Observations and Measurements of Ocean Waves 



35 



the course, direction and speed of the ship. The preceding formulae undergo 

 a change if the ship and the waves travel in the same or opposite direction, 

 then the speed of the ship has simply to be subtracted or added respectively 

 algebraically to the wave velocity. If the speed of the ship is V, the wave 

 velocity and the wave length will be, respectively 



c = (l/t 2 )±V or X = t z (c±V). 



If the course of the ship is not perpendicular to the wave crests, but maxes 

 an angle a, the above formula only gives the apparent wave velocity. In order 

 to obtain the real one, it must be multiplied by cos a; the angle here must 

 be smaller than 45°, else the result is not sufficiently accurate. 



It is more difficult to determine the wave period when the ship is travelling. 

 An observer located in A (see Fig. 20) observes in t x sec the passage of n waves 



Fig. 20. Determination of wave period and wave velocity. 



(apparent period = tjn). For this observer AA' would be the last wave 

 front counted by him. For an observer who shifted from A to B, BB' is the 

 last one, as he did not count the waves on the stretch CB. Therefore, we 

 have to add to the n waves, CB\l waves. As CB = Vt x cosa, the period will be 



T = t x :(n 



Vt x COSa 



\ = ^(wA+K^cosa) 



It is easy to establish that the wave length A = cT will then be I 



to. n 



COS a, 



i.e. the wave length is found from the apparent wave velocity multiplied 

 by the apparent period and by the cosine of the angle between the course 

 of the ship and the direction of the waves (Thorade, 1931, p. 24). 



The wave length can also be measured directly by throwing out a log 

 board or a small buoy and letting the marked rope unwind long enough 



3* 



