T&T: NAV Mode and Autopilots

[Peter Bennett]

Tuesday, November 27, 2007, 7:15:06 AM, Peter wrote:

>> After all the posts on AUTO vs NAV and reconsiling my own operation, > I
PG> feel I have to make the following somewhat counter opinion to the > trend of
the discussion.>>  > NAV is simply an XTE corrected and end of course alarmed
AUTO, so if >> AUTO is okay, NAV is essentially a more accurate attempt at the
same >> goal. So do we need to always hand steer?
PG> I'd suggest that NAV mode is inherently safer than AUTO. However many skippers
PG> punch in waypoints at points where course changes should be made and at the
PG> destination point. If these points are far apart, Xtrak error (in degrees) can
PG> lead to problems. You can have a Xtrak error of 1 mile, that is still only 2
PG> degrees if the next waypoint is 30 miles off. Breaking the route into smaller
PG> chunks of 1/2 hour segments, even if that means 20 waypoints on a straight
PG> line course for a day's cruising, will keep a tighter course if set, drift and
PG> wind will play a factor.

Cross-track error is the _distance_ that you are off the intended
track, not a bearing to the distant waypoint, so there is no need to
break long straight tracks into shorter segments.

When the autopilot is in NAV mode, and following the GPS XTE
instructions, it _will_ automatically compensate for set, drift, or
leeway by adjusting the vessel heading to minimize XTE.

-- 
Peter Bennett, VE7CEI    Vancouver, B.C., Canada
Ennos 31 "Honeycomb"
GPS and NMEA info: http://vancouver-webpages.com/peter 
Vancouver Power Squadron: http://vancouver.powersquadron.ca