"Timothy.Rulon" wrote in message
...
b393capt wrote:
I am just about to buy a bunch of Raymaine instruments for my new B
393, and just found out that the ST60 is limited to using SOW, and
won't use GPS SOG & COG, to calculate true wind.

After looking thru this forum, I see some references to this issue, but
don't see either
(1) A way to get true wind on any of the Raymarine instruments, does it
work on the Tri data ??

(2) Another vendor who does not have this limitation mentioned.

Can anyone help me ?


To calculate true wind you also need true heading. SOW, SOG and COG
alone won't work as set/drift begin to effect your vessel which will
occur as you slow down. At zero speed, a GPS is likely to report
anything for COG unless it has a built in aid like a magnetic flux gate
compass. There are some expensive GPS receivers which provide true
heading by using 2 antennas and lower cost LORAN-C sensor units being
developed which does something similar. Should also work great as an
input to an autopilot rather than magnetic sensors which can get tricked
by magnetic anomolies.

I'm not aware of a recreation-priced system which performs all the above
but would very much like to hear about it for a government application.

For marine weather information, please visit
http://www.nws.noaa.gov/om/marine/home.htm

This is correct of course, but what is often referred to as "true" wind on a
sailboat as opposed to apparent, is relative to the centre line of the boat,
but corrected for speed. The T on the ST60 instrument actually stands for
"theoretical". Many nav software packages calculate the true wind direction
and speed from heading, relative wind angle, taking into account SOG and
COG, to plot a wind vector or lay lines in the chart.
Wout