Sec. 70.37 



SCREW-PROPELLER DESIGN 



629 



range of values of the skew-back at the tip is from 

 0.20 to 0.25cMax . For the ABC propeller, the tip 

 skew-back is taken as 0.245cMax or L25 ft. An 

 easy curve, representing the locus of the mid- 

 lengths of the blade-section chords, is then drawn 

 for the skew-back line, starting at the extreme-tip 

 section and approaching the pitch reference line 

 as a tangent at the hub. The expanded outline is 

 laid out, half a chord length on each side of the 

 skew-back line. From this the projected outline 

 is drawn, explained in Sees. 32.9 and 32.10 and 

 illustrated in Fig. 32. L With the projected outline 

 sketched in, the angular variation of the leading 

 edge as each blade section passes the vertical 

 plane through the 12 o'clock propeller position 

 is checked. The interval between sections for 

 the ABC ship propeller, drawn in Fig. 70.O of 

 Sec. 70.36, is as follows: 



0.2ft- 



0.3ft- 



0.4ft- 



0.5ft- 



0.6ft- 



0.7ft- 



0.8ft- 



0.9ft- 



1.5 deg 

 2.1 deg 

 2.4 deg 

 2.8 deg 

 2.7 deg 

 2.7 deg 

 4.4 deg 



This shows a fairly regular interval and is con- 

 sidered satisfactory. It may be necessary at 

 times to draw several skew-back lines before a 

 satisfactory angular interval is obtained. 



Several schemes were tried to achieve this. It 

 was finally concluded, as related in Sec. 70.16, 

 that the locus of the midlengths of the expanded 

 blade sections represents the most convenient 

 construction line. It is almost impossible to start 

 with an arbitrary projected outline and finish 

 with fair contours in the expanded outline. 



70.36 Drawing the Propeller. All unknowns 

 have been calculated or determined so it is now 

 possible to delineate the propeller. The final 

 drawing of the ABC design, following the arrange- 

 ment and details laid down in Fig. 32. F of Volume 

 I or on the SNAME PD sheet of Fig. 78.L, is 

 shown in Fig. 70.O. A large propeller having the 

 same general blade shape and the same charac- 



teristics, except that it has constant rather than 

 variable pitch, is illustrated in a photograph 

 published by The Marine Engineer and Naval 

 Architect [Aug 1954, p. 300]. 



One point needs explanation. The required 

 blade-thickness fraction to/D is 0.053 as calculated 

 from the strength considerations. The blade- 

 thickness fraction shown on Fig. 70.O is only 

 0.049. 



The axis thickness to on Fig. 70.O has been 

 determined graphically by conventional practice 

 [SNAME, Tech. and Res. Bull. 1-13, Jul 1953, 

 p. 22]. As can be seen from Fig. 70.O, this con- 

 vention gives a blade-thickness fraction that is 

 not truly representative of the thickness at the 

 hub. Thus the actual tx , equal to AB in Fig. 

 70.O, is greater than CD, indicated by the con- 

 struction lines for determining to . The propeller 

 actually has the correct thickness required by 

 strength considerations, and to/D would equal 

 0.053 if the face and back lines were straight. 

 However, for the sake of uniformity, the con- 

 ventional method for finding to graphically should 

 always be followed. 



70.37 Calculating the Expected Propeller E&- 

 ciency. The final step in the design, by Lerbs' 

 short method, is to calculate the expected pro- 

 peller efficiency. This is given by the following 

 relationship: 



1 - 2X,e 



[-(i)d 



(70.xix) 



where rjo is the propeller efficiency 



r]K is the ideal efficiency with jet rotation and is 

 equal to 0.783, from Fig. 70.E 



e(epsilon) is the drag-lift ratio of a blade section 

 or airfoil. 



A close approximation of e is given by: 



0.008 



0.008 



Ci, at 0.7ft 0.3064 



0.0261 



Cl is obtained from Col. D, Table 70.i. 

 Xj = x' tan /3, c at 0.7ft, where tan /J/ c is obtained 

 from Col. G, Table 70.g. 



7/0 = 0.783 



0.7(0.5302) = 0.3711 

 1 - 2(0.3711)(0.0261)' 



1 + 



2Y 0.026l \ 

 3A0.371I/ J 



= (0.783)(0.9367) = 0.733 

 no = 73.3 per cent. 



