Titoff, Russetsky, and Georgiyevskaya 



It is known, however, that the maximum value of the thrust coefficient, given 

 the cavitation number and the advance coefficient of a cavitating propeller, is 

 primarily a function of the blade area ratio and depends to a lesser extent on the 

 pitch. Hence the lowest permissible value of the blade area ratio should be such 

 as to secure the prescribed value of the thrust coefficient. For this purpose one 

 may use the diagram in Fig. 1 on which the value Clh of the conventional lift 

 factor is plotted as a function of the local cavitation number at the conven- 

 tional radius 7 = r/R = 0.7, which factor corresponds to the maximum value of 



the thrust coefficient kx with a given blade area ratio, 

 determined from the following relation 



The value C^t^ can be 



77 A 



Ad 



(>^l^ 



r^) cos /3 



Solving this equation for A/Ad , the needed value of blade area ratio can be calcu- 

 lated from the values of K^ and Cl given in Fig. 1. For hydrofoil vessels, the 

 needed value is usually defined not on a full- speed basis, but so as to ensure the 

 defeat of drag hump. 



Clivox 

 0,i 



o,z 



0,1 



0,1 



0,2 



0,1 



Fig. 1 - Conventional lift factor 

 Cl„ as a function of the local cavi- 

 tation number for the maximum 

 value of the thrust coefficient k^ 

 with a given blade area ratio 



As noted above, the absence of cavitation on the pressure side of the blade 

 is provided for by choosing the proper blade sections. For this purpose it is 

 necessary first to calculate the induced velocity field of a cavitating propeller. 



The propeller -induced velocities may be considered as the sum total of the 

 velocities generated by the vortices of a propeller with infinitely thin blades and 

 the velocities which are due to the thickness effect of the blades. For a subcavi- 

 tating propeller, the former component prevails, which makes it possible in most 

 cases to neglect the velocities associated with the thickness effect of the blades. 



920 



