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wrote:
Guys,

I had the discussion how one could avoid driving doglegs with a GPS.

A dogleg is the course that occurs when you "aim" for your target
waypoint while wind or current pushes you from the side ... If you
follow bearing to target you would arch around the target and finally
meet it but it would NOT be a straght line..

Question is:

What does one have to do to get to the target on a staright line? Lets
assume we dont make lots of intermediate waypoints.

My 276C has CDI (cross track thing or whatever) havent been able to
test it but wonder if thats the solution?

Matt

xpost

About the only way is to use lots & lots of waypoints. So the GPS will
go to each in turn & ensure you're path over the bottom is as intended.

Numerous "commercial" boats have come to grief for relying too much on
the GPS taking the boat to a far off waypoint, at times depending on all
sorts of things the boat can get well off track & even end up
approaching the waypoint from a totally different bearing than was
originally intended, hopefully a reef etc doesn't get in the way.

The expensive systems will maintain track but they do it by calcing
lots of close waypoints & seeing that most systems come with lots & lots
of waypoint capacity you might as well use it, even for a seemingly safe
long open water leg.

Set & drift etc are of course contributors to being off course:-) but
the GPS knows none of this all it knows is where the boat is "over the
bottom" at any given instant & the outputs to give the autopilot to take
you straight to the next waypoint from "that" point. Again as above the
result over time is you can end up a long way from where you thought
you'd be traveling even though you will get to the right place in the
end, save you don't run aground on the way.

Last thing for people with ingredient X:-) that can save time & fuel,
depending on the trip passage leg etc, & of course safety (i.e. what
reefs or other there are to hit if you're deliberately off track)
Consider disregarding tidal flow; if your trip is of reasonable length
often what the tidal flow steals from you on say the flood you'll gain
back for free on the ebb. No point fighting the tide all day just to
cover an arbitrary line across the seabed. Lots of people think the
shortest distance is the chosen track but it's really the shortest
distance "through the water", so even though your course over the ground
seems longer (as given by say GPS) your actual distance through the
water can actually be substantially less (as given by say an accurate
sumlog)

Always a bit bemused when people bemoan the inaccuracy of their silly
old low tech sumlogs because they rarely agree with the GPS, when in
fact often both are correct, the GPS records miles over the seabed, the
sumlog miles through the water.

K