Appendix D
* * * * *
Supplementary Information on the Compass

Only introductory information on the compass was supplied in the text of this manual. For landing boat crew members seeking more information the following paragraphs have been written.

As stated in Chapter VII, the LCVP and the LCM(3) are equipped with a magnetic compass and, sometimes, with a magnesyn compass.

The magnetic compass operates because a magnetized needle, placed in a magnetic field, tends to align itself with that field. The earth has a magnetic field and the needle of the compass makes use of this, pointing to the north magnetic pole.

In the usual "VP" or "M(3)" boat compass the needle or piece of magnetized steel is not visible. It is inside the disc marked with the points of the compass which show up behind the small window to indicate the boat's heading.

All types of the magnetic compass have the same general characteristics. These are:

  1. The magnetized compass needles align themselves with the earth's magnetic field.

  2. The needles are fastened to a card, disk, or cylinder marked with the points of the compass: north, east, and so forth.

  3. The card and needle are supported on a pivot. As a boat swings the needle continues to point to the magnetic pole, thus indicating the relation between the direction in which the boat is heading and magnetic north.

  4. The moving parts of the compass are contained in a liquid which partially floats them, thus reducing the weight and friction on the pivot. The liquid also slows the movement of the needle and card and keeps them from whirling wildly. Kerosene or water and alcohol are commonly used for the fluid because they do not freeze.

  5. The moving parts are contained in a bowl or similar housing with a window through which the compass card markings may be seen easily.

  6. In the window of the compass is a line set parallel with the fore and aft line of the ship. This is called the "lubber's line." The compass is read by noting the markings directly behind this line.

  7. Below the window is a deviation card which gives the compass course to steer for every 30° of the compass. Deviation is one of several errors which disturb the compass. Two of these errors must be considered in correcting a compass. These are deviation and variation which are discussed below.

Deviation: the error caused by the magnetic properties of any iron in the immediate vicinity of the compass.

Variation: the error caused by the fact that the magnetic and geographical poles of the earth do not coincide. This error is marked on charts, since it varies from place to place and from year to year.

Deviation and variation together are known as the compass error.

Before learning how to correct the compass for deviation and variation you must know the following definitions:

True course: a course plotted in terms of the north geographical pole.

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Magnetic course: a course plotted in terms of the north magnetic pole. (correcting for variation changes the magnetic course to a true course.)

Compass course: refers to a particular boat's compass on each particular heading. It must be corrected for deviation on that heading before it can be converted to the magnetic course. Both deviation and variation must be corrected for conversion of the compass course to true bearings or true courses.

Correcting for deviation and variation. The procedure followed in correcting is simple once you know the steps involved.

Errors in deviation and variation may cause the compass card to turn either to the East or to the West. An example will serve to show you how to correct the compass so as to steer a true course when you are given a compass course of 270° (due West) and know that the deviation is 10° E and the variation is 5° W.

  1. Here is the information you have been given, expressed in table form:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
    270° 10° E   5° W    

  2. Add the 10° deviation to the compass course. This gives a magnetic course of 280°. Then subtract the variation of 5° W. This gives a true course of 275° which you will steer. The total error is 5° E which is the difference between the 10° E deviation and the 5° W variation.

  3. Filled in the table looks thus:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
    270° 10° E 280° 5° W 275° 5° E

  4. The rule to follow as shown is, when correcting add Easterly errors and subtract Westerly errors.

Uncorrecting the compass. If you know the true course, deviation, and variation it is equally easy to find the compass course. The process of finding the compass course when the true course, deviation, and variation are known is called uncorrecting. In correcting you move from the left to the right of the table. In uncorrecting the direction is reversed. An example will again serve to show how this is done. You are given a true course of 180° (due south) and know that deviation is 15° E and variation is 10° W.

  1. In table form this is set down as follows:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
      15° E   10° W 180°  

  2. Add the 10° W variation to the true course. This give a magnetic course of 190°. Then subtract the deviation of 15° E. This gives a compass course of 175°. Total error is 5° E.

  3. Your table should look thus:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
    175° 15° E 190° 10° W 180° 5° E

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  1. Note that when uncorrecting you add Westerly errors and subtract Easterly errors. This is the opposite of the procedure when correcting.

To fix the procedures in your mind, work the following problems. Then turn to the answers at the end and see if you did them correctly.

  1. Correct:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
    30° 17° W   6° E    

  2. Uncorrect:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
      7° W   11° E 316°  

  3. Correct:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
    120° 2° E   14° W    

  4. Correct:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
    185° 4° E   9° E    

  5. Uncorrect:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
      11° W   12° W 70°  

  6. Uncorrect:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
      5° W 120° 6° E    

  7. Uncorrect:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
      9° W 100°   98° 11° W

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  1. Correct:

    Compass
    course
    Deviation Magnetic
    course
    Variation True
    course
    Total
    error
    110°   117° 4° E    

 

Answers:

  1. Magnetic course is 13°, true course is 19°, and total error is 11° W.

  2. Compass course is 312°, magnetic course is 305°, and total error is 4°E.

  3. Magnetic course is 122°, true course is 108°, and total error is 12° W.

  4. Magnetic course is 189°, true course is 198°, and total error is 13° E.

  5. Compass course is 93°, magnetic course is 82°, and total error is 23° W.

  6. Compass course is 125°, true course is 126°, and total error is 1° E.

  7. Compass course is 109° and variation is 2° W.

  8. Deviation is 7° E, true course is 121°. andn total error is 11° E.

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