I would like to better understand swinging the compass so that I can compile a compass deviation table for my boat. Can you help?
Sue & Larry respond:
It would be a rare boat that did not have numerous metallic, magnetic and/or electrical objects nearby in the cockpit that affect the reading of the compass needle. The error caused by these external influences to your compass is called deviation. This deviation, along with variation (the difference between magnetic and geographic poles) must be compensated for in order to get accurate compass readings.
Swinging a compass is the physical process you go through to find out what your boatís compass deviation is. Deviation changes with your boatís heading, so it should be checked for in a full circle of direction. The simplest, but least scientific way to determine your compasses deviation is to run a known course between two charted objects from which you can determine the magnetic heading lining them up and reading the chart. These objects could be two points on land that you run as a range, or two items in the water which you can run between. Repeat this procedure heading different directions around 360 degrees. As you run each course, check your compass reading against what the chart states the magnetic reading should be.
To derive more accurate readings at smaller intervals than you can easily get from the above method, you can use a device called a pelorus. To do that, make up a chart using 15 degree intervals of your boat's direction, covering the full 360 degree circle. When using the pelorus, you can actually take your readings in all directions without the need to swing the boat around. For more details on this procedure and a sample chart, check out a copy of Chapmanís Piloting.
If deviation is found to be great, say more than six degrees, youíll want to physically compensate your compass. Most boat compasses have internal compensator magnets that can be adjusted with the turn of a screw to accomplish this.
Good luck with getting to know your compass better.