Rule.-Apply leeway toward the direction of the wind, westerly variation and deviation to the right, and easterly variation and deviation to the left. FIG. 39 EXAMPLE 1.-The true course to a certain port is N 30° E, the variation is 28° W, deviation 6° E. Find what course to steer by the compass. E SOLUTION.-See Fig. 39. True course = N 30° E Magnetic course = N 58° E = Compass course N 52° E. Ans. EXAMPLE 2.-Required the compass course, when given, true course N 8° W, variation =17° 10' W, deviation=3° 20′ E. The wind is easterly and the leeway estimated to be point. 137. In finding the compass course from the true course the variation should always be applied first, so as to obtain the magnetic course; the deviation corresponding to the magnetic course given in column 3 of Table V is then applied, the result being the compass course. In case the magnetic course does not agree with any of the values given in column 3, the deviation may be found by simple proportion; in cases where the deviation is small, it may be taken out for the value in column 3 corresponding most nearly to the magnetic course. With large deviations the use of Napier's diagram is recommended as being a valuable aid for converting true courses into compass courses. EXAMPLE 4.—The true course is S 47° E, wind SSW, leeway point, variation 27° 40′ W, deviation 3° 20′ E. Find the compass EXAMPLE 5.-The true course is N 59° W; the variation is 3° E, and the deviation according to Table V. Find the course to steer NOTE.-On examining the deviation table it will be seen that the deviation for courses between N 53° W and N 64° W is constant, hence we apply a deviation of 19° W to the magnetic course. EXAMPLE 6.-According to the chart the true course from one place to another is N 1° E, the variation is 2° 30′ E. Use the deviation for the nearest, the magnetic course, Table V, and determine the compass course. EXAMPLES FOR PRACTICE Find the compass course from the following true courses: 1. = True course = N 27° 30′ W, variation=18° 30′ E, deviation: 3° 15' W, wind westerly, leewaypoint. Ans. N 45° 34′ W. 2. True course-S 75° W, variation=33° 10′ W, wind easterly, deviation 1° 50′ W. Ans. N 70° W. 3. True course=S 17° E, variation=22° 40′ W, deviation=3° 30′ W, wind W SW, leeway 14 points. Ans. S 23° 14′ W. 4. True course = N 58° W, variation=9° E, deviation=2° W. Ans. N 65° W. 5. True course = N 48° W, variation=5° E, deviation according to Table V. Ans. N 34° W. 6. True course=S 8° E, variation=2° E, deviation according to Table V. Ans. S 23° E. 138. As previously stated, when the deviations are large and changing rapidly from point to point (which very seldom happens nowadays when compasses are so carefully adjusted), the deviation should be found by proportion to a reasonable degree of accuracy. For instance, if in example 2, in the Examples for Practice, instead of a small deviation the deviation in Table V had been used, the solution would be as follows: 139. Cautionary Remarks. Compass course N 54° 20', or N WW, nearly. Learn to master the subject of course corrections thoroughly, as it is very important. A mistake in the application of variation or deviation may at times prove very costly. Not very long ago the captain of a large steamer, in shaping his compass course from the true course as indicated by the chart, applied the deviation the wrong way. After a few hours run the steamer was grounded and wedged in between solid rocks that afterwards crushed through her hull, causing a loss of thousands of dollars to her owners and the reputation and position of her master; and all this simply because of the mistake made in applying westerly deviation to the left instead of to the right of the magnetic course. It is better to apply no deviation at all than to apply it the wrong way. |