330 



THEORY OF SEAKEEPING 



24|. 



20. 



16. 



I 12- 



i- 



Qf — 



4 G 



TCsecl 



T, ' 6.S 



m 



13 = 8.7 

 1 



_1 1_ 



Tj=II.( 



I 



n 



4 6 8 10 



TCsec) 





\Z 14 



8 10 \Z 14 IS 



TCsec) 



Fig. 3 Observed frequency distribution of apparent periods in a seaway at different wind speeds 



(from Neumann, 1952a) 



plays a part in the question of energy transfer and 'rough- 

 ness'. 



"2 Tlie relatively steep, ready-to-break 'sea.' This 

 is of greatest interest to a practical man at sea. Its di- 

 mensions depend on the wind strength: c = f{U), 5 = 

 F{U). 



"3 The gently crested, swell-like advancing 'long 

 wave' for which we will take c ^^ 1.37 U. It does not at- 

 tract the immediate attention of an observer, since in the 

 wind region it can be covered by the steeper waves." 



From Xeumanu (l-1952a pages 96, 97) : "In a seaway 

 waves of difTerent height and length occur side by side 

 and are superimposed. From this the sea surface obtains 

 its pronoiuiced rough appearance, particularlj' in the de- 

 velopment stages of wave motion. But the longer the 

 wind acts and the more the .sea attains full development, 

 i.e., its greatest height, the clearer the characteristic 

 wa\-es, corresponding to wind strength, work out of the 

 wave confusion. At the same time the individual hill- 

 like wave crests unite to an increasing extent into long 

 crests: the 'sea' builds up. 



"A further sign of de\'eloping or full}' de\'eloped sea- 

 way is the occurrence of irregularities in the sec[uence of 

 'seas' following one after another as well as the group- 

 like occurrence of w'ave systems. As one tries to follow 

 for a time an isolated wave crest, one recognizes that it 

 does not exist an arbitrarily long time, but gradually 

 loses height and disappears in the wa\-e confusion, while 

 another one grows and later similarly fades awaj'. One 

 can observe the swelling and diminishing of the waves 

 from a not-too-fast ship in a following sea, when the wave 

 celerity is approximately the same as the ship speed. 

 Two wave crests alongside the ship often change their 

 heights so that the rear one grows in size, while the front 

 one fades away. Eventually the rear one also seems to 

 disappear, and at another place alongside the ship a new 

 one begins to grow. Such wave-group appearances and 

 irregularities are typical of the nature of sea sui'face. 



"The seaway in a fully or almost fully developed state 

 is not described bj' a single characteristic 'sea,' although 



15 \i 



Fig. 4 Frequency distribution of apparent periods in a seaway 



in wind speed interval 14.5-15.4 m sec (from Neumann, 



1952a) 



one appears most conspicuous at almost all wind speeds. 

 With progressive development, a category of longer and 

 substantially flatter waves apparently builds up in the 

 seawaj'. The mutual action of these main waves, which 

 are co^•ered by waves of lower order, gives the visible ir- 

 regular picture of the sea surface, and makes under- 

 standable the occurrence of characteristic wave groups as 

 well as irregularities ob.served at sea." 



Observational data used by Neumann originate pri- 

 marilj' from four sources: The material of many ob- 

 servers collected by Sverdrup and ]\Iunk (1-1946), the 

 observations on the mean water slope in wind by Palmen 

 (l-1932a, b, 1-1936), observations of Roll (l-'l951) for 

 small c/U, and very extensive observations made by 

 Neumann himself. These were made dm-ing the voyage 

 on ]\IS Heidberg starting from Hamburg on October 14, 

 1950. The outgoing course to the Caribbean Sea was via 

 the Azores, and the return passing close to Bermuda. 

 Neumann (l-1952a, page 79) writes: ". . . the number of 

 individual measurements of periods is estimated at about 



