Gevassimov, Pevshitz and Rakhmanin 



DISCUSSION 



J. C. Dern 



Bassin d'Essais des Cavenes 

 Paris j Franoe 



I have been very interested by the paper we just listened to, 

 for it explains phenomena that I have personnaly encountered during 

 model tests. I would like to make a few remarks. 



In the introduction of the paper, it is said that the mean 

 reason for the oscillations in the position of an anchored ship is the 

 instability of her equilibrium position due to the wind. However, it 

 seems to me that under certain conditions, current and waves may 

 also create instabilities. For example the instability due to current is 

 related to the course instability of a towed ship. This instability ap- 

 pears not only with ships but also with buoys. Tests on a 2 -ton coas- 

 tal buoy have shown the presence of transverse oscillations when the 

 current velocity»is less than 4 knots. Beyond 4 knots, however, these 

 oscillations disappear. 



These results are in agreement with the conclusions of pa- 

 ragraph 9. 



The authors assume that the coupled motions of pitching and 

 heaving have no influence on the motions in the horizontal plane. How- 

 ever, in paragraph 9, it is stated that the increase in the period of 

 the sway of an anchored ship is probably due to waves. It seems to 

 me that waves may have a great influence on the behaviour of an an- 

 chored ship. For example, for very long waves, the ship may, under 

 certain conditions, go up the waves and her hawser or chain may 

 slacken partially or even completely. In this case, the wave length is 

 the main parameter though it is not taken into account in the equations 

 of the paper. 



In any case, waves change the conditions of applications of 

 Liapounov's stability theory since when waves are present, the solu- 

 tion of the motion equations is an oscillatory solution whose stability 

 may notably differ from that of the steady solution of the no -wave 

 case. In some cases, waves may have a stabilizing effect such that 

 the instability oscillations disappear completely. In the case of the 



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