Sec. 60.9 



SHIP-POWERING DATA 



S71 



independently powered model Froude, wisely 

 included runs in which the single propeller was 

 placed farther and farther abaft the sternpost, 

 varying from its normal position to about 1.15D 

 astern. Because of missing information it is not 

 possible to analyze Peabody's test data in the 

 manner presently to be described. Nevertheless, 

 his Table I [SNAME, 1911, p. 95] indicates that at 

 the highest speed reached by this craft, the 

 thrust- deduction fraction / diminished from 0.35 

 to 0.077 for the range of propeller positions given. 



In a paper "Vom Sog (Thrust Deduction)," 

 H. M. Weitbrecht discussed the physical aspects 

 of thrust deduction but pointed out that it was 

 not then possible to predict the numerical value 

 of the thrust deduction for a given ship form and 

 propeller loading [Schiffbau, Schiffahrt, und 

 Hafenbau, Jun 1938; English version in TMB 

 Transl. 62 of Sep 1940]. 



K. E. Schoenherr and A. Q. Aquino, in the 

 period 1930-1940, made a careful review of the 

 existing literature on the ship-propeller interaction 

 problem, undertook their own analysis, and 

 supplemented it with plotted observations from 

 the results of self-propelled tests on a great many 

 models. Their work is described fully in TMB 

 Report 470, published in March 1940. The 

 following rules for estimating the thrust-deduction 

 fraction, published by K. E. Schoenherr in 1939 

 [PNA, Vol. II, pp. 149-150], were developed from 

 this project: 



(1) For the thrust-deduction fraction of single- 

 screw ships: 



t = kw (60 .vi) 



where k = 0.5 to 0.7 for vessels with streamlined 



or contra-rudders 

 = 0.7 to 0.9 for vessels with double-plate 



rudders with internal arms, attached 



to square rudder posts 

 = 0.9 to 1.05 for vessels with single-plate 



rudders and external arms 



(2) For the thrust-deduction fraction of twin- 

 screw ships, specifically: 



(a) Ships with propellers and shafts carried 

 by bossings 



t = 0.25W + 0.14 



(60.vii) 



(b) Ships with propellers and exposed shafts 

 carried by struts 



t = O.lOw + 0.06. 



For the transom-stern, single-screw ABC ship 

 designed in Part 4, the estimate of the wake and 

 thrust-deduction fractions posed somewhat of a 

 problem, because of the unorthodox stern shape 

 and the lack of empirical data upon which to base 

 predictions. With little information for guidance, 

 with a screw propeller of diameter larger than 

 normal, and with a tip clearance smaller than 

 normal, it was guessed in Sec. 66.27 that the wake 

 fraction w would be as high as 0.30 and the thrust 

 deduction as low as 0.20. The corresponding hull 

 efficiency r]„ of 1.143 seemed reasonable. 



When a stock propeller was selected to self- 

 propel the model, by the procedure described in 

 Sec. 70.6, the wake fraction derived by Eq. 

 (60.ii) was 0.261, using dimensions and parameters 

 corresponding to an early stage of the design. 

 The thrust-deduction fraction was derived from 

 Eq. (60. vi). The value of k for the latter was taken 

 as 0.5, because of the contra-rudder shape pro- 

 posed for the supporting horn and the underhung 

 balance portion of the rudder. It seemed reason- 

 able, further, to reduce the calculated value by 

 15 per cent, because of the very thin skeg to be 

 placed ahead of the propeller. The predicted 

 thrust-deduction fraction then worked out as 



t = kiw)il - 0.15) = 0.5(0.261)(0.85) = 0.111 



It was realized at the time that a thin skeg 

 ahead of a single propeller was Hable also to 

 reduce the wake fraction. For a conservative 

 estimate, without the 15 per cent reduction in i, 

 the predicted hull efficiency rjn was 



t 



1 - 0.131 



1 



1 



0.261 



= 1.176 



It is brought out in (2) and (3) of Sec. 78.17 

 that the thrust-deduction and wake fractions 

 derived from the model self-propulsion tests are 

 appreciably different from those derived in these 

 two sections. 



B. V. Korvin-Kroukovsky gives the following 

 equation from H. E. Dickmann for the estimated 

 value of the thrust-deduction fraction: 



t = (w.) 



I + Vi + c, 



{w,)7,i (60.viii) 



where w^ is the nominal potential-wake fraction 

 and rji is the ideal efficiency of the propeller. The 

 problem here is to find the value of Wp , for which 

 there is no simple solution. 



An entirely different prediction procedure, 



