of J have been shown. At a J value of 0.98, the experimental cavity surface coin- 

 cides with the predicted cavity surface height at aft locations on the blade. This 

 J value represents an approximate increase in angle of attack of 1.6 deg over the 

 design value of J. For the cavity surface, corresponding to a J value of 0.96, the 

 increased incidence is equal to about 2.2 deg. However, as mentioned before, the 

 cavity did not spring from the leading edge. It moved down the span as rpm was in- 

 creased. Since the water-tunnel velocity was held close to 35 fps (10.688 m/sec) 

 and since the model propeller diameter was 16 in. (40.64 cm), the difference in model 

 rpm corresponding to the J values of 0.98 and 1.037 was 88. This corresponds to an 

 increase of about 58 in full-scale rpm. It is also interesting to note that, accord- 

 ing to performance evaluation experiments, an increase of 29 rpm, over the 1000 rpm 

 of full-scale design would give the design thrust. 



At the two outer radial positions, r/R = 0.544 and 0.726, full cavitation did 

 occur at the design J (1.037), but the theory overpredicted the cavity surface 

 height. As with Propeller 4717C, the back of the blade near the leading edge of 

 Propeller 4738A was wetted to about 2- or 3-percent of chord. 



To understand more fully the discrepancy between theory and experiment, the 

 reader should recall that a point drag is a linear theoretical model of the leading 

 edge cavity thickness represented by a point singularity. Experimental results indi- 

 cate that the actual separation point at the leading edge must be carefully chosen, 

 for example, as a slope discontinuity of the blade surface, to achieve the designed 

 leading edge cavity thickness; if the predicted leading edge cavity, not just 70 

 percent of it, had been filled by a material up to 2 percent of chord from the 

 leading edge, the experimental results would have almost coincided with the theory, 

 except very near the hub, where the hub effect is important. 



CAVITATION PERFORMANCE CHARACTERISTICS OF SUPERCAVITATING 

 PROPELLERS 4717B, 4717C AND 4738A 



BACKGROUND 



Propeller 4717C was originally manufactured as Propeller 4717B. Propeller 4717B 



was identical to Propeller 4717C except for the backs of the blades, which had a 



shape to conform to the predicted cavity shape at design operating conditions. The 



primary purpose of Propeller 4717B was to determine, by observation, how well the 



33 



