The heights of waves usually vary considerably. Ob- 

 servations should be made for at least five 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 nor 

 the average of all the waves. What is needed is the 

 significant wave height, which by definition is the av- 

 erage of the highest third of the waves observed in 

 a certain time. 



When both sea and swell or two systems of swell 

 are present at the same time, observations will be 

 more difficult. You should estimate the higher system 

 of waves first, then repeat the process for the lower 

 system. 



WAVE PERIOD 



The simplest way of measuring wave period is with 

 a stop watch. Pick out on object floating on the water 

 at some distance from your ship as a reference point. 

 You can use a piece of sea weed, a piece of wood 

 thrown overboard, a bird on the water, or a patch 

 of foam which will remain identifiable for a few 

 minutes. If possible choose an object on the bow, 

 as far off as possible. Then with the watch, measure 

 the time of successive appearances of the object on 

 the crest of a wove. You should take your readings 

 OS long as the reference object remains in sight. The 

 overage value of the period observations is used 

 for the record. 



Although both wove length and wave velocity con be 

 computed from the wove period by the formulas on 

 page 2, independent measurements of wove length 

 are desirable as a check on the accuracy of the 

 wave period. 



WAVE LENGTH 



Different procedures must be used for measuring 

 wave length, depending on the direction from which 

 the waves approach the ship. They may be coming 

 from dead ahead or dead astern, at an angle on 

 the bow or quarter, or from abeam the ship. The 

 procedures which should be used in each of the three 

 cases are described below: 



waves from dead ahead or dead astern: 



1. The easiest way to measure wave length is to com- 

 pare the distance between two wove crests with the 

 length of the ship. However, experience has shown 

 that this method usually results in wave lengths that 

 are too short since the observer is apt to take the 

 crest of the wave at a point somewhat short of 

 the true summit. 



If the waves are not as long as the ship, two observers 

 can get accurate measurements of wave length by sta- 

 tioning themselves along the side of the ship a wave 

 length apart and noting the distance between them. 



2. You can also determine the wave length by stream- 

 ing a buoyant object on a line. If you pay out the 

 line until the floating object is on one crest and you 

 are on the next, the length of line you have out will 

 give you the wave length. 



3. In a special case when your ship is travelling 

 along with the waves at a speed equal to the wave 

 velocity, the wave length can be found from the 

 equation: 

 Wave Length Units = 0.5 (Wave Velocity Units)^ 

 in feet in knots 



In this case, the wove velocity is the same as the 

 speed of the ship. 



