GPS Chartplotter Compass Interference?

cghubbell · #1 (**permalink**) 06-17-2010

I was about to purchase a Garmin chartplotter when I stumbled on an installation requirement of mounting the unit 20-38" from a compass depending on model.

The only practical location for a (small) chartplotter on my boat (and I suspect many others) is mounted on the binnacle behind the compass. That's within < 12". I'm wondering how many people have experienced problems with this mounting location as I know this requirement is not respected in many cases.

The alternative unit I was considering is the Standard Horizon CP180i. I was not able to find any restrictions on mounting location in the SH installation process. This either means SH does not have a problem with interference, or that the Garmin installation manual is more complete in its requirements.

Forget about Garmin vs. SH features... They both do everything I need and more. I'm only interested in whether or not Garmin's requirement about compass-safe distances is something I should take seriously when I compare the Garmin and SH products.

Thank you,
Chris

sck5 · #2 (**permalink**) 06-17-2010

My 4210 is right where you describe (where else could it be?) and there has been no problem - they always read within a couple of degrees of each other.

wwilson · #3 (**permalink**) 06-17-2010

I agree with sck5. Working somewhat the opposite problem, I agonized, measured, and compared for weeks before mounting a compass at my nav station - surrounded with electronics (instrument repeater, VHF radio, NAVTX and a chart-plotter.) I couldn't find a repeatable degree of disparity in the compass whether in proximity or well outside their range. Still, there must be some level of magnetic interference - I suspect magnified by the manufacturer as a CYA.

cghubbell · #4 (**permalink**) 06-17-2010

Quote:

Originally Posted by wwilson

...there must be some level of magnetic interference - I suspect magnified by the manufacturer as a CYA.

My readings suggest that there is a magnetic catch on a covering, I believe for the SD card slot. I'm assuming that Garmin's compass-safe distance schedule is based on the distance within which that magnetic field can be detected. The part of the equation I don't know about is how sensitive a compass really is to that interference relative to the distance from source.

From what I've read, many people install a GPS close behind the compass and I've only found one case where someone wrote about having a subtle issue. In my case it would be about 8 inches away. I don't have (or wan) the big nav pods that add some distance.

mpickering · #5 (**permalink**) 06-17-2010

Keep flat boat cable away from the compass mount or twist primary wire around each other to cancel out magnetic interference. The distances I've read is 12 inches should be more than sufficient. I can't speak to differences between Garmin plotters or SH. Maybe the Garmins (I have a 541s) leak more RFI than the SH CP180i. Contact Standard and ask them. I think my SH GX2100 manual did make a note of compass distance mounting. I can check tonight and let you know if it says anything.

I'm fortunate in that my primary compass is mounted externally in one bulkhead (port) and my electronic instruments are externally and flush mounted in the opposite bulkhead. Only wire anywhere near the compass is the power cable for its internal light.

Matt

cghubbell · #6 (**permalink**) 06-17-2010

Quote:

Originally Posted by mpickering

Keep flat boat cable away from the compass mount or twist primary wire around each other to cancel out magnetic interference. The distances I've read is 12 inches should be more than sufficient. I can't speak to differences between Garmin plotters or SH. Maybe the Garmins (I have a 541s) leak more RFI than the SH CP180i. Contact Standard and ask them. I think my SH GX2100 manual did make a note of compass distance mounting. I can check tonight and let you know if it says anything.

I'm fortunate in that my primary compass is mounted externally in one bulkhead (port) and my electronic instruments are externally and flush mounted in the opposite bulkhead. Only wire anywhere near the compass is the power cable for its internal light.

Matt

12" would not be possible for a binnacle mount unless you are using a large navpod at its extremity. I'm literally looking to install *right* behind the compass so that the GPS is in between the pedestal support rods.

My concern is not about mis-installed power cables. I'm aware of the twisting process, and that's part of all the proper binnacle installations. The concern here is for magnetic interference caused possibly by a component of the Garmin GPS. This concern exists only because of their documented installation process.

I looked at the SH CP180i installation manual and found no mention of a compass distance.

At this point I'm just looking for practical experience where the GPS chartplotter is mounted in very close proximity to the ship's compass so that I can determine how seriously to take Garmin's recommended compass distances.

wwilson · #7 (**permalink**) 06-17-2010

Quote:

Originally Posted by cghubbell

At this point I'm just looking for practical experience where the GPS chartplotter is mounted in very close proximity to the ship's compass so that I can determine how seriously to take Garmin's recommended compass distances.

Then go read post #3

carl762 · #8 (**permalink**) 06-17-2010

I've been wondering about this very problem. Thanks for the information, Wayne. Just purchased a 441s. Might install it this weekend.

olson34 · #9 (**permalink**) 06-17-2010

This Garmin problem started when they got into a "newer" generation of chartplotters a couple years ago. Their lawyers made them put the warning in the install instructions, but one one model I looked at the 39 inches away from the compass direction was on something like page 29 or 39.....
As a local techie told me "What the h_ll were they thinking?"

My now-getting-older model 3005C has the friction catch chart door, so it's ok.
These newer ones all have a powerful permanent magnet holding the little chart chip door closed.

Strictly IMHO, but their market has changed/evolved to the point where very few of the power boaters have a compass or care, and the niche sailing market with binnacle-mount instruments is a smaller niche all the time.
For those who say they are not experiencing any problem, one needs to remember that changes in compass variation and effects of nearby magnetism will vary with course and angle of heel.

There was always the in-use risk we would assume with buying from Garmin because their stuff is cheaper... now there is real potential danger.

Next time around I shall go with Furuno or Raymarine, FWIW.

L

cghubbell · #10 (**permalink**) 06-18-2010

Quote:

Originally Posted by olson34

Next time around I shall go with Furuno or Raymarine, FWIW.
L

Don't forget Standard Horizon. The CP180i / CP300i does not have a compass distance requirement and both are very competitive with Garmin's prices.

I had assumed that the interference would be minimal, and probably only discernible by swinging the compass multiple directions and watching very closely. However, even a (very) tiny impact is significant over a few miles. It may not impact some boats, and on other boats it may simply not be of concern because many folks don't steer by compass any more.

I've made my decision. I'm going with the Standard Horizon (can't afford the Raymarine!). I guess its my curse as an engineer to feel compelled to respect product specifications. If nothing else it's my personal dollar vote against a stupid design specification. Why would anyone think it's a good idea to replace a friction catch with a magnet in something that installs near a compass?