ENTRAINED AIR 



457 



Figure 9. Underwater view from starboard quarter of a PT boat traveling at 19 knots showing propeller cavitation. 



the depth of the screw of the launch. In fact, no 

 stream of bubbles could be detected as emanating 

 from the screw, which was clearly visible since the 

 launch approached the camera as closely as 12 ft; 

 presumably a speed of only 2 or 3 knots was insuf- 

 ficient to reach the cavitation limit. 



Figures 6 to 11 are selected frames from an under- 

 water motion picture showing a PT boat, outfitted 

 with three screws, and its wake. These pictures were 

 taken in water about 40 ft deep, near the Dry Tor- 

 tugas; the choice of this location was dictated by the 

 need for considerable optical transparency in the 

 ocean. The motion picture camera was mounted, 

 slanting upward, on a steel tower firmly anchored 

 on the ocean bottom, and was operated by a diver. 

 The distance from the camera to the ocean surface 

 was about 15 ft. 



At the left in Figure 6, a small amount of entrained 

 air is visible along the water line. Moreover, a sharply 

 outlined cavitation spot is conspicuous; unfortu- 

 nately no attempt was made to ascertain what sort 

 of unevenness on the hull caused this cavitation. In 

 Figure 7 a large amount of entrained air is seen cov- 

 ering the hull. Both Figures 6 and 7 were made as 

 the vessel traveled at a speed of 9.5 knots; there is 

 no explanation of why the amount of entrained air 

 differs so greatly in these two illustrations. 



Figures 8 to 11 illustrate the progressive develop- 

 ment of propeller cavitation as the speed increases. 

 They furnish an instructive corollary to Figures 1, 

 2, and 3, and show that tip-vortex cavitation caused 

 by the screws of a vessel under way at high speeds 

 has the same appearance as that behind a laboratory 

 propeller driven by a stream of moving water. 



