Sec. 70.16 



SCREW-PROPELLER DESIGN 



603 



(a) Van Lammeren, W. P. A., RPSS, 1948, pp. 226-227 



(b) Schoenherr, K. E., PNA, 1939, Vol. II, pp. 157, 162. 



On page 157, 1st col., there is given a formula (119) 

 which produces a practical form of expanded blade 

 outline that is generally elliptical in shape. Although 

 not stated there, this is for a propeller not subject 

 to cavitation. 



Comments concerning proper blade widths are 

 given in Sec. 70.17. 

 70.16 Selecting and Applying Skew-Back. 



The hydrodynamic reasons for applying skew- 

 back to screw-propeller blades are explained in 

 Sec. 32.15. Repeated briefly, skew-back is used 

 to prevent certain corresponding points on the 

 blade sections at every radius, or at a group of 

 adjacent radii, from passing simultaneously 

 through a region of high wake velocity directly 

 abaft a skeg ending, a bossing termination, or a 

 large strut. The necessity for skew-back in the 

 design of a surface propeller is perhaps somewhat 

 easier to understand. Here it is not desirable 

 that the entire length of the leading edge of a 

 blade, or an appreciable portion of that edge, 

 should swing downward and strike the water 

 surface at the same instant. 



The actual application of skew-back is handi- 

 capped, however, because it is not yet known 

 just what part of a blade element requires to be 

 offset successively, in time or in angular position, 

 from the high-wake region. This part is probably 

 not the leading edge, nor the trailing edge, nor 

 the locus of the midlengths of the expanded 

 elements. It is possibly the locus of the centers of 

 pressure of the elements, possibly the locus of the 

 points of maximum thickness, or better still the 

 locus of points opposite a certain part of the pres- 

 sure field of each element, as yet not known. 

 Lacking this information, the locus of the positions 

 of maximum blade-element thickness is probably 

 the best but, as explained presently, the use of 

 this line is neither convenient nor practical. 



Little is yet known about the rate at which 

 the backward offset of the selected skew-back line 

 should change with blade radius or, in other 

 words, what should be its shape, defined in 

 Sec. 32.15. Manifestly, the basic reference line 

 or plane for estabUshing skew-back is the position 

 of the region of maximum wake velocity, at or 

 near its intersection with the plane of the pro- 

 peller disc. For a vertical, symmetrical, center- 

 plane skeg ending this is the vertical plane 

 through the propeller axis. For a contra-guide 

 skeg ending the locus of the maximum-wake 

 positions in the plane of the disc is not known 



Arc for Meosurinq Ancjles 



of Rotation 25^''( 

 First Position of |g \ 

 Tronsporent /\ J>^^\. 



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Ending-, 



40 45 50 



ted Si^ew-Bock Locus 

 EntireN Schematic 



Graphical Method fob Checking 

 Skew-Back Line 



exactly, because of the oblique flow occurring in 

 the fore-and-aft clearance space between the 

 skeg ending and the propeller disc. 



Lacking better data, such as a comprehensive 

 wake survey would give, it appears satisfactory 

 to assume that the basic or reference trace for 

 skew-back is that of the actual skeT; ending, pro- 

 jected axially aft to the propeller disc. Such a 

 line appears on Fig. 70. C, showing the elevation 

 from aft of the upper half of a skeg ending with 

 contra-guide features. 



Summarizing, the trace serving as the basic 

 reference, when establishing skew-back, may be: 



(a) Straight in the disc plane, as for a symmetrical 

 skeg ending or bossing termination 



(b) Curved in the disc plane, as for a contra- 

 guide skeg ending 



(c) Curved, with modifications, as when the 

 deflected flow abaft a contra-guide ending is 

 projected back to the plane of the propeller disc, 

 or to some other position. 



The locus of the propeller may be: 



(d) Straight in the disc plane, along a radius, as 

 in the elliptical-outline blades with ogival sections 

 of a half-century ago 



