Ochi 



etc. When weather was worsening, shocks were felt but the impacts on the fore- 

 body did not induce any reaction among ship's officers until at a certain moment 

 they mentioned in the log book: "le navire travaille et fatigue," the ship works 

 and there is fatigue. At this moment the whipping stresses in the main deck 

 stringerplate amidships were 0.5 t per sq in. and the impact pressure on the 

 pressure transducer located at 0.15 Lpp from FP was about 10 psi. The ship 

 was in nearly full- loaded condition and the location of the pressure transducer 

 was not exactly the same as the location 0.1 Lpp indicated by Dr. Ochi. Unfortu- 

 nately I have no impact data of this ship in light- loaded condition hitherto, but 

 the nice agreement between the threshold of whipping stresses and impact pres- 

 sure established on our cargo ship in nearly full-loaded condition and the 

 threshold of velocity established by Dr. Ochi is certainly an encouragement to 

 believe in his prediction of slamming from model results. 



This prediction of slamming is very well presented in Table 2 for a Mariner 

 ship. Looking at the number of slams in a 30 minute operation there are in a 

 moderate Sea State 7 only 12 slams in full-loaded against 60 in light-loaded 

 condition in head waves and they are again reduced when the captain changes 

 course 45 degrees. This might indeed give the picture of what happens on the 

 bottom at the forebody and the danger of damage there. But modern cargo liners 

 are longitudinally framed and often reinforced in the forebody beyond classifica- 

 tion requirements, so today bottom damage is more seldom stated after a cross- 

 ing in severe weather condition. Whipping stresses however are excited in the 

 main girder and they might increase to a certain extent the longitudinal bending 

 stresses and be a source of fatigue. Therefore I think that perhaps more than 

 the number of slams these whipping stresses ought to be considered. At each 

 slam there is a vibration in the ship main girder and an initial whipping stress. 

 Summing up these initial whipping stresses for let us say again a 30 minute op- 

 eration and dividing by the number of low cycle stress oscillations a slam num- 

 ber is obtained which might as well give the intensity of the effect of slamming 

 on the hull girder. Establishing this slam number, whipping stresses less than 

 0.4 t per sq in. were ignored. I had these whipping stresses measured in a sea 

 state about the mild 7 Beaufort of Dr. Ochi's paper, once in light-loaded condi- 

 tion on a cargo ship of 446 ft in waves Hj^ j^ - 27 ft at 12.5 knots, on another 

 occasion in nearly full- loaded condition on a cargo ship of 480 ft in waves 

 ^1/10 = 33 ft at 9 knots, and in this nearly full- loaded condition the whipping 

 stresses and the slam number representing their intensity were larger than in 

 light- loaded condition. In light- loaded condition the severe slams are heard 

 like a gun shot whereas in full- loaded condition they are more like far-off thun- 

 der. In light-loaded condition the slams are more conspicuous and captains are 

 keen to reduce speed. That is perhaps one of the reasons why the slam number 

 is not larger in light- loaded than in full-loaded condition. As long as not too 

 much green water is shipped the captain of a full-loaded cargo ship goes ahead 

 in high waves and modern cargo ships with a long forecastle and a fair fore 

 freeboard maintain a good speed in these high waves. 



And here I should like to ask Dr. Ochi why he has taken the same speed of 

 10 knots for his comparison light-loaded and full- loaded? Has he any informa- 

 tion as to what extent captains of Mariners accept 60 slams, i.e., 2 slams every 

 minute, in light-loaded condition in waves of 31 ft significant height? As a rule 

 captains of cargo ships of 10,000 tons deadweight and 16 knots service speed do 

 not accept these waves at a speed of 10 knots, when in light-loaded condition. 



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