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used if a large scale chart is not available. When navigating in ice 

 it is desirable to compute the speed as often as possible, in any case 

 not less frequently than several times an hour. 



For determining speed in ice the navigator must fall back on the 

 method probably used by Columbus, which is as follows: 



The lookout, standing in the bow of the ship, throws to one side onto 

 the ice ahead of the vessel a chip of wood, a piece of clinker, or a sim- 

 ilar object. At the moment when this object passes abreast the stem, 

 the lookout gives a signal by hand whistle and the navigator starts 

 a stop-watch. A third hand in the stern of the ship notes the passage 

 of the object past the stern post and at that instant gives a signal on 

 which the navigator stops his watch. By this means the time during 

 which the vessel covers a distance equal to the length of its hull between 

 the stem and the stern post (or between any two widely separated 

 frames of the ship) is determined. 



Suppose for instance, that this distance is 325 feet. The stop watch 

 shows that the distance is passed in 66 seconds. The distance covered 

 in one hour (3,600 seccaids) or the speed, would be : 



325X3,600 ^^^ ^^^^ hour = 2.9 knots 



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It is not necessary to make this calculation every time ; a table should 

 be prepared in advance for the ship, by which after determining the 

 time as above the corresponding speed can be obtained at once. 



A second method of determining the speed is to use a variant of the 

 chiplog, by attaching a weight to a line on which any length, say 100 

 feet, may be marked. Having thrown the weight overboard onto the 

 ice, determine with the stop-watch the time taken to pay out the meas- 

 ured length of the line. If this is 20 seconds, the speed of the vessel 

 is shown to be : 



100X3,600 ,o^^^. , 1 oni f 

 — ^ =18,000 feet per hour = 3.0 knots 



By actual determinations of this kind, the navigator can calibrate 

 liis revolutions vs. speed curve. Thus, one ship found that her speed 

 in pack ice could be found by deducting 1 knot from the revolutions 

 per minute speed curve, except in the heaviest pack when 4 knots 

 should be deducted. 



Plotting the positions of large icebergs by radar ranges and bear- 

 ing and using these relative positions to compute the ship's speed 

 have been reported to give good results. However, this method as- 



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