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Title: Wild swings on Nav—not NMEA related!
#1
I love using the nav setting on my autopilot, which normally keeps the boat within .02NM of the proper course to the active waypoint. In the past I had some wild course swings on nav that were finally traced to a NMEA feedback loop.

I still have one situation where I get wild course swings. Many times (but not always) when following a route, I will reach a waypoint, and the boat will make a dramatic turn way off course before correcting itself and coming back to the proper track line. I thought it was due to the large error in the steering compass.

A few days ago I fixed the compass error. I used nav to go to one waypoint, then canceled and selected a route in MacENC before switching back to nav on the autopilot. When I reached the next waypoint I should have turned from roughly 110° COG to 90° COG but the heading on the autopilot was at 31° by the time I switched from nav. After putting the boat back on the proper course I switched to nav again and the autopilot display instantly started dropping. I gave up on nav for a while.

About 10 minutes later, I tried switching to nav again. This time it steered the boat almost perfectly along the route, through a sharp turn of more than 100° and finally into the harbor channel to the point that .02NM was too much error to let it stay on nav. While doing the sharp 100° turn the boat overshot the track line, spun around almost 160° to starboard, then quickly corrected back to port to settle onto the new track line. The overshoot went to .07NM XTE, but when the boat corrected back to port it only went to .02NM XTE. Other than the one sharp turn, all of the waypoint turns occurred almost exactly as one would hope.

What could possibly be wrong to make the boat turn dramatically off course at one waypoint, yet 15 minutes later allow the laptop/autopilot to steer the boat almost all the way to its harbor stall? This behavior occurs at a different time than it did with the NMEA feedback loop. The feedback loop caused sudden swings while traveling in a straight line and this problem only occurs when advancing to the next waypoint on a route. I thought it was related to the large discrepancy between the steering compass and the calculated course, but now the steering compass is within a few degrees of MacENC's magnetic COG.
Edmund
 
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[No subject] - by - April 1, 2010 20:32
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