How closely should the mechanical compass, hand bearing compass, autohelm electronic compass, and handheld GPS "compass" agree on a boat? Mine do not agree exactly and I'd like to find out what is reasonable. Should one of them be considered the "reference" compass against which the others can be calibrated? What's the best way to get them in agreement (whatever that means exactly)?
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Compass calibration
- Thread starter Gary G
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- calibration compass
compass disagreements
Boy, have you opened a can of worms here!
Let's start with your sailing location. You will need to determine the amount & direction of the "Variation" in your local sailing area. This is the difference in your area between true north & magnetic north. In this respect the ship's magnetic compass & a hand bearing magnetic compass should agree within a couple degrees assuming there are not large magnetic interferiences nearby (stereo speakers, electric motors, etc.). Your ships compass should have compensating adjustments built into it's base, so if the P.O. have "swung" the compass you may get a difference there.
Now for GPS & electronic compasses. These usually give you true direction as opposed to magnetic north. Therefore you almost always have a difference between a magnetic compass & electronis devices unless the electronic compasses have been compensated via an input to take into account the local variation.
To get a much better explanation I would suggest getting a copy of Chapman's Piloting. They will explain in detail variation & deviation and how to adjust for it. Much more detailed than we have room for here!.
A piece of advice I was given early on in my boating career is that you only disbelieve your magnetic ship's compass when you UNREFUTEABLE evidence that it's wrong. Haveing been offshore in a heavy fog I can tell you it's very easy to get confused.
I hope this will help you out a little. It's a very complex subject & there isn't enough room here to touch on everything but if you have a specific question I would be happy to try to answer it.
Have fun & sail fast
Bud E34 "Escapade"
Boy, have you opened a can of worms here!
Let's start with your sailing location. You will need to determine the amount & direction of the "Variation" in your local sailing area. This is the difference in your area between true north & magnetic north. In this respect the ship's magnetic compass & a hand bearing magnetic compass should agree within a couple degrees assuming there are not large magnetic interferiences nearby (stereo speakers, electric motors, etc.). Your ships compass should have compensating adjustments built into it's base, so if the P.O. have "swung" the compass you may get a difference there.
Now for GPS & electronic compasses. These usually give you true direction as opposed to magnetic north. Therefore you almost always have a difference between a magnetic compass & electronis devices unless the electronic compasses have been compensated via an input to take into account the local variation.
To get a much better explanation I would suggest getting a copy of Chapman's Piloting. They will explain in detail variation & deviation and how to adjust for it. Much more detailed than we have room for here!.
A piece of advice I was given early on in my boating career is that you only disbelieve your magnetic ship's compass when you UNREFUTEABLE evidence that it's wrong. Haveing been offshore in a heavy fog I can tell you it's very easy to get confused.
I hope this will help you out a little. It's a very complex subject & there isn't enough room here to touch on everything but if you have a specific question I would be happy to try to answer it.
Have fun & sail fast
Bud E34 "Escapade"
Jim Payton
Inactive Member
yep I agree
electronic compasses are not as reliable as magnetic. And you do need to know the variance for your specific location (found on the latest chart's compass rose) and you need to know the deviation for your boat (found by "swinging" your boat to know fixes also found on local chart). My experience has been that a good and I mean good hand held compass (usually in the range of $100 or more) is the most acurate for finding magnetic north becuase it can be held up and away from most the interferience that is found on any boat. The problem with using hand held is that it is hand held and is not fixed in line with the keel. The ships compass should be the compass you rely on for your heading even though it has interference causing deviation.
Like Bud of "Escapade" said, this is a difficut question to answer simply. I would suggest a navagation course by the local Coast Guard Auxillary. The local group should schedule a class a couple of times per year and they are free except for class materials like books and charts (about $20 or $30).
electronic compasses are not as reliable as magnetic. And you do need to know the variance for your specific location (found on the latest chart's compass rose) and you need to know the deviation for your boat (found by "swinging" your boat to know fixes also found on local chart). My experience has been that a good and I mean good hand held compass (usually in the range of $100 or more) is the most acurate for finding magnetic north becuase it can be held up and away from most the interferience that is found on any boat. The problem with using hand held is that it is hand held and is not fixed in line with the keel. The ships compass should be the compass you rely on for your heading even though it has interference causing deviation.
Like Bud of "Escapade" said, this is a difficut question to answer simply. I would suggest a navagation course by the local Coast Guard Auxillary. The local group should schedule a class a couple of times per year and they are free except for class materials like books and charts (about $20 or $30).
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