A Survey of Ship Motion Stabilization 



the fin installation caused their removal. This vibration problem has been the 

 subject of much of the investigation conducted on bow fins in recent years; it has 

 also caused the virtual abandonment of these devices as pitch stabilizers. 



The highlights of the work conducted in this area will be the subject of this 

 section. Mention will also be made of some recent experiments not as yet re- 

 ported in the literature. 



Fixed Bow Fins 



In 1956, Pournaras [46] fitted a set of fins to a Series 60, Block 60 model. 

 The fins were flat plates with a planform area 2.5 percent of the load waterplane 

 area. The leading edge was swept back to reduce tip load and thereby decrease 

 the root-bending moment. The fins were also fitted with tip fences. From the 

 limited tests conducted it was observed that pitching motion was considerably 

 reduced, the speed range was extended, much less green water was shipped over 

 the bow, and forefoot emergence was eliminated. Of most significance, however, 

 Pournaras noted that on the downward stroke, sheets of water were forced 

 around the leading and trailing edge of the fins, closed in over the upper surface 

 and formed a whirl near the water surface as the two sheets met. Removal of 

 the tip fence caused the formation of a third sheet and added to the problem. 



In a subsequent study, Pournaras [47] tested four different fin configurations 

 on a model of a MARINER class ship. In addition to varying planform, some of 

 the fins were slotted and others had through holes in an attempt to destroy the 

 sheet vorteces. Figure 26 shows these various fin arrangements. All configu- 

 rations were fitted with fences with the exception of fin 3. 



Figure 27 shows a summary of test results obtained and indicates that while 

 substantial pitch reductions were obtained, configuration variation had very little 

 effect. The major conclusions of this study are summarized as follows: 



1. Fins operate most effectively near the synchronous range and have little 

 effect at higher or lower frequencies. 



2. Fins have little effect on the phase lag of heave and pitch but it should be 

 remembered that a slight change in phase could have a marked effect on relative 

 motions. 



3. Area of fin planform has little effect on motions. 



4. The loadings caused by the vorticity effect can be lessened by deeper 

 submergence, greater fin span, tip fences and relief mechanisms such as slots 

 and holes. 



Next, Abkowitz [48] conducted a comprehensive study on the effect of bow 

 anti-pitching fins on ship motions. This study included a discussion on the na- 

 ture of pitch damping in addition to presenting some experimental results of 

 tests conducted on a Series 60 Block 60 model, an aircraft carrier model and a 

 destroyer model. All indicated good pitch reduction trends and good agreement 

 with theoretical calculations. It was again concluded that the major effect was 

 produced at resonance. 



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