Keeping your compass accurate: our expert advice

Answering a reader question on maintaining compass accuracy. Should you keep a VHF radio and its aerial cables well away from the boat’s compass?

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Ever wondered what you can do to maintain compass accuracy?

You’re not the only one. Reader Sue Johnson sent in the following question:

I have been advised that a VHF radio should not be installed closer than 1m away from a magnetic compass. Does this also apply to the aerial lead? Is it possible to shield the compass from the radio and leads?

Most fixed compasses will have a means of compensating for magnetic influences aboard the boat.

Compass accuracy advice from our experts

James Turner

James Turner worked in the antenna business and can be found on the YouTube channel Boating Equipment Reviews.

I don’t think you can shield the compass from much. You can swing the compass and remove a lot of the errors and then quantify the remaining errors.

But I don’t think the coaxial cable is going to interfere with the compass – when there’s a signal going through it it’s held in by the coaxial shroud.

A bigger concern is that spanner in the locker. That’s the typical one – the cockpit locker with a tool kit in it, with a whole load of steel-based spanners that can affect the compass.

Ben Sutcliffe Davies

Ben Sutcliffe-Davies is a marine surveyor and regular PBO contributor.

I think I’d be more worried about the mobile phone in your pocket by the helm than the cable.

I’ve heard of insurance claims where someone has grounded the boat but sworn blind the compass was accurate – and then it turned out the boat was under engine at the time and the compass was massively out because the engine and alternator were working flat out directly underneath it. Close proximity to the engine is definitely something to think about.

When it comes to auto-helms, you’d be amazed how many people have a little red sticker in a locker somewhere saying ‘Don’t leave anything magnetic near this’.

But the way people are fitting electrics into boats faster than you can say ‘hot dinners’, they see an empty locker and think they’ll stick all that kit in there, fit the whole motherboard of everything in the same place and then wonder why their auto-helm doesn’t work properly.

So there are lots of things to affect the compass. It’s worth bearing in mind that swinging a compass for accuracy is a bit of fun, but with the engine running it’s very noticeable how different the results can be.

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