Sec. 70. •) 



SCREW-PROPELLER DESIGN 



589 



So far as known, the design methods and 

 charts developed by C. W. Dyson ["Screw Pro- 

 pellers," Simmons-Boardman, New York, 1924] 

 are no longer used by propeller designers. 



70.5 Comments on and Comparison of Pro- 

 peller-Series Charts. Some or all of the pro- 

 peller-series charts listed in Sec. 70.4 possess 

 certain disadvantages, rendering them less than 

 convenient for the use of the propeller designer: 



(a) The parent series of models possesses charac- 

 teristics known to be inferior to those of later 

 designs. Specifically, they have ogival root 

 sections, blade outlines without skew-back, blade 

 root sections that are too thin, and so on. This is 

 no fault of the chart makers but a feature in- 

 herent in their age. 



(b) It is necessary to interpolate between irreg- 

 ularly curved lines to find the proper P/D ratio. 

 The basic series diagrams or Bp and Bu charts of 

 D. W. Taylor [S and P, 1943, pp. 275-292] are 

 admirable in this respect, with their uniform 

 scales of pitch-diameter ratio, closely subdivided. 



(c) It is necessary to make preliminary calcula- 

 tions or tabulations, to draw an auxiliary curve 

 on the chart, and to locate its intersections with 

 certain chart curves before determining the 

 value of the parameter desired 



(d) The charts as reproduced in the literature 

 are too small and too crowded with lines for 

 everyday work. This situation may be remedied 

 in some cases by procuring large-scale prints of 

 the charts from the originators. 



(e) They are not usable for small values of the 

 advance coefficient J or large values of the real 

 slip ratio Sr, , as for problems involving towing. 

 The Fermann charts are in effect inversions of 

 many of the standard charts, in that the param- 

 eter values corresponding to low advance coeffi- 

 cients and extra-large real-slip ratios are at the 

 working ends. 



Considering the rather varied amovuit of pro- 

 peller information useful in the preliminary 

 design of a ship, where backing, maneuvering, 

 and operations other than propulsion must be 

 considered, the so-called logarithmic type of 

 propeller chart has much to recommend it. This 

 is on the basis that the designer does not object 

 to a rather concentrated serving of technical 

 information, all on one piece of paper. 



The logarithmic method of presentation, as 

 far as can be learned, was originated by Gustav 

 Eiffel in France and later developed by Wilhelm 



Schmidt in Germany, indicated by the following 

 references to Eiffel's work: 



(1) "Nouvelles R6cherches sur la R&istance de 

 I'Air et I'Aviation (New Research on Air Resist- 

 ance and Aviation)," Paris, 1914 



(2) "Travaux Ex^cutfe Pendant la Guerre, 

 1915-1918 (Projects Completed During the War, 

 1915-1918)," Paris, 1919 



(3) "L'Etude sur I'H^lice Aeri6nne (A Study of 

 the Airscrew)," Paris, 1920. 



The most modern and the most useful, as well 

 as the most comprehensive logarithmic presenta- 

 tion is that of C. W. Prohaska, of the Institute 

 of Technology of Denmark, in Copenhagen. 

 Many of these diagrams are based upon test 

 data from the Wageningen series of propellers 

 developed by L. Troost but there are others in 

 the group based upon tests of single propellers. 

 One such diagram is presented in Fig. 70. A. 



This diagram contains the usual propeller 

 characteristic curves of torque coefficient Kg , 

 thrust coefficient Kt , and open-water efficiency 

 7j„ , all non-dimensional. The abscissas, embodying 

 two separate scales, are 0-diml values of the 

 advance coefficient ./ and dimensional values 

 (in English units) of the Taylor advance coeffi- 

 cient S. The ordinates are a series of simple 

 numbers, ranging from 0.006 to 1.0, for the 

 0-diml coefficients and for the efficiency fractions. 

 Both horizontal and vertical scales are logarith- 

 mic. 



There are 4 diagonal scales on the diagram, 

 3 double and 1 single. The upper scales of each 

 of the three pairs represent the dimensional 

 values of the factors A, Ba , and Bp of D. W. 

 Taylor's notation, respectively. The mathematical 

 expressions for each of these, in English units of 

 tons, feet, horses, and knots, are listed in a 

 column in the upper left-hand corner of the 

 diagram. The lower scales of each pair represent 

 the 0-diml thrust-load factor Ctl , and the basic 

 0-diml factors 6,. and bg of Prohaska, respectively. 

 The fourth single scale gives values of the 0-diml 

 fraction TD/Q. 



The original Prohaska charts, such as those 

 from which Figs. 70. A and 70.B are adapted, 

 carry additional scales showing the values of 

 7vo , Kt , and tjo for / = 0, and the values of / 

 for Kq = and Kt = 0. These hmit scales are 

 omitted from the reproductions to avoid excessive 

 complication. 



The values on the three curves of Kq , Kt , and 



