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Chapter 2 Wachstanders' Equipment
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Seaman - Military manual for the Seaman rate
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Magnetic Compass Error
point faithfully and steadily to true north. But the magnetic  compass  remains  the  reliable  standby, constantly checking the gyro's performance, and ready always to take over if it fails. MAGNETIC   COMPASS The  magnetic  compass  operates  through  the attraction exerted by Earth itself. Because Earth is certain to continue to function as a magnet, the magnetic compass has an unfailing power source. The magnetic compass (fig. 2-2) is located in the pilothouse. It consists of a magnetized compass needle attached to a circular compass card, usually 7 1/2 inches in diameter. The card and the needle are supported on a pivot that is set in a cast bronze bowl filled with a petroleum distillate fluid similar to Varsol. This liquid buoys up the card and the magnet. The buoyancy will take some of the load off the pivot, thereby reducing the friction and letting the card turn more easily on the pivot. At the same time, the liquid slows the swing of the card and brings it to rest more quickly. Marked on the compass bowl is a line, called the lubber's line, which agrees with the fore-and-aft line of the ship or boat. By reading the compass card's direction lined up with the lubber's line, you can tell the direction the ship is heading. The  card  remains  stationary,  pointing  at  the magnetic  pole  which  is  a  north-south  line  lined  up  with the north-south (magnetic) directions on Earth. When you are steering, always remember that the ship turns under  the  card. Figure 2-2.— Navy standard 7 1/2-Inch compass. 2-2 The compass bowl is mounted in a system of double rings on bearings, known as gimbals, permitting the compass card to ride flat and steady no matter how the ship may roll. In turn, the gimbal rings are mounted in a stand called the binnacle (fig. 2-3). The Navy uses a compensating binnacle, on which two spheres of soft iron are mounted on arms, one on either side of the compass. The spheres are adjusted to counteract some of the deviation (covered later in this chapter). To correct for other local magnetic forces that make up the deviation,  small  magnets  are  located  within  the binnacle, directly below the compass. The binnacle is positioned forward of the wheel, where it can best be seen  by  the  helmsman. The compass card is divided into 360°, numbered all the way around in a clockwise direction. A true course to be steered can be converted into a magnetic  compass  course  by  adding  or  subtracting variation for the area and deviation for the compass on Figure 2-3.— Navy standard magnetic compass binnacle.

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