Sec. 77.14 



PRELIMINARY DESIGN OF A MOTORBOAT 



833 



ment vessel, it is imperative to i^now, very early 

 in the design, about how much shaft (or brake) 

 power is called for and the approximate weight 

 of this machinery. Perhaps because it has been 

 customary to make direct power estimates in the 

 past, as distmguished from indirect estimates 

 based upon effective power and propulsive coeffi- 

 cient, there are more shaft (or brake) power 

 estimating procedures available to the motorboat 

 designer than to the one who undertakes to 

 design a large ship. Unfortunately, these many 

 methods usually give as many different answers, 

 and sometimes the answers vary widely among 

 each other. Moreover, the observed data upon 

 which these procedures are based also vary in 

 reliability, principally because of the lack of 

 proper instrumentation and techniques whereby 

 shaft powers for small craft may be measured 

 accurately on trial. In the late 1920's and early 

 1930's Sir Charles Ross undertook a lengthy 

 research project in an effort to measure the brake 

 power of small-boat internal-combustion engines 

 by recording the rate of rotation and measuring 

 the rate of fuel consumption with extreme care. 

 Unfortunately the project was never finished nor 

 were the results of the work written up for publi- 

 cation. The designer is therefore left with the 

 hope that when a given engine is put into a boat 

 it will deliver the same brake power at the output 

 shaft at rated rpm that it did when it ran on the 

 factory test stand. 



One of the simplest direct estimates of brake 

 power is the table and the dimensional "K" 

 formula of K. C. Barnaby [BNA, 1954, Table 40 



TABLE 77. d — Vamies of the K. C. Barnaby 

 Powering Coefficient Ki in Equation (77.i) 



The values set down here arc from a table published by 

 K. C. Barnaby ["Basic Naval Architecture," Hutchinson's, 

 London, 2nd ed., 1954, p. 310]. 



on p. 310 and Figs. 132, 133 on pp. 449-450]. 

 Although Figs. 132 and 133 cover lengths of 20 to 

 50 ft, the speed V extends only to 17 kt, so that for 

 the 18-kt as well as the 24-kt ABC tenders it is 

 necessary to use Table 40 and Barnaby's formula 



IP . V'' WV^ 



V = K.yj'^ or F« = A ^ or Pb = •-^- 



(77.i) 



Speed V, kt 



Fig. 77.E Relationship op Speed to Brake 

 Power for Small Craft 



where A is the weight in long tons and Ki is 

 found from Table 77. d, adapted from the Barnaby 

 reference just cited. 



Another simple powering estimate gives the 

 dimensional ratio W/Pg of weight to brake 

 power as a function of the absolute speed V, 

 but without the use of a length parameter. Fig. 

 77. E embodies two graphs by P. Du Cane and 

 one by E. Monk, adapted from the references 

 cited in the figure. The two lines given by Du Cane 

 are intended to be the boundaries of a design lane 

 lying between them. It is obvious from these 

 graphs that the weight of boat per unit of shaft 

 or brake power, or the amount of shaft or brake 

 power per unit weight of boat, each of them often 

 quoted in the Uterature, vary widely with the 

 designed speed. 



Although the legend of P. G. TomaUn's "Nomo- 

 gram for Speed and Power" [SNAME, 1953, 

 Fig. 8, p. 601] states that it applies to planing 

 hulls, experience indicates that the power pre- 

 dictions given by it are too low except for values 



