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Safe distance between compass and chartplotter

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Joined: 28 Apr 2006
Posts: 2

PostPosted: Sat Mar 01, 2008 3:06 pm    Post subject: Safe distance between compass and chartplotter Reply with quote


I am installing either a Simrad CX-34 or a Northstar M84 chartplotter on my 30' Lyle Hess cutter and I need some help figuring out what is a safe distance between the chartplotter and the compass. My compass is installed in the starboard aft end of the cabin.

I am planning to install the chartplotter on a RAM mount on the port side of the aft cabin, next to the companion way. I can then swing it into the companion way and view it from the cockpit. When the chartplotter is positioned in the companion way, it will be about 24 - 25 inches from the compass.

I am getting conflicting information about the chartplotters' compass safe distance from the chartplotter manuals and product information on the manufacturers Websites. Can anyone give me some feedback on what their actual experiences are on installing chartplotters near compasses?

The Simrad chartplotter should be okay because the manual says the safe distance is 25".

The Northstar plotter manual gives conflicting information. In one section of the manual, it says the display unit can be 4" from the compass. But in another section there is a caution that says all parts and cables must be 1 meter from the compass. Ugh!

I already bought the Simrad unit because I got a great deal on it, but I am now thinking of returning it and getting the Northstar unit. I really want to buy the Northstar chartplotter because it can interface with an AIS receiver and it has a higher resolution screen and its faster. But I am worried that it will interfere with the compass.

I would really appreciate any feedback on real world experiences on how close I can install the plotter without interfering with the compass.

Thanks a lot.
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Joined: 20 Jun 2006
Posts: 31
Location: Hansville

PostPosted: Wed Mar 05, 2008 10:51 am    Post subject: Safe distance between compass and chartplotter Reply with quote


A fair question. Electronic items may have an effect on a marine compass, but the effect diminishes quickly with distance.

Here are the first questions I would ask. Have you swung your compass on your boat and documented the deviations? Has your compass been swung by a professional, with the boat containing all the 'big metal' objects, in their locations that you intend to cruise with?

Truthfully though, unless you are installing a strong magnetic electrical item near - and I mean within inches of the compass OR a big lump of iron (your engine or 400#'s of chain), you would get so little deviation that you would not be able to document it.
compass compensation

On my own boat, I had speakers mounted within 4' of the compass and the magnets in the speakers did not affect the compass. All other electronics at the nav station are 3-4 away from the compass. The autopilot with its compass is 5 away from my ship compass. None of these affected my compass.

If you are mounting electronics close to the compass, I would test your boat on the four cardinal directions (at least), document; then place the equipment where you want it (just make it temporary), then document, then power up the equipment and document - compare all three and then you will know if it is a real concern.

I would not rely on what any manufacture stated because you do not know their 'test' parameters - one could test through a bulkhead (as though the compass were on the other side of piece of cored fiberglass) while another tests through open air!

Greater questions would be how comfortable you feel without your chart plotter (if it were to die)? Do you feel comfortable with dead reckoning? Get to know how to calculate drift, CMG, distance/time/speed for your boat with charts and the basic equipment you have.

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Joined: 28 Apr 2006
Posts: 2

PostPosted: Fri Mar 07, 2008 7:10 pm    Post subject: Reply with quote


Thanks a lot for the response. Good suggestions. I haven't had the compass swung, but I plan to by a professional.

I took a quick glance at the link you provided. I should be able to setup a course to then determine if the chart plotter will effect the compass using the procedure you provided.

In regards to being comfortable without the chart plotter, definitely yes. I prefer a chart, parallel rules and a hand bearing compass. The chart plotter is mainly being installed for when we are in fog and for its integrated depth sounder. Got the lead line too.

Thanks a bunch.
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