Screening Steering Compass from Electrics

Appledore

Appledore

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I have 2 Plastimo Contest compasses which can be mounted on a brackets on the GRP cockpit/cabin bulkhead. This bracket is of the wedge-shaped type that just clips over the bulkhead fitting. The problem is that on the one side of the cabin side of the bulkhead I have the VHF Radio and the electrics panel and the NASA Speed Unit. On the other side I have the chart plotter and depth monitor.

I know that the electrics cause a quite considerable 'deviation' in the compass heading, so tend not to even use it, relying only on the Chart Plotter.

There's not really any way I can re-site the compass and still use it in the cockpit, nor can I remove all the electrics from the inside. I suppose I could fix a large lead sheet in between the GRP face and the inner ply lining!!!!!?? We are talking about a 22 footer.

Seriously, has anyone got a good suggestion as to how I could try to overcome this predicament? The compass is, for me, so much easier to read during the hours of darkness, but not if it means going so far off-course.

Thanks
 
I have the Sestrel Major and it is just fitted with a pair of 24V bulbs wired from a twisted pair of cables. Perhaps the twisted cables cancels out magnetic interference, I really don't know. However, there is no movement in the needle when I power up the binnacle lamps.

I notice that the other compass on the data sheet also has a twisted pair cable, maybe that is significant?

http://www.sirs.co.uk/files/library/files/PDF Files/sestrel spares diagrams.pdf
 
You cannot block magnetism by any practical method.
I suspect the worst culprit is the speaker built into VHF.
Should not have any appreciable mag. interference from very low DC currents in compass light or most solid state electronics,with the exception of (possibly) the VHF on TX.
Fuse panel-possibly,if passing a lot of current,which I doubt.
Try placing VHF on the floor to test my theory,& then operate each piece of other eqpt,one at a time.
Good luck / Len
 
Twisted cables are good.
Running GND returns close to the DC supplies to everything also helps.
Each item should have its own return.
The principle is that current flowing around a circuit loop causes a magnetic field.
Minimise the size of the loop by keeping the returns close to the feeds, and better still twist the wires to reduce the loop to a lot of very small loops, alternately going the opposite way, so they cancel out.
 
As Len says, it's not possible to shield a compass from magnetic influences - the MAIB politely mocked the lead box that a Scottish fisherman had put his compass in with the same thoughts in mind, before steering out into the North Sea and getting lost for two days. (The main problem there was actually that he'd mounted the compass back to front, but still...)

Pete
 
BlowingOldBoots said:
I have the Sestrel Major and it is just fitted with a pair of 24V bulbs wired from a twisted pair of cables. Perhaps the twisted cables cancels out magnetic interference, I really don't know. However, there is no movement in the needle when I power up the binnacle lamps.

I notice that the other compass on the data sheet also has a twisted pair cable, maybe that is significant?

http://www.sirs.co.uk/files/library/files/PDF Files/sestrel spares diagrams.pdf
Click to expand...

Yes twisted pair wiring will cancel out the magnetic field surrounding a single conductor, That is why compass illumination circuits use it. (In all probability ordinary untwisted twin flex will just about do so as well)
 
VicS said:
Yes twisted pair wiring will cancel out the magnetic field surrounding a single conductor, That is why compass illumination circuits use it. (In all probability ordinary untwisted twin flex will just about do so as well)
Click to expand...
 
prv said:
As Len says, it's not possible to shield a compass from magnetic influences - the MAIB politely mocked the lead box that a Scottish fisherman had put his compass in with the same thoughts in mind, before steering out into the North Sea and getting lost for two days. (The main problem there was actually that he'd mounted the compass back to front, but still...)

Pete
Click to expand...
While mocking fishermen is often good sport, there are things that can be done, particularly with regard to DC current affecting a compass.
Ye Noble Guild of Compass Adjusters would have some more suggestions.

A total enclosure of iron or ferrite might work quite well to isolate the compass from the radio, alas it would also isolate it from the earth's magnetic field....


But practically, think about moving the damn thing away from any ferrous metal.
A bracket on the back of the mast like dinghies sometimes use is one idea.
 
Thanks for your responses. I do know that the VHF, when fitted, will affect the compass quite a bit. Unfortunately the distance between the two is only a few inches. I can live with the problem, but it seems that there's no easy solution, not that I thought there would be:rolleyes:

I may just have to move all the electrics to one side only, and leave the compass on the other side.

Thanks again, I'm still learning, but 'letrics are still sometimes a bit of a mistery to me.

Geoff
 
hmm, I am not sure that is what is being said. You have two main types of interferance for your compass. That from permanent magnetic things such as the magnet in a speaker (may be one in the vhf) or any magnetism in large lumps of metal, and that from electromagnetism in your electrics. As far as I am aware you can shield from the electrics using twisted pairs often with a shielding of foil wrapped round them and grounded. You can also route the wires so that you are not putting your compass in the middle of a loop where it will be subject to the electromagnetic field. You cant do much to shield from the permanent magnets other than to move them away, as far as I know.
Appledore said:
Thanks for your responses. I do know that the VHF, when fitted, will affect the compass quite a bit. Unfortunately the distance between the two is only a few inches. I can live with the problem, but it seems that there's no easy solution, not that I thought there would be:rolleyes:

I may just have to move all the electrics to one side only, and leave the compass on the other side.

Thanks again, I'm still learning, but 'letrics are still sometimes a bit of a mistery to me.

Geoff
Click to expand...
 
Check to see if the compass moves or has greater deviation when the electrics are on. If they make no difference then it's mostly the speaker and your only hope is to relocate it. In practice the field produced by a even a smallish speaker magnet is likely to be huge compared to anything an electric circuit can produce unless it is coiled (or has a small power station driving it). But I agree with Camelia, you need to swing your compass first to see exactly what the error is, otherwise you're trying to remove an unknown quantity.

If re-siting the VHF is not an option then I would simply get the compasses swung properly and see if the deviation is manageable once it's been knocked down as far as possible. I regularly used to take 20 degrees deviation down to 3 or 4 degrees max on aircraft and I've down the same on a couple of boats too.

If you want to try swinging the compass yourself then have a go but make sure you can reset the compass back to where it was before in case you get confused (mark the screws and make sure you count the number of turns, etc.). There's a very simple version of the process here http://www.captaindale.com/Chip_Log_Swing_a_Compass.html - just replace the "Sun Compass" with either a known transit or some other heading reference |(gps will work fine as long as you aren't just rotating on a sixpence). You should strictly speaking use a surveyed compass swing area but I wouldn't worry too much. Just avoid powerlines, etc. and don;t forget to add in variation (from your chart). Remember to use a non-ferrous (not just non-magnetic) screwdriver to adjust the compass and to have the electrics in the state you would when sailing.

Basically if your compass reads 10W on North and 16W on South then you should be able to adjust the compass say 10E and end up with zero deviation heading N but still with 5W when heading South. Go round again and tweak it another 3 degrees and you'll end up with 3E deviation on North and 3W deviation on South. Do the same heading East and West, recheck all the way round, note the deviation at each heading on a deviation card (8 points is more than enough, just 4 is OK really), smooth the dots out into a sine wave, laminate the card and stick in front of the compass and you're done.

Word of warning - it is easy to get confused so make sure you actually understand the procedure if you're going to do it yourself.
 
Thank you Grumpy. I will copy your post and see what I can do when I'm out in Plymouth Sound. Might try to find out if someone from the Club knows about swinging a compass.

Much obliged to you and the others here.
 
As others have advised the VHF would be my main suspect for deviation. The magnetic field around wiring carrying the amount of power we are talking about here is relatively small and I suspect of little consequence. The speaker in the VHF is the problem. Probably not possible without a degree of "surgery" but removing the speaker completely from the radio and using a remote external speaker would be one option. Also the mic for the radio, will contain a magnet, and not, perhaps, being always in the same position in its holder/clip could give rise to a variation in the deviation !
 
A few comments on previous posts. The microphone may have a magnet in ti but more likely a capacitive microphone. No magnet. Confirm by waving the mic near the compass.
As said speaker in the vHF may be a problem. Whilst they have a powerful magnet in the speaker it is usually ina very tight magnetic circuit with just a small gap for the movng coil. So may not give as much problem as you might think. The VHF radio likely has steel covers which will shield the magnetic field to some degree. As said move the VHF temporarily and see if it makes any difference.
Regarding shielding. The idea of foil as above is no good at all. Foil will shield from electromagentic interference ie where the current is AC or varying. It acts like a transformer secondary shorting out the induced current. But for static magnetism only steel or other materials permeable to magnetic filed will soak up the magnetic field.
Running the wires carrying DC close to one another will help even better to twist the wires.
A wire carrying current will produce a field in a circle around the wire. Imagine doughnut shaped with wire through the hole. 2 wires pos and neg will have current flowing in opposite directions so when in close proximity will cancel out the fields. good luck olewill
 
Yes I agree with William H, the mic maybe capacitive. However a lot of the microphones these days also act as miniature speakers/earpieces so I suspect are moving coil in nature. But easily checked as William suggests.
 
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