Sec. 60.11 



SHIP-POWERING DATA 



375 



The illustrative calculation is made for the de- 

 signed speed only; this is 20.5 kt or 34.62 ft per sec. 

 (2) The speed of advance V a is the ship speed 

 V times (1 — w). In numbers, for the example 

 cited, this is V^ = 34.62(1 - 0.072) = 34.62 

 (0.928) = 32.127 ft per sec. Then 



J = 



32.127 



nD 1.502(24.22) 



= 0.883 



(3) Consulting the characteristic curves for TMB 

 model propeller 1986 used on the test in question, 

 as shown in Fig. 78. H, the value of the real or 

 working efficiency rjo for a J-value of 0.883 is 

 0.750; this value is indicated by a note and an 

 arrow on Fig. 78.H. From the general expression 

 Vp = Voivif)'nR , the relative rotative efficiency is 



0.686 



Voivn) 0.75(0.889) 



= 1.029. 



For the single-screw transom-stern ABC ship 

 the self-propulsion model tests with a stock pro- 

 peller, reported in Figs. 78.Na, 78.Nb, and 78.Nc, 

 gave a propulsive coefficient rjp of 0.761 at the 

 designed speed. For the advance ratio J at which 

 the propeller operated in this test, the value of 

 »7o from the characteristic curves of Fig. 78.Mc 

 was 0.685. The hull efficiency rin , based on the 

 thrust delivered by the model propeller, was 

 1.148. By the relationship between these four 

 sets of 77-values, 



0.761 



„„(„„) (0.685)(1.148) 



= 0.968 



This value is well below the one that would 

 have been predicted by Schoenherr. Since it is 

 less than 1.00, it works to the ship's disadvantage. 

 There is no present explanation for it. 



60.11' Determination of the Propulsive Co- 

 efficient. A great deal of guessing was involved 

 in the estimates of propulsive coefficient rip in 

 the days before model basins made tests of self- 

 propelled models. Since that time, naval archi- 

 tects and marine engineers have rehed heavily 

 upon the results of individual model tests to 

 supply them with needed information as to the 

 shaft power to be installed in the ship built from 

 a particular design. The result is a dearth of 

 systematic data by which to predict the correct 

 propulsive coefficient for any given case. One 

 might say that in the days when one had to make 

 this estimate in order to power a ship there was 

 insufficient background information to do it. 

 When the designer no longer had to make it he 



did not trouble to analyze fully all the data 

 available to him. 



The situation was well described by K. C. 

 Bamaby in the early 1940's ["The Coefficient of 

 Propulsive Efficiency," INA, 1943, pp. 118-141] 

 and it has not improved materially up to the 

 time of writing (1955), despite publication of the 

 data to be mentioned presently. 



In tables published with his 1943 paper, 

 Barnaby gave many values of t/p for a number of 

 different types of ships, based primarily on a 

 variation of ?jp with V I VL or Taylor quotient 

 r„ . However, in his later book "Basic Naval 

 Architecture" [1948, Art. 187, pp. 242-244], he 

 presents these values on a basis of absolute ship 

 length, but subdivided for single-screw, twin- 

 screw, and quadruple-screw propulsion. 



W. P. A. van Lammeren, L. Troost, and J. G. 

 Koning present values of propulsive coefficient 

 r\p for single-screw ships, for twin-screw ships, 

 and for coasters (the latter presumably all 

 single-screw vessels), based upon the rate of 

 propeller rotation n [RPSS, 1948, pp. 284-288]. 

 D. W. Taylor gives only general information on 

 this subject and that of little help to the designer 

 of a modern ship [S and P, 1943, p. 178]. 



Since the efficiency of propulsion depends upon 

 a combination of the open-water or working 

 propeller efficiency 770 , the hull efficiency t//, , 

 and the relative rotative efficiency ?;« , it should 

 respond to variations in those efficiencies with 

 the factors which control them. Among these 

 may be mentioned: 



(a) Type of propulsion device, whether open 

 screw propeller, shrouded screw propeller, paddle- 

 wheel, rotating-blade propeller, or their equiva- 

 lents 



(b) Relative position of ship and propulsion 

 device, involving tip and aperture clearances, 

 shape of hull near the propulsion devices, and 

 other similar factors 



(c) Thrust-load factor Ctl , which limits the 

 ideal efficiency 57/ and the 0.8-value of that 

 efficiency, illustrated in Fig. 34. G 



(d) Wake and thrust-deduction fractions, and 

 the combination of the two 



(e) Characteristics of the flow at the propulsion- 

 device position (s), determining the relative 

 rotative efficiency. 



Consideration of (a) leads to the conclusion 

 that entirely separate sets of prediction data are 

 required for each type of propulsion device. 



