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Does your boat suffer from deviation?


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 hot food) pretty well. In fact after the initial problems during the first couple of days, the passage was going well. On the third day we were sailing in light steady winds with the newly installed Autohelm autopilot steering for us. Soon after midday the winds began to drop and eventually we decided to furl the sails and start 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 as indicated by the shipís compass 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 in the installation instructions 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.

Compass deviation is not the same as magnetic variation. Variation is the error in your compass caused by the fact that the earthís magnetic field is not aligned exactly with the true north and south poles. Magnetic variation changes depending on where you are located: its value is shown on nautical charts. Compass deviation (if it exists) must be allowed for in addition to Magnetic Variation.

Often compass deviation will be caused by placing equipment too near a compass. One new boat I saw several years ago 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.

With an awareness of compass deviation it is usually possible for the shipís compass on most fiberglass boats to be free of deviation. All metal surrounding the compass should be non-magnetizable (brass, bronze, aluminum or non-magnetic stainless steel). So long as magnetizable metals are kept well away from the compass (three feet is ideal) then deviation should be minimal. When installing electronics at the binnacle consult the installation instructions: most give a minimum distance that they should be located from the ships compass.

However it is not so easy to ensure that electronic fluxgate compasses are deviation free. They are often located in lockers or below the cabin sole and being out of sight , it is easy to forget about them when installing other equipment or when merely loading stores. While most fluxgate compasses can be calibrated to allow for compass deviation (this usually involves maneuvering the vessel slowly in a circle while the unit is in calibration mode) be aware that this will only work for compass deviation that remains fixed. Tool boxes that get moved, wires that carry current or canned goods that get used up during a trip can change the deviation. For these reasons it is good seamanship not to rely exclusively upon an autopilot compass. If it reads the same as the ships compass, fine; if not then you may want to check out why not.

Chart-table compasses are also vulnerable to the effects of compass deviation and they should not be relied upon to give a true heading unless they have been checked and found deviation free.

Your ships compass is one of the most important pieces of navigational equipment aboard and so it is important that it gives a correct reading and that you can have confidence in it. To ensure itís accuracy you need to make sure all magnetic influences are sufficiently far from the compass and then check for compass deviation. You can do this yourself or you can pay a compass adjuster to do it for you. Doing it yourself can be just as accurate as employing a compass adjuster but it will take you considerably longer!

If you plan to check for compass deviation yourself, then the first thing you need to be aware of is that the deviation will be different on different headings of the vessel. Because of this you will need to check the deviation on a number of different compass headings 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 markers 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. When a compass adjuster checks for deviation he will often do so for all sixteen points of the compass and so you should try and do likewise.

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.

If you own a steel boat compass deviation is a serious problem and the ships compass will need to have special features to allow it to compensate for the steel of the vessel. If adjusted correctly your compass deviation should be less than a couple of degrees on all headings but you should seriously consider using the services of a compass adjuster in this case.

Whenever a compass is adjusted by a compass adjuster (itís called swinging the compass) the adjuster will do two things. Firstly they will check for the compass deviation and then, secondly, they will attempt to remove the deviation by the adjustment of small compensating magnets located in or close to the compass. Once this has been done, a deviation card is then written up showing the remaining error which you will have to take into consideration when calculating a course to steer. A deviation card can either be recorded in the form of a table or graphically as is often done by a compass adjuster.

With a 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)

If you donít like the mnemonic given here for remembering the order T V M D C, there are several others to choose from including TV Makes Dull Children or, in reverse, Can Dead Men Vote Twice. Alternatively you could make up your own!

Though on the majority of vessels the deviation should be small enough to be ignored, here is an example of how to allow for both variation and deviation. 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 Magnetic variation (in 1999) is 4 East. You would calculate the compass course to steer as follows:

290 deg (T) Ė 4 deg E variation = 286 deg (Magnetic) [Converting True to Magnetic subtract Easterly variation]

If, for the purposes of this example, we have a vessel where the compass deviation is 3 East then:

286 deg (M) Ė 3 deg E deviation = 283 deg (Compass) [Converting from Magnetic to compass, subtract Easterly deviation]

If the deviation of your compass was the same as in this example, then you would need to steer a compass course of 283 deg in order to make good the calculated True course of 290 deg.

Next time you are down on your boat check to see if you have anything magnetic too close to your compass. Check to see if your electronic compass and shipís compass agree. Then, when you are away from the dock, check this again on different headings and check your shipís compass against a magnetic heading taken from the chart. When you have done all this you will know whether you can rely on your compass to get you home!

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