compass deviation - electric wiring

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MonniotC

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Hi everyone. I am about to install a new thru-bulkhead compass (Plastimo Mini-Contest). It will go on one side or the other of the companionway hatch. The boat is an Achilles 24, and space is limited. The hatch garage is not an option as it is nowhere near big enough.
My favoured position is on the starboard side. This places it very close (two inches) to the usual nine-switch panel as well as digital battery voltage tester and cigarette lighter socket.
I am not sure to what extent the nest of 12v wiring will cause deviation in the compass. Does anyone know? If it does, would the installation of a lead shield (roof flashing) solve the problem.
Many thanks in advance.
 
Unlikly, but why not move the compas around that area and see if the needle moves about or holds a good position to known north?

Of course it may affect is more on other points than the one you are currently moored in but it would be a good test to start with.

Ants
 
Anthony said:
Unlikly, but why not move the compas around that area and see if the needle moves about or holds a good position to known north?

Of course it may affect is more on other points than the one you are currently moored in but it would be a good test to start with.

Ants
Click to expand...

Until I cut the hole through the bulkhead, It will not be that close to the wiring. Does anyone have the lowdown on this? It must be a matter of science how much magnetic field is produced by 12v wiring. It's just that I don't know. Somebody must.
 
MonniotC said:
Until I cut the hole through the bulkhead, It will not be that close to the wiring. Does anyone have the lowdown on this? It must be a matter of science how much magnetic field is produced by 12v wiring. It's just that I don't know. Somebody must.
Click to expand...
I remember a good many years ago on that on an amphora(18) that when I switched the depth sounder on my compass used to move 5 degrees!
 
MonniotC said:
Hi everyone. I am about to install a new thru-bulkhead compass (Plastimo Mini-Contest). It will go on one side or the other of the companionway hatch. The boat is an Achilles 24, and space is limited. The hatch garage is not an option as it is nowhere near big enough.
My favoured position is on the starboard side. This places it very close (two inches) to the usual nine-switch panel as well as digital battery voltage tester and cigarette lighter socket.
I am not sure to what extent the nest of 12v wiring will cause deviation in the compass. Does anyone know? If it does, would the installation of a lead shield (roof flashing) solve the problem.
Many thanks in advance.
Click to expand...

Copper wire is non magnetic so there will be no magnetic field from the various wires around the switch panel until current flows in the wires. If positive and negative wires are parallel for a circuit the magnetic effect of the current in each wire will be approximately equal and opposite and therefore will cancel out.

However, around a switch panel, the wires are not parallel and therefore, when circuits are energised there will be a small, but indeterminate, magnetic effect. If this is only 2 inches from the compass I would be concerned that it would have an effect on the compass deviation. I can not begin to estimate the size of the effect, though, without some idea of the wiring configuration and the magnitude of the currents involved.

A lead shield will have no effect on magnetism. An iron box could provide a shield but will also shield the earth's magnetic field and therefore the compass wouldn't work!

If there is room on the port side and no killer reason not to put it that side, then I would go for that as the safe option.

My only other thought is to use a small hand held (hiker's) compass around the switch panel and turn on different circuits to see if there is any noticeable effect. If there isn't then you may be OK on the starboard side.
 
I would say within a few inches of an electrical distribution panel is too close.

The effect may be small but if any high current circuits are switched on then the effect may be significant.

You can but investigate with a hand held compass as suggested but you will have to check all combinations of electrical use and on various headings as well.
 
DC wires within 3' of a compass should be twisted. I don't know what ill effects the placement so close to the panel itself will cause but would suggest the other side. Or lower down at the fromt of the footwell possibly if your boat allows.
 
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