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Old 09-05-2011, 18:16   #16
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Re: What heading am I really on?

As Cchelsea said, variation and deviation. Aside from the inate differences between the electronic compasses and the magnetic one, every skipper should have a deviation card for the boats compass, which should be included in traditional navigation and its likely that you havent included Variation in the equasion.

But even with those figures included its unlikely all compasses will match up exactly as each will have a different response time etc
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Old 09-05-2011, 18:16   #17
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Re: What heading am I really on?

Something not yet mentioned is angle of attack. That is what allows a symetrical foil(keel) to generate lift at a heading other than DDW. Dave
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Old 09-05-2011, 18:17   #18
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Re: What heading am I really on?

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Originally Posted by Don Lucas View Post
Yes, have you? I'll let you read up on which term has nothing to do with reading the various instruments and which has to do with a chart!
ROTFLMAO......Don , I have often wondered the same thing. We have four different gps, etc. on our fishing boat and each one is off by 20 deg. or more, I usually just go off the compass the auto pilot runs off.
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Old 09-05-2011, 18:29   #19
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Re: What heading am I really on?

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I have wondered this in the past; but the other day according to my different instruments I was on 3 different headings. My autopilot was set to 135 my GPS said I was COG of 145 and my boat compass was around 160-170!

I'm more inclined to believe the GPS is giving me the real course as it is taking into account drift etc. But I've never really understood why the boats compass (on 2 different boats now) always is so much different that the autopilot.
As you point out, the GPS COG is affected by drift and leeway, while the magnetic compass shows the boats heading. Nonetheless, 15-20 degrees difference is a lot, unless you were pinching or moving slowly across strong current. You can check your compass by setting a distant landmark (e.g., a lighthouse) as a waypoint in your gps, and steer directly for the landmark. Compare the magnetic compass reading with the GPS bearing to the waypoint. You should do this on a calm day with minimal current and wind. You can also use the twin lighthouses on Thatcher Island as a convenient range--the bearing from the southern tower to the northern one is 14 degrees magnetic. If you don't get a reading within a few degrees of the GPS or range bearing, you should check what is causing the deviation and swing the compass.
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Old 09-05-2011, 18:32   #20
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Re: What heading am I really on?

Don, my comment was unnecessarily caustic but in the electronic world there are actually some who might not know about such things. I apologize. However, your description of the differences are quite easily understood if the autopilot is set to read out in True, the boat is being set 10 degrees and you have significant Variation in your area along with a touch of Deviation on that magnetic compass.

The challenge is in determining where the error, if any, lies. You've had many good suggestions to follow up on. Mounting, correcting, and settings. Hopefully, you'll track down the error if there really is one.

(Btw, your response to me still makes me wonder if you fully understand but as a former ship's captain I'll just say they both affect what your instruments read. Now I'll step quietly away)
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Old 09-05-2011, 19:27   #21
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Re: What Heading Am I Really On ?

Magnetic Heading = Compass Heading plus or minus Deviation

Course Over Ground = Magnetic Heading plus or minus Leeway plus or minus Current

Note that since we're dealing only with magnetic headings here, Variation does not apply (it's only used to compute True headings).

The GPS computes Course Over Ground directly. However, as several have noted, it averages the course over a certain time period, usually adjustable, and may include certain errors.

To compare a Compass Heading with a GPS Course Over Ground readout, you'd need to convert the Compass Heading to Magnetic Heading then to Course Over Ground, as indicated in the first two lines above.

Autopilot course headings on small boats are notoriously inaccurate, sometimes because of the fluxgate compass installation and quality, but most often because they have not been calibrated and corrected properly.

Ain't this fun? :-)

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Old 10-05-2011, 03:41   #22
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Re: What Heading Am I Really On ?

If your autopilot heading was reading magnetic and the fluxgate compass has been properly aligned to the boat, then that is probably the most accurate HEADING readout you will get.

The gps will probably give you the most accurate COURSE.

The binacle compass will also give you HEADING if you have made a compass deviation card beforehand and apply that deviation.........but the fluxgate is more likely.to be more accurate.

It's normal for all three to have different readouts if there is any current or leeway.

But a 30deg plus difference between fluxgate and binacle compasses would suggest that one of them is way off. My guess would be the binacle.
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Old 10-05-2011, 03:43   #23
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Re: What Heading Am I Really On ?

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Originally Posted by btrayfors View Post
Autopilot course headings on small boats are notoriously inaccurate, sometimes because of the fluxgate compass installation and quality, but most often because they have not been calibrated and corrected properly.

Ain't this fun? :-)

Bill
Or someone sits next to the autopilot with a mobile phone in their pocket. That caused an interesting few minutes until we worked out what was going on and why we were tacking all over the place.


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Old 10-05-2011, 03:55   #24
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Re: What Heading Am I Really On ?

I2F and Ziggy have given some simple, practical ways to work out which heading is more accurate.
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Old 10-05-2011, 04:23   #25
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Re: What Heading Am I Really On ?

Hi Don,

Best way to check the compass is to find an accurate navigation transit line, put the boat on the line and have the bow pointing directly along the transit, then note and compare the magnetic and fluxgate compass headings. A less accurate method is to park the boat along a pier, note the run of the pier from chart, get boat parallel to pier and check heading (obviously a wooden pier )
I just recently calibrated my fluxgate compass and set it to True, having set the heading on a transit, and then swung the boat to calibrate the compass. It made a huge difference, as far as I can tell, the fluxgate gives a proper true reading on all headings, and the difference between the fluxgate and the mag. compass is equal to variation
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Old 10-05-2011, 04:27   #26
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Re: What Heading Am I Really On ?

If you go out at night, the leading star in Orion's belt (Mintaka) always rises due East and sets due West (within 0.3 degrees, so better than I can hold a course). Useful for checking compass headings
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Old 10-05-2011, 06:14   #27
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Re: What Heading Am I Really On ?

You're on a magnetic, true and drift corrected heading... all at the same time..... If you're navigating on your GPS then the heading on the GPS is all that matters.... If you're using the charts you need to correct for deviation and use the compass... your auto pilot is correctable.. theres a sub menu that lets you match your auto pilot heading with the compass or true heading.
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Old 10-05-2011, 06:19   #28
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Re: What Heading Am I Really On ?

Am I missing something here? Surely you need to trust your ships compass first.Run your courses to determine if any deviation on various headings,correct accordingly and set autopilot to run per ships compass (psc).I don't care what the fluxgate says or doesn't say just as long as it follows the necessary course psc.
Over time your gps should be tellimg you if all is working properly. Once sailed a steel boat offshore N.E. to Fla. with broken compass. what's to know?Go left (sunrise) Africa ,go right(sunset) America! Any clanging sound ment land was near.
Ah youth!
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Old 10-05-2011, 06:55   #29
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Re: What Heading Am I Really On ?

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Am I missing something here? Surely you need to trust your ships compass first. Run your courses to determine if any deviation on various headings,correct accordingly and set autopilot to run per ships compass (psc). I don't care what the fluxgate says or doesn't say just as long as it follows the necessary course psc.
Over time your gps should be tellimg you if all is working properly. ....
Exactly!

This thread reminds me of the famous Churchill saying, "You can always trust the Americans. In the end they will do the right thing, after they have eliminated all the other possibilities. ~ Winston Churchill."

Someone on this Board always comes up with the right answer, after....
:-)

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Old 10-05-2011, 07:28   #30
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Re: What Heading Am I Really On ?

Bill,

Quote:
Originally Posted by btrayfors;683443Note that since we're dealing only with [B
magnetic[/B] headings here, Variation does not apply (it's only used to compute True headings).

The GPS computes Course Over Ground directly. However, as several have noted, it averages the course over a certain time period, usually adjustable, and may include certain errors.
GPS will compute COG in true. Some have an internal variation table to adjust to magnetic. So even if the user doesn't see the variation adjustment it is happening and it would be wise to understand the implications.

I run everything displayed on my boat in true except--of course--the binnacle compass. I'm a cranky old-fashioned sailor as you know *grin* and since all the old maneuvering board work was done in true I try to stay as close that as I can.
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