Where did you mount your compass?

[Zavala]

The previous owner of my boat installed the compass on the cockpit bulkhead -- which I always found handy.

After converting to wheel steering, I'd like to have it on the pedestal -- but it's not worth covering the exisiting hole for now.

[DLT]

Chip,

Thin aluminum foil will not be an ideal sheilding material for magnetic fields. Period. But, I imagine that the field at issue is relatively weak. After all, we're talking about low voltage/current DC wiring. So, it might just help. Isn't it worth a try?

No, I'm not attempting to say with certainty that this will work. I'm just suggesting that it is worth a try, rather than relocating your compass, having to drill a new hole, and covering the old one... I'm just trying to save you some effort/issues...

My suggestion to ground it, may or may not help. I don't remember everything from the fields and EMI classes I took. I do remember that sometimes grounding helps and sometimes it hurts. Again, it seems pretty easy to try it both ways...



Here's another trick I learned from an old guru teaching an EMI class. You know that rat's nest of wiring you have hanging off your PC? If you tightly coil your power cords, you just might accomplish a whole lot more than making it look neat. That tightly coiled wire will not impact normal use. But, it will look like a huge inductive coil (or a choke) to a lighting strike. Am I certain that it will stop all damage from lightning. hull no, but what do you have to loose? Have you ever looked inside a surge suppressor?

[kmclemore]

Clearly I'm not an expert in this area, but I wonder if this foil works?

http://www.lessemf.com/mag-shld.html

...and that then begs the question... if one shields compass from the magnetic field of the wiring and other metal & electronic objects on the vessel, are you also shielding the earth's magnetic field and thereby making the compass basically useless? Perhaps the foil should be used only on the source of the magnetic interference, and not the compass, to avoid this problem.

[DLT]

Also, it goes without saying that you don't want to sheild the compass itself...

Yup...

[Hamin' X]

I'm not an expert in this matter by any means but I'm almost certain that the magnetic field and ferrous materials that throw a compass off is fundamentally different from electromagnetic interference - EMI. Aluminum foil, lead, mu metal and the like, can shield from EMI, but have no effect on a magnetic field. Can somebody confirm, or better yet, explain it better than I? Words of few syllables, please.

Chip,

You are basicly correct. Foil will shield electromagnetic fields of the AC variety, ie. radio waves. That is why your TV cable has a foil shield on the outside. However, it will not have any effect on a DC generated magnetic field, because that magnectic field has no electrical component to it unless it is a moving field inside a coil, or it is generated by an AC current, even though it is created via electricity.

You can do a simple experiment to show the lack of shielding by using a refrigerator magnet and handheld compass. Lay the magnet down on a surface and mark its location by outlining it with pencil. Move the compass back and forth on the same surface at a distance to determine how close you must be for the magnet to have an effect on the compass. Mark this spot. Wrap the magnet in foil and repeat. You will find that the distance does not change.

I would offer that what is happening to other people when they try different methods of shielding compasses, is that they are changing the relative positions of the wires to the compass.

[Chip Hindes]

Thank you Hamin'. This is what I suspected without knowing why or having any idea it had to do with AC versus DC.

So I won't bother with the aluminum foil. I'll make sure the wires are twisted as well as they can be. After that, my choice is (a) move the compass (fairly painful considering the large hole it will leave) or (b) move the wires (even more painful considering the whole electrical system to the panel will have to be rewired. Again.

If necessary, I'll go for plan a.

[Catigale]

THis might be one of those theory vs application gulfs, but given the magnetic field is the cross product of the current and radial vector, for a wire flowing current the mag field has to drop to 0 axially..(the radial and current vectors become colinear.)


Could you possibly re-orient the wires so that the wires closest to the compass point at it while they run away for the most part? I guess right near the fuse box you are SOL since the wires have to bend and twist to make the connections...

Ive never noticed deflection on mine, but I did install a second circuit board aft of galley for my large current items; that board is about 5x farther away from the compass than the old one.



ANyone care to comment

[James V]

Thanks for the advice. I was going to mount the compass on the pedestal. I had thought that it might be a problem. I really do not like the bulkhead. I have seen one boat where it was mounted under the campainway step, in the cockpit. A little to far down and does get kicked. I was wanting to replace the compainway board with a 2 piece 1/2" plastic. I could then have a second lower one made and put the compass in it. I would have to lock the board into position to keep it from shakeing to much. When not in use it could be stored inside.

Any thoughts?

[craiglaforce]

I put mine on the pedestal. Seemed like a good spot. You can also sight over top of it easily to get bearings and such. I have a ritchie voyager bracket mount and left the hood and light off of mine. If sailing at night, I wear one of those LED forehead lights that lets me see stuff just well enough to be functional. It has red and white LEDS so you can preserve night vision as much as possible. Just 2 little holes to drill and thats it. Works great. I also keep my help covered when not in use with a lawn chair cover and bungee cord. THey are nice looking and pretty cheap at Home depot or lowes.

[Chip Hindes]

I have three "active" devices: GPS, Depth sounder/Multi Data and autopilot, all of which have digital screens which can't be easily read form a distance, and also require me to push buttons. Add in the tachometer and tilt trim indicator, and the real estate on my pedestal is already crowded and far too valuable for a large chunk of it to be taken up by a passive device like the compass. I've considered and rejected the idea of instrument side pods because I don't want to further restrict movement past the pedestal. Others no doubt disagree.

In the bulkhead, I can read it fine from the captain's seat or anywhere in the cockpit. With a pedestal mounted compass, when not facing forward looking over the top of it, you have to do the mental math to determine which way you're going, really. Even so, I'd have mine on the pedestal if there were room for it.

Reading the compass with a flashlight isn't something I want to do, so I'd hook up the lighting regardless of where I put it. And despite having digital direction available on both the GPS and the autopilot screens, I find I use my compass constantly for general direction (i.e., "southeast", rather than 235 degrees) and for tacking angles. An analog compass is better for these. If you put it in the hatchboard and leave the lighting out, you're relegating it to occasional/ emergency use only. If I were content with that I'd skip the mounted compass completely and opt for a good hand bearing unit with battery lighting. In fact, I want one of these
as well; it's on "The List".

Here's a shot of mine which happened to be taken at sunrise. You can even see the red glow of the night light.

[ralphk]

COULDN'T DECIDE WHERE TO PUT IT!

My compass normally snaps to a pair of little bases on the port bulkhead. Unfortunately, I found that this interfered with my one of my favourite leg stretching, beverage sipping positions. Last summer, I mounted an additional pair of base plates to an alternate location, on the vertical surface center of the sliding hatch cover. Though it's a liitle hard to read, I still prefer having the compass inline with centerline of the boat. In the event that the crew goes up and down the companionway a lot, I move the compass back to it's port bulkhead location. When we close up, I remove it along with the fishfinder and store it below.

The P.O. had bought a compass with a surface mounted bracket that can be attached to any vertical or horizontal plane. It took a little surgery and my trusty pop riveter, but found another application for 3M Command Jumbo hooks. Though initially intended as temporary mockup only, after five months of weather exposure, that darn adhesive is holding brilliantly . The little white plastic bases aren't bad looking either.