A Survey of Ship Motion Stabilization 



DISCUSSION 



Peter DuCane 



Vosper Limited 



Portsmouth, England 



The authors of this interesting paper mention that model tests of anti- 

 rolling fins either alone or on ship models have not been reported. However, 

 we at Vosper have, in fact, carried out quite a number with what we consider to 

 be a useful degree of success so far as the actual results are concerned. 



We claim that in the case of the nonretracting low aspect ratio fin we can 

 produce a fin section which can be equally, or more, effective than the flapped 

 fin for the same area. 



Without entering unduly into details it could be mentioned that in many cases 

 it is clear that the greatest percentage of roll reduction does not of necessity in- 

 dicate the most comfortable condition so far as roll amplitude is concerned. 



Quite small rolling amplitudes in certain complex wave patterns can give a 

 most disappointing result on the basis of roll reduction with fins on against fins 

 off. However, these cases do not really matter to the passenger and there is 

 probably still quite a possibility of saving power in the operation of these fins 

 by area reduction. 



The fin sizes can be substantially reduced without much loss of effective 

 performance in their true capacity as roll dampers if, instead of 3°-4° being 

 aimed for, say 6°-7° double amplitude is aimed for under the same conditions — 

 no passenger could reasonably complain at this. 



At the same time I believe it is a short sighted policy to ignore the possi- 

 bility of, and in fact reported occurrence of, quite large rolls in "stabilised" 

 ships under certain circumstances. 



The situation is, of course, that the activated fin is not in truth a stabiliser, 

 or if it is ordered to act as one by an amplitude signal in the control system it 

 is a very poor stabiliser. By far the most important function of an anti-rolling 

 activated fin is to act as a damper controlled from a velocity signal. 



As the fins are usually designed on an empirical basis to cause a heel of 

 5°-7° when at full incidence and full cruising speed it can well be understood 

 that this means little in restoring effect when considering a roll induced by yaw 

 at practically no frequency when leading up to conditions of broaching such as 

 are met by even the largest liners in a quartering sea. 



While acting as a damper a sluggish fin movement can cause an important 

 phase lag leading towards the case where the fin is helping the roll. I do not say 

 for one minute this is a normal state of affairs but in nearly all installations 

 this can happen despite the fact that most of the time the fitting of anti-rolling 



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