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Old 08-07-2014, 16:13   #16
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Re: Autopilot Calibration

OK. Our compass manual calls the turning the circle thing "compass calibration", which it says compensates for magnetic interference and any fixed offset in the heading readout.

It then allows one to manually adjust the compass for "mounting offset" - which is the difference between where it is mounted and the lubber line through the center of the boat.

I guess it is this last one you are referring to as calibration.

Mark
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Old 08-07-2014, 16:39   #17
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Re: Autopilot Calibration

Quote:
Originally Posted by colemj View Post
OK. Our compass manual calls the turning the circle thing "compass calibration", which it says compensates for magnetic interference and any fixed offset in the heading readout.

It then allows one to manually adjust the compass for "mounting offset" - which is the difference between where it is mounted and the lubber line through the center of the boat.

I guess it is this last one you are referring to as calibration.

Mark
It all falls in the category of 'calibration'.

2 procedures:
1) Linearise (or 'swing the compass')
2) Align

for #1, RM manual says:
Quote:
The magnetic deviation correction procedure (commonly called “swinging the compass”) involves turning your boat in slow circles so the autopilot can automatically determine the deviation and apply any correction required. This procedure reduces deviation errors to a few degrees.
#2 is done using the GPS, then manually offsetting for leeway
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Old 08-07-2014, 17:48   #18
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Re: Autopilot Calibration

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Originally Posted by DotDun View Post
It all falls in the category of 'calibration'.



2 procedures:

1) Linearise (or 'swing the compass')

2) Align



for #1, RM manual says:





#2 is done using the GPS, then manually offsetting for leeway

I wasn't aware #2 used GPS. Direction using GPS is notoriously unreliable

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Old 08-07-2014, 18:00   #19
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Re: Autopilot Calibration

Our manual says to "steer toward a mark with a known bearing from the boat and adjust the mounting offset to agree with the compass".

I don't know how one would easily get their boat into a position to have a known bearing to a mark (lighthouse with charted bearing lines maybe?), and then after doing so would simply adjust the offset to agree with the compass.

Seems to me you can do that last part just sitting at anchor…

Mark
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Old 08-07-2014, 18:16   #20
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Re: Autopilot Calibration

Quote:
Originally Posted by colemj View Post
I don't know how one would easily get their boat into a position to have a known bearing to a mark (lighthouse with charted bearing lines maybe?), and then after doing so would simply adjust the offset to agree with the compass.


The only surefire way I've found of doing this is to find a bay or inlet with a chartable transit between two points (light house on one side of the bay, church steeple on the other, or a fixed marker in the bay) then run the boat between the two hoping for little to no cross-current or wind.
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Old 08-07-2014, 18:17   #21
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Re: Autopilot Calibration

Quote:
Originally Posted by colemj View Post
Our manual says to "steer toward a mark with a known bearing from the boat and adjust the mounting offset to agree with the compass".

I don't know how one would easily get their boat into a position to have a known bearing to a mark (lighthouse with charted bearing lines maybe?), and then after doing so would simply adjust the offset to agree with the compass.

Seems to me you can do that last part just sitting at anchor…

Mark
That won't work because you need a calibrated compass to know you are on the correct radial.

To get a known bearing you basically have to have 2 fixed and charted points - say a light on a rock and a tower onshore. Draw a line between and transfer the radial from the compass rose on the chart to get magnetic bearing.

Then you line both items up - one behind the other - making sure the bow is also point along the radial.

The "wet" compass suffers from a few laws of physics, acceleration and dipping errors plus installation effects.

Cheap compasses have no adjustments, mid range compasses may have adjustment screws to balance the deviation among 4 cardinals.

Super fancy compasses may be gymballed to attempt to offset dipping errors.

For the average boater creating a compass deviation card my not be worth the effort as few will steer accurately enough (particularly with small compass cards) to hold a +-3 degree heading.

Installation effects can have the biggest impact - installing a compass near electronics or large metal objects can significantly impact the accuracy.

I have swung my compass and adjusted it on N-S/E-W. It rarely matches the GPS or autopilot heading (within say 5 degrees) while the A/P and GPS track each other pretty well.
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Old 08-07-2014, 19:12   #22
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Re: Autopilot Calibration

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Originally Posted by goboatingnow View Post
I wasn't aware #2 used GPS. Direction using GPS is notoriously unreliable

Dave


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Correct, I doubt the fluxgate, wet compass, and gps will ever agree.

to finish, from the RM manual:

Quote:
Aligning compass heading

Once the deviation is displayed, press disp to move to the Align Heading
(ALIGN HDG) page, then:
1. Manually steer the boat on a steady course at a speed sufficient to hold the
course.
2. If you have a GPS connected to your SPX system:
• Increase the boat speed to more than 3 knots.
• Press auto. The SPX system will then set the heading to agree with the COG
(course over ground) heading received from the GPS.
As many factors can cause a difference between heading and COG, you must fine tune the heading alignment to match the boat’s steering compass (or a known transit bearing).
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Old 08-07-2014, 20:49   #23
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Re: Autopilot Calibration

Worrying about the a/p compass being accurate and agreeing with any other compass is pretty much a waste of time, at least in practical value. I doubt if many steer their boats by simply setting a course on the a/p and tuning out. The common and useful practice is to set a course and then note COG or XTE as you proceed, making adjustments to the a/p to correct errors. In most situations you simply don't have enough data about currents and leeway to trust even a perfect compass to get you to your destination without some course correction along the way.

Of course, if you are crossing the English Channel and wanting to do some sort of "course to steer" correction, much beloved of some on this forum, then absolute compass accuracy may help. Fortunately, most of us don't have that singular problem to deal with, but rather have little or no current info (and quite possibly poor charts too), and ad hoc course adjustment is a necessity of life.

We have two magnetic compasses in the cockpit as well as the readouts from three a/p systems. I have spent some time swinging and auto-correcting, but I doubt if three out of five have ever agreed within +/- 2 degrees. Actually, it is damn hard to tell, for they all have different time constants in their responses, and differing reactions to pitch and roll, so it's several moving targets. Yet, we nearly always get where we are going... so far!

Cheers,

Jim
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Old 08-07-2014, 21:06   #24
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Re: Autopilot Calibration

Quote:
We have two magnetic compasses in the cockpit as well as the readouts from three a/p systems. I have spent some time swinging and auto-correcting, but I doubt if three out of five have ever agreed within +/- 2 degrees.
Why three ap?
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Old 08-07-2014, 21:40   #25
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Re: Autopilot Calibration

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Originally Posted by Flying Knight View Post
Why three ap?
One is a tiller pilot (ST4000). Very low power consumption, not much grunt. Only good for light conditions.

One is an Auto-Nav, installed by the PO in Canada. Neither of us could ever get it to stop hunting around the desired course. The company has gone under and there is no product support. It does steer the boat, albeit poorly, so I kept it as a backup.

The last, and most used one, is a NavMan 3100. Works and steers well, but like all things electronic, could go tits up at any time... hence the backups!

Both of the larger pilots work a Whitlock radial drive of vast power and equally vast electrical appetite. It is sized for a largish fishing trawler and is way overkill for our boat. I think the PO (who built the boat) got a deal on it. I'd love to trade for one of their smaller units...

Anyhow, that's why three!

Jim
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Old 08-07-2014, 22:03   #26
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Re: Autopilot Calibration

Quote:
Originally Posted by Jim Cate View Post
One is a tiller pilot (ST4000). Very low power consumption, not much grunt. Only good for light conditions.

One is an Auto-Nav, installed by the PO in Canada. Neither of us could ever get it to stop hunting around the desired course. The company has gone under and there is no product support. It does steer the boat, albeit poorly, so I kept it as a backup.

The last, and most used one, is a NavMan 3100. Works and steers well, but like all things electronic, could go tits up at any time... hence the backups!

Both of the larger pilots work a Whitlock radial drive of vast power and equally vast electrical appetite. It is sized for a largish fishing trawler and is way overkill for our boat. I think the PO (who built the boat) got a deal on it. I'd love to trade for one of their smaller units...

Anyhow, that's why three!

Jim
Well I guess that makes sense. At least you have an explanation.
Keeping three ap on board seems as silly as obsessing over a few degrees deviation.
Who says you can't have your cake and three aps? Lol!

My ap is quite a bit more than a few degrees off, however, hence my question. I may be inclined to agree with you and the other who posted the same thing you did, but then again I may not. Then again once I've finally got three autopilots on board I may be singing a different tune.
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Old 08-07-2014, 22:53   #27
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Re: Autopilot Calibration

Flyin' K,

I certainly think you should go through the calibration procedure for your pilot, for it does more than just align its readout with the ships compass. But all this talk about needing to have super accurate transits and that aligning with a GPS not being good enough... I don't think it is very important.

YMMV.

And a final comment about the multiple a/ps: Ann and I do do ocean passages, just the two of us. Sadly, this boat doesn't have a wind vane steerer, so we rely upon the A/p to keep us from watch on watch hand steering. Been there and done that, and even when we were many years younger it was a bitch. Quite important to us to keep some form of auto steering going, and while I have used sheet to tiller affairs successfully in the past, this boat isn't a good candidate for that method. So, multi pilots seem a good hedge. If I were richer I would ditch the AutoNav and the Navman and install two identical systems (specific type unknown). Spares and the possibility of cannibalization would be easier then.

So, despite the appearance, I don't think our array of pilots is silly!

Cheers,

Jim
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Old 08-07-2014, 23:05   #28
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Re: Autopilot Calibration

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Originally Posted by Jim Cate View Post
But all this talk about needing to have super accurate transits and that aligning with a GPS not being good enough... I don't think it is very important.
I don't think that was anyone's point, but I see what you're saying. I don't think there's any reason to come out against it though.

Quote:
So, despite the appearance, I don't think our array of pilots is silly!
Oh, I'm sure they're not silly, except when you have all three on at the same time.
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Old 09-07-2014, 03:16   #29
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Re: Autopilot Calibration

personally , since i don't slave my AP to a "route", I don't really care what compass the AP is displaying, I usually deduce the required bearing, and point the point the boat and hit auto, at that point the AP display could be pointing to timbuktu for all I care. I then plot and chart my progress as appropriate.

If I am on a slaved system, at most I use the "drop waypoint ahead" method


once sailed across the altantic , with the AP compass out by 90 degrees to the main compass, just required an additional math step in ones minds thats all.

Dave
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Old 09-07-2014, 05:45   #30
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Originally Posted by goboatingnow View Post
personally , since i don't slave my AP to a "route", I don't really care what compass the AP is displaying, I usually deduce the required bearing, and point the point the boat and hit auto, at that point the AP display could be pointing to timbuktu for all I care. I then plot and chart my progress as appropriate.

If I am on a slaved system, at most I use the "drop waypoint ahead" method


once sailed across the altantic , with the AP compass out by 90 degrees to the main compass, just required an additional math step in ones minds thats all.

Dave
Well you know a heck of a lot about calibrating the autopilot for someone who doesn't use or believe in them... You went to pretty great lengths to try and prove colemj wrong about the meaning of the word calibrate for someone who doesn't even calibrate their equipment.

That's impressive though. Was the additional step addition or subtraction?
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