Prediction of Ship Slamming at Sea 



As can be seen in the figure, the shortest time interval between two succes- 

 sive slams is 7.7 sec, a value very close to the natural pitching period of 7.6 

 sec. Although periods shorter than the natural pitching period were observed 

 between two wave encounters, no slamming was observed for these cases. 

 Hence, it may safely be assumed that the natural pitching period is the minimum 

 time interval between two successive slams. 



Figure 13 was prepared to verify that slamming is a sequence of events oc- 

 curring in time following a Poisson process. In preparation of this figure, the 

 number of slams occurring during 20 sec intervals was counted from the time 

 history (Fig. 12), and the experimental frequency for each number was obtained. 

 To determine the Poisson distribution curve, the expected value (mean) of slams 

 for every 20 sec was computed from the frequency. By using this value (0.89), 

 the Poisson distribution was obtained by the following formula: 



P(X=r) - 



(19) 



where 



^ = expected value, 

 r = integer. 



The result is included in Fig. 13. From evidence shown in the figure, 

 slamming may be considered as a sequence of events occurring in time following 

 a Poisson process for at least a size of sample (93 observations) shown in the 

 figure. 



5" 0.3 



°- 02 



I 2 3 



Number of Slams in 20 Sec Observation 



Fig. 13 - Comparison between the 

 probability density for number of 

 slams in 20 sec observation and 

 Poisson distribution 



567 



