Any good words on the best way to box (calibrate) the compass? What is the best thing to use for detecting NSEW (GPS?) against which to make adjustments. What tolerance is good enough? If it makes any difference, I just installed a new Danforth compass with both a NS and EW adjustment. It took me quite a while to get it right (if I got it right), using the GPS. I could get the GPS and the compass reliably to within a degree or two, but never on the mark in eight directions.
More than you want to Know
Pre-GPS , you steamed between two known points and marked deviations ( N to S then S to N , etc.) using a peloris , then you made a correction card or table of deviations . I used to help my Grandfather , back in the 1950's, do this on his ship.
The adjusters are for compensating for large metal objects and electrical fields . On an old fashioned compass/binacle , you had what was know as 'navigators balls' ( 2 iron balls P & S of the compass ) .They could be moved P & S to affect the compass card .
A true course could be 80, magnetic 92 and compass course 100 degrees, so your deviation would be 8 degrees form Mag N and 20 degrees from a compass rose on a chart .
BTW, A compass can be off E or W of magnetic N
A point of trivia;
Boxing the Compass , is to name all the points of the compass starting at North and giving all the quarter points back to north, clockwise or counterclockwise . I used to be able to recite it , now I have to write it down.
Swinging the Compass , is to "correct" the compass or create a deviation card for a vessel's compass(es).
A magnetic compass points to magnetic N , a gyro compass points to true N and I would guess that a GPS is giving you true N. Those little magnets in your small boat compass are not capable of correcting to true N , they are to get your error between mag N and your compass reading as small as possible or correct for being too close to the engine or electric panel. Go get a fluxgate compass if you feel the need for more accuracy.
Last edited by Mike Goodwin; 05-30-2005 at 03:52 PM.
Very interesting and thank you for the info. The fluxgate on the autohelm is what needed to be adjusted yesterday. That meant that the Danforth compass I needed to swing first. Which meant that I had to be moving fast to stabilize the GPS reading. So I have one of each type (fluxgate, GPS and regular compass) and getting them to read the same was just the problem I spent a couple hours doing yesterday. Incidentally, we are at +4 degrees magnetic deviation, so I have set everything to magnetic (including the GPS).
Why is a fluxgate more accurate? How does a fluxgate compass that makes it more accurate? That means I should probably have "swung" the autohelm to the GPS and then set the compass to the autohelm. Rigth? Wrong?
Is matching it all up within a degree or two about as well as can be done?
Fluxgate is not supposed to be affected by the motion of the boat. I've seen compass cards just go spinning on a heaving and pitching boat
But, from what I read on the web (Google>fluxgate compass), the Fluxgate compass is particularly sensitive to angular error - which I interpreted to mean that if it is not level, it is particularly prone to error. Which makes me wonder about an autohelm - if they have an automatic leveling device inside.
If your worried a couple of degrees you could always get a casio digital watch compass for $59.00 rather than spending more than that to box it in.
John
Thanks for the suggestion, but I've been told that I have nerves of steel, and that would goof up any wrist compass, wouldn't it? (Bad joke)
Hear, Hear,
Laughs and chuckles I second that.
John
To "Swing" your compass {Unless you've changed it Ariel's have a magnetic one.} anchor fore and aft, then convert your anchor lines to springs by connecting them outside of your stays taking a turn on both winches. Set a magnetic compass (Should be a good one!) on the cabin top centerline. Write down the readings listing both the portable and the boat's. Turn the boat one point (32 degrees) by turning one winch (you may have to release the turn on the other winch) again write down the readings, the difference is the primary deviation; back up to the first reading, both compasses should be the same as the initial reading. Go past your initial reading one point, check the compasses again, write it down, the difference between the two compasses is your deviation. If the difference port to starboard is not the same amount then your compass is probably in need of replacement because of internal wear and a table of deviations would be meaningless. You cannot use GPS to swing your compass since it uses polar stellar coordinates giving an adjusted offset (Thank the US Military) mercator projection which is notmally three meters off (Aprox. 1 NM after traveling 10 NM).
Anyone out there know of a home remedy to clean the compass lens....mine(bulkhead mounted) is a bit foggy and it's not the fluid...i remember reading about it somewhere and never wrote it down....i want to say vinegar can clean the lens, but just don't remember...
pk
This may or may not be a similar problem with a similar solution, but anyone out there know how to make a foggy cover plate for a depth sounder clear again?
Assuming these lenses and plates are acrylic.
there are buffing kits available that you can
probably use with a drill and some cloth wheels.
Even crazed plexi can be made clearer but the
mini cracks, when I did it once, can't be erased.
Try (Kleenmaster) Brallianize first to see if it's just soiled somehow.
Replacement?
Thanks for the advice...yeah i'll try cleaning it first...the solvent you said "brallianize"...not familiar...can i pick that up at a west marine/defender store...also, i have some 40 micron grit buffing sandpaper i might try....
pk
Sorry,
Brillianize. It's only a mild plastics cleaner. Works for lexan too. They say it is anti-static, so it may leave something on the surface, so maybe you wouldn't use it on something you've taken apart and have to caulk back together. Think you'd find it in most hardware stores.
Materials for buffing fog and small scratches you probably have to go to a plastics supply store. Or a glass place that also glazes acrylic.
Make sure that you clean plastic with a non-scratching soft cotton cloth. The fogging on the outside of a 'lens' may come from many cleanings in the past like with papertowels or basin cleaners, or windex or solvents, etc.
Last edited by ebb; 02-20-2006 at 02:36 PM.
Keep your compass covered when not in use and avoid the problem , it is UV degradation .
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