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post #1 of 36 Old 3 Weeks Ago Thread Starter
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Compass woes

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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post #2 of 36 Old 3 Weeks Ago
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Re: Compass woes

1st thought is the fluid
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Re: Compass woes

No way to know what is 'bedeviling' your compass... (Oct 31 humor attempt), but given the age of the boat I suggest it's time for a rebuild. In our situation the rebuild was cheaper than a replacement, by half.
Link: https://www.ericsonyachts.org/infoex...ss-Restoration

May the Force be with you, and your Deviation be minimal!

1988 Olson 34, by Ericson Yachts.
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Betamarine 25 (new 2018)
RCYC, Portland, Oregon
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Re: Compass woes

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).

pay attention... someone's life depends on it
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Re: Compass woes

The description lead me to think dampening prob
50 years ago we had cheap hand compasses...sometimes hard to settle down.
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Re: Compass woes

Thank you all. Im going to take the compass home next week and contact the manufacturer about a rebuild or replacement.
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Re: Compass woes

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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Deep Blue C, 2002 C&C 110
Mt. Sinai, NY

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Re: Compass woes

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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Last edited by Damon Gannon; 3 Weeks Ago at 06:42 PM.
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Re: Compass woes

A bit off topic but I have to ask. Why would a cooler have a speaker?
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Re: Compass woes

Good question.

It had an AUX cable so you could play music from your phone, ipod, mp3 player (this was a few years ago before bluetooth).


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Originally Posted by paulinnanaimo View Post
A bit off topic but I have to ask. Why would a cooler have a speaker?

Barry Lenoble
Deep Blue C, 2002 C&C 110
Mt. Sinai, NY

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