I'm planning on installing a Garmin GPSMap 498 on my 1978 O'Day 32' CC, and I'll install a pedestal guard to mount it on. Since the pedestal guard is stainless, I imagine that it probably won't affect the compass too much, but how about the unit mounted on the pedestal? It seems like current running to the unit is likely to cause problems. I just read an article in Good Old Boat where someone mounted a fishfinder on their pedestal guard, but it didn't discuss the deviation that might be caused. Since the wiring will be in the guard, it seems like only the couple inches of exposed power lead, and the unit would be a problem. Any thoughts or advice?
Magnetic deviation with compass
- Thread starter jknappsax
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If you are asking if the stainless steel pedestal mount will affect your GPS? I will say no, because it is not a magnetic instrument. If the connecting wires are insulated I can't see how the current can be affected.. besides.. would they make stainless steel mounts if this were a big problem? What other mounting solution had you considered if the stainless were an issue.
Joe, I was talking about the GPS unit affecting my binnacle-mounted compass, not the pedestal interfering with the GPS. I mentioned the wiring in the pedestal guard, since the metal guard would act as a Faraday shield until the wires exit the guard to connect to the unit.
my advice is you are in the same "boat" with all the rest of us as most boats with a charplotter have it mounted in close proximity to the compass with no adverse effects.
It's always a good idea to swing the compass periodically anyway so "trust but verify".
It's always a good idea to swing the compass periodically anyway so "trust but verify".
The GPS, even without any power to it, may affect the compass. There are often metals in the circuitry that can throw it off. With your boat in a stationary position move the GPS close to the compass and watch for any movement of the card. Next run a temporary power wire to it, power it up and move it close then away.. As Ross said any wires run up the pedestal should be twisted pair, preferably with a shield, though this is often tough to find in a marine grade wire..
When adding anything electronic or something that has any sort of metal in it you should always calibrate your compass after wards or have a pro do it for you. I pay to have mine professionally swung and I find it money well spent. Of course if you sail on a small lake it wouldn't matter much but you sail on the Chess which can see fog..
My compass is swung with the instruments, plotter and AP on as they usually are when we're moving. You will always have some sort of deviation and should have a deviation card.
When adding anything electronic or something that has any sort of metal in it you should always calibrate your compass after wards or have a pro do it for you. I pay to have mine professionally swung and I find it money well spent. Of course if you sail on a small lake it wouldn't matter much but you sail on the Chess which can see fog..
My compass is swung with the instruments, plotter and AP on as they usually are when we're moving. You will always have some sort of deviation and should have a deviation card.
My gps can read magnetic and true. I use that feature to check my binnacle compass on any heading. It also allows me to check the set caused by current when my compass heading and my gps course are at odds with each other.
I have the same GPS
I have a 498 also, with the depth finder attached. Mounted on the pedestal, in front and slightly above the compass. Mine is mounted on a mount made from a starboard like material. As far as I can tell there is minimual or no effect on the compass reading. Possibly one degree. And I for one, cannot steer that accurate a compass course anyway.
I have a 498 also, with the depth finder attached. Mounted on the pedestal, in front and slightly above the compass. Mine is mounted on a mount made from a starboard like material. As far as I can tell there is minimual or no effect on the compass reading. Possibly one degree. And I for one, cannot steer that accurate a compass course anyway.
Re: I have the same GPS
My Richie compass has 5 degree marks and when the bay is lumpy that is the best I can do. When visibility is good I steer by landmarks on shore. I set a course and when my headstay is on a landmark I steer for that.
My Richie compass has 5 degree marks and when the bay is lumpy that is the best I can do. When visibility is good I steer by landmarks on shore. I set a course and when my headstay is on a landmark I steer for that.
When using a GPS I just point the goto button and when th eGPS says that I am on track I notice the magnetic compass heading. Then usually I turn off the gps and steer by compass.
Way back in the bowels of the Garmin user manual, there is a spec that is "Safe Compass Distance" . For my 545, it is 3 feet if I recall! I don't know what it is for the 498. I didn't believe them, so mine is mounted on an Edson stainless arm hanging off the pedistal guard, about 8 inches from the compass. When I put the 545 on its mount, the compass veers about 8 degrees.. that is the same with power on or off..What I mean is that when the power is on there is no difference in the mag compass. I was surprised.. Interestingly, on Lake Pontchartrain the mag north and true north are the same right now.
Thanks for all the information about your experiences with similar setups. What I expect in theory, and what happens in the real world are frequently different. As they say, theories are great, in theory!
I would worry more about the cell phone and or the portable VHF being placed near the compass or the auto helm . These will definitely swing the compass .
Re: Some photos
Mainesail,
If you didn't turn the table twenty degrees,
then the gadgets are a serious problem.
Impressive demonstration.
Magnetic fields vary as to the cube of the distance, so I would think that 3 feet of separation would be enough. Anybody want to bet their boat on it?
Mainesail,
If you didn't turn the table twenty degrees,
then the gadgets are a serious problem. Impressive demonstration.
Magnetic fields vary as to the cube of the distance, so I would think that 3 feet of separation would be enough. Anybody want to bet their boat on it?
Maine-
I'd be interested in seeing how the error falls off with the distance, say at 6" and 1', versus the right next to the compass you've got in the current photos.
I'd be interested in seeing how the error falls off with the distance, say at 6" and 1', versus the right next to the compass you've got in the current photos.
Wild a** scientific guess 15-18 degrees at 6 " and 10-12 degrees at 1'Maine-
I'd be interested in seeing how the error falls off with the distance, say at 6" and 1', versus the right next to the compass you've got in the current photos.
Present spacing seems to be about 4-6 inches to the center of mass on the gadget.
- Jan 22, 2008
- 519
Some beverage cans/cups in the pedestal drink holder can also be an issue affecting the compass.
I've found that anything with either ferrous metal in it, or a battery or magnet of any kind, has the potential to cause deviation in a magnetic compass. I used to keep an ordinary air horn in a foam holder attached to the pedestal on my boat. Distance from the binnacle compass was about 8 inches. Yet I learned (the hard way) that the metalic cannister of the horn caused about 6-deg of deviation in the compass. Always a good idea to check everything near the compass to see if it impacts its readings in any way. I also believe it may be futile to try to swing your compass to completely agree with the magnetic course your GPS may be showing as I believe these devices are measuring two different things....
The process of rolling steel orients the molecules enough to make the steel slightly magnetic. Try this with your kids or grand kids. Cut the end from a tin can and mark a line across the center of it and float it in some water in a glass dish. The lid will slowly turn until the line points in a fixed direction. It probably won't be north-south but it will always come back to the same direction if disturbed.
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