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How to Use a Marine Compass

16 Nov 2021

Within a modern world loaded with technological innovation, it would be easy to imagine that the trusty marine compass is little more than a relic. However, this magnetic masterpiece is still something that no sailor should be without. After all, being prepared for adventure means being prepared for things to go wrong!

Electronics inevitably fail from time to time. That fancy GPS might conk out on you, leaving you with little more than the stars to work with unless you've got a marine compass on board. With that in mind, you'll likely also want to know how to use a marine compass, as well as how to store a compass so that it remains useful when you need it.

The history of the marine compass

The marine compass is a form of magnetic compass, a gadget that humanity has been relying on for a very long time indeed. The earliest clear record of the use of this kind of compass carries us back to the 11th century, thanks to Chinese military records, although many speculate that the Chinese may have developed the magnetic compass as far back as two millennia ago. Marco Polo is said to have navigated using such a device in the 13th century, while in medieval Europe, based on the notion that compasses held magical powers, the Church denounced their use as wicked.

Admittedly, I'm a little off course. We're not here to talk about history, not for too long at least. Instead, we're here to talk about how to use a marine compass, how to store a compass, and how to ensure its accuracy. Happily, developments in the making of marine compasses over the years have made them less vulnerable to being tilted or shaken, and easier to read. That said, they aren't entirely accurate - and those using them need to take this into account. The earth's true north and magnetic north are not entirely the same. Today, the geographic North Pole and the magnetic north pole are around 800 miles apart. Did you know, they're actually getting closer together? Either way, we'll come back to the accuracy of marine compasses in a moment.

What is a marine compass?

Marine compasses work their so-called magic because the permanently magnetised needle within them always points to north, no matter which way the boat may swing. A mounted compass within a yacht or other sea-faring vessel will be aligned to the centreline of the boat, so that you can always look to it and know which way your craft is facing. The component of the compass that allows you to read it is called the “card”, displaying the full 360° (degrees) of potential direction. If the compass is mounted correctly and all is well, a directional line at 90° would indicate you were travelling east, 180° south, 270° west, and so on.

As you figure out how to use a marine compass, it can be helpful to know that there are a few different types to choose from. The right one for you depends upon where the marine compass might be mounted - fundamentally, from which angle you are likely to look at it.

  • A flat card compass is sometimes used at sea, although more often by those navigating on land. These have a horizontal dial, which you look down on to read. The directional line will always face forwards, so usually away from a yacht's helmsman.
  • A direct read compass is more common on the water, and usually has a spherical display. In this case, degrees are displayed on a conical surface, and the directional line reading can be seen on the near side of the compass, making them ideal for a higher mounting, or viewing when seated.
  • A hybrid dial marine compass will have a direct read display on its side, and a flat card display on its top, allowing you to look at it from any angle, as long as you know what you're looking for.

You'll spot that a marine compass is filled with fluid. This not only allows the magnetic element to turn freely, but also moderates the needle's responses to sudden shocks and movements. Meanwhile, many marine compasses have an internal gimballing system which allows the display to stay close to level and stable, no matter how much the boat rocks and rolls.

Installing a marine compass

If a marine compass is mounted within your vessel, you'll want to ascertain your lubber line. This will be a line that is perfectly parallel to the centreline of the boat, intersecting where the marine compass will sit. The compass will need to find a home where it is readily available for use, and without any problematic objects nearby (another thing we'll return to!)

An accurate lubber line will ensure that your compass is correctly aligned with the vessel. Keep in mind: If your compass were only 5 degrees off, after 11 miles travelled, you'd be a full mile off course! Whether you'll be using a mounted marine compass or a hand-held compass, knowing how to store a compass properly is a must, so read on.

Marine compass variation

Variation is the term used to describe how the difference between true north and magnetic north can throw a spanner in the works when navigating by compass. Irksome as it is, there isn't a universal fix that we can apply. Variation, I have to report, varies in different locations, due to differences in the Earth's structure. However, you'll notice on nautical charts that you can see two compass roses, one inside the other.

The outer compass rose indicates true north, and the inner one indicates the magnetic north in that area, so you can calculate the difference when plotting the route ahead. The difference might only be a few degrees, or none at all, but as we already covered, it doesn't take much to send you sailing off in the wrong direction.

Marine compass deviation

As you figure out how to use a marine compass, you might also notice that they can behave strangely if you chuck your smartphone down next to them. This is because anything around a marine compass that has a magnetic field; think radios, chart plotters, portable speakers, magnetic clothes fasteners, or even large metal objects, can throw off the reading it gives. Worse yet, a strong enough magnetic field can even reverse the magnetic polarity of your compass, potentially leading to chaos.

A good rule of thumb, when mounting, using, or even keeping a compass in the cupboard, is to keep it a decent distance from anything metallic, electronic, or magnetic. For seafarers who need to keep 'how to store a compass' in mind, the “rule of 36” is a popular discipline. This dictates that nothing is placed or carried near a marine compass within a distance of 36 inches - which is around a metre, so easy to remember.

If you are intending to rely on a marine compass over the course of an extended passage, it may be worth hiring a professional compass adjuster to “swing your compass”. This describes adjusting it to take into account minute deviation caused by metal on board, like a yacht’s engine, for example.

The best way to be confident with a marine compass when you cast off for cruising is to practice using it. With proper care and a little understanding, your marine compass will always be there for you, no matter what else fails. Beyond anything else, compass navigation is another worthy aspect of becoming a sailor through and through, so why not make it part of your seafaring repertoire?

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Author: Jimmy Green