Is Your Compass Lying?
Brought to you by coastalnavigation.com
On most recreational boats the magnetic steering compass is the primary, if not the only, navigational device; so you might be surprised to find out that it's probably giving you inaccurate information. But don't toss it overboard just yet. Instead, read on.
The direction a magnetic compass is pointing is indicated on the compass card. This disk-shaped card is attached to small magnets. Mounted on a spindle, the card rotates as the magnets align themselves with the magnetic field of the Earth. Or at least the magnets try to align themselves.
As any boating enthusiast will confess, boat's can be a big distraction. They're distracting for magnets too. Metal objects like anchors, holding tanks, engine blocks and keels have their own magnetic fields. So do batteries, electronics and wiring. While these fields are minutely small compared with the Earth's, their proximity to the compass have an effect. As the magnets on the compass card rotate, they're pulled out of alignment by these onboard magnetic objects. This influence is called deviation.
Knowing the influence of deviation on your boat's steering compass is important if you want accurate information about the direction you're travelling. Deviation is unique to each boat, and it varies depending on the direction the boat is pointing. This is because the position of onboard magnetic objects change relative to the Earth's magnetic field. Depending on the heading, deviation can be East or West. So, a compass heading of 180° may be off by 4° East while a heading of 350° produces 8° of West deviation.
Finding your boat's unique compass deviation on different headings involves a little investigative work. With a helper, a hand-held compass, pencil and paper, and about a half hour on the water, you can create a deviation table to correct readings from your steering compass.
Visit www.coastalnavigation.com to find out how to make a quick-and-easy deviation table for your boat. A little time spent creating a deviation table will help you stay on course and out of trouble.
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