0.6. The pins at the 10 percent of chord locations were filed to a height corre- 

 sponding to the multiplication factor used for the other pins in the set being 

 tested. 



A thin coat of international yellow paint was applied to the tips of the pins 

 prior to testing to aid in visual observation. The experimenters could thus locate 

 the pins easily when the propeller was revolving. 



EXPERIMENTAL PROCEDURE 



The propeller rpm was increased until the cavity enclosed all the pins. This 

 procedure was begun with the set of brass pins that protruded highest. While the 

 pressure and velocity in the 36-in. variable pressure water tunnel was held constant, 

 corresponding to the design cavitation number, a = 0.34, the propeller rpm was 

 gradually reduced in decrements of 10. Each time the propeller rpm was reduced, a 

 hand-held strobe unit was used to observe visually whether the pins were in or out 

 of the cavity. These observations could be made rapidly, although the pins that 

 barely touched the upper cavity surface required more attention than did the others. 

 When all pins protruded through the cavity surface, the procedure was repeated with 

 another set of pins. A depth micrometer was used before and after each test to 

 measure the height of the pins above the back of each blade. This was done to ensure 

 that the pins were at the correct height and had not moved during testing. 



As the pins began to break through the cavity surface, a furrow or small 

 groove formed in the surface, accompanied by some spray or cavitation behind the 



pin. A directional strobe unit with variable light intensity made these furrows 



19 

 much more visible. This phenomenon has been recorded in several color photographs. 



Large, international-yellow numbers painted on the backs of the blades proved 



invaluable during testing. Also, each propeller hub was coded with a series of dots, 



the number of which corresponded to the number on each blade. 



DISCUSSION OF EXPERIMENTAL RESULTS 



Figures 29-31 compare linear theory predictions of cavity height with experi- 

 mental results for Propeller 4717C. These figures show three experimental upper 

 cavity surfaces corresponding to three values of J, one of which is the design J 

 (1.037). Note that the following relationship gives the advance angle, 3, at each 

 radial blade section, 



31 



