TM No. 377 



There does seem to be a small decrease in the covariance with time* However, 

 no shifts in the peak positions of the spectra were obtained* Further discus- 

 sion of the results of the spectral analysis are presented in chapter V. 



These preliminary results showed that this type of instrument^, however 

 crude s can definitely sense and discriminate periodic oscillations associated 

 with relatively small amplitude and high frequency (i.e., low energy) wind 

 waves. Thus,, the auto-spectra and cross -spectra of wave motions can be 

 examined for their relationship to the physical processes occurring within 

 the waves. 



Further measurements of waves to substantiate these preliminary results 

 could not be made from the Navy pier. Reconstruction made the pier facilities 

 unavailable shortly after the above experiments were complete do Work was then 

 concentrated on calibration (discussed in chapter II ) and on possible wave meter 

 modifications. It was also decided to explore the possibilities of making wave 

 measurements under open sea conditions. 



BBELS as a Platform for Open Ocean Mea surements 



The preliminary wave measurements in Narragansett Bay were far from perfect. 

 The location of the pier^ near a breakwater and large Navy docks <, provided a 

 rather unique environment in which the waves generated were likely to be quite 

 different from those in the open ocean. It was therefore judged worthwhile 

 to make measurements using the wave meters where local boundary effects would 

 be minimised. 



A suitable platform was the first requirement. The use of a ship as a 

 platform presents numerous problems,. The greatest is the distortion of the 

 measured wave motions caused by ship response to the wave. Another involves 

 the effect of vessel dimensions upon the field of motion around the vessel. 

 This is a particularly complex problem if one is trying to measure a scale 

 of fluctuating motion of smaller 'dimensions than those of the vessel (which 

 is certainly the case for moderate sized wind waves). Tucker (1956) makes 

 use of ship motions to record larger ocean waves by judicious use of vertical 

 accelerometersj, hull-mounted pressure gauges, and a shipborne computer. But 

 this method is rather ineffective for measuring smaller wind waves e 



The ideal platform from which to make measurements of motions associated 

 with surface waves should: (l) provide minimum interference with the waves; 

 (2) be fixed relative to the ocean bottom.; and (3) be located far enough from 

 the coastline to provide an approximate open sea environment, (See chapter II.) 

 If there is an interaction of the platform with the environment, it should be 

 quantititively assessed so that the effects can be neglected or subtracted out 

 of the data. Also 3 since the majority of locally generated wind waves have 

 wavelengths that are smaller in magnitude than the water depth; measurements 

 should be made in a depth of water commensurate with the existance of relatively 

 large deep water waves. Finally, at any site chosen to measure waves , one should 

 have a general knowledge of the gross climatic and oceanographic environment. 



