Issues with fluxgate compass

Cardo

Cardo

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We have a Simrad/Robertson autopilot with a dedicated fluxgate compass. I have the compass information passed to the Raymarine chartplotter via NMEA 0183.

Everything seems to work fine, aside from the compass headings.

I find the headings can be out by up to around 20º depending on which way we're pointing.

I've calibrated the compass by running the calibration whilst turning in a big circle a couple of times, and whilst things improved after the second calibration, we still have some issues.

Now, when I calibrated, the conditions weren't perfectly calm and the turn wouldn't have been perfectly round due to small wavelets/gusts. Could this have caused the problems I'm seeing? Do I need to calibrate when conditions are near enough perfect/flat?

Or does the compass, which is around 10 years old now, need replacing?

Thanks for any advice!
 
Cardo said:
We have a Simrad/Robertson autopilot with a dedicated fluxgate compass. I have the compass information passed to the Raymarine chartplotter via NMEA 0183.

Everything seems to work fine, aside from the compass headings.

I find the headings can be out by up to around 20º depending on which way we're pointing.

I've calibrated the compass by running the calibration whilst turning in a big circle a couple of times, and whilst things improved after the second calibration, we still have some issues.

Now, when I calibrated, the conditions weren't perfectly calm and the turn wouldn't have been perfectly round due to small wavelets/gusts. Could this have caused the problems I'm seeing? Do I need to calibrate when conditions are near enough perfect/flat?

Or does the compass, which is around 10 years old now, need replacing?

Thanks for any advice!
Click to expand...

1) It used to be more accurate?
2) No magnetic objects has be placed close to the compass?
 
knuterikt said:
1) It used to be more accurate?
2) No magnetic objects has be placed close to the compass?
Click to expand...

1) Don't know. Only had the boat just under a year so only have experience of it since we relaunched in March.

2) Only thing that's close is the prop shaft which is around 50cm below it. However, isn't the reason for the calibration to counteract compass deviation? (Or is it variation?)
 
Cardo said:
1) Don't know. Only had the boat just under a year so only have experience of it since we relaunched in March.

2) Only thing that's close is the prop shaft which is around 50cm below it. However, isn't the reason for the calibration to counteract compass deviation? (Or is it variation?)
Click to expand...
What is the distance to the engine (horizontal)?

The calibration is there to compensate for deviation, but there are limitations to how much error that can be compensated for.

At what courses is it 20º off, does it happen to be on two opposite courses?
 
knuterikt said:
What is the distance to the engine (horizontal)?

The calibration is there to compensate for deviation, but there are limitations to how much error that can be compensated for.

At what courses is it 20º off, does it happen to be on two opposite courses?
Click to expand...

The compass is at the same height as the engine, although about 1.5 metres away. Would this cause issues?

I haven't checked whether the error is at opposite courses. I'll check it out. I definitely know that it's off around 270º.
 
Cardo said:
The compass is at the same height as the engine, although about 1.5 metres away. Would this cause issues?

I haven't checked whether the error is at opposite courses. I'll check it out. I definitely know that it's off around 270º.
Click to expand...

1.5 m should be OK, note that magnetic objects in the same horizontal plane as the compass is having most influence.

This is from the Raymarine installation manual http://www.raymarine.eu/WorkArea/DownloadAsset.aspx?id=1265
Note: To check if the intended mounting location is free from magnetic influence, tape a handheld
compass to the chosen location, then turn the boat through 360°. The readings on the handheld
compass and your boat’s main steering compass (if accurately aligned) should not differ by
more than10° at any heading
Click to expand...
You can also use COG from the GPS as reference (corrected for variation).
Select a time / place with no current, and do this procedure while motoring.
You must give the GPS time to settle on each new heading before you read the fluxgate course.

If the location of the compass is wrong - there is no help in buying a new one (if you can get one that matches the old equipment)

If you are having most deviation on 270º (and 90º) any magnetic object creating the deviation is probably in front or aft of the compass.
 
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knuterikt said:
1.5 m should be OK, note that magnetic objects in the same horizontal plane as the compass is having most influence.

This is from the Raymarine installation manual http://www.raymarine.eu/WorkArea/DownloadAsset.aspx?id=1265

You can also use COG from the GPS as reference (corrected for variation).

If the location of the compass is wrong - there is no help in buying a new one (if you can get one that matches the old equipment)

If you are having most deviation on 270º (and 90º) any magnetic object creating the deviation is probably in front or aft of the compass.
Click to expand...

How will COG help?

Unless he knows the leeway and tide accurately I would think COG will be almost useless to calibrate a magnetic compass.
 
Cardo said:
1) Don't know. Only had the boat just under a year so only have experience of it since we relaunched in March.

2) Only thing that's close is the prop shaft which is around 50cm below it. However, isn't the reason for the calibration to counteract compass deviation? (Or is it variation?)
Click to expand...

Interesting concept and I don't know if it matters in reality but consider that if the prop shaft is under the compass then on a port tack with heel the prop shaft will be to the right of the compass and heeling on the other tack will put the prop shaft to the left of the compass as related to true vertical. If the prop shaft has magnetism it could influence the compass in different ways on different tacks. Just speculating olewill
 
Buck Turgidson said:
How will COG help?

Unless he knows the leeway and tide accurately I would think COG will be almost useless to calibrate a magnetic compass.
Click to expand...

Sorry should have been more specific - did not think it was SOP to swing the compass while sailing (leeway).

I would of course select a time and place with little or no current.
 
William_H said:
Interesting concept and I don't know if it matters in reality but consider that if the prop shaft is under the compass then on a port tack with heel the prop shaft will be to the right of the compass and heeling on the other tack will put the prop shaft to the left of the compass as related to true vertical. If the prop shaft has magnetism it could influence the compass in different ways on different tacks. Just speculating olewill
Click to expand...

It would also be interesting to know under what conditions the fluxgate is off and how this is measured.
 
Try re-swinging it - we installed a Raymarine unit a few months ago, swung it as instructed and it was well out on some bearings. We swun it again under perfect conditions and it is now far better. Try to find the most enclosed, tide and wind free area you can to do it. Our marina is locked - so absolutely no tide - and has a large lagoon at one end that is completely empty - a great location. We waited for a calm day and then for a period with no other boat movements, then went for it. The difference from the helm compass is now 5 degrees or less on all bearings.
 
move the compas to another location and re calibrate, preferable place would be about 1/2 way of hull length. well thats where I would start.
 
knuterikt said:
Sorry should have been more specific - did not think it was SOP to swing the compass while sailing (leeway).

I would of course select a time and place with little or no current.
Click to expand...


You cant get a GPS COG unless you're underway. You could eliminate leeway by steaming but you still need to take into account current. You may also find that deviation changes engine on and engine off depending on the position of the flux gate and the ships electrical equipment.
 
You don't say which AUTOPILOT you have.I have an AP 45 (which is more big ship stuff) and it has worked perfectly for 15 years but now and again it decides that it has a different compass so I have to pull out the manual ,which if over 100 pages long,find secret buttons,and eventually manage to reprogram it..allbeit it French last time but I daren't touch it in case I loose the lot again!
 
Thanks for all the comments, all. I think I'll try re-spinning the compass in "perfect" conditions first and failing that I'll move it to a different location. Hopefully one of those should do it.

The pilot is an AP-22 btw.
 
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