I dunno about being better off in NAV mode. It seems to me in NAV mode, the computer is thinking we got to get to X,Y so I'm going to tell the autopilot use a heading H. The computer watches progress and occasionally sends a different H for course correction. As long as the autopilot is using the compass for the H it is subject to the effects of passing under a bridge. -- Jim Sent from Android tablet. Expect swypos. On Jun 3, 2015 5:07 AM, "Randy Pickelmann via Great-Loop" < great-loop@lists.trawlering.com> wrote: > _______________________________________________ > > http://lists.trawlering.com/mailman/listinfo/great-loop_lists.trawlering.com > > To modify your Great-Loop subscription options (change email address, > unsubscribe, etc.) go to: > http://lists.trawlering.com/mailman/options/great-loop_lists.trawlering.com > > > ---------- Forwarded message ---------- > From: Randy Pickelmann <rwp_48@yahoo.com> > To: "great-loop@lists.trawlering.com" <great-loop@lists.trawlering.com> > Cc: > Date: Wed, 3 Jun 2015 07:06:56 -0400 > Subject: Autopilots and Bridges > Ken is correct, the autopilot should not be effected if operating in NAV > mode as long as the GPS has a clear view of the sky. However, if in > COMPASS mode bridges will definitely effect the compass autopilot. A few > years ago I was helping a friend deliver his Whaleback from Annapolis to > Solomons. As we passed beneath a bridge on the Severn River the compass > swung a full 30*. > Personally, I wouldn't rely on the autopilot in that situation. Hand > steer...it's fun! > > Regards, > Randy Pickelmann > Morning Star > lying Calvert Marina, Solomons, MD > > Sent from my iPad > >

The reason the use of COURSE mode is better is that the heading is not derived from the compass. The system computer is watching the sequential lat/lon positions streaming in from the GPS and calculating the heading made good from the series of positions, therefor magnetic perturbations from a bridge (for example) have no effect. The fluxgate compass plays no role in this mode. Ken Bloomfield MTOA 2062 AGLCA 3529 On 6/3/2015 7:23 AM, Jim Ague wrote: > I dunno about being better off in NAV mode. It seems to me in NAV mode, the > computer is thinking we got to get to X,Y so I'm going to tell the > autopilot use a heading H. The computer watches progress and occasionally > sends a different H for course correction. As long as the autopilot is > using the compass for the H it is subject to the effects of passing under a > bridge. > > -- Jim --- This email has been checked for viruses by Avast antivirus software. https://www.avast.com/antivirus