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Did you know that the compass on your boat may not work properly in other parts of the world? I didn’t, until just recently. Inspired by something that I read in the book My Old Man and the Sea, and a comment by our friend Wade on this post, I did a bit of research on the subject of how compasses react to changes in latitude. What I came across was a Wikipedia article that talked about Magnetic Dip.

Magnetic dip, dip angle, or magnetic inclination is the angle made with the horizontal by the Earth’s magnetic field lines. This angle varies at different points on the Earth’s surface. Positive values of inclination indicate that the magnetic field of the Earth is pointing downward, into the Earth, at the point of measurement, and negative values indicate that it is pointing upward. The dip angle is in principle the angle made by the needle of a vertically held compass, though in practice ordinary compass needles may be weighted against dip or may be unable to move freely in the correct plane.

I’m going to admit it, I didn’t know any of this!

Both the guys in the book that I mentioned, and the folks that our friend Wade came across, were ultimately able to fix their compass problems. This explains the problem a bit more:

Because the Earth’s magnetic field’s inclination and intensity vary at different latitudes, compasses are often balanced during manufacture so that the dial or needle will be level, eliminating needle drag which can give inaccurate readings. Most manufacturers balance their compass needles for one of five zones, ranging from zone 1, covering most of the Northern Hemisphere, to zone 5 covering Australia and the southern oceans. This individual zone balancing prevents excessive dipping of one end of the needle which can cause the compass card to stick and give false readings.

I believe the problem can be fixed by opening the compass and adding a small, calibrated weight to the needle. How specifically, I have no idea. At this point, I don’t know anything beyond what I’ve written. I wonder if our friends who have travelled to the southern latitudes have experienced anything like this. I’ll need to talk to them.

9 Comments

  1. It is funny that you posted this today- I was just reading about compasses in binoculars that may or may not work in the Southern Hemisphere. Do you know anything about this?

  2. Mike,
    Here in Alaska we deal with magnetic declination of 18 to 20 degrees (east) from the magnetic bearing. For the most part you don’t have to adjust anything, you just have to account for the declination in your bearing calculation. There are maps that show the isogonic lines indicating the declination in the area you are cruising.
    Cheers

    • Unless I’m mistaken, declination and dip are not the same thing. Declination can be mathematically accounted for. Dip is the physical lowering of the compass needle and at extreme angles, it causes the needle to stick. I am open to being shown that I am wrong in this.

  3. I believe the issue is that the compass card won’t sit level on the pin it rotates on.
    Adding a weight levels it. If one doesn’t they I guess the card is a little more sluggish and perhaps wears away the pin more rapidly. At least that’s what I recall.

    However it’s kind of a moot point these days what with autopilots having flux-gate compasses and ubiquitous GPSs. I really don’t glance at my compass much these days.

    It’s possible to order a compass dipped for the southern hemisphere.
    I did it once for an extended sailing trip to the South Pacific (NZ and Oz).

    Just contact Richie or one of the other manufactures.

    • You’re right about GPS but I still think having a working compass is pretty important.

    • Flux gate compasses can stick too. I met a German cruising couple who rounded Cape Horn 7 years ago, and now spend their time between New Zealand and Fiji. En route from Cape Horn to Easter Island both their binnacle compass and flux gate failed. The binnacle compass stuck in one position, due to dip, and the flux gate compass was also sticking and erratically steering the boat. They jury rigged a solution by put a small magnet near the flux gate, steering 60 degrees off to avoid the sticking point – and following the result on their chart plotter.

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