Compass Deviation

bastonjock · Post by **bastonjock** » Thu Feb 28, 2008 12:48 am

Has anyone worked out the deviation for the 26x? or is it so small its not worth the hassle

baldbaby2000 · Post by **baldbaby2000** » Thu Feb 28, 2008 2:37 am

I depends on how your compass is mounted. On my M it's on the pedestal and before I adjusted the compensation it was quite a bit off. After compensating it's pretty close. There are normally 2 adjustments for compensation. I used GPS track to align mine. Make sure there's no current and little wind if you do it that way.

Highland_Piper · Post by **Highland_Piper** » Thu Feb 28, 2008 4:07 am

Go to ukdivers net - The compass has what you need to know and do.
Rich

Catigale · Post by **Catigale** » Thu Feb 28, 2008 7:01 am

Usual procedure is to turn on what would likley be turned on when navigating - eg VHF radio for one. You want the compass to be most accurate under your typical operating conditions.

Paul S · Post by **Paul S** » Thu Feb 28, 2008 7:25 am

every boat will be different. It should be adjusted by 'turning' the boat, as the deviation may be different at different points due to variation.

A coastal navigation course will teach you how to turn/swing a boat...and make a deviation table (at least the USCG aux course does)

One of many websites that explain how to turn a boat http://www.hshyachts.com/html/compass_deviation.html
or
http://www.sailtrain.co.uk/navigation/deviation.htm

Retcoastie · Post by **Retcoastie** » Thu Feb 28, 2008 8:24 am

Many small airport have compass roses for checking aircraft compasses.You can often get to them and do a very complete setup. It may be off a fraction because of the trailer, not enough to be a problem at the distances we navigate on the compass.

bastonjock · Post by **bastonjock** » Thu Feb 28, 2008 8:50 am

thanks for the replys,it explains it in my day skipper course,guess ill have to do it one day,like its been said though,we dont go far enough away from the coast for deviation to have a huge impact

Paul S · Post by **Paul S** » Thu Feb 28, 2008 9:21 am

bastonjock wrote:thanks for the replys,it explains it in my day skipper course,guess ill have to do it one day,like its been said though,we dont go far enough away from the coast for deviation to have a huge impact

Yea til you hit that big rock

j/k

At least it seems like some of us still use the compass!

dennisneal · Post by **dennisneal** » Thu Feb 28, 2008 9:45 am

I don't know if this procedure is 100% correct, but I set my compass while the boat was in calm water and by adjusting the compass so that it agreed with sightings taken with a hand-held military-type compass. I pointed the boat in each of the primary directions according to the hand-held compass. The compass displays magnetic direction, rather than "true" direction. It "off" by a couple of degrees in some directions, but that's close enough for my purposes. It was fairly easy.

Morimaro · Post by **Morimaro** » Thu Feb 28, 2008 10:08 am

Would it not be possible to callibrate at obtain a correction card for the Mag Compass comparing it to the GPS heading reading, which should be the equivalent of a Gyro Compass for accuracy on True readings?

Catigale · Post by **Catigale** » Thu Feb 28, 2008 11:05 am

You guys in the courses know this, but the deviation of Compass heading from True Heading is not a fixed thing - it isnt unusual to run into local magnetic variations where your compass can really be quite useless.

Terry · Post by **Terry** » Thu Feb 28, 2008 11:56 am

So which is which;
we have deviation, variation & declination.
I thought it was declination that defined the difference between true north and magnetic north and it was deviation that defined the effects of other electrical infuences in close proximity to the compass. I forget what variation is... But I had to set my fluxgate compass and having installed it below the companionway ladder facing aft just inside the hold there I had to correct for 180 degrees to make it behave so i took it outfor sea trials and did the swing the compass exercise and now it agrees with all other instruments. It even works in my driveway which runs within a few degrees of east/west. All compasses now agree within a couple degrees.
So which expression is which?

Loefflerh · Post by **Loefflerh** » Thu Feb 28, 2008 12:34 pm

Terry, Variation defines the difference between true north and magnetic north, and is posted on all aviation charts. Your definition of deviation is correct: defined the effects of other electrical infuences in close proximity to the compass and in addition by metal objects.
Declination - you can forget about that for boats, unless you want to cruise the (Magnetic-) North or Southpole. Its the up - and down movement of that compass-needle; over the Poles actually the needle would point straight down. In addition while turning (quickly) your compass has some errors, very important in airplanes but for boating, forget it (caused by declination). Just remember in a turn yor compass might read wrong, it is only accurate when level and unaccelerated and if corrected for variaton (a fixed value you read off a chart) and deviation (a value you have to find for your boat / installation)
Hans

Catigale · Post by **Catigale** » Thu Feb 28, 2008 12:49 pm

I think this is also called 'swinging the compass'

baldbaby2000 · Post by **baldbaby2000** » Thu Feb 28, 2008 1:33 pm

Declination (or variation) is the difference from true and local magnetic north on the surface of the earth.

Inclination is the angle the magnetic flux lines hits the earth; you probably don't care about this.

Deviation is a function of heading is the error due to stuff on your boat that distorts your magnetic field; for example, you may have a deviation of say 5 degrees when going north but a deviation of say -2 degrees when going east.

MacGregorSailors.com

MacGregorSailors.com

Compass Deviation

Compass Deviation

Mag Comp correction vs GPS