HOW 



TO MAKE 

 OBSERVATIONS 



Four items are to be measured and reported: these 

 are wave height, wave period, wave length, and 

 direction from which the waves approach. Aboard 

 ship, the height and direction must be estimated, 

 the period can be measured, and wave length either 

 calculated or measured. In addition, certain obser- 

 vations of the general sea conditions should be made. 

 The following methods of observing are for use on 

 a moving vessel. No special procedures will be given 

 for use when the ship is hove to or not underway, 

 since the job then is greatly simplified. 



WAVE HEIGHT 



Estimating wave heights from shipboard is compli- 

 cated 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 

 obtained by observing another ship in company. 

 The height from trough to crest of a wave against 

 her side can be estimated as a port of some known 

 vertical distance. For example, a wove might be Va of 

 the bridge height of 28 feet or seven 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 amidships and near the center line. 

 If possible, the ship should be heading info 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 two known 

 points along the side. For example: the sea-ladder 

 fittings, the loading ports, the rubbing stroke. 



K waves are longer than the ship: 



Wait until the ship is in the trough of a wave and 

 vertical. By trial and error, move up and down on 

 the superstructure 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 vertical, otherwise the height estimate will be too 

 large. 



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