different that a successful comparison of the two methods cannot 

 be ascribed to like errors being propagated in the same way. The 

 digital spectral densities computed for this comparison are considered 

 to be representative of the data; it remains to be established that 

 the analog analyzer of the SEADAC produces similar results. 



For the sake of comparison, a set of data obtained on a Liberty 

 ship full-scale trial was analyzed by both methods. The ship speed was 

 approximately 15 knots in a head sea where the highest waves were about 

 8-10 feet. 



The seakeeping events which were recorded and analyzed are listed 

 below and appear as Figure 6: 



1. Roll 



2. Pitch 



3. Heave Acceleration 



4. Starboard Strain 



5. Port Strain 



6. Wave Height 



Examination of the graphs in Figure 6 shows that the agreement is 

 quite good so far as shape is concerned, and this is essentially all 

 that may be required of the SEADAC. Where the curves do not superpose 

 indentically, it is believed that instrumentation calibration is at 

 fault. This is discussed in the next section. An unexplainable 

 discrepancy occurs in Figure 6c, where the SEADAC shows a third peak 

 in the spectrum that does not exist in the numerical calculations. 



17 



