Wednesday, December 21, 2011

Captain John: Avoid Getting Lost

Captain John offers five easy steps to check your magnetic compass and keep your boat on course...

Guest blog by Captain John
Captain John Jamieson, a regular contributor to the Daily Boater, is author of Seamanship Secrets and publisher of the popular boating education website

Did you know that a small screwdriver can create havoc with your magnetic compass?  Are you sure you’ve mounted your compass in a place free of errors from a magnetic field?

Photo © Nautical Sites Media
Make every effort to keep magnetic material, such as knives, watches and tools clear of your magnetic compass. Every compass card has a small magnet attached to the back. And these things love to swing over toward anything metal or electric.

This plants an error called “magnetic deviation” in your compass. Forget the dopey movies where the compass swings like a roulette wheel. Uh-uh. You won’t even notice the subtle shift of the compass card toward the object of attraction. And, before you know it, you’re lost.

Follow these steps to make sure you are steering the right course to get to your destination. Note: These steps are temporary and if you will be cruising, you will want to hire a marine compass adjuster, or learn to do this yourself.  In any event, know the steps below for safety’s sake!

1.    Buy a Hand Bearing Compass
This miniature tool looks like a small hockey puck with a neck lanyard. You sight over the compass to take a compass bearing to check your position or your steering compass deviation.

The best hand bearing compasses are rubber coated for shock resistance, waterproof, and can be used at night. A hand bearing compass (HB compass) has no deviation, as long as you follow the next three steps.

2.    Turn on Marine Electronic Equipment
Get underway on a calm day. Turn on all normal operating equipment, such as VHF radio, radar, depth sounder. This simulates the magnetic field that surrounds your compass when sailing. Doing this insures your test will yield accurate results.

3.   Remove Personal Magnetic Influences
Check yourself and remove jewelry from ears, neck and wrist.  Make sure to keep knives, cell phones and tools at least 36” away from the compass. Twist the wiring on any electronics mounted near the compass to lower their magnetic field. Now your ready to begin the test.

4.    Find Your Boat's Magic Mark
Stand in the cockpit, on the centerline, away from metal awnings or dodger frames. Use the hand bearing compass to sight onto an object 1-2 miles away. Note the compass bearing.

Have the person on the helm turn in a slow, tight circle. Check the bearing to the object again. If the bearing reads the same, you’ve found your “magic mark”. For the best accuracy, stand on your mark anytime you take bearings with your HB compass.

5.    Check the Steering Compass
Have the steerer steady onto a course. Stand on your magic mark and sight down the centerline onto the forestay or bow pulpit. Call out “mark” with the helmsman and compare magnetic compasses. If they are different, align the boat to the hand bearing compass. Then, steer the course you see on the steering compass.

Example: Course is 190 Magnetic.

Helm steers: 190 Magnetic; HB Compass reads: 193 Magnetic
Swing the boat until the HB compass reads 190 Magnetic.
Glance at steering compass: it reads 188. Steer 188.

Check your steering compass often for deviation when coastal cruising. It takes less than one minute, and could mean the difference between making and missing a far away destination.


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  1. As a Compass Adjuster I have to be a little critical. Deviation is not linear on any vessel. It will be greatest on E-W headings and can be sourced to things like engines on the longitudial axis. On N-S the source is usually electronics or items on the athwartship axis. To be useful you must do the procedure on the cardinal and intercardinal headings and produce a small table. On Government vessels we do 24 points around the compass. The built-in correctors on most compasses can be used to eliminate up to 20 degrees of error but are best left to a professional.

  2. Thanks for your input Bill.

    I think we can agree that Captain John calls attention to the fact that many boater's compasses may not be accurate, and as John points out in his article:

    "These steps are temporary and if you will be cruising, you will want to hire a marine compass adjuster, or learn to do this yourself."