Has anybody else noticed this effect? It would be useful to know how common it is. It is more likely to affect tiller pilots, where the compass is likely to be close to the pocket containing the mobile, than wheel pilots, where the compass is likely to be hidden away from the cockpit.

In my Windy 37 Motor Cruiser we always travel with the mobiles on the dashboard. We have a Raymarine RL80 combined with a ST 6000 autohelm and have had the same experience of the boat under autopilot without warning taking a violent course change for no apparent reason (quite exciting at 30 knots). We have also noticed on the chart plotter, without the autohelm engaged, that the boat position symbol spins in a clockwise direction. Having had a sotfware upgrade on the sytem last year this not seem to occur so regularly and having spent much time ensuring that there is no metal close to the fluxgate compass (and spending out on a new one) the mobile phone effect may well be the answer. I look forward to giving it a good test.
SRT

Two of us have had this unexplained problem. My cousin has an autohelm control box mounted on the cockpit face. Inside the saloon is a shelf where he keeps his mobile some 60 cms away. I have a tiller pilot and every now and then the mobile is in my pocket. David Evans may have stumbled across a revelation. What do Raymarine/Simrad say?
DMH Du B

Well in my now elderly sharp autopilots manual it warns of anything electrical/cables nr.the sensor -a no go area of at least 3 ft.-also warns of having sensor near any other compass etc.
Also in case of power boat/motor sailor proximity to windscreen wipers/any other type of electric motor?-and mobile phone signals can be heard interfering with radio at home!Then there is the interference that can be generated by the engine alternator.
Whether these can effect modern computer controled systems I do not know.
Phil