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Old 23-04-2011, 20:22   #1
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Compass Dip when Crossing Equator

Magnetic compasses are balanced for either Northern Hemisphere or Southern Hemisphere, and if you take a compass balanced for one hemisphere to the other hemisphere, the card will not be level. How big a problem is this for world cruisers? Do you rebalance your compass when crossing the equator? Do you have two compasses? Is the problem so minor that you ignore it? Looking for reponses from experienced world cruisers. Thanks!
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Old 24-04-2011, 00:19   #2
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Re: Compass dip when crossing equator

For flat sealed units such as hockey puck hand bearing compasses, the dip will cause the card to ground out on the casing giving random erroneous readings. There is nothing to adjust on these compasses. If you are desperate you might be able to hold the compasses tilted at some appropriate tilt and get a good reading.

For ship's compasses gimballed inside a ball, the card will not be level but the reading should be correct or very close to correct. Because of the tilt the gimballing may ground out at less heel on one tack or the other. Most ship's compasses are adjustable and you should be able to readjust the compass for the correct hemisphere.

Directions for compass adjustment are in the 1984 Bowditch, I don't know about newer editions. There is an online edition of unknown vintage available for free at
Bowditch Online

or just the compass chapter at http://www.irbs.com/bowditch/pdf/chapt06.pdf.
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Old 24-04-2011, 03:12   #3
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Re: Compass Dip when Crossing Equator

You will find that there several different areas with magnetic differences throughout the world, not just North and South of the equator. A normal ships compass shouldn't have any problems but as earlier mentioned hockey puck compasses and binoculars with a compass fitted will have trouble as the bowl isn't big enough and therefore the card grounds out on the body. I got caught with a set of binoculars bought on line from a well known US firm but wasn't told that they were set up for use in the states area and not in the South Pacific. I had expected that they would send the appropriate ones but apparently they only had the one setting. Still good Bino's though, just can't take accurate bearings.
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Old 24-04-2011, 03:26   #4
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Re: Compass Dip when Crossing Equator

Full explanation of magnetic compass adjustment can be found in the Handbook of Magnetic Compass Adjustment, here:
https://www1.nga.mil/ProductsService...s/default.aspx
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Old 24-04-2011, 04:45   #5
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Re: Compass Dip when Crossing Equator

Quote:
Originally Posted by richard_wb View Post
Magnetic compasses are balanced for either Northern Hemisphere or Southern Hemisphere, and if you take a compass balanced for one hemisphere to the other hemisphere, the card will not be level. How big a problem is this for world cruisers? Do you rebalance your compass when crossing the equator? Do you have two compasses? Is the problem so minor that you ignore it? Looking for reponses from experienced world cruisers. Thanks!
We have cruised from the arctic to the antarctic using the same ships compass and the same in-binocular compass. They both worked fine everywhere.
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