Spinning magnetic compass - what's up?

[willk]

While cruising along recently, I glanced at my compass (an Offshore 75) and was more than a little surprised to see it quite busily spinning (clockwise IIRC) at a rate of 5-10 RPM - it was fairly flying around at times. I slowed and circled back, ran a similar course again at a similar pace and the same happened. I didn't have much time to check it and didn't want to spook the missus, so continued on my course. After a spell the compass settled down and behaved for the remainder of the day. All I can add is that yes, I had a phone in my pocket, and I switched off the VHF and plotter and there are no other speakers or motors near the compass. In any case, it wasn't a deviation, but spinning - like it was driven by an internal motor.

I've googled this and the only plausible thing I've read is that this can be caused by a certain vibration of the compass mountings. I must say that I was holding a very constant speed over a light ripple that was noticeable as vibration of the boat in general. The rest of the day was a little smoother.

Anyone have any experience of this happening to them? Any explanations?

[Steve509926]

There are certain areas in the Lakes where I walk that a compass can react in exactly the same way, Bow Fell probably being the best known.
Years ago I had to lead off a team of RAF Trainee Mountain Rescues from North Wales out on exercise on Bow Fell. The mist came down and they naturally turned to their compass and map, it was only after they had passed me , having lunch, for the 3rd time they asked for directions. As I was going down it was easier and safer for them to follow.
So it's a possibility that you passed through a very strong earth magnetic field .

[Fenlander]

Folks report this on land in the mountains from time to time as Steve has experienced. But at sea I'd like to think submarine if deep enough or someone hit the wrong button in a military EW aircraft.

[willk]

Thanks for the replies. To be clear, the youtube video below is the sort of thing I experienced, albeit the video is a fair bit faster. Not THAT faster though, mine was flying around.

https://www.youtube.com/shorts/c7qwy-C7Hlc

[Steve509926]

Now I've seen it, I don't think my explanation could cause it to spin that much.

[willk]

It was pretty eye-opening TBH. I've always 'trusted the instruments' over my inclinations, but it was spooky to watch. It just spun like a top for a couple of minutes.

[Brinormeg]

A strange one for sure. I see a few others have experienced the same….with the expected comments of the bermuda triangle or alien invasion

I did wonder about some military/naval equipment in the area.
I’ve known drones being messed up during flights for this reason, although to alter magnetics to the amount that would be needed, and from any great distance, might seem a tad far fetched. But you never know!

https://youtu.be/AhcjhGooZrQ?si=yI97JJAa0hkxoR1z

[Pikey Dave]

There are certain areas where “local magnetic anomalies” exist. These are marked on the charts, I’ve experienced them off the west coast of Scotland & in the Channel Islands. They tend to cause the compass to wander though, not give the symptoms you describe.

[jeffstevens763@g]

You see it on charts stating magnetic anomalies large iron deposits probably

[willk]

I managed to reproduce this today at a different location, in similar wave and speed conditions. I going to chalk it up to vibrations. I got a video this time in case the X Files want a copy.

[Fenlander]

Could the special vibration absorption system (I assume something other than fluid) in that model have failed?

[willk]

Originally Posted by Fenlander

Could the special vibration absorption system (I assume something other than fluid) in that model have failed?

I guess anything is possible. Interesting that Plastimo make a big play of the vibration thing, I'd not heard of it previously. I won't sweat it too much, now I know (I think) what it is I can mitigate against it, if I really need the compass.