Compass Deviation Problem ???

[mica]

Has anyone come across the same compass deviation problem that we have recently discovered ?

Having had several attempts at producing a compass deviation card, and getting different readings everytime but found the pattern of the curve very similar. Finding some huge fluctuations in the actual readings, as much as 12 - 18 deg. W, some 5 - 10 deg E, so we decided to get to the bottom of the problem.

First we decided to remove the cockpit speakers. Yes they were magnetic, yes they did have an effect on the readings, but on trying again with them removed found the same curve on the deviation card, with still huge variations from our previous readings.

So next, we wondered about the instrument pods mounted either side of the compass. To our amazement we established that this was the cause of the problem. When moving the pods back and forth, the compass changed as much as 5 degrees. Furthermore, we found that when the wind indicator arrow moved around the dial this also affected the compass reading from anywhere between 0 and 10 degrees. This occurred both when the instruments were switched on or off, so it is not caused by the electrical activity in the mechanism, but would be still be an unacceptable situation even if it were.

How can manufacturers get away with making these instruments, designed for mounting on the pedestal in close proximity to the compass and maintain that they are a suitable piece of equipment for the job. We have yet to receive a satisfactory answer from them to our complaint.

Questions:
1) Does anyone know of a non magnetic marine speaker ?
2) Has anyone come across the same deviation problem from their instruments ?
3) Has anyone got any suggestions on how to overcome the problem ?

 

[Guest]

Most moving needle instruments work on the moving coil principal this means that the signal is used to induce a magnetic field which moves the needle. Hense the disruption to the compass.
The only realistic option is to use a display which does not use this type of indtrument ie. a digital one. If this type of instrument is properly installed with correctly shielded cables and grounded cable ways the interference will be kept to a minimum. Seatalk and other data line instruments are a good example of instrument that can minimise the risk of interference.
All electrical circuits will affect the compass and all the manufacturers know this and usually print warnings to this effect warning installers to take proper precautions. The fault really lies with the boat designer cutting corners either for asthetic or financial reasons.
You don't say how old your boat is but under the CE regulations the manufacturer has a responsibility to install the instruments in accordance with the recomendations to comply with the CE marking. It is an offence not to. So it may be worth throwing the ball back into his court.

 

[Strathglass]

THe easist way out of your problem is to fit a fluxgate compass and you can mount the sensor inside the hull well away from any ferous metal. Several are available one of the least expansive being NASA.

 

[incognito]

Another thing you can do is put a ferrous (magnetic material) canister around the instrument which is causing the problem. It will act as a 'keeper' for the magnetic field. The field will still vary, but very slightly.

 

[incognito]

PS to my last...

...the canister can be fabricated from a tin can. As long as it is a closed loop. If you can fashion it so that you are using a can without a top, so the intstrument is not impeded, and the bottom of the can has holes for your connectors, I would be incredibly surprised if your compass was affected at all.

I have just done a little experiment in my office with/without the tin-can, and the result is phenomenal.

One has to ask what is the casing of the instrument, plastic I guess!!