A Survey of Ship Motion Stabilization 



Abkowitz concluded that the horizontal vibration at the bow was due to the 

 large angle of attack on the fin during the downward motion giving rise to a low 

 pressure on the upper-fin surface. Just before the downward stroke begins (fin 

 near surface) the fin is near the surface and the low pressure area causes a 

 suction and possible ventilation. As the fin goes deeper, the bubble collapses 

 causing a large pressure impact. He further concludes that since port and star- 

 board fins do not ventilate uniformly, a differential pressure impact situation is 

 created. He is at variance with the conclusions of Pournaras in that tip fences 

 will be harmful rather than beneficial since they increase aspect ratio which in 

 turn will increase the low pressure on the upper surface and lower the angle of 

 attack at which the breakdown occurs. 



Abkowitz further conducted some comparative experiments between trumpet 

 shaped fins and hydrofoil fins located at the keel and concluded that hydrofoil 

 fins would produce less horizontal vibration and when separation did occur, only 

 small horizontal vibrations were produced. 



In 1959, Becker and Duffy [49] presented the results of full scale sea trials 

 conducted on the COMPASS ISLAND. It was concluded in this study that pitch 

 was reduced but the magnitude of reduction could not be properly established 

 because of lack of an exact "without fin" correlation condition. Vertical and 

 transverse vibrations were excited in the ship, very seriously in heavy seas. 

 While it was possible to calculate the fundamental mode of the vertical hull 

 stresses (±11,000 psi) the transverse vibration was not measured. This was 

 unfortunate since it was apparent that the transverse mode was excited at a 

 more moderate sea state than the vertical, and all previous studies have indi- 

 cated the transverse mode to be the probable problem area. 



Stefun [50] continued the effort by conducting an interesting experimental 

 study investigating the effect of planform area and aspect ratio. Table 3 pre- 

 sents a summary of the various fin configurations. This study again reaches 

 the general conclusion that fins are effective mostly at near synchronous condi- 

 tion and again significant pitch reductions were achieved. Aspect ratio is indi- 

 cated to be a significant parameter. Pitch reduction is 30 percent less for a fin 

 with an aspect ratio of 0.5 compared to a fin with an aspect ratio of 2.0. It was 

 further indicated that while an increase in planform area was helpful in achiev- 

 ing increased pitch reduction this increase was not in direct proportion to the 

 increase in this area. The fins decreased ship resistance in waves of between 

 75 and 120 percent of the ship length. Heave was increased in long waves while 

 in intermediate waves, heave was increased at low speed and decreased at high 

 speed. 



Finally, this study indicated that the use of tip fences reduced pitch by an 

 additional 5 percent. This is the result of apparent increased aspect ratio due 

 to the addition of tip fences. It should be noted that this particular study used 

 tip fences for the reasons stated by Pournaras [10,11]. 



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