contained no face cavitation over the range of cavitation numbers and advance co- 

 efficients covered. The sole exception was at an advance coefficient of 1.2, where 

 some leading edge face cavitation was observed at cavitation numbers of 0.75 and 

 lower . 



CONCLUSIONS 



At design speed coefficient and design o, Propeller 4717B contains practically 

 no back cavitation. If advance coefficient is reduced slightly, at design a, the 

 backs of the blades are covered by sheet cavitation from the blade tip to 50 percent 

 radius. This indicates that the predicted cavity shape over this part of the pro- 

 peller blades is quite accurate. 



Neither of the designed propellers, 4717C and 4738A, had face cavitation at the 

 design operational points. Propeller 4717C essentially had full back cavitation and 

 Propeller 4738A had back cavitation from about 35 percent radius to the tip of the 

 blades at the design operational point. 



The propeller theory slightly overpredicted the available thrust for both pro- 

 pellers. Propeller 4717C would require 6 percent more rpm and 8 percent more power 

 than predicted to reach design speed. Propeller 4738A would require 5 percent more 

 rpm but 4 percent less power than predicted to reach design speed. If the operating 

 point is defined as the speed and rpm where the propellers absorb the available 

 maximum power, Propeller 4717C would operate at V = 58.7 knots and rpm = 1016, and 

 Propeller 4738A would operate at V = 61 knots and rpm = 1054. The propeller 



efficiencies at these conditions are 66 percent and 67 percent, respectively. 



23 

 It has been recognized that the nonlinear effects on lift and drag of cavi- 



2 

 tating foils are approximately equal to -0.5 C /(1-hj) and -0.5 C C /(1+a). There- 

 fore, propellers designed according to the linear theory would produce less thrust, 

 as indicated in the experimental results. However, the leading edge cavity thickness 

 was slightly smaller than the design thickness due to the unmatched separation point 

 and 70 percent filling of cavity thickness. Thus the drag should have been a little 

 less than predicted by the linear theory, as was found experimentally for Propeller 

 4738A. Because of the decreased efficiency of Propeller 4717C, however, more care 

 may be required in calculating the cavity drag due to the blunt leading edge. 



All in all, the design theory predicted the cavity thickness and the propeller 

 performance quite well at the design point, within the bounds of error to be 



35 



