Chartplotter distance from compass

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Plevier

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Garmin give a compass safe distance of 32" from their chartplotters such as the 750.
Lots of them are mounted on the instrument/grab rail in front of the wheel about 12" above the binnacle compass.
Anyone had a problem or is the 32" way OTT?
 
Bearing in mind that magnetic fields decay as the square of the distance, then the field 12 inches from the chart plotter is going to be just over 7 times stronger than it is 32 inches away. It really depends on what Garmin regard as safe limits - but it would have to be safe by an order of magnitude for 12" away to be OK.

However, much depends on the direction of the field from the chart-plotter. If it's nearly all vertical, then it won't affect the compass (much), though it may cause the card to be deflected from the horizontal. The only real way of checking would be to swing the compass, checking the boat's head by using a hand bearing compass distant from the chart plotter, or using fixed marks ashore.

A quick check is to watch the compass as the chart-plotter is switched on. If the card moves, then it's too close! There probably aren't any permanent magnets in the chart plotter, so there will be a difference in filed when it is on from when it is off.
 
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Slow_boat said:
Should I leave it switched on whilst on passage or save power by switching it off until I'm is into pilotage?
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Up to you. Personally I don't like using a plotter for navigation (as opposed to pilotage) so leave the one on KS stowed below until approaching an unfamiliar port where it may be useful. But some people leave theirs running all the time, and I don't think there's anything wrong with that. Plotters with bigger screens can eat a lot of power, though - worth being aware of how much yours uses compared to how much battery you have.

Pete
 
Slow_boat said:
It strikes me that I may as well switch it off and stick to charts on passage, then.
It seems to be a bit of kit for pilotage and position plotting rather than navigation anyway.
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Plotters are incredibly useful for navigation on passage. Shame to have one and switch it off!
 
pvb said:
I reckon people with the plotter at the helm probably rarely use the binnacle compass.
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I'm sure you're right, but on the only plotter I've used at the helm of a boat - a Raymarine - I had to squint at the screen from about 3" to read the heading. I'd like to have the compass working too!
Found that Raymarine say 3ft separation but when you dig deeper they have a drawing showing safe distance in various directions from the plotter. The shortest is if the plotter is vertically above the compass in which case 12" is OK.
 
I have a Garmin 750, currently mounted at the chart table. I have thought long and hard about a second mounting position either on the coachroof under the sprayhood or over the compass (wheel steering). I too was worried about its effect on the compass if monted on the binnacle, so I rigged up a power lead so that it could be switched on near the compass. When it was held level with the compass and quite close it had very major effect on the compass which varied as it was moved round-if you see what I mean. Cannot remember how much deviation it caused, but I think at least 30 degrees, could have been more. However when held over the compass, in the position it would be if permanently mounted, the effect was just visible, maybe 1 or 2 degrees.

So my finding agrees with Troubador!

HTH
 
I tested before installing a Garmin 3005C - at about 12" away from compass - up and forward - I could find no alteration more than about half a degree to the compass deviation, so fitted it there. If you steer to half degrees I'd rather you looked where you were going. Other plotters may differ.

I suspect that the greatest effect is from currents in the 12 volt supply wiring rather than anything much generating magnetic fields inside the plotter. Avoiding magnetic fields from wiring is why leads to compass light bulbs are twisted.
 
jwilson said:
If you steer to half degrees I'd rather you looked where you were going.
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:)

I once had a Norwegian pilot giving me courses to the half degree when steering Stavros up a fjord. Seemed a bit excessive, but I mostly managed to do what he said. Gyro repeater on a 600 ton ship is considerably easier than the average yacht compass, though!

Pete
 
Hi, fitted a 750 last year at the helm quite close to the compass. I've had no problems with the installation using it on an extended cruise, the only problem was with AIS seeing my yacht as a target but that's another problem
 
I have a SH 180 on a lead and a bracket at chart table and at the helm, next to the compass, about 6" away on the edge of the table and when switched on gives about 10 degrees deviation. When crossing the channel use the compass to follow the route taking account of both lots of tide so I get a nice flattened sign wave as my track, surely others do this?
 
Troubadour said:
Garmin give a compass safe distance of 32" from their chartplotters such as the 750.
Lots of them are mounted on the instrument/grab rail in front of the wheel about 12" above the binnacle compass.
Anyone had a problem or is the 32" way OTT?
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For many modern single wheel boats, even 12" is optimistic. The instrument pod on our Jeanneau puts the bottom of the plotter no more than 4" above the compass. I could, of course, put it in a separate pod bolted on top of the built-in binnacle pod, but that would be very cumbersome and leave a great big expanse of empty panel below it.
 
lustyd said:
No probs that you're aware of, or have you verified no probs?
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I have used it for the last two years like this, sailed from Pwllheli to Milford, to Padstow, to Swansea inc several all nighters and Back to Holy head. I didnt hit Skokholm, the Bishops or any nastys so I think I can safely assume that it doesnt affect the compass.
Stu
 
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