YES! We do Magnetic Compass Adjusting -- What you need to know
YES! We Do Magnetic Compass Adjusting
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In an age of global positioning systems and computer navigation who needs a magnetic compass?
Your old-fashioned mechanical magnetic compass is that reliable, nearly failsafe backup every navigator needs to insure that if the power fails, the computer crashes, batteries die, the government switches off GPS, lightning strikes, or streams of ions from a solar flare disrupt satellite communications, you will not be lost or stranded. GPS will give an indication of direction, but only if you are under way. The properly compensated magnetic compass is the most reliable source of heading information.
How about relying on an electronic or fluxgate compass as my magnetic compass? The new technology electronic magnetic compasses have many nifty convenience features. But they are still electronic devices subject to the same potential for failing when you need them most that are described above. You may want to have an electronic compass as a convenience just as you have a other electronic navigation aids, but you still need a reliable mechanical magnetic compass.
What is compass adjusting or compensation? Why do I need it? A mechanical magnetic compass, in good repair, is wonderfully reliable. In almost any emergency storm or power failure it just keeps on working independently reacting to the earth’s magnetic field. However, it will also be influenced by local magnetic fields caused by steel or iron in the vessel’s hull or equipment or electromagnetic fields caused by electrical equipment and wiring near the compass. The compass cannot be shielded from these influences and still be able to sense the Earth’s magnetic field. Therefore, it is necessary to compensate for these spurious magnetic influences by placing magnets near the compass in such a position as to neutralize or offset these on-board magnetic fields. Most modern compasses have built-in compensating magnets that can be adjusted to provide this compensation. In other situations compensating magnets may need to be mounted on or under the shelf, console, or pedestal supporting the compass. In the case of vessels with steel hulls or large pieces of steel or iron gear near the compass special compensating devices such as quadrantal spheres or “balls” may also need to be installed near the compass and adjusted for proper effect. Unless the vessel and it’s equipment is largely nonmagnetic, if this compass adjustment is not done, the compass will give incorrect readings. This error is called “deviation”. If you run courses per a chart and find significant differences between what the chart, or your GPS shows and what the compass shows (after allowing for the difference between true and magnetic north) then your compass is probably in need of compensation or repair. Remember, the direction and amount of compass deviation will likely differ according the boat’s heading. The compass may be quite close to correct on some headings, but have significant deviation error on other headings. A compass that is not adjusted may not be very useful in time of need. Remember always, when using a magnetic compass, magnetic north is not the same as true, or geographic north. In the center of each compass rose on your charts is a notation of the difference between true and magnetic north. In Seattle, today, magnetic north is about 20 degrees east of true north.
How do I go about getting my compass adjusted? There are, of course, goods books on navigation that describe techniques you can use to compensate a compass. When you read through these you may realize that there is a fair amount of skill involved to do it right and not spend all day at it. The typical navigator would not do a compass adjustment often enough to develop skill at it. The professional mariners who man commercial ships, and Navy and Coast Guard vessels typically hire a professional compass adjustor rather than try to do it themselves. For over 100 years Captain’s Nautical Supplies has offered professional compass adjusting services in Seattle for both recreational and commercial vessels. Captain’s staff adjustors have adjusted every type of vessel imaginable from trailerable boats to the huge aircraft carrier USS Nimitz.