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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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  #21  
Old 11-23-2010
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Garmin vs. Humminbird

Looks like this stirred up some well deserved attention. Some references were made to electrically induced magnetic fields of wiring near the compass, but that is usually not a problem and in any event, the Garmin unit's permanent magnet is really the issue. The good news is that my old, completely non-magnetic Garmin GPSMAP 185 (with sounder) came back to life and seems to be working fine. But I also researched Humminbird and they seem to have some units with better display resolution and better sounder features (model 588c?) for about the same price as the Garmin 541s. I just sent a question to their tech site asking about save compass distance - we'll see what they say, but does anyone here use a Humminbird GPS/sounder and how has the reliability been?
I thought also that, why do I need an accurate compass reading as long as I'm just GPSing to the next waypoint? And if the GPS dies and I have to use the compass and DR, then I just remove the GPS from it's quick-mount on the pedestal. Actually if I'm in someplace unfamiliar then I usually do at least some checking on the waypoint courses using the compass, visual aids and paper charts. For this I could use a hand bearing compass to get the boat's heading.
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Old 11-23-2010
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The magnet is (was, in my case) internal in the Garmin 546 and it definitely caused major deviation when the GPS was mounted on the binnacle .... the most suitable location on many boats. It is advantageous to pivot/tilt the GPS to a position facing the helmsman, and of course this changes the deviation.

The 546 is a very good chartplotter. I expect Garmin is aware of this thread and it would be relevant for them to explain their rationale here. They might at least take a more cooperative stance with customers. When I called them and asked politely for them to remove the compass from a unit I just purchased their "help line" was of no help. Garmin is not the only manufacturer who specifies a "compass safe distance" because of inductance.... sort of an easy out CYA stance. But the thing is... inductance from chartplotters really is not a source of compass deviation in many cases from what I can see. Or is it?
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Last edited by FishSticks; 11-26-2010 at 01:20 PM.
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  #23  
Old 11-23-2010
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"And if the GPS dies and I have to use the compass and DR, then I just remove the GPS from it's quick-mount on the pedestal. "
Not so easy. if the compass is regularly and continuously exposed to a magnetic field from the GPS, or anything else, it will be remagnetized and no longer point to magnetic north. Even when that extra field is removed, the compass needle will still be showing you a false reading. Maybe not by much--but still off course.
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Old 11-23-2010
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Some people here should have a Compass adjuster to come down to their boat and have their compasses adjusted. The power boaters and commerical boats have a nice overhead to mount most of their electronics above the console.
On sailboats your friendly Compass Adjuster would add compensating magnets.
1. to compensate for the ambient magnic fields around your compass.
2. to reduce the boat's deviation to as small as possible. Then your Deviation card will be acturate. Then it up to you to check its acturacy with ranges, natural & man made.
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  #25  
Old 11-24-2010
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Yes, we can adjust the compensating magnets, but it's probably best to attack the root of the problem if it's a 15 degree swing we're talking about.
I did get an immediate but completely bogus answer from Garmin product support. They say the problem is the EMI from the color display, which they say is much more than from the monochrome display in the older models. I believe this is false. In the first place EMI (electromagnetic interference) is basically a radio wave and requires for it's creation, either a rapidly moving magnet or a time-varying current. We don't have either in this case, since the compass swing when the unit is placed within one foot of the card, happens when the unit is not connected to any wiring, ie. straight out of the box (you can even leave it in the box and get the same effect). So the compass swing is simply caused by permanent magnetism somewhere in the chartplotter unit (memory card latch has been proven to be the culprit according to other messages in this thread).

I doubt the screen has any permanent magnetism associated with it, but I'm not an expert in LED or LCD technology. I'll check it by grabbing my hand bearing compass off the boat, and moving it close to a flat screen TV, to see if it has any effect. There may be some magnetic materials in the connectors and the base of the flat screen TV, but near the top edge should be a good enough test.

Humminbird tech support recommends 1 foot distance of chartplotter to center of compass. I can get close to that with my present mounting configuration. Would appreciate anyone using a Humminbird chartplotter with quick mount, to check if it swings the compass when moved near it.
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  #26  
Old 11-24-2010
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Quote:
Originally Posted by RichH View Post
I have a Gamin 545 and a very sensitive compass. I ultimately had to remove that damn magnet; I caulked the 'door' shut with 3M 4200 and then covered the 'seams' with 'gorilla taop' - problem solved.
STUIPD idea to put a strong magnet in a device that normally will be used in proximity of a compass.
Rich--

How did you remove the magnet? In my cursery examination of the door assembly on the 441 it appears that removing the magnet would also destroy the seal for the card slot. If one hopes to use the card slot it would seem that having to caulk the door closed would be counter productive, no?

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Old 11-24-2010
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He fixed the seal by caulking the door shut with 4200 and then taped over it all with duct tape. Makes removing the card a bit problematic, but it prevents it from leaking.


Quote:
Originally Posted by svHyLyte View Post
Rich--

How did you remove the magnet? In my cursery examination of the door assembly on the 441 it appears that removing the magnet would also destroy the seal for the card slot. If one hopes to use the card slot it would seem that having to caulk the door closed would be counter productive, no?

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  #28  
Old 11-24-2010
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Quote:
Originally Posted by sailingdog View Post
He fixed the seal by caulking the door shut with 4200 and then taped over it all with duct tape. Makes removing the card a bit problematic, but it prevents it from leaking.
SD--I realized all of the foregoing at the time I penned my enquiry. I was wondering about Rich's thoughts via-a-vis the card slot.

I too am now confronted with the need to replace a plotter at the helm and have been very unhappy about the possibility that newer generation Chart-plotters will adversly effect our Compass which we do use and rely upon. Unfortunately, I have no convenient spot to relocate the plotter to that is sufficiently far from our Compass hence pulling the magnet may be the only possibility tho' that is a somewhat nonsensical solution to this issue, no?

N'any case...
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You may be able to deamgnetize the magnet without needing to pull it out. Or weaken it enoug so that it won't interfere.

Magnets can be neutered by heating them (i.e. with a soldering iron) or by applying an AC magnetic field, i.e. a tape head demagnetizer. Whether either one will overcome the Garmin magnet, would depend on how strong the Garmin magnet is.
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HS makes a good point. Most magnets can be neutralized. However, then you'll need to make provisions for sealing the card slot in another way.
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