I. Moo. 



SEAWAY 



.3. Mod. 



l.Mod. 



-0 1 0.1 -0.4 -0.2 0.2 0.4 0.6 -0.2 -O.I 01 0.2 



Po 



2 m 

 Fig. 7 Plot of air pressure versus wave elevation (from Motzfeld, 1937) 



I 2 3 4 5 G 7 8 9 10 II 12 13 14 15 16 17 18 19 20 21 22 23 24 



Shape of the Wave 



Fig. 8 Measurements on paraffin model of wind-generated wave (from Thijsse, 1952) 



from' these the slope of the water surface was obtained. 

 The horizontal drag force in this case includes both the 

 dynamic and frictional drag. The mean wave height 

 and length also were measured. The results of these 

 measurements are summarized in Table 2. 



The data in Table 2 are approximate. The values of 

 'K/H were read from the mean curves given on the graphs 

 since no tabulated data are given in the reference. In 

 calculating the drag coefficient the assumption was made 

 that V- = (V — c)-; i.e., the wave celerity of small 

 waves was neglected in comparison with wind velocity. 

 The maximum and minimum \'alues of the total drag 

 coefficient C^* agree well with those for the idealized 

 models of Motzfeld. It is clear, however, that the Cj of 

 wind-driven waves is not a unique f miction of the mean 

 value of the \/H ratio. These wa\'es are \'ery irregular 

 and while the mean wave is tabulated, lower and higher 



waves are found in reality. In particular, sharp and 

 breaking crests occur fairly freriuently. The sudden- 

 ness with which the Cj increases for the sharp-crested 

 profile is probal)ly the most significant result of Motz- 

 feld's tests. It appears that the mean drag of the ir- 

 regular-wave system may be primarily governed not by 

 the mean \/H ratio, but by the frecjuency of occurrence 

 of sharp crests or of steep wave slopes, since the drag of 

 the sharp-cre.sted waves is so much higher than that of 

 the steep trochoidal waves of \/H = 10 ratio. At the 5 

 m/sec wind ^'elocity in P>ancis' tests, sharp crests prob- 

 ably were absent and the drag was very low. With in- 

 crea.sing wind strength the sharp crests must have oc- 

 curred more often and the C\ increased correspondingly. 

 The measurements of Johnson and Rice (1952) were 

 also made in a wind flume. Because of the high wind 

 velocity used, the mean waves are exceptionall.y steep. 



