Garmin Autopilot Heading Way Off

The heading displayed on the Garmin GHC-10 control unit for the Garmin GHP autopilot installed in my 2012 R27 is way off- 20 degrees or more. Best I am able to tell, to correct this issue involves re-running the Sea Trial Wizard. Has anyone dealt with this issue? If so, did running the Sea Trial Wizard solve the problem?

The autopilot CCU in our 2014 r27 is in the cave and often gets bumped by things we store in there. That can easily throw off the heading as you describe. While the Sea Trial wizard is one approach and probably the best in the long run you can quickly get it back to a close approximation by loosening the thumb screw on the CCU mount and rotate the ball until the heading is correct. Turning on radar and set the boat heading line to auto helps in this process. You can then line up the radar image with the gps image and match the autopilot heading with the chart plotter heading. When you have more time then you can run the sea trial wizard to correct the remains variance.

Curt

CaspersCruiser,

On my previous 2012 R27 the Autopilot CCU (the approximate 4" diameter black ball) was located in the cave (mid-berth). If you accidentally bumped this black ball and re-oriented it that could explain why your autopilot heading is off. Or, you might have placed something magnetic or a significant amount of stuff containing iron that could cause it to be maladjusted. This CCU can be adjusted by turning it, but you should consult the manual for the details of doing so.

Keep in mind that your GPS heading can and usually is different than your compass heading. I know you are a pilot and know this, but the compass heading is the direction that the bow is pointing and the GPS heading is the heading that the boat is traveling. In flying terms this is called crabbing. At times when wind and current affect direction the correction can be significant with a difference between the two proportionately. Then add into that compass error and it can be worse. Put something near the compass like an Iphone and you can really see a big difference. So, before I would change anything I would swing the compass and get that adjusted before comparing between the two. The most accurate way would be to do it while the boat is on the trailer similar to using a compass rose at the airport. I matched mine up using an independent hand held compass and my iphone compass app. Once you have the compass swung then I would do a comparison on a current free windless area.
I did my chartplotter heading as described above. I had my wife at the helm and as I turned the ball slowly and asked her to tell me when the two headings matched.
Now add another element to this, variation. I think, but I am not sure that you can set the chartplotter to pick up either true north or magnetic north. I would check this out before making any changes. Depending on where you are in the world that variation can also be a significant number. Where I am in the Northeast it is a 15 degree difference.

Thanks, everybody. The heading discrepancy has been present since day-one with this boat. At first I thought it may have been a case of heading vs. track, but as I gained experience with the boat and verified that the ship's compass was fairly accurate, it became apparent the autopilot compass was in error. Tomorrow I'll investigate the autopilot CCU. Mike, GREAT idea about putting boat on trailer and calibrating the ship's compass. It does need it, but it's only off five degrees or less.

Rerunning the Wizard is probably in order. It is my understanding that if you run the Wizard, say in the PNW then the auto pilot is good for that general area of boating for being accurate. However when that same boat gets trailered east the auto pilot will have a discrepancy based on where it was set vs the magnetic pull being different in its new boating grounds... like the Ohio River. As I stated this is my understanding so somebody please collaborate my statement or correct as needed.

Jim F

Jim,

I would say your comment is correct. With that said, the compass will always point to magnetic north. If the Chartplotter is using true north then there will always be a discrepancy between the two. It is somewhat unfortunate that with today's technology (GPS) all the numbers become somewhat insignificant. Even if the setting on the Chartplotter is wrong the boat will go to to your waypoint. I think it is most important to make sure your magnetic compass is accurate and one should keep track of their heading on the magnetic compass in case their electronics go out and there is a need for paper charts. Having a backup plan and a redundant GPS means is always good.

Mine was way off after 1) moving boat from PNW to Texas and 2) the power(?) cable to the ECU came undone and the chart plotter/autopilot could not find it. After reconnecting the power cable I ran the compass and rudder calibration, called for by the autopilot. It appears to be working (at least much better) now.

Todd