Observations and Measurements of Ocean Waves 



49 



The division of the frequency maxima in two regions shows perhaps the 

 border between wind waves ( > 1 : 35) and swell ( < 1 : 35). The average steepness 

 of the wave profile on the leeside and the windside are within a narrow range. 

 They are of the magnitude 005, which corresponds to an angle of inclination 

 of about 3°. But on short distances we have sometimes extraordinary large 

 waves inclinations, as is shown by the maximum values in the table. Values 

 of 0'4 (about 22°) have been observed, and 0-2 (about 12°) are frequent. 

 These values are very important in relation to the stability of ships. 



It is of interest to discuss the angle formed by the wave crests. The ob- 

 servations available are too few to be grouped e.g. according to swell o.a. 

 Mostly the angle is in excess of 120°. Of 52 values we have 41, which is 

 about four-fifths, lying between 149° and 165°. Schumacher could find only 

 very few angles smaller than 120°, which corresponds to the limits of a Sto- 

 kes's wave. Weinblum concurs that the waves measured by him did not 

 have any angle formed by the crest smaller than 120°. 



The wave profiles derived from the contour lines have not a simple ma- 

 thematical form. An approximation to a cosine form could not be found, 

 and even a trochoid profile could only be established in a few well-shaped 

 waves. Looking at the profiles as presented by Schumacher, one has the 

 impression of a very irregular appearance of the sea. But by grouping and 

 averaging and by elimination of the very numerous irregularities, he gets 

 for well-shaped wave trains a profile which is very similar to the form of 

 a trochoid. In Fig. 29 several profiles have been compiled and compared 



200 



Fig. 29. Comparison between averaged waves and trochoid profile, (mean values of several 

 profiles, wave length reduced to 200 m). Dotted lines: trochoid (height scale exagerated 



10 times). 



with the corresponding trochoid of similar height. One camrecognize a certain 

 similarity, especially on the windward side. On the leeside, the real wave 

 profile is below the trochoid. Such deviations, which make the crests steeper 

 and the troughs flatter, have already been observed in the pictures of Kohl- 

 schuetter taken on the "Planet". The deviation on the leeside, which cor- 

 responds to the Stokes theory, seems to be characteristic of the wave profile 

 for well-developed waves (see p. 80). 



The stereophotogrammetric wave maps are morphological instantaneous 



