Cavitating Prop Design and Screw Prop Development 



e<=t 



0,il 



o,J 



0,1 



o,i 



I'i-ji 



Fig. 4 - Thickness of a cavity on the trailing 

 edge of a blade versus nominal angle of attack 

 at infinity for various geometrical elements 

 of the blade 



much difficulty, because the convergence of process is rapid enough, provided 

 the first approximation is reasonably chosen. 



Following the determination of the flow parameters in way of the blades, 

 one could proceed directly to choosing blade-section elements. It is clear that 

 the curvature and the nominal angle of attack should be chosen so as to provide 

 a prescribed value of the lift coefficient Cl for the element in two-dimensional 

 parallel flow; subsequently, these values should be corrected, with allowance for 

 the curvature of flow. Generally, the solution is not unequivocal, since one and 

 the same value of c^ can be obtained with various relations between the blade 

 curvature on the pressure face and the nominal angle of attack. 



It is easily shown that the deceleration of flow before the propeller due to 

 the presence of cavities involves a decrease in propeller inductive efficiency 

 which is the greater, the greater the thickness of the cavities. The design 

 losses will also increase with the increase of cavity thickness, and the latter 

 will result in the deterioration of the hydrodynamic quality of the sections. 



Thus, to ensure the maximum efficiency of a cavitating propeller, it is es- 

 sential that the relation between the curvature and the angle of attack should be 

 such as to reduce the cavity thickness to a minimum. An additional requirement 

 restricting the greatest value of the pressure -face curvature is the absence of 

 cavitation on this side of the blade. 



Accordingly, the section elements are defined from two equations: 



aj, = f ( §2, S , cr) , ■ •' •' - 



923 



