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

wrote in message
oups.com...
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


Well, couldn't you make some assumptions for set and drift of current and
expected time to travel, then plug in a waypoint that matches the new
corrected magnetic course? No, I guess not, since that fancy gps thing will
continue to correct you back to an arc and your false target instead of
allowing a straight line course that brings you to your real target.

I've only used paper, pencil, erasers, plotters and rulers to sail anywhere
(still haven't even used an autopilot) and never thought about the dilemma
created by gps in the presence of set and drift. I guess a gps controlled
autopilot would make so many corrections that the doglegs would be
insignificant. Cant wait to hear the answer.

my suggstion was to point to the target and leave the wheel straight
and go.

current will turn you to one side and the angle will be displayed on
GPS as off course you then have to steer the same angle to the other
side of your direct line course.

This should get you on a straight line to the target but is a little
cumbersome ...

Matt

wrote in message
oups.com...
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


Not sure what kind of GPS you are using, but I assume it is a chart plotter.
Most I am familiar with will draw a line (track) from your position to your
destination or waypoint. If you are drifting due to current or wind, your
position cursor should show that you are moving off the line, even though
your bow is still heading toward the waypoint. At least that's how the
Raymarine and Furuno systems I have used work.

Eisboch

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

Karen, thanks for the info ...

btw. you still owe me some ETec info

I havent ruled out that motor yet, no maintenance, light, little fuel
use, quiet and lotsa power is just soooo tempting ...

Would you please email me (real addy posted)? ( I think your posted
email here is eithr fake or not checked)...

Matt

wrote:
Karen, thanks for the info ...

btw. you still owe me some ETec info

I havent ruled out that motor yet, no maintenance, light, little fuel
use, quiet and lotsa power is just soooo tempting ...

Would you please email me (real addy posted)? ( I think your posted
email here is eithr fake or not checked)...

Matt




Thanks Matt, as for the other GPS replies I mostly agree but they all
assume you know the direction & quantum of set/drift etc (believe it or
not most smaller boats don't even track true through the water & it
varies with speed, sea state, etc), then there's a further assumption
that the quantum AND direction of off track forces will remain constant
throughout the leg; both rarely happen.

All assumptions are a big ask & pretty much negate the perfect beauty
of the GPS itself because you still end up with a ded reckon element in
your navigation (much reduced yes but why have any???) Given how easy it
is to put a series of points along your displayed track it's easy enough
to avoid. I have assumed you are using the GPS to drive an autopilot???
but if you're standing there at the wheel then the highway display can
help & most you can even set the width of that highway so it doesn't
drive you mad by being too sensitive.

Someone said you might not get to your destination but alas that's not
right & indeed the essence of the problem:-) the GPS & autopilot will
"always" get you there, but you might be approaching a long off waypoint
from the wrong & possibly a dangerous direction:-)

Sorry about the Ficht E-Tec thing Matt:-) I think you can safely buy an
E-Tec as you could most any other brand & certainly you shouldn't be
influenced by just one voice (no matter how loud:-))

I'm certainly a bit touchy on the subject, particularly when before I
even enter the subject I get personally denigrated by "believers" who
are nothing more than that & will not even try to address the technical
issues when raised. Since 98 we've pretty much been on our own against
the Ficht & Optimax technology & sadly it's always degenerated into a NG
personal slinging match, mostly me against the selling dealers (do they
have a motive to silence me??) & the usual NG suspects (owners or dealer
spruikers, do they have a motive in silencing me??; see below:-)). I
have to say since OMC rolled into a ditch under a pile of dead Fichts I
see lots of people now who suddenly "knew" they would fail & even can
tell you exactly why:-)

Other, than noting every answer & maybe the grounds for it, you should
always balance the pros & cons to your own satisfaction.

K

Tom,

Have been comparing Lowrance LMS-337CDF vs Garmin 198C. Results have been
interesting,

Many swear by Garmin, any Garmin, while a small minority seem to prefer
Lowrance. Probably depends on where you started given the reliability of
both brands. I currently have a Lowrance LMS-160 purchased in 1999 with
which I've been pretty happy overall. I've never had any issues until last
Spring when I started getting vertical lines on the display. The unit was
remains useful but I needed an excuse to upgrade to a WAAS unit and now I
have one.

I down loaded the virtual 337 and owner's manual and have been playing with
it for about a week. I also compared both units side by side at my local
Boater's World store. The display on the Lowrance is superior with a
480X480 display but a map chip must be purchased at around $100 for a
Navionics Gold. The Garmin has a complete coastal Blue Map installed but
who needs all the extra mapping? Lawrance also has a chip for about $80
(???) which has good coastal coverage for a wide area and even covers our
Mobile Bay Delta very well. The Garmin is more expensive but by the time a
couple of chips for the Lowrance have been purchased the prices are similar.
The Lowrance includes a dual freq transducer with temp and water speed
included. The Garmin xducer must be added and if dual freq, temp and speed
are included it gets pretty expensive. On balance the Lowrance is a little
less expensive and has a better display, IMO.

The downloadable virtual 337 is a great teaching device. It also has a
couple of Navionics maps included to give the user an idea of the detail
possible on the unit.

I'm not in love with either owner's manual but the Lowrance manual seems
slightly easier to understand. Both companies could take lessons from the
military on how to write manuals.

I believe I'll be buying the 337 from Lowrance. The display quality and the
fine detail of the Navionics Gold maps are the major discriminators but the
lower price is also attractive.

Butch
"K. Smith" wrote in message
...
wrote:
Snipped!

On Mon, 26 Dec 2005 14:00:59 +1100, "K. Smith" wrote:

there's a further assumption
that the quantum AND direction of off track forces will remain constant
throughout the leg; both rarely happen.

=================================

Not true. If you monitor COG vs bearing to mark and keep them equal,
the "off track forces" can vary all they want, same thing if you are
monitoring (and minimizing) XTE.

The old fashioned way was to estimate current set over the entire leg,
plot a graphical solution, and steer a constant offset. At the end of
the leg you got to find out the accuracy of your estimate and
calculations. With GPS you get constant feedback.

wrote in message
Guys,

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

Matt....

What you're talking about is the difference between "homing" and "tracking".
The GPS unit ought to be able to handle this auto-magically for you, if
you're coupled to an autopilot. The concept is that you input a correction
right from the start, so you're constantly compensating for the forces
driving you off course. You don't need a bazillion waypoints, or anything
like that. The correction factor is adjusted (as required) to maintain your
track to target. It's actually quite easy to do manually, and most GPS
units will at the very least give you a screen that displays track, so you
can steer a correction yourself. Check a navigation text, (or take a
course - CGAux, for example) to be sure you understand the concept and
mechanics of track and correction, then check your GPS manual to see how to
set it up.

Merry Christmas!