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Old 07-24-2007, 05:22 PM   #1
sgold44
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Default Compass problems

I've got a problem w/my compass. There is about a 30 degree variance between my compass reading that's factory installed in my dashboard & the reading on my GPS. I beleive the problem is the location of the compas in my dashboard. My dash is curved, w/all the guages spread out throughout the curve. The compass is in the 2nd to last spot on the starboard side of the dash. the red line on the compass is centered on the compass, but the compass is not centered on the boat. The red line faces off to the right about 30 degrees. It's been driving me crazy, but I truly think this is the reason for the variation. Any input?? ny one else have this issue??
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Old 07-24-2007, 08:10 PM   #2
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Default Compass thoughts

Quote:
Originally Posted by sgold44
I've got a problem w/my compass. There is about a 30 degree variance between my compass reading that's factory installed in my dashboard & the reading on my GPS.
I can account for 15 degrees of that. The difference between true north (your GPS) and magnetic north (your compass).

To see if the other 15 degrees is related to positioning in the boat you could remove the compass and check the reading on land or various other places in your cockpit. Odds are that the compass is near some metal that is throwing the reading off. Even the engine could through it off. There usually is a "tweak" of some sort on the compass that allows you to make some adjustments to compensate for some amount of variation. Check your owners manual.

I'm sure others will chime in on this.

P.S. My factory installed compass kept breaking. I replaced 3 or 4 while under warranty. The last one lasted longer but finally broke. I haven't replaced it for several years. I figure that on the lake you need to know where you are before the compass helps you. If I already know where I am, why do I need the compass? Is it open water or an island at 250 degrees? Depends on where you are.

OK, I do carry a boy scout compass on board just in case I ever find a need for one.
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Old 07-24-2007, 09:46 PM   #3
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Most likely is its location and the location of other metal objects in the vicinity, as noted in the previous post. Its also true that there is usually a way to make adjustments. Hope that helps.
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Old 07-25-2007, 06:49 AM   #4
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Check the settings on the GPS. There's often an option for magnetic north or true north. Use magnetic north to check the accuracy of your compass.

The compass does not need to be on the centerline of the boat but the lubberline needs to be parallel with the boat's centerline.

You really need to know if your compass deviates 30 degrees from magnetic north at every heading or at just some headings. If it's every heading, you may be able to compensate with magnets or by moving some wiring or other metal objects away from the compass. If it the deviation varies dramatically with heading, there's not much you can do to fix it and your best bet is to make table of the deviation at 10 degree intervals or so. Less expensive compasses will have larger variations, but all magnentic compasses have some variation.

A fluxgate compass may be a great solution if you really care about headings. They are very accurate but don't come cheaply.
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Old 07-25-2007, 09:09 AM   #5
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Default It keeps changing!

Just watched a show the other night that showed the magnegtic pole is moving even faster than previously observed.I thought they said somewhere around 100-200 feet a year.A team travels every year to find and plot the actual magnetic pole.
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Old 07-25-2007, 02:17 PM   #6
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Quote:
Originally Posted by SIKSUKR
Just watched a show the other night that showed the magnegtic pole is moving even faster than previously observed.I thought they said somewhere around 100-200 feet a year.A team travels every year to find and plot the actual magnetic pole.
Somehow I think the Dems will find a way to blame George W for this SS!
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Old 07-25-2007, 05:56 PM   #7
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Default Let's get deviant....

Sgold:

A magnetic compass has two inherent errors, VARIATION and DEVIATION.

Variation is caused because the magnet(s) in the compass align themselves with this big blob of metal which is almost at the North Pole. In fact it is about 15 degrees west of the pole as seen from here. If we were in Florida it would be in alignment and would read true! It is interesting that this blob is moving from east to west. When Jesus walked the earth it was in Scandinavia!

So for us, VARIATION is 15 deg. WEST.

The other error is DEVIATION. And this is a bit trickier. Deviation is caused by magnetic influences in the boat. They can be metal objects or electronics. On the Misty Blue I have a 15 degree compass swing that moves along with the windshield wiper mounted above it. Fun on a rainy night when you need it!

The thing about deviation is that it is a product of your boats heading in relation to north. For example: You are heading true north. Now start to turn to starboard slowly. The pull from magnetic north is constant but the relative location of all of the metal things near the compass is changing as they rotate around the compass card. Remember the compass card does not rotate! The boat rotates around it!

As you do this you will note that the deviation (typically) will be the greatest when you are heading due east 090deg. and due west 270deg. This is because on a small boat the engine block is the critter with the most influence on the compass. When heading north or south the block is in alignment with north and has less deviate pull on the compass.

You have already been given good advice by the fourm on what to do. I will reiterate:

Remove the compass and take it to a place that is free of metals such as a field or possibly a big parking lot. Know where magnetic north is from your location, say from a map. Somewhere on the compass you will find two brass screws. These are adjustment screws that move the deviation balls or magnets located in the comapss. (Those are the same as the "cannon balls" that you see on the compss at long john silvers seafood place. Yup, now you know what they are for!)

Use a NON-MAGNETIC screwdriver, something made of brass, copper, wood or stainless steel, to adjust the screws until your compass reads North when you are pointing at magnetic north. Congratulations! You have just zeroed your compass!

Now, are you ready for this sailor, we are going to "Box the compass".

Mount the compass in the boat with the fixed line known as the "lubber's line" directly in line with the keel of the boat so that it is directly fore and aft.

Now get out a pencil, paper and your GPS. You are about to make a "diviation table". (Cut out the jokes kids, this is serious!) It should look something like this:

MAGNETIC COURSE DEVIATION

000
015
030
045
060
075
090
105
120
135
150
165
180
195
210
225
240
255
270
285
300
315
330
345

Now set your GPS for MAGNETIC north and steer course 000M. (Remember you must be moving through the water to figure out your heading) Any difference between the GPS and the compass is diviation. If the compass reads let's say 355deg you have 5 degrees "west" deviation. If it reads 005deg you have 5 degrees of "east" deviation. If it reads 000 you have 0 deviation. Log this under DEVIATION above, ie. 5deg W.

Do the same at 015deg, 030deg and continue until you spin the boat through 360 degrees 15 degrees at a time logging the error for each heading. Well YO-HO-HO yo have just boxed the compass!

Still got the pencil? Good. You'll need it for the math.

True
Virtue
Makes T V M D C Add WEST and subtract EAST when going from left
Dull right.
Company Subtract WEST and add EAST whrn going from
right to left.

True is true heading as in true north.
Virtue is for Variation usually 15 deg W. in our parts.
Makes is for Magnetic course.
Dull is for deviation as found in you deviation table above.
Company is the Compass course that you will see on your compass.

Well I have gone on a bit. We'll do some examples tomorrow.

Hope this helps.

Misty Blue.
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Old 07-25-2007, 06:10 PM   #8
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Misty, that was one great lesson on compass use. I don't have a boat but I do like maps (charts) and just dropped in on this post and found your explanation of compass adjustment and use to be extremely clear and consise. Thanks for the lesson. Well done.
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Old 07-25-2007, 07:31 PM   #9
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Default Tates compass

I had a Tates compass it was inacurate and unreliable.
That's where the saying comes from















He who has a Tates is lost
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Old 07-25-2007, 07:36 PM   #10
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Default Thank you

Your info was extremely helpful. Thank you all.
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Old 07-26-2007, 05:09 PM   #11
Tired of Waiting
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Question Can it be adjusted?

Quote:
Originally Posted by sgold44
Your info was extremely helpful. Thank you all.
Hi sgold44,

Just another thought.

I have a compass in my boat. It has two adjustment screws on it. Thay allow me to adjust the compass to compensate for any error from the boat and to adjust to true north. I have mine adjusted so it reads true north. And is very close to my GPS. It reads less than 5 degrees from the GPS.

Don't know if yours has this ability but a look see might reveal the adjustment screws.

ToW
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Old 07-26-2007, 06:02 PM   #12
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Thanks. I'm going to check it out tomorrow as soon as I get up there.
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