessel has with a low centre of buoyancy and low metacentric height (see Fig. 6, page 9), brought about by a heavy displacement obtained by depth of body ballasted with lead. Her metacentre falls below the load water-line ; yet, owing to her great depth of body, and the great weight of lead stowed in that depth, her centre of gravity is low, and is, in fact, an inch or two below her centre of buoyancy. Still the metacentric height
is limited, and at small angles of heel the righting lever (e G, Fig. 6) is short ; but that lever continues to increase up to the time that Jullanar would be on her beam ends, when it is more than double what it would be at an inclination of 30°—a condition which renders Jullanar absolutely uncapsizable. She thus has the finest seagoing elements possible—low metacentric height for ease, and illimitable range of stability for safety. Jullanar's success in extremely strong winds, as compared with her performances in light winds, is directly attributable to the peculiarities of her stability curve. Florinda is an example of a yacht with less depth of body than Jullanar, and a higher centre of gravity. Under lower sail, with a wind force of 1.71b. per square foot of canvas, Florinda would heel 15°, whilst Jullanar would heel 17'; but if the force were increased to 31b. per square foot, Florinda would heel to 35°, whilst Jullanar would only fall over 30°. It will be seen by referring to the annexed stability curves, that at 24° (about the deck edge) the stability of Jullanar and Florinda are equal; at any further inclination the stability of Jullanar makes a very rapid gain on Florinda. At 20° inclination Florinda's deck begins to be immersed, whilst none of Jullanar's deck is put under until she reaches an inclination of 26°. This fact, coupled with her depth of body and her extraordinary low centre of gravity, can be regarded as the cause of her long range of stability, and the cause why in strong winds Jullanar is able to beat Florinda.
The Seabelle is a vessel somewhat similar to Jullanar so far as the vertical position of her centre of buoyancy is concerned, and her curve of stability resembles that of Jullanar. The Rose of Devon, on the other hand, like Florinda, represents a shallower type of vessel, and the curves of stability of these two are of similar character. Rose of Devon, however, has a high centre of gravity as she is ballasted wholly with iron; her curve, with 20 tons of lead on her keel, is shown by the dotted line. The character of the carve remains the same, but owing to the greater length of righting lever due to the lower centre of gravity the " righting power in foot tons " is largely increased for any angle of heel.
It has been sufficiently shown that sail-carrying power is a very large factor in the elements which make up a vessel's success in competitive sailing; and generally, as that power cannot be increased by adding to beam and length without such additions being adequately taxed for competitive sailing, the designer exercises his ingenuity in depth and ballasting. Thus, if we find two vessels of equal length and breadth, and one exhibits an advantage over the other, that advantage will be nearly always traceable to greater sail-carrying power. The conditions upon which this power is dependent have already been explained, and the importance of well considering them has been sufficiently exemplified by the reference to the relative stability of such distinctive types as Seabelle and Columbia, Jullanar and Florinda.