Q&A: Magnetic Compass

Question: Do you use a magnetic compass to augment your navigation process?

Answer: Yes, we do. We have a large Ritchie compass mounted on top of our binnacle. Despite having two flux gate compasses (it’s a long story why two) we like to check the course with the magnetic compass, which we treat as the master compass. It’s also much easier to take a bearing sighting over it, particularly in rough weather, than using a hand bearing compass, since it has much better damping.

We have the Ritchie professionally adjusted for deviation any time we make any changes to the location of metals around it, or about every 3 or 4 years. I also check it occasionally via sun azimuth.

We would not consider relying totally on flux gates since their deviation drifts quite a bit as our latitude changes, particularly in the high latitudes.

An accurate compass is still important to us since we do not just blindly follow a GPS waypoint without checking that the course makes sense. By doing this, any error in entering a waypoint becomes apparent before it can do any harm.

Finally, the magnetic compass, together with an old Walker log and sextant, is an important part of our equipment to allow us to navigate in the event of a total power failure. Unlikely I know, but Murphy is always lurking at sea.

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Meet the Author

John

John was born and brought up in Bermuda and started sailing as a child, racing locally and offshore before turning to cruising. He has sailed over 100,000 miles, most of it on his McCurdy & Rhodes 56, Morgan's Cloud, including eight ocean races to Bermuda, culminating in winning his class twice in the Newport Bermuda Race. He has skippered a series of voyages in the North Atlantic, the majority of which have been to the high latitudes. John has been helping others go voyaging by sharing his experience for twenty years, first in yachting magazines and, for the last 12 years, as co-editor/publisher of AAC.

4 comments… add one
  • Clint May 4, 2014, 2:21 pm

    My question is regarding our Constellation magnet compass. Another sailor was making an argument that a compass calibrated for the northern hemisphere is weighted such that it will not operate properly in the extreme southern latitudes? He said that I needed to either have my compass re-calibrated for southern latitudes or purchase a new compass already designed for southern latitudes. Is there any validity to this? I would much appreciate your advice and any suggestions.
    Sincerely,
    Clint & Reina Burnett
    s/v Karma

    • John May 5, 2014, 4:55 am

      Hi Clint,

      The person you were talking to is quite right, compasses have different cards for the south and northern hemispheres. The issue is dip, or the vertical angle to the earth’s magnetic pole, which changes depending on how far north or south you are. As I understand it, compass cards have a small weight to keep them level but that weight needs to be changed depending on the the dip angle.

      There is a handy chart at Ritchie Compasses that shows the areas that each compass card will work well in.

      I don’t believe you need to change compasses for every zone, but I’m pretty sure you will have trouble with your northern hemisphere compass getting sluggish, due to binding on the bearing, if you go a long way south.

      When we bought our boat, she had a southern hemisphere compass installed because the previous owners had been around South America as far south as Cape Horn, and we did indeed have trouble with it in the north until we had the card changed.

      • Clint May 5, 2014, 4:58 pm

        John
        Thanks for the info, is there similar problems with fluxgate compass for autopilot?

        • John May 6, 2014, 3:49 am

          Hi Clint,

          No I don’t believe that a fluxgate has the same issue.

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