Course heading disagreement

[Submariner]

We have an 2021 RT-27 OB.

I’m moored on the dock. The compass reads 256 degrees. The autopilot reads 229 degrees.

Who do I believe? And what do I adjust to get them into agreement? Or, is it normal to have a 25 degree disagreement?


Sent from my iPhone using Tapatalk

 

[CaspersCruiser]

It is not normal for there to be a large discrepancy between magnetic compass and autopilot heading. Max should 10°or so. Use the compass app on your iPhone to break the tie. Then find out why the device with the incorrect heading is that way.

If it’s the magnetic compass that’s off, check for items nearby that may pull it off the correct heading— a phone, flashlight, tool, etc. Check to make sure the compass is aligned with the centerline of the boat. Your boat is new. The factory may have screwed that up. Lastly, the compass may need to be compensated. I actually don’t know if a boat compass has that adjustment capability. I suspect it does. Airplane compasses do.

If it’s the autopilot heading that’s off, just as with the magnetic compass, look for something placed near the autopilot heading sensor that may cause the error. When the autopilot heading on my boat suddenly became 30° off, the culprit was a magnetic tip screwdriver that had been left near the heading sensor when working on something else.

If there’s nothing near the autopilot heading sensor, then the heading sensor may need calibration. Your autopilot is a couple generations newer than the one on my 2012 R27, but it may have a utility similar to the Dockside and Sea Trial “Wizards” on my Garmin autopilot. The first function of the Sea Trial Wizard is compass calibration. When I first took delivery of my boat from the previous owner, the autopilot heading was WAY off. Calibrating the autopilot heading sensor via the Sea Trial Wizard made it spot-on.

Good luck!

 

[marcham]

Find a known line of position, such as a track between two markers or follow a pair of daymarks. Adjust the true tracks for magnetic variation and then you'll known which of your compasses is off. For best results, test them in all 4 cardinal directions. Then read the respective setup and calibration instruction for each compass source.

Is the dock made mostly out of metal? Do you have the same discrepancy when 30ft away?

Sent from my SM-G970F using Tapatalk

 

[knotflying]

There are quite a few recommendations here. However there are several factors that will affect a liquid compass. First, there is the difference between True North and Magnetic North. “True north” is the northern axis of rotation of the Earth. It is the point where the lines of longitude converge on maps. “Magnetic north” is the point on the Earth's surface where its magnetic field points directly downwards. ... The difference between Magnetic North and True North is called “Declination” or “Variation”. I seem to remember that there is a setting on the chartlotter for either True or Magnetic, but I can be wrong so check out how the chartplotter is set up. Variation changes depending where you are in the world. Where I am, in the northeast, it is a 15 degree difference. So if your Chartplotter is set differently from your liquid compass you will see a difference. Now, assuming there are no items around your liquid compass that will affect it, all compasses have errors. To correct for this one has to "swing" the compass. There are adjustments in the compass to do this. I will not get into how to do that here. You will never get a liquid compass to be exact and this is called deviation. You always have to add or subtract to get the "real" heading. Since you are on a mooring both chartplotter and compass should be close, but when moving your chartplotter will show a reading based on your course not what direction your bow is facing. Sooooo....... With all this technical mumbo-jumbo here is ,IMO, reality. You are not using a chart and a Sextant to navigate. Any error in you compass will be insignificant. You are relying on your chartplotter to get you to where you are going. If that should fail I am sure you have backup on your phone or Ipad. You are not flying a plane where the liquid compass may become critical.
I would make sure my heading sensor for the chartplotter is properly set and you did the setup wizard. After that enjoy cruising and live with the dependency between the compass and chartplotter.

 

[FWTMD]

Knot gave the declination for the PNW at 15 degrees. To give a sense of scale to how that differs by location, its about -11 degrees on the Chesapeake. Declination changes over time because Magnetic North moves.

https://www.ngdc.noaa.gov/geomag/calcul ... eclination

I am an old-fashioned compass and paper chart navigator. I have gray hair. When I started boating folks just didn't have electronic navigation and auto-pilot. So understand I have some ingrained distrust of the gadget approaches.

When setting a route with waypoints, how do you KNOW its going to steer you clear of trouble? The given accuracy on these things is 10 feet. More than enough in open water. In close quarters? When approaching known rocks or markers? How close is your plotter taking you to turn points?

That 10 feet is on a perfectly calibrated and perfectly functioning system.

Yet, pay attention to double checking it with radar overlay. Begin at the 13:00 mark

https://www.youtube.com/watch?v=yraYqKlfv5Y&t=50s

If your radar is telling you a mark's location or the shoreline is in a different spot than what the Garmin chart is telling you, then you have a calibration error somewhere in your system. Over-reliance on the electronics can then give you a false sense of security while it either auto-pilots you into trouble or guides you to manually steer it over submerged trouble.

If you want to trust it, verify it. Understand how much potential location error needs to be accounted for.

Back in the day with paper charts, you begin knowing there is an element of guesswork. Electronics, not so much.

"It ain't what you don't know that hurts you, its what you think you know that happens to be wrong."

 

[FWTMD]

The Garmin GPS location antenna sells for $200

Simrad sells various models of them, in a range from $450 to $2600.

I have wondered why.

Forgetting about brand differences, I wondered why someone with a Simrad system would pay $2600 when a $450 option exists. There has to be a good reason. One day I'll find a good electronics expert to answer it.

 

[CaspersCruiser]

A lot of terms are getting mixed in here. The original poster said his autopilot heading differs from that displayed by the magnetic compass. I’m on my boat now, tied up in my slip and I have powered up my navigation system. The autopilot heading says 160° magnetic. The compass says 164° and, of course, that is magnetic. Close enough for me in the type of waters I cruise.

One poster mentioned the chartplotter. My Garmin 5212 chartplotter displays magnetic course made good, unless for some reason it is set up to display true course. Garmin confuses the issue by labeling the magnetic course made good on the 5212 as “GPS Heading”. It’s NOT a heading. It is a course made good or, more accurately, a track. In later model chartplotters, like the one on the original poster’s boat, Garmin fixed that annoying misnomer and changed it to (I think) “COG” or course over ground.

The heading of a boat is influenced by wind and current and the resulting track is the course made good. For example, crossing a river, your boat’s heading may be 090°, but a strong current may have the boat tracking a course of 080°and that value is what is displayed on the chartplotter. Do not confuse track with heading.

Magnetic variation was also mentioned. The only time magnetic variation comes into play is if you plot a course on a chart and you want to navigate over that course using your boat’s magnetic heading instruments. In that case, magnetic variation must be applied as well as compass deviation. If one is comparing the autopilot compass magnetic heading display with the magnetic compass, magnetic variation is irrelevant.

 

[FWTMD]

Correct! In trying to provide clarity, more confusion was created! I'll own that. 🙂

I think the simplest things to say are:
1) There is a difference between True North, and Magnetic North. Know which one you are looking at, especially so that when you look at the other there is no confusion.
2) Know the declination in your area. That's what the difference is supposed to be between something showing True vs Magnetic. Its easy math to make the adjustment.
3) Either here or some other site, I can't recall, someone's chart plotter was way off. Something like 20 degrees or more. No settings adjustments helped. It turned out to be something metal, like a thermos coffee cup, sitting next to the plotter. It was throwing everything off somehow. The good news is the effect was so big as to be obvious.

I'll stick with the point that its worthwhile to verify that your system is showing you a position that is accurate.

 

[Submariner]

I downloaded a compass app on my phone as suggested. The compass is correct. The auto pilot is what’s off. Nothing is near the autopilot to interfere with it.

I verified that that the autopilot was set for magnetic north. I toggled it between true north and magnetic to see the difference. It did change. But is still not close.

I’m at anchor currently, and the heading on the autopilot is still about 10-25 degrees less. The compass also appears more stable in reading while the autopilot heading fluctuates a lot. Sometimes it’s within 5 degrees, other times they are 25 degrees off.

I’ll dig out the instruction for how to calibrate the autopilot heading. Maybe the autopilot display just doesn’t update as quick as the compass does. (We are swinging a bit on the hook, we are just anchored while crabbing, 3:1 scope).


Sent from my iPhone using Tapatalk

 

[FWTMD]

Logical conclusion:

If you are confident there is no interference, then something simply isn't working correctly. Whether it is set to True or Magnetic, it should not swing between an error of 5 to 20 degrees. Its not a setting.

Two options: 1) I see you have a brand new boat. Turn the problem over to the dealer to fix. 2) Get behind it and check all electrical connections. Snug? Others report connections being merely finger tight, or loose. Check cable connections.
That's worth doing anyway, because a loose connection creates heat which is dangerous. I would think a loose connection would create the intermitent error you are seeing.

 

[CaspersCruiser]

>> Maybe the autopilot display just doesn’t update as quick as the compass does.<<

The autopilot display should be as stable as the boat heading and update with every degree of heading change. The magnetic compass, not so much. Since your boat is new, it’s a near certainty that your problems will be solved by calibrating your autopilot compass. I helped a local guy with a new R27 a couple years ago with the same exact problem. He had newer electronics, but it was a fairly easy process to calibrate his autopilot compass. The process involves getting into the “Sea Trial Wizard” and just follow the prompts. You’ll make two or three 360° turns at a slow rate until the Wizard says stop. Then a straight line course is made to find north. You’ll need some smooth, open water to accomplish this.

Concerning making turns with the magnetic compass, be aware that it has inherent errors. I don’t want to start a helmet fire for you, but when using the magnetic compass and turning away from a northerly heading, the compass will initially indicate a turn in the opposite direction! I want to be sure that you understand I’m talking about the magnetic compass floating in liquid at your helm and not the autopilot compass. The autopilot compass washes out all the turning and acceleration errors.

For those interested in learning about magnetic compass errors, here’s a good resource. It relates to compasses installed in aircraft, but the principles apply to boats:

https://www.boldmethod.com/blog/learn-t ... ass-works/

 

[Submariner]

Thank you all so much for the info! I’m not looking to navigate by compass, necessarily.

This pic from my chart plotter yesterday is actually what got me looking at the auto pilot heading. Most of the time it looks like I’m driving sideways. The boat being brand new, I’m still figuring out what’s “normal” and what what’s not.


Sent from my iPhone using Tapatalk

 

[Red Raven]

Current can sometimes cause “crabbing” like that. More likely you just need to run the Sea Trial Wizard as described above.

Curt

 

[CaspersCruiser]

>> I’m still figuring out what’s “normal” and what what’s not. <<

That ain’t normal. Calibrate your autopilot compass and you’ll be all fixed up.

 

[bill46]

Autopilot fluxgate compass can be physically adjusted to read correctly. Must rotate the reactor. Behind cabinet drawers aft of refrigerator on one 31, need long arms and mount is delicate.

 

[GZigmont]

Here are a few practical steps for you:

1. Verify with your dealer that the magnetic compass has been swung and that both the dockside and seatrial wizards were completed to set up your autopilot. If not, have the compass swung and either you or the dealer can do the wizards to properly set up the autopilot.
2. When the above is done, make sure that the dealer did not leave your autopilot in demo mode. The autopilot will behave as you described when it’s in demo mode.
3. PM me if you need more detail.

Greg

 

[IFLY2]

Jumping in here, the unit (reactor) is located basically behind the microwave in the 'cave'. Do make sure that no metal objects are near it. If you look back into the dark cave you'll see a green blinking light. Worth Checking out. Have enjoyed the conversation of magnetic compass. I spent my life flying aircraft and even navigating all over the country by a magnetic compass and its a learning curve to say the least! On the water a map and understanding of the workings of the compass is is very important, but should be practiced to get a better understanding just in case! Enjoy....

 

[Cutwater28GG]

my autopilot compass is different to the heading reading on the chartplotter! its never made sense to me

 

[CaspersCruiser]

my autopilot compass is different to the heading reading on the chartplotter! its never made sense to me

The autopilot compass displays the magnetic heading of your boat (if it’s set up to display magnetic, which it should be). In simple terms, it’s the direction the bow is pointing.

Your chartplotter displays the course that your boat is traveling in degrees magnetic (again, if it’s set up to display magnetic, which it should be). In simple terms, it is the direction the boat is moving.

If the autopilot heading is properly calibrated, current and wind acting on the boat are what cause the the heading and course to differ. If there is zero current or wind, they should read the same.

 

[Submariner]

I'm the original poster on this topic. On the new Ranger Tugs, the autopilot is a Garmin GHC-20, with a Garmin Reactor 40.

The CCU, which is the magnetic compass for the autopilot, is located in the cave, directly under the microwave, near centerline of the boat. I have cleared out all metal from within 5 feet of that area. The heading still reads 25 degrees less than the magnetic compass does. I've used an app on my phone to break the tie and realize the magnetic compass is close to correct, the autopilot CCU is way off.

My next step is to calibrate the autopilot magnetic compass. On the GHC-20 helm display for the autopilot, the "dealer setup" option is not there. It is hidden and must be unlocked first. Select Menu -> Setup -> System -> System Information, then press the center button and hold it for 5 seconds. You will see "Dealer Mode" appear. This will open up the autopilot setup wizards and calibration options. (must be done every time you power cycle the unit).

Alternatively, you can access the autopilot setup wizards (Dealer Mode) via the Garmin chartplotter. On the chartplotter press Settings-> My Vessel -> Auto Pilot setup. It'll give you a warning "Are you sure you want to configure the Autopilot?". Once in there, the same options are available to calibrate the CCU (magnetic compass).

The instructions state that compass calibration requires flat, calm water, little to no wind. You will need to spin the boat 1.5 times in either direction without pitch or roll of the boat. Then you will need to travel in a straight line for 45 seconds. The compass calibration can be done without having to go through the entire sea-trial wizard.

So, I'm waiting for a chance to find some flat, calm water, with little to no wind, outside of Everett to perform the compass calibration. Hopefully this will correct my problem.

(For those following this thread, the issue I'm trying to solve is that with my autopilot compass off 25 degrees, I do get to where I'm going, and the autopilot and everything works. But my "Heading up" setting on my chartplotter puts the boat facing up with my course at a 25 degree to the right. It looks like the boat is driving sideways.)