28 



WAVE DIMENSIONS 



are tlie longest yet on record (Kriimmel, 1011, p. 40. and Cornish, 1010, 

 p. 92). 



Table 11. — Lengths of storm waves observed in different oceans 

 [Adapted from Gaillard] 



Information as to the lengths of waves is scant for narrower seas. 

 They appear to average somewhat shorter in the China Sea than in the 

 open ocean under similar conditions of wind and weather. And this 

 probably applies also to the Mediterranean, where the longest waves 

 yet recorded in print were of about 3:28 feet, although higher and hence 

 probably somewhat longer waves have been reported in winter gales. 



THE STEEPNESS OF WAVES 



The lengths of waves concern the seaman in two ways chiefly : first, 

 as governing the number of individual crests and troughs across which 

 a ship of a given length will extend in different conditions of wind and 

 weather, and second, and more especially, because the length of a wave 

 combined with its height determines its steepness. Steepness may be 

 expressed in two ways, as the ratio of height to length, or as the 

 ratio of length to height. Both methods of expression are used here. 

 The steepness of waves is a matter of very direct concern, if one's 

 vessel is heading into a sea of any considerable size, or if she is run- 

 ning in the trough of the sea. 



It is a matter of common knowledge that waves average steeper in 

 the earlier stages of a blow than they do later on (i. e., that they are 

 "choppier"). And theory is in accord with observation in this respect, 

 as appears from table 12. 



This tabulation is in line with observations at sea. Off the Cape of 

 Good Hope, for example, a wind blowing for 4 days in a uniform di- 

 rection has been reported as lifting the average heights of the waves 

 only from 20 to 23 feet on one occasion, though their average 

 lengths increased from 370 to 770 feet (observations by Lieutenant 

 Paris, cited from Kriimmel, 1011, p. 64). In another published in- 

 stance for the Atlantic Trade Wind Belt near the equator, when the 

 largest waves grew from about 5 or 6 feet high to abont 10 feet in 

 height, their average lengths increased from abont 33 feet to more than 

 ion feet, i.e., tripled (Kriimmel, 1011, p. 64). 



