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Discussion Starter #1
My 1991 Catalina 28 has three compasses: on the GPS, the autopilot, and the old school magnetic one in the binnacle. The GPS and the autopilot rarely agree what course we are steering, but they are within a few degrees of each other.

The wildcard is the magnetic compass. Whenever the boat is moving, the compass is swinging back and forth, sometimes 20 or 30 degrees on either side of the actual course. At first, I thought it might be that whoever wired the gps and the autopilot failed to shield them properly; both are mounted on the binnacle and the wiring for them runs next to the compass. But I did a couple of experiments, sailing with both gadgets off, and then off at the breaker panel. No difference with the swinging compass.

Can a compass go “bad”? Any ideas on which gremlin is messing with me?
 

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GPS does not have a compass. It computes the course from the most recent fixes.

An AP will have a fluxgate compass.

I am confused by your terminology. If you don't have a reliable compass... how do you know what heading you are steering?

ALL compasses are not happy with interference of nearby magnets... If you have something magnetic near a compass it will not read properly... and your AP will not steer well.

My C Plathe binnacle compass will move a bit because the card is floating. It takes a bit of watching it to figure out the heading (magnetic direction boat is pointing)

I have a KVH sailcomp which displace digital direction.

Because of current an leeway the course made good will rarely be the same as the heading. In fact... you can see the difference on the plotter which shows both a HEADING line (out to infinity) and a COURSE line (out to infinity). They will be different because of current and leeway.

If you want to STEER by the plotter... turn the boat.... AP or helm until the COURSE line intersects where you want to go to (buoy for example).
 

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Discussion Starter #6
Thank you all. I’m going to take the compass home next week and contact the manufacturer about a rebuild or replacement.
 

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Hey,

Before you take things apart and bring them home I suggest you buy a real cheap toy type compass and put that near the compass mounted in the binnacle. If the cheapie does the same thing then I suggest you have an interference problem. If the cheapie is stable then get the good compass repaired or just buy a new one.

A few years ago it seemed that my compass and autopilot would sometimes just go crazy. Eventually i figured out the problem - I had a soft cooler that had a speaker in it. If the cooler got close enough to the steering pedestal the magnet in the speaker would affect the compass and autopilot!

Barry
 

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As someone noted above, your GPS may not have an actual compass (but some do have an internal fluxgate compass). Most GPS units calculate heading from a series of locations over time. So to get a reasonably accurate heading from your GPS you need to be moving at a reasonable speed and in a straight line. Also, most GPS units can give you either true or magnetic heading. The difference between true and magnetic heading is called variation. Variation results from the fact that Earth's magnetic north pole is not located at the geographical north pole.

The fluxgate compass on your autopilot should be correct, assuming it is aligned properly and that it has been calibrated. It is probably set to display your heading in true degrees.

Magnetic compasses can only give you the magnetic heading (plus deviation). All magnetic compasses are subject to the earth's magnetic variation and to the magnetic deviation that is unique to the particular location of the compass on your boat (ferrous metal, such as your engine, and electrical wiring cause deviation). You can make a deviation table by swinging your boat in an arc, lining up landmarks and using a chart to find the actual magnetic headings to those landmarks. The difference between your compass heading and the magnetic heading you get from your chart is the deviation. Deviation varies depending on the direction in which your boat is facing (as your boat turns in an arc, the location of your engine and other ferrous objects relative to magnetic north changes). Therefore, you need to determine deviation for at least the 4 cardinal directions.

I bet if you make a deviation table and you convert all of your headings into a common currency (e.g., degrees true), you will find that all three of your compasses are in pretty close agreement. There will still be some small differences because your autopilot and magnetic compass tells you the heading in which your boat is pointing and the GPS gives you the heading in which your boat is actually moving.

Unless the compass dome is crazed or there is an air bubble inside the dome (indicating that oil has been lost), I wouldn't bother removing it from the boat for servicing.
 

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Discussion Starter #11
As someone noted above, your GPS may not have an actual compass (but some do have an internal fluxgate compass). Most GPS units calculate heading from a series of locations over time. So to get a reasonably accurate heading from your GPS you need to be moving at a reasonable speed and in a straight line. Also, most GPS units can give you either true or magnetic heading. The difference between true and magnetic heading is called variation. Variation results from the fact that Earth's magnetic north pole is not located at the geographical north pole.

The fluxgate compass on your autopilot should be correct, assuming it is aligned properly and that it has been calibrated. It is probably set to display your heading in true degrees.

Magnetic compasses can only give you the magnetic heading (plus deviation). All magnetic compasses are subject to the earth's magnetic variation and to the magnetic deviation that is unique to the particular location of the compass on your boat (ferrous metal, such as your engine, and electrical wiring cause deviation). You can make a deviation table by swinging your boat in an arc, lining up landmarks and using a chart to find the actual magnetic headings to those landmarks. The difference between your compass heading and the magnetic heading you get from your chart is the deviation. Deviation varies depending on the direction in which your boat is facing (as your boat turns in an arc, the location of your engine and other ferrous objects relative to magnetic north changes). Therefore, you need to determine deviation for at least the 4 cardinal directions.

I bet if you make a deviation table and you convert all of your headings into a common currency (e.g., degrees true), you will find that all three of your compasses are in pretty close agreement. There will still be some small differences because your autopilot and magnetic compass tells you the heading in which your boat is pointing and the GPS gives you the heading in which your boat is actually moving.

Unless the compass dome is crazed or there is an air bubble inside the dome (indicating that oil has been lost), I wouldn't bother removing it from the boat for servicing.

The problem isn't that the three amigos disagree on headings or courses; my comment about the GPS and the Autopilot not being completely in agreement was more of a throwaway comment than a complaint. And I fully understand the difference between headings and courses, as well as how compass deviation works. My problem is that my magnetic compass swings wildly as I steer a straight course. I don't think it's interference, but I like Barry's idea about seeing if a second compass has the same problem before I start disassembling things.

Again, thanks to all.
 

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If another compass were to behave properly I don't think it would prove anything necessarily. If it were me, I would remove the compass and hold it in other locations.
 

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Whenever the boat is moving, the compass is swinging back and forth, sometimes 20 or 30 degrees on either side of the actual course.
mstern seems to be not saying that his compass is incorrect; I think his complaint is about its wide swings and slow recovery. One theory about why he'd get a wide swing is that the magnetic field in the compass pointer is weak; that would mean there is a very feeble signal available to bring it back to truth after a disturbance.

I do know that there are ways of maltreating a horseshoe compass (say) that cause it to weaken; what I don't know is whether straight-line encased compass pointers actually do weaken with time or abuse. Does anyone have enough experience with this to be able to say?
 

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Discussion Starter #19
Has it always been like that, or is this a new phenomenon? If new, did it come on gradually or suddenly?
The boat is a 1991; I bought it a year ago, and have used it just this past one season. I don't know if the PO had the same problem, but the compass has been like this since I splashed the boat in May.
 

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IT sounds to me as though you have some device that produces a magnetic field that is turning on and off in the neighbourhood of your compass. I can't speculate what it could be, but I'd be looking toward the displays mounted near the compass, or in a locker nearby. I know you said you'd shut down all of them, but there may be some other unit in the area you may not know is there, like an old auto pilot compass or drive motor? If you have a wheel pilot it could be a source of intermittent power.
Anywhere else on the planet it's called electricity, but aboard a vessel of any kind, I call it electrickery. Good luck.
 
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