Driving Doglegs
 

"Dan Krueger" wrote in message
ink.net...
Gene Kearns wrote:
On Mon, 26 Dec 2005 12:33:13 -0500, Wayne.B
wrote:


On Mon, 26 Dec 2005 00:33:08 GMT, "Butch Davis"
wrote:


IMO, for the gps to compute a compass course to compensate for wind and
current it would have to have some means to acquire the inputs.

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

Exactly right.


Anybody know of a GPS with this capability?


I'm not sure what you are trying to accomplish, but inputting wind speed
and current wouldn't help since both affect different boats in different
ways. A smaller, shorter boat, for example, is less affected by wind than
a taller, longer boat.

The purpose of a GPS is to do more than a compass. It will help you
correct for wind and current if you simply use it to get to your waypoint.

Dan

I guess I am missing something here. Assuming the discussion is about a
decent chartplotter type GPS, why all the questions about wind and drift?
As I recall using the Raymarine system on my Navigator, I would transfer my
planned track or route from the paper charts to the chartplotter in the
morning before getting underway. The route would consist of several
waypoints, usually placed in the area of a navigation buoy on the paper
charts.

I'd then just follow the tracks from waypoint to waypoint, either by
manually steering the boat or by autopilot. If the boat marker started
drifting away from the track due to wind or current, I'd see it and correct
for it. The autopilot can be used in two modes. One simply steers to a
course heading with no correction for wind or drift. The "Navigate" mode
keeps the boat on the intended waypoint track, automatically correcting for
wind or drift and automatically steering the boat to a new course as
required by the waypoint settings.

Worked fine for 1500 miles to Florida. So what am I missing?

Eisboch

Driving Doglegs
 

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.

Driving Doglegs
 

On Tue, 27 Dec 2005 00:11:51 GMT, Gene Kearns
wrote:

Anybody know of a GPS with this capability?

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

Some of the better sailboat systems like Ockam or B&G offer direct
digital readout of current direction and speed. They are calculated
numbers based on observing SOG vs speed through the water, and COG vs
magnetic compass heading. The instrument system resolves all the
vectors and spits out the numbers. It is fairly slick but requires
constant calibration and cleaning of the speed sensors which are
mechanical flow transducers in through hull fittings.

My Furuno Navnet, integrated chart plotter/GPS/RADAR, displays a
heading vector and a separate COG vector on the RADAR screen. It has
an electronic compass sensor to provide heading information. Since it
knows your heading, it can also overlay chart data with the RADAR data
which can sometimes be useful. It needs heading information to rotate
the chart into the proper orientation. I find this function
particularly useful for quickly determining whether a RADAR target is
a NAVAID such as a buoy, or another boat.

Driving Doglegs
 

On Mon, 26 Dec 2005 19:55:05 -0500, "Eisboch" wrote:

Worked fine for 1500 miles to Florida. So what am I missing?

Nothing, that's the way most folks do it. The GPS displays COG
(Course Over Ground) which is the combined effect of Boat Speed
through the water, Heading, Current Speed and Current Direction, plus
any set due to a crosswind. It is impossible to separate the various
components without having a very sophisticated instrumentation system
such as the ones that many racing sailboats carry.

If you set both a "From" waypoint (sometimes done automatically), and
a "To" waypoint (destination), then the GPS can also calculate XTE
(CrossTrack Error). Most GPS units today can direct the autopilot to
minimize XTE and thus maintain a straight course regardless of current
or wind.

Driving Doglegs
 

"Wayne.B" wrote in message
...
On Tue, 27 Dec 2005 00:11:51 GMT, Gene Kearns
wrote:

Anybody know of a GPS with this capability?

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

Some of the better sailboat systems like Ockam or B&G offer direct
digital readout of current direction and speed. They are calculated
numbers based on observing SOG vs speed through the water, and COG vs
magnetic compass heading. The instrument system resolves all the
vectors and spits out the numbers. It is fairly slick but requires
constant calibration and cleaning of the speed sensors which are
mechanical flow transducers in through hull fittings.

My Furuno Navnet, integrated chart plotter/GPS/RADAR, displays a
heading vector and a separate COG vector on the RADAR screen. It has
an electronic compass sensor to provide heading information. Since it
knows your heading, it can also overlay chart data with the RADAR data
which can sometimes be useful. It needs heading information to rotate
the chart into the proper orientation. I find this function
particularly useful for quickly determining whether a RADAR target is
a NAVAID such as a buoy, or another boat.

I had very mysterious readings for a while until it was discovered that the
electronic compass (I think it's called a flux gate compass) had been
mounted in the engine room about 6 inches away from the electric motor for a
washing machine. If I deviated more than 10 degrees off course, the boat
marker would swing around so it appeared we were doing 19 knots in reverse.

Another neat feature of the overlays (haven't tried it yet though) is that
you can set up the Raymarine system to automatically command the autopilot
to change course if the radar shows something big and solid (like another
boat) on a collision course and within a programmed range. I am sure the
Furuno system has something similar. I briefly had an Egg Harbor with the
Furuno setup and never really learned the system. I find the Raymarine to
be much more intuitive.

Eisboch

Driving Doglegs
 

the original question was that if a computer (GPS) could work its magic
and do a no brains naviagtion by telling you where to steer to go from
point A to B WITH cross wind/current.

WITH NO MANUAL INPUT OR READING THIS PARAMETER OR THAT ERROR

Simply (and only) by giving you degree where you need to point your
bow.

Matt

Driving Doglegs
 

wrote in message
oups.com...
the original question was that if a computer (GPS) could work its magic
and do a no brains naviagtion by telling you where to steer to go from
point A to B WITH cross wind/current.

WITH NO MANUAL INPUT OR READING THIS PARAMETER OR THAT ERROR

Simply (and only) by giving you degree where you need to point your
bow.

Matt


My answer: Yes, *BUT* it may take you a hell of a long time to get there.

Eisboch

Driving Doglegs
 

On Mon, 26 Dec 2005 22:07:16 -0500, "Eisboch" wrote:

the original question was that if a computer (GPS) could work its magic
and do a no brains naviagtion by telling you where to steer to go from
point A to B WITH cross wind/current.

WITH NO MANUAL INPUT OR READING THIS PARAMETER OR THAT ERROR

Simply (and only) by giving you degree where you need to point your
bow.

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

That's not a GPS, it's called a navigation computer or calculator. It
will still require manual inputs, and they will be estimates at best.

The question as stated is of theoretical interest only since the
constraints imposed are quite artificial in the real world.

In the real world your GPS will in fact tell you where to point your
bow but it is a feedback mechanism, not a one shot calculation. Even
NASA makes mid course corrections based on observed vs calculated.

Driving Doglegs
 

On Mon, 26 Dec 2005 20:57:02 -0500, "Eisboch" wrote:

I had very mysterious readings for a while until it was discovered that the
electronic compass (I think it's called a flux gate compass) had been
mounted in the engine room about 6 inches away from the electric motor for a
washing machine.

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

The flux gate sensor should be mounted as far away from iron, steel or
electrical wiring as possible. I ended up putting mine in a locker
where I store only life jackets.

I think the Furuno user interface may have improved in recent years.
I had very little difficulty teaching Mrs B how to use the target
tracking functions and she is still struggling with computers and the
internet. Target tracking with the ARPA functions is way cool,
everybody loves it.