Cavitating Prop Design and Screw Prop Development 



Such an assumption will lead to considerable errors, if applied to cavitating 

 propellers whose hydraulic sections are blocked up by blade cavities rather 

 than by the blades themselves. 



We now turn to the methods of calculating the components of the velocities 

 induced by the cavitating propeller. Theoretical principles for calculating the 

 vortex component of the induced velocity were presented in 1948 by N.N. 

 Polyakhov, who demonstrated, for the case of a developed cavitation, the rela- 

 tions existing between the lift and the circulation on a subcavitating section. 

 This made it possible to apply the basic relations of the vortex theory to the 

 design of the vortex component of the cavitating propeller velocity. The differ- 

 ence in the design formulas is that the lift coefficient proves to be the function 

 of one more parameter, viz., the cavitation number, and the incident angle dCy/da 

 is taken to be less than that for a subcavitating section. In the case of propeller 

 design, the propeller is also considered as being optimum, according to Betz; 

 such an assumption in the case of the finite length of cavities can be made to an 

 accuracy of the magnitude of cavity drag. This approach is widely used both in 

 the USSR and in other countries. 



Practical calculations in which account is taken only for the vortex compo- 

 nent of the velocity are, however, in bad agreement with the experiment, i.e., 

 the pitch ratios and blade section curvatures prove to be underestimated. 



This circumstance gave impetus to a number of investigations aimed at 

 solving the problem of blade flow for a cavitating propeller, taking into account 

 the finiteness of cavities which develop on the blades. Such a solution, based on 

 using the acceleration potential, has been obtained by V.M. Lavrentiev. Accord- 

 ing to this solution, given the distribution of pressures over the suction side and 

 the load, the distribution of singularities (sources) defining the configuration of 

 the blade and cavity is due to the solution of Fredholm's integral equation of the 

 first kind. A similar problem was later solved by G. Cox (1). 



Unfortunately, the design diagrams based on these methods have found no 

 practical application as yet, and accordingly consideration is given below to ap- 

 proximate methods of making allowance for cavities in the design of a screw 

 propeller. 



If we now turn to the performance of the cavitating propeller as shown in 

 Fig. 2, we can see that the blocking up of the hydraulic section by cavities brings 

 about (a) a decrease in the mean velocity of fluid inflow to the propeller disk, 

 and (b) an increase in the flow velocity in between the cavities (blades). 



The first of these circumstances can be taken into account for solving a 

 three-dimensional problem of cavitating propeller performance, and the second 

 for the flat-plate theory, considering the blade cascade at various relative radii. 



In the practical design of decelerating the flow before the propeller, a solu- 

 tion found by V.F. Bavin (2) for an ideal cavitating propeller is used, by assum- 

 ing that the cavity sizes are predetermined. To define the induced velocities in 

 the way of the blades, it is assumed that cavities are responsible only for addi- 

 tional axial velocities which can be calculated by the equation for the uniformity 

 of flow through the hydraulic section of the cascade. Hence, the local velocity 



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