irlcal relationships, which may lead to a satisfactory approximation. 

 Let 



A := Ajj + A^ » p'v3p[Y^(p) + Yn(p)(l - 0)^]| (12) 



the combined action of both wind force components or 



A = p'v^p.C(p) . (13) 



The effective factor C(p) with the meaning of a resistance 

 coefficient may be estimated empirically and determined as a function 

 of p. The hydrodynamical characteristics of the rough wavy sea sur- 

 face are now implied in the dimensionless quantity C(p), and its value 

 is related empirically to the wind velocity at "anemometer height," 

 say, 10 m above the mean sea surface level. This means that C is 

 determined by observations in such a way that it refers to the wind 

 velocity at a certain height by definition. This empirical method 

 is not a very satisfactory one, but steady pursuit in this direction 

 may in the future yield a means of determining better approximations. 

 At present this empirical way seems to be the only approach. 



For estimating the resistance coefficients, several different 

 methods have been tried. As already mentioned, H. Jeffreys [18] 

 considers only the normal wind force components, and assumes for 

 the effective wind pressure a priori a certain distribution. Let 

 the effective pressure component be p*j then we have according to 

 H. Jeffreys 



p* = sp'(v - <T)^ dv/ix "%■-'■ ^"' 

 H. Jeffreys calls the coefficient of proportionality, s, the "shelter- 

 ing coefficient" ("streamlining coefficient" according to Sverdrup- 

 Munk [1]). The work done by this force per unit surface area and 

 unit time on a wave, will be 



57 



