Sec. 60.14 



SHIP POWERING DATA 



III I 



TB" 



0:3 . 05 0.6 3.7 0.6 0.9 \J0 I I.I 



T-r.^ 



T 



250 

 1) 

 ■200: 



Weiqht-Speed- Power Foctor- 6.88 — p — 

 where W is in lonq tons, V m kt, fg in horses 



£ 60 

 ' 30 



:e: 



Loke Tank- Corqo Ships, 



reiqhters ers Liners, Corners 



Destrouers and Destrouer Escorts 



±J 



"0 0.02 0.04 0.06 0.08 0.10 O.ia 0.14 0.16 0.18 0.20 0.22 0.24 026 



Froude Number squared - {y/'^aL.) 



Fig. 60.U Tentative Mbanline for Selecting the Weight-Speed-Poweb Factor for an Average Vessel 



M = 0.61 



LPs 



(16,573)(20.5)' 

 (510)(13,243) 



12.895 



This is nearly 36 per cent greater than the mean- 

 line value given by Fig. 34.1, indicating that the 

 merit-factor method of estimation still requires 

 considerable development. 



An alternative merit factor makes use of a 

 somewhat simpler O-diml relationship in the 

 form of 



WV 

 Ps 



Weight-Speed-Power Factor = 



(60.x) 



This is, in fact, the Telfer merit factor M divided 

 by the Froude number squared, indicated as one 

 of the terms in Eq. (34.xxiv) on page 517 of 

 Volume I. When WV/Pg is plotted on a basis of 

 F^ , as in Fig. 60.U, it indicates a dispersion for 

 vessels of supposedly normal design as great as 

 that evidenced in Fig. 34.1 for the Telfer merit 

 factor. Indeed, with a plot having a uniform 

 instead of a log scale of ordinates, the dispersion 

 in both would be much greater. It is apparently 

 true that the propulsive merit of some vessels is 

 vastly superior to that of others. 



Using the tentative meanline of Fig. 60. U as 

 an indicator for estimating the probable shaft 

 power of an ABC ship design of average merit, 

 the estimated power is worked out in Sec. 66.9. 



60.14 Shaft-Power Estimates by the Ideal- 

 Efficiency Method. A method of deriving the 

 propulsive coefficient jjp from the ideal and real 

 efficiencies of a screw propeller under a given 

 thrust-load condition is described fully in Sec. 

 34.11 of Part 2, Volume I. An ampUfied version 

 of that description is given here for the con- 

 venience of the reader. From the effective power 

 and the propulsive coefficient the shaft power is 

 found. This method also enables the designer 

 to select a suitable P/D ratio for the propeller, 

 and to approximate its rate of rotation n. 



Assuming that this procedure is to be used in 

 the early stages of a ship design, it is necessary 

 to estimate or determine the total ship resistance 

 Rt , and to select values of the wake and thrust- 

 deduction fractions w and t, and of the relative 

 rotative efficiency tjr , which appear reasonable 

 for the design in question. The ship speed V is 

 assumed to be known, as are the number of 



