Sec. 76.19 



DESIGN OF SPECIAL-PURPOSE CRAFT 



785 



Spai'kman and Stephens, Inc. On this body 

 "plan there is drawn an inclined waterUne for a 

 heel of 30 deg, when the main deck edge touches 

 the water on the low side, for the condition in 

 which the inclined volume V equals the upright 

 volume V. 



Diagram 2 of Fig. 76. N gives a half-waterline 

 shape with the yacht upright; diagram 1 shows 

 the whole waterline shape in the inclined position 

 just described. This shape, corresponding to the 

 intersection of the hull with the level of the 

 undisturbed water at rest, is definitely unsym- 

 metric, even about a diagonal line drawn from 

 the stem to the extreme after end of the inclined 

 waterline. For this particular yacht, the inchned 

 WL has very nearly the same length as the upright 

 WL, when measured parallel to the ship axis. If 

 measured along the diagonal broken line in 

 diagram 1 of Fig. 76.N it is about 1.012 times 

 the upright WL length. 



The foregomg analysis apphes to the static 

 case only, with the water surface undisturbed 

 by any waves whatever. The dynamic situation 

 when sailing is far different, even in smooth water. 

 At their top speeds these craft generate a very 

 pronounced Velox-wave system, with high crests 

 at bow and stern and a deep trough amidships. 

 The immersed length of the inclined waterline is 

 then considerably longer than that shown in 

 diagram 1 of Fig. 76.N, and its shape is probably 

 much different. There is a similar difference 

 between the at-rest inchned section-area curve 

 and the corresponding yl-curve when underway. 

 This difference may be greater than between the 

 upright and inclined A-curves of diagrams 4 

 and 3, respectively, of Fig. 76.N. 



While considerable research has been and is 

 being carried on relative to the overall resistance 

 and propulsion aspects of saUing-yacht design 

 and performance, this should be extended to 

 cover the wave profiles and lines of floAV at various 

 angles of heel. Put in another way, attention 

 should be focused as much on the flow patterns 

 around the hull when underway as on its form 

 coefficients, section-area curves, and other param- 

 eters. All too often the latter apply only to the 

 upright condition, with the vessel at rest. In the 

 few modern books on saUing-yacht design there 

 is little discussion of the hull characteristics in 

 the inclined condition [Skene, N. L., "Elements 

 of Yacht Design," 5th ed., 1944, Fig. 45, p. 75]. 

 Sec. 29.8 mentions a much older paper by R. C. 

 Allen covering some phases of this subject 



Waterline Offsets Are Measured from Intersection of Plane of 



Undisturbed Woter Surface and Plane of Symmetry of Yocht 



401 S> 



3j 

 2.0 

 1.0 



Shape of Desiqne 



Waterline with 



Yocht Upriqht 



Station^ 



-13 



The Waterline Lenqth is the Station Lenqth m the Upriqht Position. 

 For the 30-de<j Heel Position the Reference Lenqth L3 is olso theL^L 



Fig. 76.N Inclined and Upright Waterplanes of 

 Sailing Yacht op Fig. 76.M, with Coreesponding 



4-CuRVES 



["Naval Science," 1875, Vol. 4, pp. 89-93]. A. B. 

 Murray cites the case of two six-meter yachts 

 where the difference between the upright and the 

 heeled performance was an important factor in 

 the racing abilities of the respective craft [Yacht- 

 ing, Dec 1946, p. 60]. In his words, ". . . although 

 Jack had lower resistance with zero heel angle, 

 her resistance in a heeled-over, close-hauled 

 attitude was higher than JilVs." 



In fact, it is probable that too little attention 

 has been given to the inchning forces and mo- 

 ments, involving -wind forces high up on the sails 

 and rudder forces low down on the hull. There is 

 an appreciable down-pitch moment, comparable 

 to that encountered in motorboats driven by 

 airscrews, due to the high position (on the sails) 

 of the line of application of the thrust compared 



