786 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76.20 



to the center of resistance of the underwater hull. 



Manj^ saUing yachts have transom sterns. On 

 these craft, where the propulsive power is limited 

 to that which can be derived from the wind, the 

 shape and position of the transom may be of 

 greater relative importance than on a high-speed 

 motor yacht or a destroyer. At slow speeds the 

 transom must definitely be kept out of water, to 

 avoid the separation drag abaft it. At higher 

 speeds it may be possible to accept the added 

 drag, especially if some of the deflection drag 

 behind the bow-wave crest can thereby be elim- 

 inated. 



Several new ratios appear in the design of sailing 

 yachts. Among them may be mentioned the 

 ballast ratio and the sail-area to wetted-surface 

 ratio. In its simplest terms, the ballast ratio is the 

 ratio of the total weight of ballast, both inside and 

 outside (portable and fixed in the keel), to the 

 total scale weight of the yacht. It ranges from 

 0.25 to 0.35 or more, depending upon the beam 

 and other factors. There are rather elaborate 

 ways of defining ballast, by modern racmg rules, 

 but the scheme remains the same. 



The saU-area to wetted-surface ratio appUes 

 primarily to small sailboats which run fast 

 enough to generate large dynamic lifts and which 

 approach planing speeds, although it is also a 

 factor in the design of purely displacement types. 

 It has probably been assumed by some yacht 

 designers that the hull resistance of a sailboat 

 in the semi-planing range is due largely to friction, 

 so that speed and actual wetted area are the 

 major factors. They are indeed factors but it is 

 doubtful that they are the major ones. They are 

 certainly not the only factors. 



D. Philhps-Birt presents a discussion which, 

 though appUed to a sail-and-power craft called a 

 motor sailer, embodies a number of useful 

 comments on saihng-yacht design in general 

 [Rudder, May 1955, pp. 12-15, 46-55]. 



As an indication of the type of unpublished 

 data available on saihng-yacht design, and of 

 those to be expected in the not distant future, the 

 followmg is quoted from an article by A. B. 

 Murray of the Experimental Towing Tank, 

 Stevens Institute of Technology [Yachting, Dec 

 1946, pp. 62, 110]: 



"However, a system of correlation has been worked up 

 which provides a useful yardstick of performance for sailing 

 yachts of all sizes. Upright resistance, speed-made-good, 

 leeway angle, stability, and balance are put into coefficient 

 forms which eliminate the effect of differences in length 



and displacement. Upright resistances of the full size boat 

 for a few selected fixed speed-length ratios are divided by 

 the displacement in tons. The resulting ratios are of the 

 same order of magnitude for all lengths of boats, tending 

 to be smaller as the boat length is increased. Plots of the 

 ratios against load water fine make a convenient chart 

 for comparison of hull resistance. Similar coefficients are 

 worked up for other performance characteristics. For 

 instance, an excellent basis on which to compare per- 

 formance to windward is obtained if close-hauled speed- 

 made-good is divided by \/Z for a few fixed values of 

 true wind speed. By similar methods, stability, leeway 

 angles, and balance may be compared. Charts of all these 

 characteristics have been prepared for a large number of 

 Tank tested boats. These charts give immediately a means 

 for evaluating a new design, besides indicating performance 

 trends. 



"Another step was taken when a method was set up to 

 compare three important characteristics on the basis of 

 hull alone without . effect of sails or center of gravity 

 position. This compares hull resistance, leeway angles and 

 longitudinal center of lateral resistance for three heel 

 angles at a standard speed and stability which are func- 

 tions of boat length. By separating hull characteristics 

 from the sail power, a more critical analysis may bo made 

 of the problem." 



76.20 Brief Bibliography on Sailing-Yacht 

 Design. While no attempt has been made to 

 collect all the published and unpublished refer- 

 ences relating to the design of sailing yachts the 

 reader may find the following brief bibliography 

 useful and interesting from a historical and general 

 information standpoint: 



(1) Harvey, J., "On the Construction and Building of 



Yachts," INA, 1878, Vol. 19, pp. 150-158 



(2) Kemp, Dixon, "A Manual of Yacht and Boat Sail- 



ing," Cox, London, 3rd ed., 1882 



(3) Kemp, Dixon, "Fifty Years of Yacht Building," 



INA, 1887, Vol. 28, pp. 232-246 



(4) Nixon, L., "Yachts in America and England," 



SNAME, 1894, pp. 261-277 



(5) Kemp, Dixon, "Yacht Architecture: A Treatise on 



the Laws which Govern the Resistance of Bodies 

 Moving in Water; Propulsion bj' Steam and Sail; 

 Yacht Designing; and Yacht Building," Horace 

 Cox, London, 3rd ed., 1897. This is a veritable 

 treatise on naval architecture in most of its phases. 



(6) Crane, C. H., "Some Thoughts on the Design of 



Modern Steam Yachts," SNAME, 1903, pp. 

 57-65 



(7) Erismann, M. C, "The Effect of the Universal Rule 



in Recent Yachts," SNAME, 1906, pp. 223-241 



(8) Warner, E. P., and Ober, S., "The Aerodynamics of 



Yacht Sails," SNAME, 1925, pp. 207-232 and 

 Pis. 133-146 



(9) Fox, Uffa, "Sailing, Seamanship, and Yacht Con- 



struction," 1934 



(10) Baier, L. A., INA, 1934, pp. 107-108. Gives a brief 



discussion of form variations for sailing yachts. 



(11) Stephens, W. P., "Yacht Measurement," SNAME, 



1935, pp. 7-42 



