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Old 11 January 2007, 12:25   #1
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magnetic compass problem

hi all
I have a ribcraft which i thought had a faulty plastimo 105 compass but after fitting a new one found the siting of the compass the problem or more to the point the equipment around it.
The console is a large chart table one with the usual stainless grab bars,garmin plotter vhf and dials which as far as i can work out do nothing to the compass.
However fitted to the console is a stereo system (minus the front unit of the stereo ) with four speakers ,two on the back edge of the console and one either side These are not wired up at present but after moving the compass around the speakers i'm 90% sure its these causing a huge magnetic field.
I reckon if i could tip the boat in iron filings she'd look like a hedgehog..!!

So is there anything i can shield the compass with?
The nearest speaker is about 50-60cm below it but i wonder whether by sheer fluke they're aranged in such a way to cause a magnetic field.
The compass points west ("go west young man" ) all the time.
I've tried the compensators in the compass and they are not enough.
I have a bracket now so will try mounting it up a bit but it seems odd that the compass is fine until i place it within the confines of the console.
The only other thing i thought it may be is the compass is quite close to the hydraulic steering helm and next to that is the ignition key and kill cord switch.

Also the compass is right next to the filler/breather on the side . ie the filler goes through the console just under the compass.

Is there anything i can shield the compass in?
do plastimo do a binnacle for the 105?
If i wire up the speakers will this loose some magnetism?

Things i'm going to try is:

: to take out a speaker at a time and see what happens?

:Put the compass on the bracket (but i'll have to fill the hole it was in)

Anyone got any thoughts and had experience of stereo speakers and compasses?

Worst case for me is having to fill four 8" holes in the side of the console

Or paying for a professional compass adjuster to come over and fix it.

help any advice welcome
pete









I've got a mini c compass as well and this does the same thing.
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Old 11 January 2007, 15:11   #2
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I would be 99% certain it is your speakers causing the problems. Most modern plotters vhfs etc are magnetically shielded and are safe around a compass.
I put a compass on a the top of a jockey console. The boat had twin engines (hondas) and so all the dials were cramed in the front of the narrow console. The compass actually touched the bag of the rev counters. At the time the engines were pretty old and the dials made the compass head south permenantly. Got a compas swinger in and he corrected it to within 4degrees on all headings by fitting small magnets to the console.
Last year I replaced the engines (Mercs) (again the compass touches the dials) and again the compass was all over the place due to the new rev counters. I removed the magnets which had been put on by the swinger and the compass worked fine.
Before christmas I had the swinger back to check another boat and asked him to do a quick check on this boat and he found it to be correct to 2 degrees which will do for me!
There is also a gps plotter mounted within a couple of inches of the compass and this seems to have no effect.
I should imagine a vhf may have some magnets in but nowhere near the size of the ones in your four speakers.
First thing i would do is leave the compass where it is and take out the speakers to see if its them and resite them if necessary perhaps in seat pods or somewhere.
I would say the siting of your correctly reading compass is more important than your sound system if you out in the nasty stuff!!
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Old 11 January 2007, 16:15   #3
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It's definatly the speakers. All Speakers have Fkn great magnets in the back Look here:- www.bullnet.co.uk/shops/test/images/DCP00824.jpg

Resite them as far away as poss. Remember also your VHF will have a magnet in the speaker and Microphone.

Hope this helps
Brian
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Old 11 January 2007, 17:16   #4
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remove the speakers

did you consider removing the speakers to see what happens?

rgds
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Old 11 January 2007, 17:24   #5
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Quote:
Originally Posted by behavin View Post
.......Things i'm going to try is:

: to take out a speaker at a time and see what happens?

Yes, I think he did
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Old 11 January 2007, 18:47   #6
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Can you not buy shielded speakers for your Radio? I'm pretty sure I've seen some for sale somewhere, they use shielded speakers in TV's and AV speakers.

I'll have a search later.
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Old 11 January 2007, 19:31   #7
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A lot of people think a product is safe because it says "magnetically shielded". What they never mention is electromagnetic shielding.
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Old 11 January 2007, 20:28   #8
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Yeh, but that's not a problem in most speakers. More of an RF electrical issue with the instruments and most manufacturers are pretty well sorted in that area. Check for wiring as well, the bulb in my compass had the two wires twisted together. This caused a deviation in the compass, untwisting them sorted it. The only other problem I had with my compass was after I installed and new seering wheel, an 8 spoke rubber rimmed "stainless steel" design. When I turned it, the compass went all over the place. I took the wheel off and demagged it at work. It's No longer a problem.
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Old 11 January 2007, 20:29   #9
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Don't rely on the compass. Deviation is another factor to consider. Every boat is different with regards this. Certainly because of the many different outside influences on a small fast boat I don't think you can get much out of a standard compass on a RIB. Best bet in this day and age is a GPS chartplotter backed up with the standard boat compass and a handheld compass. One of them will point you in the right direction. Certainly as with most things if you are having problems it can always be sorted when you chuck money at it. This will sort you problem but are you prepared to chuck money at it. If a compass reading is your major perogative on your boat might be best off to rip all your other electronics out. Electronics upset standard compasses. As I said maybe you should take a handheld compass and stand at the back of the boat/or front when you want to try and take a decent fix.
Personally I always try and remember where the coast is. Even better don't go out if visibilty is going to be bad.
Must admit I have RIBCRAFT and the compass is quite good. Not that I have had to use it but it doesn't wander all over the place and seems quite stable. I'm not too sure if I would be happy to do anysort of navigation with it though. I would probably make Christopher Columbus look good with my traditional navigation skills.
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Old 11 January 2007, 20:34   #10
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Well you only really need to know where North, East, South and West are. You'll hit land eventually .
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