Sec. 70.10 



SCREW-PROPELLER DESIGN 



597 



Lerbs short method in Sees. 70.21 through 70.38 

 with paragraphs involving considerations of 

 engineering instead of analysis for hydrodynamics, 

 there are given here some brief comments on the 

 selection of certain physical features as governed 

 by the problem in hand. 



70.9 Selection of Propeller Diameter. In 

 both the series-chart and the Lerbs short-cut 

 exaritiples worked out in this chapter the propeller 

 diameter is selected in advance. If there are 

 operational or other limitations on diameter, the 

 designer has Httle or no freedom of choice. He 

 must do the best that is possible under the 

 circumstances, but with a clear understanding on 

 the part of everyone concerned that it is not the 

 best that could be done if he were free of these 

 limitations. 



If he has freedom of choice as to diameter, he 

 proceeds on the basis that the propeller is the 

 most important unit in its part of the ship, and 

 that it can have whatever diameter best fits it 

 for the task to be performed. If propulsive effi- 

 ciency is not a primary requirement then almost 

 any available propeller can be used and only 

 the sketchiest of design procedures is necessary. 

 Sec. 34.2 describes and illustrates the reasons 

 why screw propellers with the largest diameter 

 and the lowest thrust loading may be expected 

 to work at the highest real efficiency. Some years 

 ago K. E. Schoenherr said ". . . in general, that 

 propeller is the best choice which has the largest 

 diameter admissible in the propeller aperture" 

 [PNA, 1939, Vol. II, p. 166]. F. M. Lewis removed 

 the latter restriction by saying that "In nearly 

 all cases of modern ships the optimum diameter 

 will be the largest that is practical, . . ." [SNAME, 

 1951, p. 619]. If the ship hull is designed to 

 accommodate the propeller, rather than the 

 other way around, the latter will be properly 

 guarded against air leakage from the surface, 

 against racing at sea, against rotating through a 

 region of excessively high wake, and against all 

 other ills to which a large propeller is supposed 

 to be subject. 



The principal aim in the prehminary design of 

 the ABC hull was to swing the largest possible 

 single propeller. This was the reason for develop- 

 ing the arch type of stern, which permitted a 

 propeller diameter D of 24 ft on a draft of only 

 26 ft. The arch recess afforded effective shielding 

 from air leakage and (it was hoped) equally 

 effective protection against racing when the 

 ship is pitching. The reason for using the large 



propeller was to keep the thrust-load coefficient 

 low and to obtain a wheel that would work in a 

 region of the highest possible ideal and real 

 efficiencies, the latter represented by a value of 

 about O.Sjjj . Another reason was that pointed 

 out by E. K. Sullivan and W. G. Scarborough in 

 their paper on "Machinery Design of the Schuyler 

 Otis Bland" [SNAME, 1952, pp. 467-503]. There 

 (on page 474) they stated that, as a result of 

 their studies, "the largest propeller that could be 

 accommodated would have the least total annual 

 operating cost." 



Objections against large casting and shipping 

 sizes and weights can be overcome by develop- 

 ments in detachable blades and welded assemblies 

 described in Sec. 70.43. It is assumed in the 

 foregoing that the rate of rotation n of the 

 propeller shaft can be and is adjusted to suit the 

 optimum diameter. If not, n becomes a given 

 quantity and D is one of those to be found by 

 the design procedure. 



Means of determining the proper diameter, 

 and comments on optimum screw-propeller diam- 

 eters, are given by: 



(1) Schoenherr, K. E., PNA, 1939, Vol. II, pp. 159, 165, 



170, 172 



(2) Van Lammeren, W. P. A., RPSS, 1948, pp. 232-233 



(3) Lewis, F. M., SNAME, 1951, pp. 619-620 



(4) Van Manen, J. D., and Troost, L., SNAME, 1952, 



pp. 446-448 



(5) Edstrand, H., "Model Teats on the Optimum Diam- 



eter for Propellers," SSPA Rep. 22, 1953 (in EngUsh). 



If the propeller power and the ship speed are 

 low, the friction and pressure drag of the blades 

 becomes large in proportion to the thrust. Some 

 efficiency 7nay be gained by reducing the propeller 

 diameter below the optimum given by the 

 orthodox design procedures. However, the designer 

 is cautioned against any reduction of this kind 

 for higher powers and higher speeds, where it 

 may result in an actual loss of efficiency. The 

 arguments against this are set forth by H. 

 Edstrand, in reference (5) preceding, especially 

 pages 24-27 and Figs. 13 and 14. His diagrams 

 illustrate clearly the reasons for decreasing D 

 in one case and holding it in another. 



70.10 Determining the Rate of Rotation. 

 Closely related to the selection of propeller 

 diameter is a determination of the proper rate 

 of rotation. On the basis that the propeller designer 

 has the same freedom of choice as for propeller 

 diameter, and that optimum propulsive perform- 

 ance is desired, the rate of rotation n should be 



