Prediction of Ship Slamming at Sea 



Symposium on Naval Hydrodynamics ten years ago: Is it not true that the time 

 has come for a scientific evaluation of the freeboard problem of a ship on the 

 basis of wetness considerations? It would seem that Dr. Ochi's paper as well 

 as that of Mr. Goodrich in this Symposium both provide essential evidence that 

 we are properly equipped to undertake such an investigation. 



REPLY TO THE DISCUSSION 



Michel K. Ochi 



David Taylor Model Basin 



Washington, D.C. 



Professor Lewis mentioned that the probabilities presented in this paper 

 are based on assumed stationary conditions, i.e., ship speed, heading as well as 

 sea conditions are constant. The assumption of stationary conditions, however, 

 is considered to be a proper approach in the analysis. Since voluntary reduc- 

 tion of speed or change of course angle are entirely dependent on the personal 

 judgment of ship operators, it is appropriate not to include human elements in 

 establishing the statistical rules. 



He also discussed that the aftward velocity of the green water would have a 

 considerable dynamic effect on the pressure on the deck. The pressure on the 

 deck reported in this paper is the vertical component of green water flowing 

 over the deck from the top of the stem. Pressure records obtained in the ex- 

 periments have shown that pressure normal to the deck is not an impact type 

 and that the pressure magnitude approximately corresponds to the static water 

 head experienced at the stem. Judging from these results there is no reason to 

 believe that consideration of the dynamic effect of the aftward velocity is neces- 

 sary for the vertical pressure on the deck. This consideration is of course 

 necessary for the horizontal component (aftward direction) of pressure on the 

 deck, since the green water would crash at the front face of the deck super- 

 structure with considerable velocity. 



Mr. Swaan remarked that the bow wave of a ship usually decreases the 

 probability of slamming and increases the wetness. For this reason, he said 

 most reliable way to obtain the relative motion is to measure it at the location 

 considered. Consideration will be given to his remarks in future experiments 

 by the author. 



Professor Aertssen asked why the same speed of 10 knots was used for 

 comparison of frequency of occurrence of slamming for light and full draft 



'''See discussion by Pierson to paper by Ogilvie and discussion by Tick to paper 

 by Cummins and Smith. 



595 



