Sec. 77.15 PRELIMINARY DESIGN OF A MOTORBOAT 835 



TABLE 77.e — Values of tub Piiillips-Birt Powering Coefficient Ki in Equation (77.iii) 



The values set down here arc taken from a table published by D. Phillips-Birt [The Motor Boat and Yachting, Jan 

 1954, p. 29). 



IV. Method of D. Phillips-Birt, Eq. (77.iiia), 

 where K^ from Table 77.e is 3.38. Then 



Pb (horses) = 



V^ (kt) W (long tons) 



(24)^(8.482) 



= 427.7. 



(3.38)' 



V. Method of N. L. Skene, where C = 185 and 

 Eq. (77.iva) is 



W (lb) y (mph) 



Pb (horses) = 



19,000(27.6)' 

 (185)' 



= 422.9. 



The results from three of the four methods are 

 surprisingly consistent, considering that only the 

 speed, weight, and waterline length enter as 

 variables. They indicate a brake power of about 

 430 horses, for which two HN-10 diesel engines 

 with large injectors appear ample. They should 

 be able to deUver, at full throttle, a total of 2 

 times 225 or 450 horses at rated brake power. 



A more accurate power estimate, at a later 

 stage in the design, is described in Sec. 77.26. 



As a check on the foregoing one may use the 

 dimensional formula of F. L. Marran and H. R. 

 Shaw, derived from an analysis of full-scale data 

 on many motor-driven small craft ["Motor Boat 

 Powering," The Rudder, Jun 1950, pp. 16-17, 48] 



V = 



242.5 



+ M, 



(77.V) 



Here V is the predicted speed in kt, TF is the scale 

 weight of the boat in lb, Pb is the total rated 

 brake power of all engines, and Ml is a length 

 correction factor, for which the authors give a 



table and a graph in the reference cited. For the 

 ABC full-planing tender, where W is 19,000 lb, 

 Pb is 430 horses, and Ml for a 35-ft length is 7.10, 



242.5 



V = 



/ i9,ooo y- 



\ 430 / 



+ 7.1 = 24.2 kt. 



The agreement, for this 24-kt design at least, is 

 remarkably good. 



77.15 Selecting the Hull Features; Section 

 Shapes. Whether the 24-kt speed can be 

 achieved with the engines selected is principally a 

 matter of keeping the total weight within the limit 

 of 19,000 lb. To be sure, the form of the hull 

 chosen for the planing craft under design has an 

 appreciable effect upon its performance, especially 

 as to dryness, controllability, and behavior in 

 waves, but the overall weight still remains the 

 controlling factor in its smooth-water speed. This 

 weight can not be estimated more closely until 

 the huU is shaped, the lines drawn, structural 

 features determined, and arrangement drawings 

 prepared. 



Considering first the type of section to be used, 

 there are four basic forebody or entrance types, 

 diagrammed in Fig. 77.G. The concave section at 

 2, with a relatively low chine and small rise of 

 floor, gives excellent smooth-water performance. 

 The wetted area at planing speed is usually a 

 minimum because the water is thrown abruptly 

 off the chine with httle or no wetting of the sides. 

 Craft with these sections usually run at a favor- 

 able trim with a desirable shape of planing surface 

 presented to the water. The result is a form with 

 minimum smooth-water resistance, having high 

 speeds at low powers and fuU planing behavior at 

 low speed-length quotients. However, the concave 

 lower sections are unsuitable for wavegoing. In 



