26 





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"^ Most Unstable Region 







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5.6 5.8 6.0 6.2 6.4 6.6 6.8 7.0 



Speed In knots 



Figure 22 - Hysteresis Loop for TMB Planing Float under Load Imposed 

 by 100-Pound Weight Towed from the Forward Towpolnt 



the loop In Figure 22, was shown to be the more stable of the two types; on 

 the lower branch, any slight disturbance in the flow causes a complete break- 

 down over the top, and the flow goes over into the type in which the water is 

 thrown forward. This is illustrated in Figure 21b, in which the flow while 

 on the lower branch of the hysteresis loop broke down when the float yawed 

 suddenly. The right-hand photograph of Figure 21b was taken immediately after 

 this breakdown had occurred; the float had not quite returned to a straight 

 course from its yawed position. 



The conclusion can be drawn from these observations that the in- 

 stability and probably also the hysteresis region will exist for any hydro- 

 foil with a well-rounded leading edge, in transition through the air-water 

 interface. It is possible that for a hydrofoil of much larger aspect ratio, 

 for which the angles of attack must be kept small to avoid stalling, this 

 instability may not manifest itself to the extent shown by the TMB float, 

 which, in the submerged condition, may be considered as a thick hydrofoil of 

 low aspect ratio. Furthermore, as was shown by the tests of the NRL Mark 3 

 buoy (5), the transition for a hydrofoil with a sharp leading edge is well- 

 defined and occurs at a single speed. For this hydrofoil, the point of sep- 

 aration of flow is fixed at the leading edge and no hysteresis loop exists. 



During the experiments in the hysteresis region, it was also found 

 that striatlons appeared in the flow configuration while the water was still 

 passing over the top of the float. A discussion of this phenomenon and an 

 attempt to explain the formation of the striatlons follow. 



