Srr. 70.4 



SCREW-PROPELLER DESIGN 



585 



and to be derived, a good screw-propeller dala 

 sheet or design chart meets the following require- 

 ments, listed in the order of their importance: 



(a) As for any graph of its kind, it should ". . . 

 show at a glance the variation of the most import- 

 ant dependent variables with the independent 

 variable" [Schoenherr, K. E., SNAME, 1951, p. 

 629] 



(b) The range of the sheet or chart should cover 

 all propulsion conditions that may reasonably be 

 expected 



(c) The most important variable to be derived 

 from each sheet should appear in the formulas 

 applicable to that sheet in its first power and 

 should occupy the principal position in the formula 

 or chart 



(d) The primary numerical design values to be 

 taken from the chart should require little or no 

 interpolation between curves to give the precise 

 engineering answer 



(e) The primary variable on at least one sheet of 

 a group should be the pitch/diameter ratio 



(f) The chart should be readily entered and the 

 desired values found without effort, confusion, 

 or misunderstanding. It is better to have separate 

 charts than to embody too many features on one 

 chart. In other words, a propeller design chart 

 "should possess graphical simplicity, permitting 

 ease of I'eading and interpolating" [Kane, J. R., 

 SNAME, 1951, p. 626]. 



(g) All chart parameters should be dimensionless, 

 with dimensions in any system of units to be 

 derived by simple substitution and calculation 

 (h) The chart should be no larger than necessary 

 for the precision required in ship and propeller 

 design but large enough for easy visual selection 

 of the data desired 



(i) Nomograms should be embodied, wherever 

 practicable, for determining the values of chart 

 parameters and certain physical quantities 

 (j) With at least three variables given, the chart 

 should yield all the data for the preliminary 

 design of a screw propeller, specifically: 



(1) Diameter, pitch, and pitch-diameter ratio 



(2) Number of blades (considering propeller 

 efficiency only), expanded-area ratio, mean- width 

 ratio, and blade-thickness fraction 



(3) Blade shape (outline) and blade-section 

 shape 



(4) Hub-diameter ratio 



(5) Maximum or actual open-water efficiency 

 or both 



(6) Possibility and effect of cavitation, perhaps 

 by auxiliary charts. 



(k) There should be sufficient charts in a group 

 to permit a propeller designer to enter them with 

 given values of any of the primary variables 

 (1) Future chart groups should embody the 

 approved symbols adopted by the International 

 Towing Tank Conference. 



Propeller-series charts must necessarily be 

 adaptable to the several variations of the design 

 problem actually encountered. These depend upon 

 which factors are known or given and which are 

 unknown. K. E. Schoenherr has set down this 

 situation in systematic fashion, from which the 

 following is adapted [PNA, 1939, Vol. II, p. 159]: 



First, Preliminary Design. 



(i) Given the designed ship speed V, the 

 corresponding effective power Pjs or total resist- 

 ance Rt , and the propeller diameter D. Required 

 to find the propeller pitch P and the rate of 

 rotation n (in rpm) for the best propeller efficiency. 



(ii) Given the designed ship speed V, the cor- 

 responding effective power Pe , and the rate of 

 rotation n of the propeller shaft. Required to 

 find the propeller pitch P and diameter D for 

 the best efficiency. 



Second, Final Design. Given the curve of effective 

 power P E as a function of ship speed V, the pro- 

 peller diameter D, the rate of rotation n (in rpm) 

 and the engine output in horses, at the designed 

 rpm, as delivered to the propeller shaft. Required 

 to find the propeller pitch P, the propeller 

 efficiency 570 , and the ship speed V obtainable 

 under the given conditions. 



Third, Analysis. Given the propeller dimensions, 

 the ship speed V, the shaft power Ps , the pro- 

 peller thrust T, and the rate of rotation n, in rpm. 

 Required to find the fractions Sr for real slip, 

 w for wake, and t for thrust deduction. 



A considerable number of chart groups are 

 now available, in one form or another, to the 

 propeller designer. These are fisted hereunder, 

 as an adaptation of a listing and a description by 

 F. M. Lewis [SNAME, 1951, pp. 612-613]: 



(I) Charts of R. E. Froude; also known as the 

 series charts of R. W. L. Gawn [Froude, R. E., 

 INA, 1892, pp. 292-294; INA, 1908, pp. 185-204; 

 Baker, G. S., SD, 1933, Vol. II, Fig. 14 and pp. 

 41-44; Gawn, R. W. L., INA, 1937, pp. 159-187; 

 van Lammeren, W. P. A., RPSS, 1948, pp. 251- 

 256]. 



