Sec. 70. N 



SCREW-PROPJiLLER DESIGN 



601 



the stem arrangement in the design stage, allow- 

 ing ample clearances in all directions, little or no 

 rake is required. There is usually no excuse to 

 be forced to excessive angles of rake in order to 

 obtain proper aperture clearances. 



70.14 Propeller-Hub Diameter; Hub Fairing. 

 Some general comments on hub-diameter ratios 

 d/D are given here. More detailed comments are 

 to be found in Sec. 70.43, under a discussion of 

 the mechanical construction of screw propellers. 



It is rarely possible to consider hydrodynamics 

 alone in a discussion of hub shape and diameter 

 for a screw propeller, especially for one at the 

 stern of a hull. Obviously the blades must be 

 attached to some sort of hub or enlargement on 

 the shaft, which can not have too small a diameter. 

 Further, the geometric pitch angle 0(phi) becomes 

 very large as the radius R is diminished toward 

 zero; too large, in fact, to make any blade lift 

 effective in producing useful thrust. Finally, 

 there is no point in making the hub much smaller 

 than the diameter of the housing for the propeller 

 shaft bearing just ahead of it. 



There have been recurring proposals, over the 

 past century or more, for propeller hubs having 

 diameters of one-third or more of the propeller 

 diameters. Some early ship propellers were 

 built in this way. While it is true that the root 

 sections of a normal screw propeller do relatively 

 little work as a rule, at least they permit the 

 water to pass through, which a large solid hub 

 would not do. 



From considerations of strength and mechanical 

 attachment, both of the hub to the shaft and the 

 blades to the hub, the diameter of the propeller 

 hub depends partly upon the diameter of the shaft 

 and partly upon the widths of the blade sections at 

 the root, where they join the hub. It usually varies 

 between 0.16D and 0.20D for solid propellers. For 

 controllable or built-up propellers the diameter of 

 the hub may be as large as 0.28 or 0.30Z), with 

 a possible maximum of 0.25D for a propeller 

 having blades that are demountable but not 

 adjustable [ME, 1942, Vol. I, Fig. 2, p. 269]. 

 It is pointed out in Sec. 70.43 that a loss of 

 efficiency of from 1 to 1.5 points may be expected 

 if a built-up rather than an integral hub is used; 

 for example, a drop in tj from 0.685 to 0.675 or 

 0.670. 



To avoid separation and cavitation abaft the 

 hub, a fairing cap is usually fitted at the after 

 end. This cap also covers the propeller nut. In 

 many cases the hub fairing cap does not rotate 



but is attached to the fixed part t)f the rudder, 

 the rudder horn, or the rudder itself, as described 

 in Sec. 74.15 and illustrated in Figs. 66. Q and 

 74.K. 



It is customary and convenient, as well as 

 good design, to shape the propeller hub and its 

 cap as a fair, tapering continuation of the barrel 

 or boss just forward of the propeller which houses 

 the propeller bearing. This means that the end of 

 the propeller hub next to the bearing has a 

 diameter about equal to that of the bearing 

 barrel. The end away from that barrel has a 

 reduced diameter, as small as is consistent with 

 the necessary mechanical strength and rigidity 

 for the type of attachment of the hub to the shaft. 



In the case of built-up propellers with adjust- 

 able or demountable blades it is rarely possible 

 to make the hub diameter as small as the bearing 

 barrel so that the fair surface of revolution between 

 the latter and the end of the hub cap has a bulge 

 in it in the vicinity of the disc plane. 



In a speed range where the smaller, pointed 

 end of a propeller fairing cap is covered with a 

 hub vortex or swirl core, sometimes having a 

 diameter half as great as that of the cap at its 

 larger end, the portion of the cap within the core 

 is obviously serving no useful purpose. It is 

 possible, in some cases, to eliminate the swirl 

 core or hub vortex entirely by cutting the cap 

 off square at about two-thirds or three-quarters 

 of its length from the after end. The separation 

 drag which occurs abaft this blunt end is almost 

 certain to be less in magnitude than the drag 

 resulting from the presence of vapor pressure 

 only inside a swirl core of somewhat smaller 

 diameter. If the pressure is low enough and other 

 conditions are favorable, the swirl core may 

 persist, even abaft a blunt or square ending 

 ["All Hands," Bu. Nav. Pers., U. S. Navy Dept., 

 Feb 1953, pp. 18-19]. 



It appears unlikely, on a fast or high-speed 

 ship, that any reasonable slope at the pointed 

 end of a fairing cap will eliminate the swirl core 

 or hub vortex entirely. Certainly the small slopes 

 of 8 and 10 up to 20 deg (with reference to the 

 shaft axis), previously used on the fairing caps 

 of the fastest vessels, such as the World War II 

 German cruiser Prinz Eugen, are inadequate for 

 this purpose. 



Swirl cores abaft the fairing caps of model 

 propellers have been observed and photographed 

 in model basins, variable-pressure Avater tunnels, 

 and circulating-water channels. However, because 



