Flux gate compass and magnetic anomalies

[KAM]

My autopilot went haywire for a short time yesterday. Checking the flux gate heading against magnetic compass indicated a large error. Looked as if the fluxgate had failed. I checked the chart and it indicated a magnetic anomaly. 20 minutes later all was back to normal. Is it possible that the flux gate was more susceptible to the anomaly than the standard steering compass. On the subject of anomalies we also sailed over an underwater spring. I hadn't even noticed this as a chart feature before. We were quite a long way offshore.

 

[Concerto]

Are you sure there was nothing magnetic close to the flux gate compass. Last year I had a problem and managed to work it out.

 

[KAM]

Yes. Small crew no foreign objects.

 

[BabaYaga]

I think a fluxgate compass may well be more sensitive.
I pass regularly over a HVDC sea cable connecting Sweden and Denmark. It is buried in the sea bed at about 15 m depth. When passing by autopilot, this will often make a 90 degree turn at this point (the effect varies from time to time, I guess depending on how much current is flowing at that particular moment).
I have kept an eye at my steering compass when this happens, without noting any significant anomalities.
NB my EV tiller pilot does not use a fluxgate compass but some more sophisticated sensor, but I would expect the effect to be similar.

 

[Bilgediver]

My autopilot went haywire for a short time yesterday. Checking the flux gate heading against magnetic compass indicated a large error. Looked as if the fluxgate had failed. I checked the chart and it indicated a magnetic anomaly. 20 minutes later all was back to normal. Is it possible that the flux gate was more susceptible to the anomaly than the standard steering compass. On the subject of anomalies we also sailed over an underwater spring. I hadn't even noticed this as a chart feature before. We were quite a long way offshore.

I have seen this happen with flux gate controlled autopilots. This happened on one boat I was sailing where the autopilot wanted to take the boat off to port however I disconnected it and after a while the autopilot returned to the correct heading so was reconnected. Later the boats owner came on watch. I described what had happened and it was nor something he had experienced. A little while later the autopilot again took us off to Port. We agreed as there was no one around to wait and see what happened We were off coarse for about 5 minutes at which point the autopilot bout us back to the correct course. Subsequent discussions ashore indicated others had similar experiences.

This was in an around the main shipping g channel between Inchkeith Island and Hound Point oil term8nal in The Forth.

 

[Porthandbuoy]

Small boys with a pocket full of neodymium magnets have a similar effect.

 

[johnalison]

Both magnets will be responding to the same forces but the fluxgate one will not show the delay evident in the other and may well respond to a transient anomaly, though this seems odd for the kind described.

 

[KAM]

Possibly a co incidence. The compass stuck again today. Fortunately permanently so I was able to fully investigate. It eventually started working again when I changed the Seatalk cable joining the compass to the rest of the system. I don't understand how a faulty Seatalk connection could cause the compass display to jam as the analogue connection from the flux gate unit goes directly to the compass display. Anyone know why the display would jam with a Seatalk fault.