AEROGRAPHER'S MATE 3 & 2 



WIND DRIVEN WAVES 



SWELL 



WIND DRIVEN WAVES PLUS SWELL 



209.422 

 Figure 10-6. — Swell and wind waves in combina- 

 tion. 



until at least 15 waves have been determined. 

 Divide the number of waves by the elapsed time 

 to determine the wave period. Do not count the 

 wave crest on which the timing of the mark 

 is started. 



OBSERVATION OF WAVE HEIGHTS. — The 

 observation of wave heights from shipboard 

 is complicated by the rolling and pitching of 

 the ship, its rising and falling with the waves, 

 and the presence of high local winds. Low 

 waves tend to be underestimated; high waves, 

 overestimated. The best estimate can be ob- 

 tained by observing another ship in company. 

 The height from trough to crest of a wave 

 against her side can be estimated as a part 

 of some known vertical distance. For example, 

 a wave might be l/4 of the bridge height of 

 28 feet, or 7 feet high. 



Without the aid of another ship, you must 

 make your observations at times and positions 

 when your own ship's motions are as small 

 as possible. Observations should be made amid- 

 ships and near the center line. If possible, 

 the ship should be heading into the waves. If 

 not, choose a time when the rolling and pitching 

 is at a minimum. 



If waves are shorter than the ship, estimate 

 the wave height by looking over the side — using 

 as a yardstick the relative heights of known 

 points along the side. 



If waves are longer than the ship, wait until 

 the ship Is in the trough of a wave. By trial 

 and error, move up and down on the super- 

 structure until the wave crests are on a line 

 with the horizon. Then the distance your eye 



is from the ship's water line is the height 

 of the wave. If the ship is rolling, care should 

 be taken to line up the wave crest with the 

 horizon at the instant the ship is on an even 

 keel otherwise the height estimate will be too 

 large. 



The heights of waves usually vary consider- 

 ably. Observations should be made for approxi- 

 mately 5 minutes and mental estimates made 

 of the higher waves in each wave system. If 

 two observers are available, one can make the 

 estimates and the other record the observed 

 values. The wave height to be recorded is not 

 that of the highest wave, nor the lowest, but 

 the average of all the waves. 



When both sea and swell or two systems 

 of swell are present at the same time, ob- 

 servations will be more difficult. You should 

 estimate the higher system of waves first, 

 then repeat the process for the lower system. 



Swell Observations 



Swell waves may be defined as being sea 

 waves which have traveled outside of their 

 generating area. 



Swell consists of wind-generated waves which 

 have left their generating area and are no longer 

 subject to the original wind action. Conse- 

 quently, the wave loses energy and changes Its 

 characteristics; height decreases and period 

 increases. Characteristically, swell waves are 

 low with rounded tops. Figures 10-7 and 10-8 

 graphically illustrate the differences In the con- 

 figuration of sea and swell waves. 



The swell waves following each other are 

 nearly of the same height, and It Is possible 

 to follow a series of crests for long distances. 

 This Is in contrast to "sea waves," which 

 are individually shaped with sharp angular tops; 

 crests are not very long; and there are many 

 small waves superimposed on the larger waves. 

 Looking from one wave crest to the one imme- 

 diately following, one can see that the heights 

 are not regular. The individual crests are not 

 lined up In the same direction. (See figs. 10-5 

 and 10-6.) There may be some places where 

 for short distances the waves appear to be 

 lined up and regular, but this Is not a con- 

 sistent occurrence. In most Instances the sea 

 waves will appear broken up and confused. 



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