Hey Selene Friends:
I had an interesting issue with my Simrad AP28 autopilot about 10 days ago that I thought you might benefit from hearing about.
I'd noticed over the past couple of weeks out cruising in Alaska that my autopilot was "wandering" a lot more than I remember it doing before. At first, it wasn't particularly pronounced---just a gentle yawing back and forth maybe 5 degrees or so.....but it was even doing that in completely flat water (which it never did before).
Of course, it got worse when we had quartering seas, as you would expect, but for whatever reason, it got progressively worse to the point that about 10 days ago I couldn't run it in Auto mode (or Nav mode) without it yawing so bad that it was making my wife sick. Even with the stabilizers on, the swinging back and forth was irritating.
I couldn't figure out what had changed and I started fiddling with settings on the AP28. I hadn't changed anything in terms of settings and I also checked the heading sensor by manually steering in a straight line to see if the heading was changing. It was not.
While I was checking the heading sensor I noticed that the little bar graph that usually is displayed at the bottom of the AP28 screen that shows the rudder position was just showing dashes--no rudder position.
OK.....that would make sense, that if the autopilot can't detect the position of the rudder, it would have a harder time steering a straight course. So....I started digging around in my spares because I knew I had a spare heading sensor.
However, then I also realized that when I'd been out on the wing stations earlier that day, I could swear I saw the rudder position indicated on the little Garmin NMEA 2000 displays that are on my wing stations.
So....I looked on the Maretron displays, and there was still rudder position showing. I even looked on the little wireless remote for the autopilot and it showed the rudder position. I knew that my rudder position sensor is not directly connected to NMEA 2000. It's an older analog sensor that connects to the autopilot computer and the autopilot computer transmits the rudder position on NMEA 2000. So.....the fact that the rudder position WAS showing up on the NMEA 2000 displays indicated that the position sensor itself was working and that it was even being read properly by the autopilot computer.
But somehow, neither of the AP28 control heads---the one in the pilothouse and the one on the flybridge--could see the rudder position!
That seemed really odd.....
I even tried going to the installation menu and manually trying to select or change the rudder position sensor input, but the AP28 didn't "see" a rudder position sensor, even though there obviously was a functioning one connected to the autopilot computer.
I'm always reluctant to make big changes when we're out cruising as I didn't want to screw things up worse, but at this point, I was having to hand steer anyway, so I decided it was worth the risk.
The AP28 has an "auto-scan" function in its installation menu that will cause it to go out and look for its inputs. I ran that procedure and it immediately "discovered" the rudder position sensor! It did require me to do a couple of other steps, including a "rudder test" where the autopilot drives the rudder back and forth a couple of times and apparently watches the position sensor.
What was so curious about this is that there's no evidence there was ever anything wrong with the rudder position sensor or even its connection to the autopilot computer, since the computer was transmitting good rudder position information on NMEA 2000.
However, somehow, the AP28 control heads had "lost" the rudder position sensor, and that seemed to cause the computer to be unable to steer a stable course.
Once I re-ran that auto-scan process and it found the position sensor, everything returned to normal. The autopilot has never wandered or yawed significantly, even in less than ideal sea conditions, so it was great to get her back to steering properly with the autopilot.
SO....if you find your boat yawing badly under AP28 autopilot control......check to see if the control head has lost its rudder position information.
Happy Cruising!
Mark Tilden
Selene 60 "Koinonia"