Are we just antiquated or is the original navigation instrument still relevant?

It has not gone unnoticed that a lot of new, small boats are being produced without the venerable steering compass fitted. I was having a conversation with a few boat captains, a marine engineer and a marine surveyor, all parties agreed this is wrong.

It points to magnetic north

A compass has only one job, it points to magnetic north (there are others that are rigged differently, but for our purposes, it´s magnetic north). The boat effectively spins around the compass. Autopilots use a compass to guide your boat, a specially gimballed gyrocompass, often fitted somewhere near the center of gravity of the boat, under the floor.

The origins of the magnetic compass as we know it go back to China, around 200BC. It is very much tried and tested technology. I think I have said elsewhere, I am by no means a luddite, I did in fact have a brief sortie to the dark side and was a computer programmer. I am fully familiar with electronic navigation. It´s just there is something comforting about having an instrument on board that has more than two thousand years of practice, a sort of lifeline or phone a friend.

Even the most expensive and technologically advanced electronics are not without their faults, they rely on electricity, knowledge on how to use them and being able to see them clearly. A compass needs very little in comparison.

North is North?

Depending upon where you boat on planet earth, you get different ways of mounting the compass, these are called gimbals. You could not use a compass gimballed for use in Australia in Europe for example. Navigation charts are drawn to TRUE NORTH.

As long as you have not got any magnetic materials too nearby (nickel, iron & cobalt) the compass will work. If you look on a nautical chart you will see a compass rose. This indicates the difference between true north and magnetic north, this changes year on year and is called variation. The magnetic forces local to your boat are called deviation and are unique to each boat. Although I have not done it in a long time, part of the commissioning process on a new boat was to “swing” the compass and draw up a deviation table. I believe this is still common practice on commercial ships.

Technology and tradition

You can make your own deviation card. Generally speaking a compass has two adjustments, north and South and east and west, the idea being that in our perfect world, 000 degrees on a GPS would show as exactly north on our compass, it won´t, there will be a difference.

You deviant

A simple way to make your own deviation card, using GPS to help.

First pick a really calm day, minimal wind or waves.

Set your GPS or plotter or autopilot to magnetic, not true.

Set your COG (course over ground) to 000 or 180 (due north or due south), if the reading on the compass is off, start adjusting the north to south adjustment screw, use the special copper or brass screwdriver supplied with the compass. I try to go fast enough but not full speed, say 8 to 10 knots.

A typical compass adjustment screwdriver, made of non ferrous metal, pencil for scale

Repeat the process, but now turn the boat to 090 or 270 degrees (due east or west).

These are the basic steps, I then like to run the boat on at least all 4 cardinal points of the compass, noting on a piece of paper the difference between the COG and the steering compass.

Using your favorite spreadsheet program you can make a deviation graph. Every time you install new navigation equipment you should repeat the compass compensation process. Navigation aids, antennas, speakers and other items usually contain a “minimum compass safe distance” sentence in their specs.

A sample deviation card for a 27 foot sports boat, 2010

“Error East Compass Least, Error West, Compass Best”

Remember that phrase, when you look at the example deviation table above, you´ll see deviation west and east, this means if you need to match your steering compass to your GPS in magnetic north mode, you would subtract for east deviation and add for west.

Its all academic

So I now make my confession to you all, even with many years of being at the helm. I simply cannot steer the boat to the same accuracy as an autopilot can. I like to have the compass as accurate as possible, but the truth is if its within a few degrees its good enough for most coastal navigation.

A good friend crossed from Melilla, a Spanish enclave in North Africa to Malaga, Spain, about 120 to 130 nautical miles in mostly open sea, he was using just the steering compass due to the GPS failing, no visual aids for much of the journey. Upon picking up a well known landmark on the Spanish mainland when he got within sight of land, he realized he was about 6 nautical miles west of his intended destination. I think that is pretty good considering he was only following a steering compass of unknown accuracy.

I still use a compass, a lot in fact, whether out of habit or because I cannot always see electronic screens as easily, a steering compass is usually mounted in a prominent position. I like to think of the humble compass as an old and very comfortable overcoat, it might not be flashy or fashionable but it´s always just right and always there.

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