46 



THEORY OF SEAKEEPING 



U = 40 Kno+5 



Neumann 



Darb^shire C.R. 



Darbyshire O.S. 



D.S.A H 



very rapidly, and then is slowly displaced towards the 

 longer periods. Strictly speaking, there is no limiting 

 stationary regime, but rather a slowly variable regime 

 which we %vill call 'quasi fully arisen sea', after a dozen 

 hours. It is this regime which is represented by the pre- 

 ceding eciuations."^^ 



No fetch or duration effects were discussed for Darby- 

 shire's (1955) (open-sea) spectnmi but it was assumed 

 to exist after a wind duration of about 10 hrs. 



Fig. 47 was prepared by Gelci, et al (1957) to show the 

 mean of i lo highest wa\-es as a function of duration of a 

 40-knot wind as depicted bj' various forecast methods.'* 

 The original report by Gelci, et al, contains similar plots 

 for 30 and 50-knot winds in which the relati\'e placement 

 of the cur\-es is in the same order. 



Each of the methods discussed so far was based on the 

 investigator's interpretation of the sea data available to 

 him. Apparently the only direct measurements of the 

 growth of waves with wind duration are found in the work 

 of Ijima (1957) from which Figs. 48, 49 and .50 are taken. 

 These figures are based on obser\'ations at the Port of 

 Sakata on the west coast of Japan, where appear well- 

 defined and often stationary generating areas of moder- 

 ately strong wind over the Sea of Japan. Ijima's paper 

 gives data on seven summer and three winter storms. 



Fig. 46 Energy spectra of fully arisen ocean waves U(T) at a 



40-knot wind according to different formulations (from Gelci, 



Casale, and Vassal, 1957) 



'' Darbyshire's wave height spectrum transformed into energy 

 spectrum in ergs/cm-/sec. 



'* The author has not examined the reference to Suthon (1945), 

 liiit believes that it is the basic material used later by Bracelin 

 (1952) who is cited in Walden's (1953-54) discussion in the next 

 section. 



14 



12 _ 



-?6 



'A'ind Ourafion 



40 



50 



Fig. 47 Growth of waves with duration of 40-knot wind according to different formulations 

 (from Gelci, Casale, and Vassal, 1957) 



