By 'general purpose" I was referring to the features built in to the typical
GPS unit. However, even software like MapTech is not pushing the bounds of
the analysis that's available. Try to find some details of the software
that America's Cup racing boats use, and you'll be amazed. And it's not
difficult to do. For instance, it should be possible to select any part of
a track and have VMG to any nominated point plotted as a line graph. This
means that you can see, for a course that may have included several tacks,
exactly when you were making best time to the mark. Marchaj introduced the
polar performance diagram in the sixties, when sophisticated additional
equipment had to be created to collect the data - now it's all available
from GPS and standard instruments, but I don't know any GPS software that
can produce a polar diagram.
--

"Matt O'Toole" wrote in message
...
"Amanda" wrote in message
...

I have the opportunity to crew on a racing yacht as tactitician and
would like to learn more about racing with a GPS. Apart from the
obvious (distance and bearing to the next mark) what other uses can a
GPS be put to in yacht racing?

I've raced dinghies and cruised yachts so I'm familiar with general
tactics and how to use GPS, but I need to put the two together.

Jeff Richards wrote:

VMG - velocity made good towards a waypoint. That will tell you
whether pinching (and slowing) or freeing (for speed) will get you to
the windward mark quicker, or exactly what course on a downwind gybe
gets you to the lee mark quickest. You will need a 'relative bearing'
(angle off the bow, or pelorus) device to tell you when to tack or
gybe onto the equivalent course for the mark. You will need to get
all the marks as waypoints, so this may mean visiting each mark and
entering the waypoint before the start.

If you can link the GPS to boat speed and direction then it can
calculate tide set and rate, or you can do it manually if they can't
be electronically connected.

The other useful feature is to be able to save the course sailed for
later analysis, but general purpose GPS software doesn't usually
provide decent analysis tools, so you may have to write your own. You
should be able to replay the whole race or selected parts,
identifying where you gained or lost ground and perhaps working out
why.

How do you mean, Jeff? I find that Maptech, etc., make this pretty clear.
With
my path drawn on the screen, I can see exactly what was going on, where I
gained
or lost ground, etc., better than ever before. A picture is worth a
thousand
spreadsheets!

PC-based navigation programs all have their warts. In fact most of them
are
pretty bad in one way or another. But for the most part they do a good
job
telling you what you need to know.

However, I'm not familiar with the software/firmware in most
self-contained GPS
units.

Matt O.