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Nigel Calder Answers Question - Ask Sail Page 2

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COMPASS TRUISM

Is it all right to use magnetic headings on my chartplotter to adjust a new compass?

KARL WESTMAN, OCEAN CITY, NEW JERSEY

TIM BARTLETT REPLIES:

In theory, definitely not. But in practice I'd have to give you a very guarded "maybe." The problem, of course, is that your heading is the direction your boat is pointing in. Your chartplotter/GPS unit cannot know this. It only knows the direction the boat is moving, known as your "track" or "course over ground."

And there can be a big difference. For example, if you are doing 6 knots with a 3-knot current on your beam, there's a difference of almost 30 degrees between your heading and your track. But if you should be lucky enough to have a perfectly calm day and there is no tide or current to affect the boat's course over the ground, the track shown by your chartplotter should be the same as your compass heading if the compass is adjusted properly.

A good way to check your compass is to aim your boat at a fixed landmark or buoy, and then take the magnetic bearing of the landmark by placing your chartplotter's cursor on it. Then compare that bearing with the compass heading. As long as the landmark is more than a mile or so away, the bearing shown by the chartplotter should be the same as the compass heading, give or take a degree.

CLEAR CHANNELI'm thinking about getting a satellite radio receiver so I can download all the marine weather products that are available these days. But I'm curious: Will I have to mount one of those softball-sized antennas above my deck?

HENRY MEYER, CHICAGO, ILLINOIS

GORDON WEST REPLIES:

Before you do anything about mounting the antenna, first check with your marine electronics specialist and make sure that the satellite radio weather products you plan to receive will be compatible with the charting program you have or plan to use. Navico's Northstar, for example, includes the Sirius weather module with Shakespeare's Galaxy softball-sized antenna system, and this antenna can pull signals in through fiberglass. Even so, if you have a deck with a teak overlay and it is covered with seawater, chances are good that reception of satellite signals will likely be blocked.

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