Giddings and Wermter 



Fixed Fins — Partially Activated 



Brief mention will be made here of various suggestions and/or studies that 

 have been forwarded to retard flow separation over a fixed bow fin. They are 

 termed partially activated because some means of flap or flow control would 

 have to be provided for their proper operation. 



In his paper, Abkowitz [48] mentions a study conducted at MIT wherein 

 boundary layer suction was used to control the pressure on the suction side of 

 the fin. While the angle at which breakdown occurred was increased it was not 

 prevented at large pitch angles. 



Stefun and Schwartz [51] recommend further study in the use of moveable 

 trailing edge flaps as devices to retard the onset of stale. Along this same gen- 

 eral line, Goodman and Kaplan [53] have recently proposed the use of a jet- 

 flapped hydrofoil as an anti-pitching fin. This device would take advantage of 

 the existence of two pressure peaks (leading and trailing edge) causing the for- 

 ward peak to be smaller than for a conventional foil for the same loading. This 

 initial theoretical study indicated the foil did not separate. It was concluded 

 therefore that the jet-flapped foil would be cavitation limited and for reasons of 

 the lower initial pressure peak considerably more lift would be produced before 

 cavitation occurred. The authors of this preliminary work plan additional ef- 

 forts in this area. 



Activated Bow Fins 



The authors were unable to find any experimental work dealing with acti- 

 vated bow fins. Abkowitz [48] discusses this type of fin from the point of view 

 of automatic control. With pitching motions as the control the fin angle would be 

 additionally increased over an already large angle caused by the large amplitude 

 of pitch. This method of control would therefore hasten the onset of ventilation. 

 This leads to the concept of negative control, that is when the pressure on the 

 foil reached a certain point, the foil angle would be decreased and thereby re- 

 tard the onset of ventilation. The results of a computer study at MIT comparing 

 this type of activated bow fin with a fixed fin indicated no difference in either 

 method. Abkowitz concludes that there is little to recommend the use of an ac- 

 tive bow fin. 



Fixed Stern Fins 



Abkowitz [48] makes mention of the use of fixed stern fins and concludes 

 that they would be much less effective than bow fins. In addition to the obvious 

 disadvantages of operating in the ship's boundary layer, the stern fin would 

 probably increase the excitation due to waves and the pitch damping effect would 

 also be reduced. The force applied to a stern fin would also produce less mo- 

 ment than a bow fin since the apparent pitch axis is generally aft of amidships. 



In his experiments, Ochi [52] fitted a stern fin of equal area to the bow fin 

 to the MARINER model. It was also found that the stern fin was much less ef- 

 fective than the bow fin in reducing pitch. Vibration was not a problem although 



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