Compass correction for speaker install

Iain C

Iain C

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My boat has a GRP wheel pedestal...fairly standard stuff with the wheel on the back, instruments and steering compass on the top and a fold out cockpit table on the front. The compass is the standard Bavaria Plastimo job with the "segment" built in sun cover.

I want to install a Fusion Panel Stereo speaker in the front of the pedestal facing the table, however holding the stereo unit up where it will be installed predictably caused my compass to swing about 30 degrees due to the magnets inside it.

Would a compass adjuster be able to adjust this out? Has anyone had this done in the Solent area and roughly what are the costs?

Thanks!
 
Last time I had this done (not in Solent) charge was £130. I agree that putting a big permanent magnet below compass is a very bad idea. Especially in a sailing yacht that spends much time heeled one way or the other, moving the magnet between below, or port or starboard of the compass.
 
I wouldnt install a permanent speaker in that position as others have said but would consider a portable speaker.
 
peteK said:
I wouldnt install a permanent speaker in that position as others have said but would consider a portable speaker.
Click to expand...

That would be even more daft, as the error would be variable.

You can get speakers which have a 'compass safe distance' quoted.
It was a while ago when I fitted some, but IIRC the spec was less than 1degree of deviation at 1m from the compass.
 
Compass safe distance came is how close to the compass you can go to get a 1 degree of deflection. Speakers give two problems, one is the large permanent magnet and the other is the electromagnetic effect when it is powered. You would be better to mount the speaker on the bulkhead. After you have found a good position, would recommend that you perform a compass swing of the boat to ascertain whether there is any effect.
When I performed a compass safe distance check with a 300W base speaker on an aircraft, we needed a minimum of 3m to get 1 degree of deflection.
 
I had the same problem. radio speaker HAD to be mounted within a foot of the bulkhead mounted compass. It is not possible to block the magnetic field/pull, but you can redirect it. I googled the problem and found that placing a round metal fence post cap over the magnet will redirect the "field". I had to cut a small slot in the cap so that the wires in the speaker were not "trapped" I used a cap with a diameter 10mm or so bigger than the magnet. My speakers are wall mounted in carpet covered rounds as used on car parcel shelves My compass is not affected that I have noticed. Some comments were made in the google article that speaker "output" maybe affected or distorted, again I have not noticed any. Cheers Peter
 
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You can buy magnetically shielded speakers. These were common in the days of televisions with glass CRT's .. as they were extremely sensitive to external magnetic fields. Modern TVs (LCD, LED etc) are not affected, but it is still possible to buy magnetically shielded speakers.

They look like this: https://cdn3.volusion.com/dzkjd.jyzrx/v/vspfiles/photos/JAMO2.5x4-3.jpg?1389772936

with a closed steel cup around the magnet.
 
rszemeti said:
You can buy magnetically shielded speakers. These were common in the days of televisions with glass CRT's .. as they were extremely sensitive to external magnetic fields. Modern TVs (LCD, LED etc) are not affected, but it is still possible to buy magnetically shielded speakers.

They look like this: https://cdn3.volusion.com/dzkjd.jyzrx/v/vspfiles/photos/JAMO2.5x4-3.jpg?1389772936

with a closed steel cup around the magnet.
Click to expand...

Many moons ago I fitted a pair of cockpit speakers to our then W33 ketch, massive deviation anywhere in cockpit (tested before holes cut), took them back and replaced them with magnetic shielded ones by Alpine as fitted by sunseeker back then, this was with a specialist ICE store in Poole who did lots of marine installs. IIRC the speakers had two magnets opposing each other allowing use within a foot or two.
 
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pij27 said:
Compass safe distance came is how close to the compass you can go to get a 1 degree of deflection. Speakers give two problems, one is the large permanent magnet and the other is the electromagnetic effect when it is powered. You would be better to mount the speaker on the bulkhead. After you have found a good position, would recommend that you perform a compass swing of the boat to ascertain whether there is any effect.
When I performed a compass safe distance check with a 300W base speaker on an aircraft, we needed a minimum of 3m to get 1 degree of deflection.
Click to expand...
As said the speaker will not be producing any more magnetic effect when powered. However those very ancient techos like myself might remember "elecrodynamic " speakers which had a huge coil to provide the magnetic field. (In the days when strong permanent magnet material was not available) This coil was used as a choke for high tension filtering and would dissipate about 5 or 10 watts.
Anyway back to the real world. One problem of a speaker with magnetic field mounted below the compass is that you can correct the compass for deviation when the boat is level. When the boat heels the speaker is effectively either to port or starboard of the compass requiring different and opposite correction. So do not try to correct the compass for the speaker unless you mount it to one side. Mount it well away or use a magnetically shielded speaker. Incidentally you might consider one of those audio driver devices which do not need a cut out but vibrate the GRP bulkhead. ol'will
 
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