44 



float, succeeds only because the wave-making and form drags are the dominant 

 contributions to the total drag. 



With respect to the hysteresis loop discovered In the drag charac- 

 teristics of the planing float In transition through the air-water Interface, 

 It was shown that there are two possible types of flow. The type In which 

 water Is thrown forward and to the sides was observed to be more stable than 

 the other type In which the water continues to pass over the top of the float. 

 The region near the speed at which transition begins Is very unstable with 

 the flow tending to oscillate between the two types. Finally, any disturb- 

 ance of the flow of the second type causes a complete breakdown, and the flow 

 goes over into the more stable type. 



The phenomenon of striation formation requires further investiga- 

 tion. It may be possible to test experimentally the hypothesis concerning 

 vortex instability by a series of high-speed photographs of a very smooth 

 sphere dropped through an air-water interface . Such photographs would dis- 

 close any transverse oscillations that may occur if the vortex near separation 

 does behave in the same manner as a free ring. It is doubtful that such tests 

 could be made with a float because of the difficulty in holding the float for 

 any length of time in the type of flow in which these striations occur. It 

 would be desirable to have the sphere highly polished to minimize the forma- 

 tion of cavities about irregularities on the surface of the specimen. At the 

 present time, however, this phenomenon appears to be only of academic inter- 

 est. If it were found that such formations are detrimental, e.g., in terms 

 of drag, the practical problem presented would probably be one of boundary- 

 layer control. 



REFERENCES 



(1) BuShips CONFIDENTIAL letter C-S8l-1 (3) (620) of 7 September 

 1943 to TMB. 



(2) "Design and Performance of the TMB Planing Float as a Towed 

 Position Indicator," by L. Landweber and Ensign P. Elsenberg, USNR, TMB Re- 

 port 540, March 1945. 



(3) TMB CONFIDENTIAL letter C-S8l-1 of 6 March 1944 to BuShips, 

 Minesweeping (620). 



(4) "Characteristics of Clark Y Airfoils of Small Aspect Ratios," 

 by C.H. Zimmerman, NACA Report 431, May 5, 1932. 



