SCIENCE AND THE SEA 



hint is based on two important factors: one, that the chart is accurate, 

 and two, that the bearings were accurately taken within a few seconds 

 of each other. 



CALCULATION FOR DETERMINING COEFFICIENT A 



DEVIATION HEADING NORTH —14° 



DEVIATION HEADING EAST —18° 



TOTALS —32° 



DEVIATION HEADING SOUTH + 8° 



DEVIATION HEADING WEST +12° 



+ 20° 



Figure 6. When possible, at least three cross hearings should always be taken 

 in obtaining a fix. If the bearing lines do not cross compass error may exist. 



In figure 6, the solid lines represent the observed bearings of 

 three fixed objects on an accurate large scale chart. The navigator had 

 properly observed and laid off the bearings and realized that the 

 triangle formed was the result of unknown compass error. After a 

 few minutes of juggling the bearings, equally clockwise and counter- 

 clockwise, the navigator found that by adding 4° to each bearing that 

 the lines of position crossed outside the original triangle, (the dashed 

 lines.) The four degree adjustment revealed a previously unknown 

 error of 4°E. in the compass. 



SPECIAL CASE 



There is a special case to be guarded against in the selection of 

 terrestrial objects to be used in the cross bearing fix. A geometrical 

 peculiarity which should be recalled is that through any three points 

 not in a straight line, a complete circle can always be drawn, and only 

 one. Now, if by chance the vessel itself is on or near this circle, a 

 seemingly perfect fix can always be obtained. This situation is 

 possible as compass error will in no way prevent the lines of bearing 

 from crossing at a common point. 



COMPASS ALIGNMENT 



The true fore and aft alignment of the lubber's line on the 

 standard compass and pelorus is relatively quick and easy to deter- 

 mine. This is accomplished by comparing the relative bearing of 

 a distant object with that obtained by careful measurement on the 

 chart when the vessel is alongside the dock and its true heading is 

 known. The correct alignment of the lubber's line on the steering 

 compass, as a rule, is more difficult to ascertain. Some mariners 

 take for granted that the alignment is correct and fail to check it. 

 This situation, however, should always be investigated when first 

 reporting aboard for duty and after yard repair or lay up. 



Cases have been recorded where the lubber's line of the steering 

 compass was off the longitudinal axis by 5 or 6 degrees. While 

 azimuths reveal errors of the compass card, they do not disclose the 

 error of a misaligned compass bowl. When the steering compass is 

 so located that it is difficult or impossible to line up with the jack- 

 staff, the error in alignment, if any, may be very closely determined 

 by the following method: 



Ascertain the deviation on the four cardinal points by careful 

 comparison with the standard compass or pelorus. Assign a (+) 

 when the deviation is easterly or a ( — ) when westerly. Add the 

 four figures together algebraically retaining the sign of the larger sum. 

 Next, divide the result of the addition by 4. The remainder thus 

 obtained is known as coefficient A and, if the compass is well made, 

 is due for the most part to a misaligned compass bowl. When coef- 

 ficient A is (+), the lubber's line should be moved to the right the 

 number of degrees indicated or if ( — ) moved to the left. 



In the above example, the lubber's line should be moved 3° to the 

 left to place it on the longitudinal axis of the vessel. 



MECHANICAL DEFECTS 



The compass, like any precision instrument, is subject to various 

 mechanical defects which can easily go undetected for some time. 

 Most common among these are broken or blunted pivot points, 

 punctures or roughness of the jewelled cap, card not moving freely 

 in the bowl, and excessive weight on the card itself. 



The mariner, with the aid of a small magnet, can quickly check 

 his compass for defects of this nature by the following simple 

 procedure: 



Note the compass heading, then using the small magnet draw 

 the north point of the compass card about 15° to the right. Next, 

 remove the magnet and record the heading when the card comes to 

 rest. Repeat the process this time drawing the card about 15° to 

 the left. If the card comes to rest each time on the original heading, 

 the compass is free of the mechanical defects mentioned. A final 

 heading differing from the original indicates one or more defects are 

 present and that repair is needed. 



RETAINED MAGNETISM 



A change in course after a vessel has been steaming or lying on 

 the same heading for some time is always attended by compass error. 

 This error is caused by the retained magnetism induced while the 

 vessel was on that heading. The exact amount of error can only be 

 determined by observation, but can be expected to throw the vessel 

 in the direction of the last course. The general rules regarding the 

 error to be expected are: 



1. After steering for some time on westerly courses, expect: 



(a) Westerly error if you turn north ; 



(b) Easterly error if you turn south. 



2. After steering for some time on easterly courses, expect: 



(a) Easterly error if you turn north ; 



(b) Westerly error if you turn south. 



Retained magnetism induced in a vessel after lying or steaming on 

 the same course for some time always causes the compass to hold back in the 

 direction of the last course. 



LOCAL MAGNETIC DISTURBANCE 



There are a few locations in the world where the charts show 

 areas, usually very small in extent and located in relatively shallow 

 water, where local magnetic disturbance of the compass is caused by 

 magnetic mineral deposits on the bottom. Although numerous 

 reports have been received concerning local magnetic disturbances. 



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