MERIDIAN COMPASS CORRECTION WITH THE NAUTICAL ALMANAC

[HenryC]

Deleted 12/10/09 - HC

[Bob23]

Henry:
   Thanks so much. I'm printing your post even as we speak.
Bob23

[nies]

HENRY, SHAMWOW......MY BOY SCOUT COMPASS NEEDS UP GRADED.  LOL . MANY OF US SAIL INLAND LAKES AND DONT FEEL WE NEED A COMPASS OR BASIC KNOWLEDGE OF NAV. ,TRUST ME, WHEN ITS DARK AND THE FOG ROLLS IN OR EVEN  IN DAYLIGHT BEING ABLE TO FIND YOUR WAY HOME SAFELY IS WHY WE NEED THE BASICS . "OH, I KNOW MY LAKE" IS LITTLE COMFORT WHEN YOU CANT SEE TWO FEET IN ANY DIRECTION. YOUR TOPIC GOT ME TO REVIEW MY VERY BASIC NAV. SKILLS TO  BE SURE  THEY WERE AT LEAST TO THE LEVEL NEEDED IN MY AREA OF SAILING...........THANKS NIES

[HenryC]

Thanks for the vote of confidence, gentlemen. 

But keep in mind it isn't always necessary to to spend an afternoon wrestling with the the Nautical Almanac.  Most sailors can check their compass right from the dock.  If your location is on the chart, and you can see several (distant) objects from where you are that are also on the chart, just aim your boat at them and write down your steering and bearing compass headings to those objects.  Compare them with the magnetic bearings derived from your chart, and you will have a record of your compass deviation in different directions.  If there is no metal in your boat affecting the compass, the error should be constant to every object, plus or minus so many degrees.  If the error varies and you can rule out nearby interference from ferrous metals, then your compass needs adjustment.  And don't forget there is a totally separate error introduced by where on earth you are.  In Florida, it is near zero and I usually ignore this, but when I sailed in California variation was close to fifteen degrees, and I almost lost a boat by sailing her into Point Lobos in a fog because I forgot.

As I mention, compass adjustment is best left for professionals.  Most quality steering compasses have little internal magnets which can be adjusted to compensate for errors due to electromagnetic fields in your boat, but compass correction is an iterative process, you fix the error in one direction and it just moves to another. A pro will not only spread this error out evenly as much as possible, he will also provide you with a compass card, which will give you the corrections for all points of the compass. If you spent some cash for a really good compass, it makes sense to have this done, or spend a frustrating weekend with the manual doing it yourself.

A quality hand-held compass without internal adjustments, (like the poplar French Vion "hockey puck" I mention in my article) should have a constant error on all headings, although if your boat contains ferrous metals or electrical equipment, it can be affected too.  As it turns out, those little adjusting magnets in your binnacle compass will also introduce error if they are not properly set.

Also, make certain your compass is tuned for the proper hemisphere.  Yes, there is a vertical component to earth's magnetic field, and this is usually set at the factory.  A northern hemisphere compass will work south of the equator, and vice-versa, but it will not be quite as precise and it will be harder to adjust.

Sailing in a thick fog with only a compass is as scary as it sounds, even if you have taken some of these precautions, (unless  your boat is small enough that you can jump out and push).  There are already enough things to push you off course without having to worry about your compass, too.