456 



FORMATION AND DISSOLUTION OF AIR BUBBLES 



FxGtjRE 8. Underwater view from port quarter of a PT boat traveling at 6 knots showing propeller cavitation. 



acoustically opaque enough to blank out the bottom 

 of San Diego Bay, where all the experiments were 

 made. The wake thickness did not differ significantly 

 from that observed in runs made with engines only, 

 with the same vessel under comparable weather 

 conditions; the average thickness was 12.4 ft with 

 engines, and 13.0 ft under sail. As far as this scanty 

 evidence goes, the geometric form of the wake seems 

 to be determined primarily by the shape of the hull 

 of the vessel and its speed, and it seems to be imma- 

 terial whether the bubbles are produced by entering 

 surface air or by propeller cavitation. 



A novel direct approach to the visual study of the 

 subsurface structure of wakes has been made pos- 

 sible by the recent development of underwater mo- 

 tion pictures at the David Taylor Model Basin. This 

 technique should also prove most useful for repealing 

 the distribution of entrained air around the hull. 

 For instance, when a small power boat passed with 

 a speed of 2 to 3 knots over the underwater camera, 

 mounted on the bottom in shallow water, the film 

 shows a strongly foaming, shallow stern wake ex- 

 tending backward from the hull wake over a con- 

 siderable distance. This wake did not reach down to 



