60 WAVES 



fathom contour. And the growth of their inshore ends is still further 

 hindered by the effects of refraction, as explained on page 155. The 

 end result is that, while waves generated by winds blowing parallel 

 with the coast are only a little lower near the land than they are 

 farther out during the first stages in their development, the difference 

 in height between the inshore and offshore sectors becomes progres- 

 sively greater as they continue to advance, depending on the strength 

 and direction of the wind, on the length of the fetch, and on the angle 

 of slope of the bottom. 



METHODS OF MEASURING WAVES 



The lengths and periods of waves can be estimated from shipboard 

 with little difficulty and with a fair degree of accuracy, if the waves 

 are upwards of 20 feet or so long. A great number of measurements 

 of this sort have been taken in various parts of the world. 



The simplest way to measure the lengths of waves is with an old- 

 fashioned chip log. If this is payed out over the taff rail until the chip 

 is at the crest of one wave when the stern of the ship is on the crest of 

 the next, the length of line outboard is equal to the length of the wave, 

 provided the ship is running at right angles with the crests. If she is 

 not, the angle of her course can easily be allowed for by the traverse 

 tables that are included in every navigational handbook. Another 

 method is to record the frequency with which the waves overtake the 

 ship, and the time required for each crest to run her whole length from 

 bow to stern. If, for example, it takes a wave 10 seconds to run from 

 the stern to the bow of a vessel 300 feet long, running in the direction 

 of the waves, and if the waves overtake the ship every 18 seconds, it 

 means that the ship is only ten-eighteenths as long as the waves, i. e., 

 that the length of the wave equals 300x18 or 540 feet. 18 But an allow- 



10 

 ance must be made, in this case also, if the ship is running at an angle 

 with the waves. 



Measurements, from shipboard, of the velocities of waves demand a 

 knowledge of the speed of the ship through the water. If she is lying 

 motionless at anchor, and the time is recorded for the crest of a wave 

 to run along her side for a known distance, the velocity of the wave is 

 equal to the distance divided by the time. If, for example, observers 

 100 feet apart were to note that it required 5 seconds for a wave crest 

 to advance from opposite the one to opposite the other, its velocity 

 would be 100/5 or 20 feet per second, corresponding to 11.8 knots. If 

 the ship is running with the waves, the velocity of the latter is equal 

 to the distance divided by the time, plus the speed of the ship. If she 

 is running against the waves, the velocity of the latter is equal to the 



18 Adapted from Marnier, H. A., 1930, The Sea, New York, p. 182. 



