PRINCIPLES OF CAVITATING PROPELLER DESIGN 

 AND DEVELOPMENT ON THIS BASIS OF 



SCREW PROPELLERS WITH BETTER 



RESISTANCE TO EROSION FOR HYDROFOIL 



VESSELS "RAKETA" AND "METEOR" 



I. A, Titoff, A. A. Russetsky, and E.P. Georgiyevskaya ..; 



Kryloff Ship Research Institute 

 Leningrad, U.S.S.R. 



It has been found that appreciable erosion develops when cavities collapse 

 on screw propeller blades, the area of collapse shifting in the course of revolu- 

 tion along the blade chord because of the nonuniformity of the velocity field. At 

 high flow velocities characteristic for screw propellers, the intensity of the 

 erosion process is so great that it is useless to attempt prevention of failure of 

 the material by improving its mechanical properties. Accordingly, to decrease 

 or fully eliminate erosion damage it is necessary to provide for a suitable form 

 of cavity development, i.e., collapse outside the blade. 



In principle, cavitation may occur on both the suction and pressure sides of 

 the blade. The latter form of cavitation, however, is observed only when the 

 adopted curvature of the blade section is excessive or the edge pitch is too 

 small. Thus, the conditions required for the design of a propeller may be stated 

 as (a) the absence of cavitation on the pressure side, and (b) the collapse of 

 cavities beyond the trailing edge of the blades. The first of these conditions is 

 mainly provided for by choosing reasonable blade sections, and the second by 

 choosing the blade area ratio. 



The collapse of cavities beyond the blades cannot be obtained for all speeds. 

 There inevitably exists some range of speeds within which the cavities will col- 

 lapse directly on the blades. 



It is well known, however, that the intensity of erosion is proportional to ap- 

 proximately the sixth power of the flow velocity. Consequently, the erosion in 

 this range of speeds will develop much slower than at full speed, should a simi- 

 lar form of cavitation exist under the latter condition. Moreover, fast ships, 

 especially hydrofoil vessels, are for the most part operated at speeds approxi- 

 mating full speed, and hence the amount of intermediate speeds in the total pe- 

 riod of operation is insignificant. Accordingly, it is more advantageous in at- 

 tempting to decrease erosion damages that the conditions under which partial 

 cavitation of the blades may occur should be observed at the lowest possible 

 speeds, and hence the value of the blade area ratio should be as small as 

 possible. 



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