COMPASS DEVIATION

By

 

Jeremy R. Hood

We were sailing across the Gulf aboard a thirty foot catamaran on a delivery from Clearwater Florida to Clear Lake, Texas. The voyage had not been without incident though we were coping with the minor problems (such as the lack of beer!) pretty well. Then the winds died and we found ourselves motoring westward in the left over seas which produced a pretty awful motion aboard  the boat. Well the point of relating this incident is that we were using the newly installed Autohelm at the time we started the engine. Within seconds we found our course had changed by a little over ten degrees. At the time we merely reset the autopilot so that we were back on course but we soon began to realize that each time we started the engine our course would change. Eventually I realized why. The fluxgate compass which the Autohelm used had been installed low down on the center line of the vessel as recommended but the actual location placed it close to a bunch of electrical cables. One of these was the main charging wire from the alternator to the batteries. When the alternator was charging the current in the wire was producing a small but significant magnetic field which was affecting the fluxgate compass.

All magnetic influences aboard a boat will affect any compasses, steering, hand-bearing or fluxgate, and cause them to have an error in their reading. The combined effects are generally referred to as compass deviation. That is the amount a compass will deviate from its correct heading (the magnetic north pole) because of magnets or magnetizable metals within range of the compass. Often compass deviation will be caused by placing equipment too near a compass. One new boat I saw last year had a pod of new instruments surrounding the compass binnacle; depth, log, loran, VHF radio. Having the VHF so close to the steering position was great but clearly whoever had installed it had given little thought to the loudspeaker with its large magnet inches away from the steering compass. That compass did little more than point to the radio! Similar problems can occur with bulkhead compasses when equipment is installed on the inside of the cabin without regard to the compass on the outside. Offending items could be stereo speakers, galley items such as cast iron skillets, tools (such as those pliers you have handy for a recalcitrant shackle) or electrical cables carrying high currents. Pay particular attention to the location of electronic fluxgate compasses which can suffer from the out-of sight, out-of-mind syndrome. Many boats are fitted with a small chart table compass and on one trip I did this I used this extensively until after pulling out the fold-away pilot berth the steel support tube rendered it useless. In this case the error was so large as to be instantly recognizable; a smaller effect could have led to serious navigational errors

Your ships compass is perhaps the most important navigational equipment aboard and so it is important that it gives a correct reading and that you can have confidence in it. And so the first thing to do is to check that you have no magnetizable metals near the compass, no steel; screwdriver left behind under the compass (I saw it once), no equipment hanging from or mounted on the binnacle that could affect the compass. Once this is done you can check the deviation of your compass yourself or you can pay a compass adjuster to do this for you. If you plan to do it yourself you will need to check the deviation on a number of different compass headings as it will be different for each heading. One simple way to do this is to find a pair of channel markers aligned in the required direction and then to calculate from the chart what the course is between them. If allowance is then made for the Magnetic variation applicable in the area you will have a magnetic heading. When actually travelling between these buoys your compass should be reading this value if the compass deviation is zero. Make a note of the actual compass heading and then move to another pair of marks and check the deviation along that bearing.

Another method is to tow your inflatable dinghy (minus outboard and all things metallic) behind your vessel on a calm day. If you have someone in the dinghy with a hand bearing compass then you can steer your boat on a number of different headings and get your accomplice in the dinghy to record these headings from the hand-bearing compass. With zero deviation these should be identical to your headings on the steering compass.

Unless you own a steel boat your compass deviation should be less than a couple of degrees on all headings and often you will find that it is zero on all but a couple of headings. When you have this data you can record it in the form of a deviation table or graphically (Figure 1) as is often done by a compass adjuster. With your deviation card you will then be able to allow for this amount in calculating a course to steer from a true course derived from the chart. Most navigators remember the method of applying Magnetic variation and compass deviation using the mnemonic True Virgins Make Dull Company. Add Whiskey!

True The true heading from the chart (where 0 degrees is the true north pole)

Virgins The magnetic variation that applies in the area where you are

Make The magnetic heading (after you have allowed for variation)

Dull The compass deviation that applies to the magnetic heading

Company The actual compass heading you will have to steer so that after allowing for variation and deviation you will be heading on your true course

Add Whiskey Converting from True to Magnetic or Magnetic to Compass ADD Westerly corrections (and conversely subtract Easterly)

To take an example familiar to all those who sail in Galveston bay, the true course from the #59 buoy on the Houston Ship Channel to the #2 marker on the Clear Lake entrance channel  is 290. What would be the compass course to steer if the Magnetic variation is 5 E and the compass deviation is as shown in Figure 1.

290 deg(T) - 5 deg E variation = 285 deg (Magnetic) [Converting True to Magnetic subtract Easterly variation]

Using a value for deviation  when on a magnetic heading of 285 as 2 deg West (as an example: every boat will be different).

285 deg (M) + 2 deg W deviation = 287deg (Compass) [Converting from Magnetic to compass, add Westerly deviation]

If the deviation of your compass was the same as that in figure 1 then you would need to steer a compass course of 287deg.

Next time you are out sailing check out your compass deviation and then allow for it in steering a course. If you ever get caught out in fog or poor visibility you will be pleased that you took the time to familiarize yourself with this aspect of navigation!

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