Sec. 70.22 



SCREW-PROPELLER DESIGN 



611 



(a) Determine the number of blades, the pro- 

 peller diameter, the hub diameter, the rake, if 

 any, and the rate of rotation 



(b) Calculate the required thrust-load coefficients 

 and advance coefficients 



(c) Determine the ideal efficiency with jet rota- 

 tion from Kramer's charts 



(d) By successive approximations, calculate the 

 hydrodynamic pitch angle and the thrust distri- 

 bution over the blade which allows the propeller 

 to develop the required thrust 



(e) Determine the lift-coefficient product, apply 

 the lifting-surface correction to the hydrodynamic 

 pitch angle, and calculate the hydrodynamic pitch 

 distribution 



(f) From strength considerations determine the 

 blade-thickness fraction and the maximum-blade- 

 thickness distribution 



(g) Choose the type of meanline and thickness 

 form to be used for the blade sections 



(h) Using cavitation criteria, determine the maxi- 

 mum camber of the meanline and the chord 

 lengths of the blade sections from appropriate 

 charts 



(i) Fair the blade outline and determine the 

 final chord lengths, camber ratios, and lift co- 

 efficients 



(j) Apply the curvature correction to the camber 

 ratio 



(k) Correct for viscous flow by adding an angle 

 of attack at the various blade sections. Calculate 

 the final pitch distribution. 



(1) Determine the amount of skew-back, if any 

 (m) Draw the propeller 



(n) Calculate the final propeller efficiency cor- 

 responding to the design conditions. 



Owing to the intricacy of all existing design 

 methods for screw propellers, based upon the 

 vortex or circulation theory, it is necessary to 

 employ a number of special symbols. For the 

 Lerbs method described and illustrated in the 

 sections following, all the special symbols, addi- 

 tional to the standard symbols listed in Appendix 

 1, are defined when they are first used. Neverthe- 

 less, these special symbols are listed here in one 

 place, with brief titles for each: 



{Ctl)s Thrust-load coefficient based upon the 



ship speed V instead of upon the usual 



speed of advance Va 



i Tangent of one half the angle of rake 



of a screw-propeller blade, based upon 



the notation of D. W. Taylor [S and P, 

 1943, p. 134] 

 k Ludwieg-Ginzel curvature correction, 

 applied to the camber ratio 

 m.Yo Maximum section camber, corrected 

 Sc Allowable stress in a propeller blade, 

 based upon the notation of D. W. 

 Taylor [S and P, 1943, p. 137] 

 Ti Non-viscous thrust, as in a perfect 



hquid 

 x' Ratio of local radius R to tip radius 



^Max of propeller 

 iSi- Average local wake fraction 

 Wj' Average local wake fraction, corrected 

 to match the effective wake 

 (3/ c (beta) Corrected hydrodynamic pitch angle 



e Drag-lift ratio of an airfoil or blade 

 (epsilon) section 



Tjff(eta) Kramer's ideal efficiency, with jet 

 rotation 

 X Absolute advance coefficient, equal to 

 (lambda) yx/(7mD) 



Xs Absolute advance coefficient, based 

 upon ship speed V instead of speed of 

 advance V a 

 ju(mu) Viscous-flow correction 



(Ts Cavitation number based upon ship 

 (sigma) speed V instead of the resultant inci- 

 dent velocity F^ on a blade section. 



70.22 ABC Ship Propeller Designed by Lerbs' 

 1954 Method. As a help in understanding Lerbs' 

 method, and as a practical illustration of its use, 

 a screw propeller is designed here for the transom- 

 stern ABC ship. The design is based upon the 

 wake survey diagrammed in Fig. 60. M. 



Although cavitation is not expected on this 

 ship, and the wake-velocity distribution is in no 

 way unusual, the design procedure is carried 

 through as though these two features presented 

 real problems. 



When selecting the model propeller to be used 

 in the self-propulsion model tests of the transom- 

 stern ABC ship designed in this part of the book, 

 an estimated propeller thrust of 193,476 lb was 

 used. This led to the conclusion presented in 

 Sees. 70.6 and 78.4 that the stock model propeller 

 should have a P/D ratio of 1.02, four blades of 

 moderate width, and airfoil sections along the 

 inner radii. The calculated rate of rotation was 

 109.2 rpm at the designed ship speed of 20.5 kt. 

 The corresponding advance coefficient J was 

 0.703, and an open-water efficiency of 68.0 per 



