GL: Swinging a compass

[Rich Gano]

As others have said, do not attempt to use a GPS to "swing ship."  GPS gives
you course over the ground (only when it is moving).  What you want is the
boat's heading, which can be different from the COG.  GPS heading is a good
double check to make sure the compass is in the ball park, but that's about
it.

The United States Power Squadron publishes a booklet with several methods on
how to "adjust" a compass.  I have a copy and would be willing to make a
copy of it and send it along.  Chapman Piloting is another excellent source.

Before I went to the trouble of "swinging ship," I'd  a well-installed,
quality compass.  Then ensure the bearing is in good shape by using a magnet
to pull it off course about 30 degrees to either side and observing the
result.  If the compass card does not return to its original position when
the magnet is pulled away (6-10 feet to be safe), you need some compass
repair.

The compass magnets (the ones that come installed in the compass base) can
be zeroed by removing the compass from the boat and placing it on a surface
marked with four 90-degree marks.  Chapman Piloting provides the details.
Don't do this if the compass seems fairly close.

I personally prefer to use my US Army Corps of Engineers sun compass because
you can run a deviation card in about an hour or two with one well set up.
This is what the real pros use, and because real sun compasses are far and
few between and fewer people know how to use one, this is why it is costly
to have done.  The "shadow pin" method uses some of the principles of the
sun compass, but it's not the same because the sun compass gives you actual
true headings.

Buying a relatively cheap plastic pelorus and temporarily mounting it for
use while crossing back and forth over a known geographic range (not
necessarily a navigation channel range but just something you can find that
is convenient on your chart) is probably the simplest and most accurate
method for the average bear.

Rich Gano
CALYPSO (GB-42 #295)
Southport, FL (near Panama City)