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redhead78 06-17-2010 01:38 PM

compass differences
Hello all, I have noticed this year, my pedastal compass, and GPS headings are very close however, the readout on my autohelmis like 40 degrees off. There is nothing metal or magnetic by it, is it an adjustment?? Red

Boasun 06-17-2010 01:51 PM

There are two things that affect Magnetic compasses normally.
1. variation
2. Deviation

One other minor detail; there could be a magnetized tool, screw or bolt next to it. Or a speaker/mike for a radio next to it. If your Autohelm is using a magnetic compass. Oh! someone might have reversed a compensating magnetic that is being used for correcting that compass. That happens every now and then.

DwayneSpeer 06-17-2010 01:53 PM

I'm surprised that your pedestal and gps agree at all as the pedestal should read magnetic north and your gps true north. See if your gps has a setting that causes it to read magnetic north instead of true.
You don't say where you are or what the declination is for your area but if it's near zero then your autohelm is wrong. In any case you have a real conundrum.
Good luck.

hellosailor 06-17-2010 02:51 PM

"pedestal should read magnetic north and your gps true north."
Not surprising at all, since GPSes can be set up to read EITHER true or magnetic north. In my experience, that's a setup option on all of them.

But a 40 degree difference far exceeds the maximum that can be explained by any setup choices, unless someone has incorrectly programmed in a huge deviation setting manually, or the autopilot has had its memory corrupted.

I would check the settings (variation, deviation, true/mag) in the autopilot and if possible do a complete reset on it. Then make sure the magnetic compass in it has been properly calibrated, typically this requires making one or two slow and even 360-degree circles with the boat.

Most likely though, there is SOMEthing magnetic near the fluxgate sensor that the autopilot is using, or the sensor is installed incorrectly. If there's still nothing wrong--it is defective.

Take a hand compass on board the boat. See what it reads in the cockpit. Now move down near the fluxgate sensor for the autocompass, and see if the hand compass also goes 40 degrees out of whack. If it does--there's something interfering with both of them. (And the hand compass SHOULD be disturbed somewhat when it gets real close to the fluxgate sensor, in any case.)

tomandchris 06-17-2010 09:45 PM

This is getting way to difficult. If you auto helm is Raymarine then just make the adjustment. They get out of whack all the time. Pick the one you like, binnacle compass or gps, and adjust to it. Hopefully you will be watching as auto steer you to your destination.

jerryrlitton 06-17-2010 10:10 PM


Originally Posted by redhead78 (Post 614227)
Hello all, I have noticed this year, my pedastal compass, and GPS headings are very close however, the readout on my autohelmis like 40 degrees off. There is nothing metal or magnetic by it, is it an adjustment?? Red

I have to step in here a moment. Your GPS is not showing heading at all. It is showing a direction across the ground. Boats are like aircraft in that respect. Where the nose is pointing is not always thew way we are going. We travel in a moving air mass. Unless we are going directly upwind or downwind our direction we are headed will be different then our ground track. I suppose in a sailboat you can throw in leeway in addition to water currents. In addition to all of that you can throw in magnetic deviation and variation.


Omatako 06-18-2010 01:09 AM

First before you do anything else (and some posts above have touched on it), make sure that when you compare the GPS to a steering compass (binnacle) that you are motoring in slack water with no known currents. Else the GPS is not providing course, it's providing direction and that is a totally different thing.

Secondly, if your auto pilot still holds a course even with the compass reading being way off then the fluxgate is probably OK. If the fluxgate is dodgey the auto pilot will not hold a course.

Thirdly, if the fluxgate appears to be dodgy (won't hold a course), check it's location. If it is anywhere near the engine, move it because the engine's magnetic field changes every time you run it. Also check that you haven't got stuff (belt buckles, binoculars, etc.) in lockers near the fluxgate that are moving and causing a change in local magnetic field.

If the auto pilot holds a decent course, find the calibration instructions for your auto pilot and reset the fluxgate. Driving round in circles is not going to cut it.

redhead78 06-18-2010 12:34 PM

Thanks, I will look into it tomorrow. Jerry good point I may have been looking at heading, I will go to the compass page at check it. After all your input, maybe the auto pilot is correct, the GPS was a heading, and my 35yr old binnicle compass is wrong, lots of good stuff to look into, thanks all...Red

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