Sec. 77.26 



PRELIMINARY DESIGN OF A MOTORBOAT 



8-17 



Rotio of Chine Lenqth Lq to Mean Chine Beom B(. 



Fig. 77. N Variation of Chinb-Area Ratio 

 Ac/V^'^ FOR Optimum Resistance, on a Base 

 OF Chine Ratio LcIBq 

 The solid line represents a combination of ratios indi- 

 cating minimum resistance, based upon the available data. 

 For the EMB Series 50 the LOG was at O.OllLc abaft the 

 center of area of the projected chine planform, at zero deg 

 initial trim. 



A moderately loaded utility-type planing boat 

 about 40 ft long would, by reference to Fig. 77. C, 

 probably weigh in the vicinity of 23,000 lb. This 

 would involve a length greater than the proposed 

 35 ft to meet other optimum design conditions. An 

 increase in the mean chine beam from the 9.205 

 ft of the actual craft to the 11.29 ft derived in 

 the preceding paragraph would likewise involve 

 a heavier boat. On the basis that a full-planing 

 craft of modern design produces somewhat more 

 dynamic lift for a given overall resistance than 

 the planing forms of EMB Series 50, no attempt 

 is made here to increase the chine area and the 

 chine dimensions of the boat under design to 

 correspond to the meanhne values given by 

 Clement's graphs of Fig. 77. M and 77. N. 



77.26 Second Estimate of Shaft Power, Based 

 Upon Effective Power. With a body plan of the 

 24-kt, full-planing type of ABC tender laid down 

 and the principal dimensions and parameters 

 selected, the next design step is to make a second 

 estimate of the shaft power. The method em- 

 ployed here is that described by A. B. Murray 

 ["The Hydrodynamics of Planing Hulls," 

 SNAME, 1950, pp. 658-692], based upon the 

 factors and relationships discussed in Sees. 53.2 

 through 53.7. It embodies a calculation of friction, 

 residuary, and total resistance based upon flat- 

 plate values of specific friction coefficient and 

 upon wetted-surface and resistance data derived 

 from experiments on model planing hulls and 

 surfaces. 



This will be recognized as one of the methods 



used for predicting the power of the ABC ship 

 itself, in Chap. 66. Nevertheless, the indirect 

 Froude procedure, when applied to the powering 

 of motorboats, lacks an equivalent degree of 

 reliability because of the unknown effect of the 

 propulsion devices in changing the trim (and the 

 resistance) and because data on propulsive co- 

 efficients i7p(eta) for motorboats are extremely 

 scarce. Adequate apparatus and proved tech- 

 niques for the self-propulsion of motorboat 

 models are almost nonexistent, as are reliable data 

 on the brake or shaft powers required to drive 

 full-scale craft. To make the situation still more 

 uncertain for the designer, little is known, in 

 specific values, of the effect of the propulsion 

 devices on the running trim of any particular 

 boat. This change of trim when self-propelled, 

 beneficial or otherwise, can change the resistance 

 by a large percentage. Applying an estimated 

 propulsive coefficient to so variable a factor 

 would normally not be recommended. It serves 

 here, for the present, because nothing better is 

 available. In a way, it serves also for the future 

 on the basis that the development of techniques 

 for predicting motorboat performance will follow 

 the same fines as for larger vessels. 



An example of the method recommended by 

 Murray in the reference cited early in this section, 

 developed by the staff of the Experimental 

 Towing Tank at the Stevens Institute of Tech- 

 nology, is given by him on pages 668-671 of his 

 paper. This is based on a tentative planing-boat 

 design and is carried through in comprehensive, 

 step-by-step fashion. 



The data required for calculating the total 

 resistance and the effective power of the full- 

 planing type of ABC tender, using the method 

 described by Murray, are taken from the sections 

 preceding and from the dimensioned lines drawing 

 of this tender in Fig. 77.K. Summarizing, these 

 are: 



Weight W = 19,000 lb 



Chine beam Be at midlength of the designed 

 waterfine, Sta. 5, = 9.96 ft 



Chine beam at the transom, Sta. 10, = 

 0.9(9.96) = 8.96 ft 



Rise-of-floor angle at midlength, Sta. 5, = 

 18.25 deg 



Rise-of-floor angle at the transom, Sta. 10, = 

 3.75 deg. 



For the after portion of the underwater body, 

 which makes up most of the planing surface at 



