Raymarine SPX30 Autopilot loses calibration - SailNet Community
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Raymarine SPX30 Autopilot loses calibration

I am having a problem with our auto pilot fluxgate compass losing calibration. We have only just started getting used to using the autopilot since we have never had one before. At first we just hit the "auto" button to hold our current course, which seemed to work fine. Then I tried to steer to a waypoint and found the boat steering around 30 degrees off course. I then noticed that the fluxgate bearing was about 30 degrees different from GPS and binnacle compass readings. I went through the calibration procedure that had me driving the boat in slow circles, as well as syncing to the gps, and everything seemed to work fine for the rest of the day. The next day I noticed the compass was 30° off again. I did a quick gps sync on the way out of the harbour, and again everything seemed fine. We were motoring along and I set the AP to hold our current course, which it did for a while, then suddenly it made a 30 degree turn! The fluxgate was suddenly out of calibration again.

I don't know if the problem is new, or if it has always been there. We have only had the boat for 1 season, and only just started playing with the AP. My first thought is that something metal might be interfering with the fluxgate so I went looking for it, and finally found it right next to the battery bank. That doesnt seem like a good place to me, but when I looked up the installation guide, which is little more than a pictoral diagram, it did not specify to keep it away from batteries specifically, just engine, vhf and magnets. Having said that, will be dc current flow in the cables when charging or discharging.

Could the fluxgate compass location be my problem, or might there be something else going on here?


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Re: Raymarine SPX30 Autopilot loses calibration

That would be my first guess. Easily relocated to find out.

FWIW, mine behaves the same way. My fluxgate is in the back of a hanging locker, with metal hangars. Pretty sure that's what screws it up and I've not gotten around to doing any better.


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Re: Raymarine SPX30 Autopilot loses calibration

I notice on this installation pictograph from Ray there's what looks like a wiring run as something requiring distance.
https://www.sv-zanshin.com/r/manuals...ationguide.pdf

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Re: Raymarine SPX30 Autopilot loses calibration

I just looked on a sister ship to mine, and his compass is mounted high up on a bulkhead. The installation instructions show mounting at waterline level. I wonder how critical that is?

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Re: Raymarine SPX30 Autopilot loses calibration

I’ve for the same TP. Definitely could be the location. In fact, I had to relocate mine after it failed to calibrate properly. Seems to me I had it located close to the batteries.

My fluxgate is quite a bit higher than the waterline, and close but not centre of my boat.

I would try moving the compass. It shouldn’t be hard to do. If that doesn’t work, then you probably have a more difficult problem — but I bet this is it.

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Re: Raymarine SPX30 Autopilot loses calibration

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Originally Posted by SchockT View Post
I just looked on a sister ship to mine, and his compass is mounted high up on a bulkhead. The installation instructions show mounting at waterline level. I wonder how critical that is?
For these older simple fluxgate compasses like shown here, it is fairly critical to keep them low and centered. For more current compasses that have multi-axial rate compensation, their physical location in the boat isn't critical at all, as long as they aren't near strong magnetic interference.

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Re: Raymarine SPX30 Autopilot loses calibration

Does this only happen when motoring? A charging current to the batteries will cause a magnetic field near the batteries and could interfere with the compass in that location.

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Re: Raymarine SPX30 Autopilot loses calibration

The OP might take a hand bearing compass (iPhone would work) and move it around the area of the fluxgate compass to detect anomalies. If that doesn't show anything, it could be current flowing in the battery cables generating enough of a magnetic field to throw off the fluxgate compass.
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Re: Raymarine SPX30 Autopilot loses calibration

I tried using a compass app on my phone and placed it right above the fluxgate, and it read within a couple of degrees of what both my binnacle compasses say. I will still try to relocate the fluxgate when I have time. Unfortunately the wiring was run through a raceway so I have to get the wiring disconnected and pulled out in order to relocate. Would rather not do it if location isnt the issue.

I looked at the manual calibration and found that somebody had already entered a -100 offset. I had to increase it to -132 in order to get the fluxgate to agree with the binnacle compass. The next day I rebooted the instruments, and the compasses still agreed, but then I watched as the fluxgate began to tick up, 1 degree every few seconds! In a short time it went from a correct reading of 60° to 67°!

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Re: Raymarine SPX30 Autopilot loses calibration

So I just went through the whole commissioning process again, and after I was done the fg compass matched the GPS and binnacle compasses. After I did the slow circles to build the deviation tables the instruments indicated only a 1° deviation which is well below the maximum of 10° that the manual deems acceptable. My understanding is that the good deviation numbers indicate that the location of the fluxgate is acceptable and free of interference. Having said that, I still wanted to see what happens when the battery cables were carrying higher amps. To test this I did not plug the boat in while we were away this weekend, and instead ran on the batteries and discharged them quite a bit. The compass read correctly at the dock, and continued to read correctly as we motored home, even though the alternator would have been putting quite a bit of charge back into the batteries. So it seems the proximity to the batteries does not affect the compass.

The whole way home the compass was completely accurate, and everything is working. It is still a mystery why the calibration was drifting so badly...time will tell if the problem returns!

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