754 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 76 J 



are of interest to a designer. These craft did not 

 plane in a strict sense, despite the T^ values of 

 4.0 or more. Midsections of some of them are 

 shown in Plate 190 of the reference. Deck plans 

 and waterUne planforms are drawn on Plates 

 191 and 192, while photographs of some of the 

 boats underway are reproduced on Plates 193-197. 



Other successful yachts and launches of this 

 tj'pe were designed and built by N. G. Herreshoff. 

 A few of them are described and illustrated by 

 L. F. Herreshoff [Yachting, Sep 1950, pp. 26-27]. 



While these vessels had shapes no longer con- 

 sidered stylish or efficient, they represented, and 

 still represent the furthest point reached in the 

 development of fine-ended displacement-type 

 craft. Their shapes are almost necessary for hulls 

 which are not permitted to create large surface 

 disturbances when moving rapidly ["Speed With- 

 out Fuss," The Motor Boat and Yachting, Sep 

 1956, p. 423]. These shapes may very well become 

 useful for certain requirements not yet presented 

 to the naval architect. Speaking of the future, the 

 requirements of the early years of mechanical 

 propulsion for a craft which could be driven 

 swiftly yet easily and smoothly, such as a pleasure 

 launch or yacht, may be expected to continue as 

 long as mechanical propulsion is utilized. When 

 another cycle of human behavior rolls around, the 

 former demand for a quiet craft, gliding grace- 

 fully yet rapidly, may well be repeated. 



Jet and rocket propulsion may, in the years 

 ahead, be appUed to slender displacement forms 

 for certain particular duties rather than to the 

 skimming and planing forms now associated with 

 high speeds over the water. Pounding and slam- 

 ming on planing craft, even in small waves, may 

 well set a limit on the ultimate speed at which 

 their hulls will hold together. 



Except for the additional wetted surface un- 

 avoidable with large L/B or L/H ratios, and the 

 extra friction resistance involved, the optimum 

 shape for a hull, to reduce the pressure resistance 

 due to wavemaking and separation to a minimum, 

 is one which is definitely fine and slender. The 

 L/B ratio may then exceed 10 and even approach 

 15. This is not easy to accomplish in a small 

 craft, which needs space for the crew, passengers, 

 propelling machinery, and some useful load but 

 must have metacentric stabihty as well. 



A satisfactory compromise between length- 

 beam ratio and absolute length is determined by 

 the speed-length quotient T^ or the Froude 

 number F„ at which the craft is expected to 



make its maximum speed. The lengths and pro- 

 portions which were developed through the years 

 when this was the predominant type for small, 

 high-speed craft are given by C. H. Crane 

 [SNAME, 1904, pp. 321-326; 1905, p. 369]. Some 

 of them are listed in Table 76. c. The B/H ratio 

 is frequently amenable to some variation but it is 

 only rarely, if at all, that the propulsion perform- 

 ance can be improved by departing from the 

 proportions listed in the table. 



With the length-beam and beam-draft propor- 

 tions likely to be found the optimum, the draft 

 may become a small proportion of the length; so 

 small, in fact, that it is difficult to provide 

 sufficient rudder area in an efficient shape of 

 blade except by projecting it well below the 

 baseplane. 



With the large L/B ratios mentioned here it is 

 easy to keep the waterline run slopes below 11 or 

 12 deg and thus to eliminate all possibiUty of 

 separation at the stern. Attempts to save resist- 

 ance by cutting off the stern and its wetted surface 

 are rarely successful unless the transom so formed 

 is kept out of water at all speeds. Working reverse 

 curvature into the buttocks and terminating them 

 tangent to the at-rest WL is good design provided 

 the occasional wave slap under the stern can be 

 accepted. However, any curvature of the stern 

 buttocks, convex downward, develops — Ap's 

 under them and drags the stern down, with all the 

 disadvantages of excessive trim by the stern. 



Special problems are involved in the design of 

 high-speed craft running at so-called "inter- 

 ference" speeds in the range of T^ = 1.3 to 

 1.8, F„ = 0.39 to 0.54, where a practically con- 

 tinuous resistance hump is shown by Fig. 66. B. 

 The design problems involved are discussed at 

 some length by E. Rolland, on the basis of 

 designs of former years by N. G. Herreshoff and 

 E. W. Graef [ATMA, 1951, Vol. 50, pp. 443^62; 

 an English translation of this paper is available 

 at the David Taylor Model Basin]. 



76.3 Ultra-High-Speed Displacement Types. 

 Despite the insistent modern demand for ultra- 

 high-speed ships to carry cargoes or other useful 

 loads, reckoned by the hundreds or the thousands 

 of tons, relatively Httle thorough and systematic 

 investigation has been devoted to the displace- 

 ment type of hull driven at speed-length or Taylor 

 quotients T^ exceeding 2.0 or 2.2, F„ greater than 

 about 0.60 or 0.66. 



The 100-ft Turhinia of C. A. Parsons made 34 

 kt in the 1890's, and the 100-ft steam yacht 



