Calibrating a compass

[cbadmin]

From: C-LionRay (Original Message) Sent: 12/24/2002 4:15 PM
My compass on the Sea Lion doesn't seem to want to move. Maybe it needs calibrated (I'd like to at least verify that it's accurate). Given that the thing don't move I'm thinking there is either a trick or it's broke.
Either way I'm gonna have to calibrate one so how do ya do it?
Thanks in advance...



From: stevej Sent: 12/26/2002 7:32 AM
Ray check out http://www.ritchienavigation.com/service/compensation.html
Look at the FAQ also I think it will answer most of your questions. Remember that we need to subtract somthing like 12 or 22 (listed on the NOAA charts) degrees of variance from the reading for this area.
Have to admit that I tend to reference my GPS's more than my compass but I do check all three (2 GPS 1 compass) against each other most every time out.

steve

[rascal]

the below thread can be used to calibrate a compass this thread was used before hope it helps ---chapmans has a section on it also......ras
So, do you know exactly how much error you have in your magnetic compass? Can you follow your compass on a designated course home in a dense fog with confidence? You can if you've done a good job of correcting your magnetic compass for on board deviation and worked up a deviation table. By the way, this discussion is for magnetic and not for flux gate electronic compasses that have a different method of correction.

I did this on my new Cruiser, Kestrel, as we were commissioning her a few weeks ago and just within the last week on Capt Darrell's Hunky Dory. Here's what we did.

We used a sun compass. This is a compass rose set on a flat fixed base with a tall thin needle set vertically in the center of the rose. In this way when put it out in the sun at 10:00am or 2:00pm you will get a nice shadow on the compass rose from the needle. The purpose of the sun compass is to help us run exactly reciprocal courses for error correction and then to give us data to calculate a deviation table.

First, we get out as much error as possible. Start by running a due north compass course by your on board compass. When the helmsman is exactly on course he tells the person reading the sun compass. The sun compass is turned to throw the shadow on 000 (due north). Then the helmsman turns the boat until the sun compass shadow reads exactly 180 degrees. The person reading the sun compass tells the helmsman when the sun compass shows exactly 180 course. The helmsman reads the on board compass to check the amount of error. Let's say it's reading 195. Oops, that's 15 degrees of error. So with a nonmagnetic screwdriver the helmsman adjusts the N/S compensating magnet to remove 1/2 the error or about 7 - 8 degrees bringing the compass to about 187.

Now the helmsman returns to a due north, 000 degree course by on board magnetic compass. And the whole maneuver is repeated, North to South by sun compass and removing half the error until little or no error remains. Once you've got the N/S reciprocal course error minimized the same thing is done on E/W reciprocal courses using the E/W compensating magnets in the compass. When you're done on E/W do a quick recheck on N/S just to be sure you didn't do anything that screwed up that adjustment.

Once you've gotten the error out as much as possible, set the sun compass so that 000 degrees on the sun compass is pointed directly in line with the bow of the boat. Now the helmsman runs magnetic courses beginning at 000 and going around the compass rose, stopping at each 15 degree increment, i.e. 000, 015, 030, 045, 060, 075, 090, etc. etc. all the around to 345. At each 15 degree increment the shadow on the sun compass is read and noted. Now this part needs to be done within about a 2 minute duration so that sun shadow does not move appreciably due to movement of the sun. So you'll wind up with five columns:
Compass/ Relative / Compass / Magnetic /Deviation
Reading/Bearing-Sun/Bearing-Sun/Bearing-Sun/
000 084 084 086 2E
015 070 085 086 1E
030 056 086 086 0
045 042 087 086 1W
. . . . .
. . . . .
. . . . .
345 100 085 086 1E

Compass Reading= Taken from ships compass
Relative Bearing-Sun= Sun's Shadow
Compass Bearing-Sun= CompHeading+RelBearing-Sun
Magnetic Bearing-Sun=Avg of all CompBearing-Sun
(total divided by 24)
Deviation=CompBearing-Sun - MagBearing-Sun
(- = east or + = west)

Now once you've got all this done you've got a nice deviation table to correct your magnetic heading to a compass course using:
T rue
V irgins
M ake
D ull
C ompany
the TVMDC correction formula. Subtract easterly and add westerly going down and the opposite going up.
So you've got a magnetic heading of 000 (due north) you want to go. Subtract 2degrees easterly deviation we get 360 - 2 = Steer Compass course of 358.

Looks like a lot of work for 2 degrees. But, of course in practice you may have 3, 4, 5 degrees or more of remaining deviation shown in your table. Can make quite a difference over a 10 mile distance when you're looking for a 300 foot wide inlet.

And, as involved as the directions make it sound, it is really quite easy. I did it on Kestrel and again on Hunky Dory in about an hour each.

If the directions are not clear, try the description in Chapman's. It may well help you out. Especially if MSN screws up the nice columns I've tried to make to show how this is done. Good luck. And, enjoy the new found confidence you'll have in your magnetic compass when you finish with this excercise.

Capt Terry.