592 



HYDRODYNAATICS IN SHIP DESIGN 



Srr. 70.6 



required of the ship projieller is greater than the 

 predicted total ship resistance i?r by the ratio 

 1/(1 — 0- Further, the propeller efficiency t/b 

 behind the ship is greater (or less) than the open- 

 water efficiency ijo by the relative rotative effi- 

 ciency Tjfi . These three unknown factors may be 

 estimated from analyses of trial data on similar 

 ships [Davis, H. F. D., ASNE, Aug 1932, pp. 

 332-352; Schoenherr, K. E., PNA, 1939, Vol. II, 

 Chap. Ill] or, as is usually the case, they may be 

 determined from tests of a self-propelled model. 



70.6 Preliminary-Design Procedure, Employ- 

 ing Series Charts. For the screw-propeller- 

 design procedure set down in this book several 

 of the series charts are utilized in the preliminary- 

 design stage, particularly for determining the 

 characteristics and for selecting a suitable stock 

 propeller to be used in the first self-propulsion 

 tests of the ABC ship models. This is not to be 

 taken as an indication that series charts are 

 suitable for making only first approximations in 

 the early stages of a ship design. In fact, by far 

 the greater number of propellers designed in 

 practice and manufactured for ships are worked 

 up from these charts, insofar as the propeller 

 features can be determined from them. 



The references of Sec. 70.4 in which the various 

 propeller-series charts are published usually 

 describe in considerable detail the procedure to 

 be followed .for the particular problem at hand. 

 In many cases they also contain examples worked 

 out to illustrate these procedures. 



As examples of the methods of using propeller- 

 series charts there are given here the steps 

 employed and the calculations made for the pre- 

 liminary design of a propeller for the transom- 

 stern ABC ship, leading to the selection of a 

 stock propeller for self-propulsion tests of the 

 first model. 



The following three methods were used: 



(1) That of K. E. Schoenherr, based upon tests 

 of EMB series propellers, as set down in PNA, 

 1939, Vol. II, pp. 158-168, including propeller- 

 design charts 1 through 4 



(2) That of F. M. Lewis, based upon tests of 

 Wageningen B-series propellers, as described in 

 SNAME, 1951, Vol. 59, pp. 618-620 



(3) That of C. W. Prohaska, based upon logarith- 

 mic charts embodying the test data of the Wagen- 

 ingen B series of model propellers. 



derived in Chaps. 66 and 67. The designed ship 

 speed, for wliich the propeller is to give optimum 

 performance, is 20.5 kt. The ship resistance at 

 this speed is estimated in Sec. 66.9 as 171,830 lb, 

 or say 172,000 lb. The corresponding effective 

 power Pb is 10,827 horses. The wake fraction w 

 is estimated from Eq. (60. ii) in Sec. 60.8 as 

 0.261. This figure is different from the 0.255 

 worked out as the illustrative example in that 

 section because it is calculated at an early stage 

 of the design, using preliminary dimensions and 

 parameters instead of the final ABC values in- 

 serted in the illustrative example. • 



The thrust-deduction fraction / is estimated 

 by the method described in Sec. 60.9, applying 

 a 15 per cent reduction to the value derived from 

 Eq. (60. vi) because of the very thin skeg con- 

 templated ahead of the propeller. This gives a 

 predicted value for ^ of 0.111. 



To keep the propeller loading as low as possible, 

 consistent with good performance, four (4) blades 

 are to be used. This means that the blade width 

 and blade thickness can be small, with a resulting 

 rather high efficiency. The mean-width ratio 

 Cm/D is taken tentatively in the range of 0.20 

 to 0.25. The blade-thickness fraction t^/D is 

 assumed to be of the order of 0.04 to 0.05. These 

 values are typical for 4-bladed propellers [PNA, 

 1939, Vol. II, p. 157] and are considered reasonable 

 for the first approximation. Since adequate 

 clearance is allowed in the design of the propeller 

 aperture on the transom-stern ABC ship, and 

 since the skeg ahead of the propeller is relatively 

 thin, the inclination of the streamlines in the 

 inflow jet with reference to the propeller axis 

 should not be unduly large. There appears to be 

 no need, therefore, of raking the blades, especially 

 as they would then have to be thicker to with- 

 stand the offset centrifugal forces. 



The Prohaska preliminary-design procedure is 

 described and illustrated first. Prohaska's pro- 

 peller-design chart, as contrasted to the propeller- 

 data chart of Fig. 70. A, is somewhat more intricate 

 and is used in a somewhat different manner. 

 Fig. 70. B, adapted from one of these charts, 

 contains five sets of Kq graphs, five Kt graphs, 

 and five -qo graphs, one each for a given P/D 

 ratio, plus the four sets of diagonal scales of 

 Fig. 70.A. In addition there are three maximum- 

 efficiency graphs for use when: 



Certain data were taken as basic for all three (i) The tbrust-load coefficient Ctl or the factor 

 methods, using a propeller diameter D of 20 ft, .1 is known 



