Loss of Compass Alignment

[dawntreader]

Yesterday afternoon whilst on passage I noticed that it appeared we were heading into shore. After a mild panic attack, rush up top and a look outside it was obvious we weren't and that my wife, who was on watch, had everything under control. However, both heading and wind direction, as shown on the screen, were both about 120 degrees out of true (see photo). After a series of checks I concluded that we had 'lost' the compass alignment on one of the electric compasses (the Smart Heading compass for autopilot was fine  . We slowed and re-aligned the compass as per instructions on the Raymarine C80 unit we have. It is now ok again. Question is, is this a 'warning' of something breaking down? I have had the C80 unit from new and it has behaved perfectly for over 10 years.

[Salty]

Have you left any tools, spanners, screwdrivers with magnetic tips to the blades etc., anywhere near your transmitting compass, albeit by mistake? Makes a huge difference once the interference has been removed !!

[Lyra]

I encountered this once on my ST4000+ unit about 3 years ago - Most probably it is due to loss of calibration parameters but I could not detect why it happened (battery was a bit low so maybe there was a power "glitch" although I believe the calibration parameters are stored in a non volatile memory).
Nevertheless since then it worked perfectly OK so probably it is not something that starts to go wrong inside the unit.

[dawntreader]

Have you left any tools, spanners, screwdrivers with magnetic tips to the blades etc., anywhere near your transmitting compass, albeit by mistake? Makes a huge difference once the interference has been removed !!

One of my 'daily checks' prior to leaving port is to ensure that the autopilot digital compass reading is the same as the magnetic compass. This fault occurred whilst on passage - that is, everything had been normal but the alignment failed/changed whilst we were motoring along not touching anything. One minute the heading and course were similar, the next the heading and wind was 120 degrees off. The best part was that COG was accurate.

[Salty]

Have you left any tools, spanners, screwdrivers with magnetic tips to the blades etc., anywhere near your transmitting compass, albeit by mistake? Makes a huge difference once the interference has been removed !!

One of my 'daily checks' prior to leaving port is to ensure that the autopilot digital compass reading is the same as the magnetic compass. This fault occurred whilst on passage - that is, everything had been normal .....................
1.     but the alignment failed/changed whilst we were motoring along not touching anything............................
2.     The best part was that COG was accurate.

1. Could anything that might have had a magnetic influence have moved, perhaps caused by the boat rolling, might you have put something in the vicinity of your transmitting compass, and then later moved it away without realising the affect it had on your compass?
2. With respect, COG surely would have been determined by your chart plotters calculation between successive GPS positions rather than by any input from your transmitting compass.

Hope you find the cause of the glitch, and if you do, please let us know. Good luck.

[Lyra]

One more thing - check that the magnetic flux sensor (probably installed on one of the bulkheads inside) did not get loose or moved for some reason?

[dawntreader]

Thanks for all the input guys:

• Having had interference in the distant past, one of my printed 'daily pre-passage checks' includes ensuring there is nothing metallic on the shelves or in the vicinity of either of my flux compasses that would then (or later) interfere with their operation
• Both compasses are firmly fixed to the bulkheads on either side of the boat - one is the original for the chart plotter, the second for the autopilot/smart heading system
• Yes, COG is determined by GPS and I turned off Wind and HDG when the fault occurred. I was actually looking at the chart plotter when it happened and noticed the wind had 'changed' direction. Hoping we could turn off the diesel sail, I went topsides where my wife was on watch. She said the wind was still on the nose  When I went back down HDG was pointing straight into shore - but the boat wasn't.

At least the 'fix' is easy to implement 
[/list]

[Craig]

I have banned bras on "Shirley Valentine" as we almost collided with another boat while on auto-helm.

The wire in the bottom of the bra had metal that deflected the fluxgate compass by 70 degrees. Our fluxgate compass is located in the forward cabin, in a locker my wife kept her bras in.

Not the same problem you appear to have but serves as a warning about metal objects near fluxgate compasses.

There should be a warning sticker "No Metal, No Magnets" near where the fluxgate compass is located.

Craig
"Shirley Valentine"
Gold Coast
Australia.

[Salty]

Not the same problem you appear to have but serves as a warning about metal objects near fluxgate compasses.

There should be a warning sticker "No Metal, No Magnets" near where the fluxgate compass is located.

Yep, got the notices, but there's nothing like a bit of absent mindedness to make a complete mockery of them.
The other day I was asked to help a friend whose autopilot was giving him some aggravation. On the day I noticed that his fluxgate compass was located just in front of a large chunk of metal called the engine, and next to it he had placed a portable battery charger where he was recharging his starting battery while in the marina. I'm no expert on battery chargers, but don't they have coil windings inside used to create a magnetic field !!  So I said I'll come back another day when the charger is not connected. What a difference that made