358 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 60.4 



be considered typical for medium and large single- 

 screw vessels as a class. 



Thei-e are in the archives of many ship-model 

 testing establishments numerous sets of graphs 

 similar to those of Figs. 60.B and 60. C. In these 

 the effective powers, and sometimes the friction 

 powers and other factors, are given for selected 

 ship models when run at widely different dis- 

 placements and trims. For cases similar to these, 

 where the light displacements vary by some 30 

 and 45 per cent, respectively, from the heavy or 

 normal-load displacements, the range is rather 

 large for making estimates of effective-power 

 changes by the graphs of Fig. 60.A. 



60.4 Methods and Factors Involved in Pre- 

 dicting Shaft Power. The preliminary design of 

 a ship can not proceed very far until the shajt 

 power Ps needed to drive it at the designed speed 

 is determined in some manner. It is customary to 

 estimate or to predict this power by one or more 

 of a series of methods, involving successive 

 approximations. However, to afford a better 

 understanding of the procedures underlying some 

 of the early approximations, the later approxima- 

 tions are described first. The discussion here is 

 Umited primarily to screw propulsion. 



The shaft power Ps is obtained directly from 

 the effective power P^ by dividing the propulsive 

 coefficient T?i.(eta) into the latter. This is simple 

 but estimating the proper value of tip is not. 

 From Eqs. (34.xv) and (34.xvi) of Sec. 34.7, 



Vp = Voiv hJvr = Vo^ 



I — w 



The value of 770 is known for the working range 

 of a considerable number of screw propellers 

 suitable for driving a wide variety of ships. 

 Published data can be supplemented by informa- 

 tion obtained from model basins which have 

 tested many propeller models. However, the 

 working range of the advance coefficient J is 

 also rather large, and tjo may vary rather rapidly 

 with J in that range. A J-value may be chosen, 

 for a propeller not too heavily loaded, just under 

 (less than) the J-value for maximum 170 . 



Estimating the hull efficiency rja involves 

 estimates of both the thrust-deduction fraction t 

 and the wake fraction w. Methods of accomplish- 

 ing this, in advance of or without self-propulsion 

 tests of the ship model, are described presently in 

 Sees. 60.8 and 60.9. Estimating the probable value 

 of the relative rotative efficiency jjk is described 

 in Sec. 60.10. 



It is possible, with experience, to estimate the 

 propulsive coefficient rjp directly, as is done for 

 the first approximation to the shaft power of the 

 ABC ship in Sec. 66.9 of Part 4. It is extremely 

 difficult, however, to analyze this kind of experi- 

 ence and to set it down as a design rule. An 

 attempt to do it is set down in Sec. 60.11. 



A procedure for the preliminary estimate of the 

 shaft power of merchant ships recently described 

 by V. Minorsky [Int. Shipbldg. Prog., 1955, Vol. 

 2, No. 9, pp. 226-229] is in reality a dimensional 

 version of the Telfer merit factor described in 

 Sec. 34.10. It takes the form 



Powering factor = 



V 



V'Ps 



as compared to the 0-diml 



gLPs 



Telfer merit factor M 



WV 



(34.xxiv) 



The latter is assumed in Fig. 34.1 on page 518 of 

 Volume I to vary in some manner as Fl , with 

 further variations for better-than-average or less- 

 than-average performance. The expression of V. 

 Minorsky uses only a single average factor, which 

 varies somewhat with block coefficient Cb and 

 speed-length quotient "F/vL. 



G. Deparis, in his paper "Etude Comparative 

 des Cargos; Puissance des Moteurs (Comparative 

 Study of Cargo Vessels; Propelling-Plant Power)" 

 [ATMA, 1955, Vol. 54, pp. 499-549], describes 

 methods whereby "guestimates" may be made of 

 propelling-plant power and speed on a basis of 

 useful load, in an early stage of the preliminary 

 design. The factors given are based upon a study 

 of the characteristics of many ships, including 

 undoubtedly the inefficient as well as the efficient 

 ones. 



J. E. Burkhardt discusses several methods of 

 estimating ship power [ME, 1942, Vol. I, pp. 

 22-28] but all of them are covered in the present 

 book, in one form or another. Descriptions of 

 other methods of predicting shaft power are 

 embodied in Sees. 60.13, 60.14, and 60.15. 



60.5 Axial-Component Wake-Fraction Dia- 

 grams at Propulsion-Device Positions. Charac- 

 teristics of the wake at propulsion-device positions 

 are discussed in Chap. 11. Methods for indicating 

 the situation graphically with respect to wake 

 velocity and direction over the whole thrust- 

 producing area are described there, specifically 

 as applying to a screw propeller. 



There is a great amount of published data on 



