a steady state has been reached, that is at any given point the 

 waves do not change, no natter how long the '';ind lasts, but beyond 

 35 miles the waves continue to grow for a length of time v/hich 

 depends upon the distance from the coast. After 10 hours a steady 

 state has been established to a distance of 85 miles, after 15 

 hours to a distance of l60 miles, and so on. In Figure 8 the full- 

 drawn and dashed ciarves show the steady state. Parts of the curves 

 and the horiz.ontal lines represent wave height and period as func- 

 tions of the distance from the coast at 5 to 35 hours after the con- 

 stant wind of 30 knots started to blow. 



The fetch sho^/'m in Figure 8 can be limited either by the pre- 

 sence of a coast line or by the characteristics of a wind system 

 over the open ocean. It may be seen from the figure that for a 

 given wind velocity the time needed to establish a steady state 

 depends only upon the length of the fetch. For a given fetch this 

 time depends, however, on the wind velocity and is longer for weak 

 winds than for strong winds. This time is called the minimum dur - 

 ation and is measured in hours, Plate I shows the minimum duration 

 as function of mnd velocity and fetch. 



Plates II and III show wave heights and periods as functions of 

 fetch and wind velocity when the dui'ation is longer than the minimum. 



If the time is shorter than the minimum duration, the waves at 

 the end of the fetch depend on the wind velocity and the duration in 

 a manner similar to that shov/n for a 30 knot v/ind in Figure 8. For 

 practical use Plates IV and V show ^^-ave heights and periods as func- 

 tions of wind velocity and duration. 



23 



