DESIGN AND OPERATIONAL CONDITIONS FOR WARTIME SERVICE 



In the discussion of the statistical background, it was pointed out that the distribution 

 patterns readily give the probability of exceeding any given magnitude of the motion or stress 

 and that the distribution pattern can be utilized as a load spectrum for endurance strength 

 calculations. In this section the methods will be applied to determine design and operational 

 conditions for wartime service. These determinations are based on the following assumptions. 



1. The vessel will be operating in the North Atlantic Ocean. The observations of sea 

 conditions at weather station C (52°N37°W), see Figure 2, are considered typical of conditions 

 in the North Atlantic and are assumed to represent the conditions the ships will encounter in 

 service. 



2. The ship operating speeds for the different sea conditions are taken to be the average 

 of the estimates obtained from ship's officers of a number of ships; see Table 1. 



3. All headings of the ship relative to the predominant wave direction are assumed equally 

 likely, excepting only that seas coming approximately off the beam are considered unlikely for 

 combinations of high speeds and rough seas. 



"LONG-TERM" DISTRIBUTIONS OF SHIP MOTION, HULL BENDING 

 MOMENT, AND WAVE HEIGHT 



The charts of Figures 7 through 10 give the probability of exceeding and of not exceed- 

 ing any given value of stress or motion if all the motions or stresses are considered to which 

 the vessel is subjected over a period of a number of years. For example, only 3 percent of 

 all variations in roll angle would, on the average, exceed a value of 10 deg peak-to-peak; 

 see Figure 9. Figure 2 gives similar data for significant* wave heights to be expected in the 

 North Atlantic Ocean. These distributions may be considered valid up to maximum variations 

 of 25 deg in pitch, 56 deg in roll, 0.8 rad/sec^ in pitch acceleration, and 40,000 ft-tons in 

 bending moment. 



PREDICTIONS OF SHIP RESPONSE TO WAVES FOR GIVEN CONDITIONS 



It is difficult to make reliable estimates on the basis of the available data because the 

 specification of the sea state and of the relative heading of the ship to the sea are somewhat 

 indefinite. Nevertheless the following formulas may be used to make estimates which should 

 be fairly good if the environmental conditions are similar to those for which measurements 



were made. 



The most frequent magnitude of the variation will be 0.707 \JE. The average magnitude 

 of the variation will be 0.886 \JE. The significant magnitude (average of the upper third of the 



*The significant wave height is used to denote a sea state. It is estimated as indicated in the footnote on 

 page 3. 



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