606 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 70.19 



Airfoil sections with what is known as lifted 

 leading and trailing edges, set back from a base 

 chord passing through the main portion of the 

 face, are necessary in way of the roots and the 

 inner radii to obtain good flow through the 

 regions where the blades are close together; see 

 the blade-spacing diagrams at the lower left-hand 

 corners of Figs. 70.O and 78. L. If the leading 

 edges are not lifted sufficiently, cavitation and 

 erosion are liable to occur on the faces, close abaft 

 those edges. 



There are many types of airfoil section suitable 

 for screw propellers. Some of the most satisfactory 

 forms are those developed by the National 

 Advisory Committee for Aeronautics, based upon 

 the principle of superposing the ordinates of a 

 symmetrical hydrofoil upon a curved or cambered 

 meanline, with or without minor modifications 

 here and there. The merits of these NACA 

 sections and the advantages of using them are 

 described in detail by I. H. Abbott, A. E. von 

 Doenhoff, and L. S. Stivers, Jr., in NACA Report 

 824, issued in 1945, and entitled "Summary of 

 Airfoil Data." The manner in which these sections 

 are developed for a particular case is described 

 in Sec. 70.31. 



70.19 Shaping of Blade Edges and Root 

 Fillets. In former years many propeller drawings 

 did not specify the detailed shapes for the blade 

 edges but this procedure is no longer compatible 

 with good design. One method of doing this, for 

 sections with circular noses and tails and setback 

 as well, is shown by W. P. A. van Lammeren 

 [RPSS, 1948, Fig. 126, p. 190]. 



The leading edge of a blade must be thick 

 enough to: 



(a) Withstand the impact of small objects without 

 nicking or deforming the blade edge permanently 



(b) Avoid local cavitation, either on the face or 

 back, when the angle of attack changes from its 

 predicted value; that is, when the direction of 

 the incident velocity shifts with respect to the 

 blade. This change can occur throughout one 

 revolution because of local circumferential varia- 

 tions in the wake velocity or it can occur for the 

 whole propeller because of changes in displace- 

 ment, in ship resistance, and in thrust loading 

 over the disc area. 



(c) Render the blade section reasonably invul- 

 nerable to changes in the angle of attack, due to 

 variations in the local speed of advance, in the 

 direction of flow, and other factors. 



Both leading and trailing edges must be thin 

 enough to: 



(d) Eliminate excessive dynamic pressures along 

 the leading edge because of the relatively large 

 velocity with which the blade passes through 

 the water. This is particularly true for the 

 sections at the outer radii. Although screw pro- 

 pellers are rarely designed for partial immersion, 

 there are large dynamic pressures due to impact 

 when the exposed blade portions strike the water 

 surface. 



(e) Eliminate losses due to separation drag at 

 the trailing edge 



(f) Prevent the formation and shedding of eddies 

 and the lateral vibration which causes noise and 

 singing. 



For the leading-edge shape, a circular arc is 

 simple and satisfactory. This is achieved as 

 indicated in the axial view of Fig. 70.O by 

 bringing the face and back section outlines in to 

 tangent points on a small circle. For the trailing 

 edges much smaller circular arcs are used. If, 

 however, the blade sections are rather thick at 

 the extremity of the run, they are given a chisel 

 shape, illustrated by Fig. 70.P in Sec. 70.46. 



It is possible that some full or rounded form of 

 edge at the blade tip may be found which will 

 diminish the intensity of the tip vortexes by 

 increasing the diameter of the vortex cores. 



In the working drawing of a final propeller 

 the exact shapes of the leading and trailing edges 

 and of the tip edge are to be shown by an adequate 

 number of large-scale details, such as those given 

 by R. H. Tingey [ME, 1942, Vol. I, Fig. 6, p. 280, 

 Detail "A"]. W. Henschke gives a tip radius (in 

 thickness) of 0.0015Z) and a trailing edge radius 

 of 0.008c ["Schiffbau Technisches Handbuch," 

 1952, p. 145]. 



The root fillets, also lacking details in days 

 gone by, are now of the constant-stress shape 

 diagrammed by R. H. Tingey in Fig. 16 on page 

 292 of "Marine Engineering" [Vol. I, 1942]. This 

 produces a form resembling that given by nature 

 to the bottom of a tree, where the trunk joins 

 the roots and where large bending moments are 

 to be resisted. 



70.20 Partial Bibliography on Screw-Propeller 

 Design. Although they are not all quoted in this 

 and other chapters relating to screw propellers, 

 there is given here a partial bibliography of the 

 principal references on the design of screw pro- 

 pellers for ships, most of them dating from about 



