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Old 21-01-2012, 12:51   #1
RDW
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Compass and Autopilot Adjustment

My compass, gps, autopilot and chartplotter are not is aggrement. I have ignored this in the past because it is a relative small difference but in trying to improve my skills I would like to have everything in better agreement.
Can someone outline or refer me to a good source on where to start and in what order and in what way to make adjustments so everything is in agreement? I would assume I would start with my compass and check the deviation. I would assume that I would use a hand held compass or the compass in my binoculars as the true heading in degrees C.
Thanks for any advice.
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Old 21-01-2012, 13:56   #2
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Re: compass and autopilot adjustment

Depending on your setup, your GPS and Chartplotter will most likely use true north for your heading determined by GPS, but that will be the direction the boat's going, and not necessarily the way the boat is "pointing." The compass and the autopilot usually will be compass based to magnetic north and will show the way the boat is pointing, rather than where it's heading. It's important to get your compass calibrated (there's lots of reasons it could be wrong, and it may take a pro to figure it out), but it will not, and should not, show the same as the GPS based devices. Your devices may be different from this setup, but I hope this helps. I probably would forget about the binoculars compass...they are generally not precise enough, although they are very helpful when you're using them to determine how many degrees a sighted object is from another.
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Old 21-01-2012, 15:34   #3
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Re: compass and autopilot adjustment

You can check the magnetic compass by steering on a known transit line and comparing the compass with the transit (bow of boat pointing directly at the transit marks), or, tied up to a charted dock of which the direction of the dock is known (not a steel dock), and boat secured parallel to the dock. You'll have to allow for variation, but you will be able to determine deviation for what ever heading your on.
Do that over a sufficient number of headings and you can produce a deviation curve, and from that decide whether it is worth correcting.
You cannot really compare GPS to a compass, the GPS only provides a filterd course over ground.
Autopilot, I assume you have followed the makers guide to calibrate the flux compass
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Old 21-01-2012, 17:12   #4
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Re: compass and autopilot adjustment

Thanks for your comments and help.
What should I use for the most accurate device from which to judge my instruments?
My thought is probably a good chart and a distant landmark.
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Old 21-01-2012, 17:31   #5
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Re: compass and autopilot adjustment

use known bearings.old school. line up 2 markers with known locations ie church steeple and water tower., or day marker and pier etc... you steer so they line up and check your compass. there is a power station in longg island that has 3 stacks aligned north south great for checking a compass. there are probably lots of local references to sighy your compass against.
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Old 21-01-2012, 18:08   #6
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Re: compass and autopilot adjustment

I apply a slightly different method:

- I let the magnetic (steering) compass be,
- I make sure all electronics are based on the same reference (preferably the fluxgate compass),
- when fluxgate compass available, I will forward the signal to all instruments,
- show GPS based data only on the GPS unit (now often - the plotter),
- if AP and the plotter are the same make, I will link them.

Now the only issue remaining is the magnetic compass often being in slight misalignment with the fluxgate (which I let be, unless the difference is not acceptable and something is OBVIOUSLY wrong with any of the compasses).

A very neat solution then is a fluxgate readout for the driver (which is hardly ever present). However, when the AP control head is at the helm (which is most often the case) then this issue is solved.

One look at the plotter and any difference btw the GPS data and the magnetic data is obvious. Note that today most plotters are at the helm too.

Now the big secret: many owners NEVER go thru the proper AP tuning procedure (as described in AP manuals). This is then probably about the only thing I have to do on boats (other than forwarding the fluxgate info to the network, unless already done) to get all things synched up.

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Old 22-01-2012, 05:32   #7
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Quote:
Originally Posted by RDW
Thanks for your comments and help.
What should I use for the most accurate device from which to judge my instruments?
My thought is probably a good chart and a distant landmark.
RDW
The mag compass with have different deviation at different points of sail. The mag compas is affected by large metallic mass, electronics operating in proximity and internal differences. You cannot "remove" the error. The procedure is basically -

- use known bearings - two lines at least 90 degrees apart. More bearing lines is better but not strictly necessary.
- with normal electronics on and running align with first bearing. Record mag heading
- turn 180 and record mag heading.
- using trim screws remove half of error on first bearing. Check second bearing, repeat if necessary. The idea is to equlaize the error
- do the same on the cross bearing.
- prepare a deviation card for all major and minor headings. Attach the deviaiton card to the compass. The deviation card simply says, "when mag reads 90 we are actually 92. This is recorded on the card as +2 (for 90deg only) - if you did it properly 270 might read -2. That is you equalized a 4 degree east west deviation.

Note - most people dont really do this. The errors are small and not many folks are sailling long distances on mag alone. The boat swings a bit and one probably cant hold accuracy af helming within accuracy of compass anyway. However it is recommended. I swung my compass to equalize error but did not prepare a deviation card.

The second bit of kit to calibrate was the flux compass on the autopilot. Procedures vary. Mine is to get into cal mode and make slow 360s until it decides it is calibrated. It is within a couple degree of my mag compass.

I dont know of any gps that has a calibration. The difficulty with gps is the mode that it is tracking. The gps will not generally tell you where the bow is pointing. It tells you which direction the boat is traveling. A current set will require you to steer up against the current. The mag heading and autopilot heading will be different than gps heading.

The thing is these three devices will never read spot on to each other. The trick s knowing that they are off and why they are off from each other.

In practical terms I use the gps as the truth for bearings - that is if I put the waypoint on a buoy, inlet or dock the mag bearing from the gps is the one I use as the truth. That is the gps heading is the one I initatilly enter in the autopilot in heading mode. After a while I monitor my xte and adjust a/p heading.
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Old 22-01-2012, 06:30   #8
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Re: Compass and Autopilot Adjustment

At work, correcting the ships magnetic compass is one of my favourites tasks.
There is a real sense of achievement to swing the ship, establishing the deviations, working out the co-efficients A, B, C, D, and E, then split B, and correct with magnest and soft iron, and get deviations down to less than 2 degrees on all headings.
Sadly, another dying art in the maritime world
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Old 22-01-2012, 08:17   #9
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Re: Compass and Autopilot Adjustment

RDW, Any GPS derived headings will almost never agree with your compass headings. The GPS headings are affected by set and drift of the boat and all of that information is factored in. Your compass derived headings will show the direction the boat is pointed and not the direction you are moving. So you need to correct the compass if possible, or build a deviation table and keep it at the helm. Be sure all GPS headings are set for magnetic, I know it has been suggested in previous posts, and decide which you will be primarily using for a transit. Not sure why you would need them all reading the same and getting everything to do this under every kind of condition is going to be next to impossible. In over 20 years of extensive cruising it has never been an issue on any of our boats. We are usually more interested in the GPS readings than anything else and the compass is used to point the boat at something and maintain a compass course, even if it is not completely correct. The autopilot is set for the direction we want to travel and engaged. The correctness of the autopilot compass reading has seldom been a factor. But with many autopilots, once it has been calibrated the compass can be adjusted internally. Chuck
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Old 22-01-2012, 08:43   #10
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Re: Compass and Autopilot Adjustment

The first thing I will do is swing my compass and hope like hell it has very little deviation. I do not know if I have the guts to try the corrections but I can learn.
Thanks
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Old 22-01-2012, 09:50   #11
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Quote:
Originally Posted by Waterway Guide
Depending on your setup, your GPS and Chartplotter will most likely use true north for your heading determined by GPS, but that will be the direction the boat's going, and not necessarily the way the boat is "pointing." The compass and the autopilot usually will be compass based to magnetic north and will show the way the boat is pointing, rather than where it's heading. It's important to get your compass calibrated (there's lots of reasons it could be wrong, and it may take a pro to figure it out), but it will not, and should not, show the same as the GPS based devices. Your devices may be different from this setup, but I hope this helps. I probably would forget about the binoculars compass...they are generally not precise enough, although they are very helpful when you're using them to determine how many degrees a sighted object is from another.
Urgh, this is really a bit misleading. Let's slow it down:

Waterway guy is referring to COG - course over ground. Yes, don't compare this to your compass. COG is NOT "heading determined by GPS". It is different data which will deviate from heading as a result of currents, leeway, other factors. Let me say it again - COG is not any kind of heading. To give you a crude example, if your bow is pointed due north, you have a 0 degree heading. If you're motoring in reverse, straight and with no current, leeway, or effect of prop walk, you have a COG of 180 - the direct opposite of your heading. Don't mix them up. They are incomparable.

No, it will not necessarily read true north. It depends on how you set up your system. My system is set to read everything in magnetic, both COG, and also HDG.

Your system does not only give you COG - it should also give you HDG - heading, which is the compass data from your fluxgate compass, unless you have a very simple system where the pilot and plotter don't talk to each other. HDG in magnetic should agree with your steering compass - it is the same data. Just make sure you're reading HDG in magnetic.

If HDG data from your nav system and your steering compass agree, then you should be good to go. If not, then start by swinging your fluxgate. Read the instrucs to see how to do this. On my system, you put it in calibration mode and sail three slow circles.

If that doesn't do the trick, then have your steering compass swung by a pro.

Don't give up until you get agreement within a couple of degrees. It's very important to good navigation for your data to be in harmony.
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Old 22-01-2012, 10:21   #12
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Re: Compass and Autopilot Adjustment

Fixing this can be a real difficult job First the magnetic North and Geographic North are not the same (Variation) and the magnetic North is moving - so much so that recently some airport runways have been relabled. Some think that its movement is accelerating and may be a precurser to the Earth's magnetic field reversing. They think that this may be what the Mayans were predicting for next December. Secondly "swinging" a magnetic compass is a dull task and it produces a Deviation Table that must be manually applied with Variation to your compass course to establish True North. Most modern autopilots will allow you to "swing" the electronic compass and establish true north by running the boat in circles and then some other manuvers. So - the electronic compass if properly positioned on the boat should be your best compass. I recently had a new AP and ploter installed on my power boat and everything worked great until I engaged th engine synchronizer (a solenoid control). The compass immediately went bonkers. Repositiong the fluxgate fixed the problem. As stated above, GPS heading is course made good over the ground, not the direction you are steering so dift, current, lee way and other factors will make this course different than your steered course. Bottom line - your magnetic compass, your flux gate compass, and your GPS diection should never exactly agree.
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Old 22-01-2012, 10:53   #13
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Re: Compass and Autopilot Adjustment

The real lost art is to use the shadow pin on your steering compass and compare it to the calculated azimuth of the sun.

For the modern world, this is a pretty good start How to Adjust Ritchie Magnetic Compass | eHow.com
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Old 22-01-2012, 13:14   #14
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Re: Compass and Autopilot Adjustment

Quote:
Originally Posted by donradcliffe View Post
The real lost art is to use the shadow pin on your steering compass and compare it to the calculated azimuth of the sun.

For the modern world, this is a pretty good start How to Adjust Ritchie Magnetic Compass | eHow.com

Have never seen a shadow pin for a yacht compass, but we still have them for the gyro bearing repeaters and the magnetic compass.
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Old 22-01-2012, 14:37   #15
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Re: Compass and Autopilot Adjustment

The previous owner of my boat bolted an irridium sat phone antenna four inches away from the compass, it has some rather large deviation issues...... the fluxgate for the autopilot was way out, till i removed the fire extinguisher he had placed right next to it!

The autopilot is now within a few degrees of the gps course (we have 3 degrees west variation here, the compass is about 20 degrees off (lots o deviation), bugger trying to make em all read the same.
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