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Thrillsbe

Red Hot Compass Deal

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Bracket mount, I have built a small ply box that sits on the thwart against the mizzen tabernacle.  Painting today, pics on Southern Express 's build thread when presentable.  I plan to compensate for the variation by rotating the compass in it's mount and deviation will be ok, it is just a little plywood boat, very little ferrous material around anyway! 

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On 6/19/2017 at 7:25 PM, Drew said:

No, not snarky Don, many people get the terms confused. I teach aeronautical navigation (having served as a navigator in the navy and being a pilot) and students constantly wrestle with the concept. Variation, otherwise known as declination, is of course the difference between True and Magnetic, even in the land "down under", Deviation should remain constant on a vessel due to magnetic interference, although it might be slightly different for each cardinal point of the compass. Hence the need for a correction card and swinging the compass to check the deviation occasionally. I generally find that compasses built in the northern hemisphere need correction to operate in the southern, although I still have not figured out why. I have one that is unusable because it has no adjustments but reads about 30 degrees off.

Indeed, nautical compasses are built for different hemispheres. Some are "universal" but may still need readjustment when changing hemispheres.  Partly because the needle dip is different north and south, which can cause friction on the pivot.  Ritchie calls it "zone balancing".  Ritchie Compass Balancing 

 

BTW, my big boat has a 40 year old Ritchie and they rebuilt it for me a few years ago for a great flat rate and it was as good as new.  Did have to reswing it, but not too hard.  I race Wednesday night races on a friend's boat and we just use the compass bearings (2 identical bulkhead compasses port and starboard).  His are dialed in to a couple of degrees.  No GPS.

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