These results are made even more interesting by considering the values 

 obtained by summing the columns in Table 11.6 where the effect of a disturb- 

 ance from a distance has been removed. These values can be plotted against 

 a Neumann spectrum for 18.7 knots and against the wave pole spectrum as 

 shown in figure 11.24. One could not ask for much better agreement between 

 theory and observation than is shown between the theoretical Neumann spec- 

 trum and the frequency spectrum obtained from the directional spectrum. 

 The agreement between the wave pole spectrum and the theoretical spectrum 

 is actually a little (but not much) better than shown because the contribution 

 of the swell for k equal to 11 and 12 will reduce the sharp peak. One dis- 

 advantage of wave pole data is evidently that there is no way to see the 

 swell if it has the same frequencies in it as the local sea. 



Another question to be asked before entering into a discussion of the 

 above results is what would the wave pole calibration have had to have been 

 in order to provide agreement with it and both the theoretical Neumann spec- 

 trum, and the directional spectrum. This result can be obtained by dividing 

 the values for the directional spectrum, including swell, by the values for 

 the wave pole spectrum before multiplication by the calibration curve. The 

 result is shown in figure 11.25. There is the possibility of some sort of 

 amplified response in the wave pole, undetected by still water damping and 

 resonance tests, as |a. equal to 2tr(15)/96. The agreement between the two 

 spectra would be fairly good if something like one of the dashed curves 

 were used for calibration instead of the original theoretical curve. 



209 



