Wout B. wrote:

Especially at low sailing speed and strong current, this can make all the
difference.
The Navman wind instrument can use either SOG or water speed for true wind
calculation.
You can also "fool" any NMEA wind instrument by converting the NMEA RMC
sentence from the GPS (COG) to VHW (speed thru water). A Brookhouse
multiplexer will do this for you. You can load a simple script for this
conversion. You may need a multiplexer anyway if you are building an
integrated instrument system.
Hi,

You can fool any NMEA wind instrument. But if you have a SeaTalk only
instrument, you have to "fool" the SeaTalk device.
If someone is interested in playing with it have a look at
www.tklinux.de

Frank

I think you are confusing true wind direction with magnetic wind
direction. To find true wind you need apparent wind (speed and
direction) from your masthead and boat speed from your paddle wheel.
To find magnetic wind speed/direction you need an instrument system
that accepts info from a fluxgate compass to do the calculations.
Signet SmartPak system for example.

I don't understand the reference to the paddlewheel rgnmstr. True wind
direction = wind direction (true north) if the boat was anchored at a
point on the water, e.g. not relative to the boats movement or water
current. To achieve this (with the boat in motion), you need the
apparent wind & speed, SOG (from the GPS) and ship heading (true north)
from your flux gate (COG from GPS introduces error) to get a close
approximation, and a little more accuracy can be had if you include
adjustments for angle of heel error, heading error (compensate for mast
& sail induced air flow), a yaw sensor on the mast, etc ... oh yea, and
you need an instrument package that uses this, some like Raymarine wont
use SOG from the GPS.

Interesting paper published that explains true wind, and notes that 80%
of marine vessels calculate it wrong.

http://www.coaps.fsu.edu/woce/truewind/paper/

Better squeeze up for the Brooks & Gatehouse...

I have an open suggestion to Raymarin to incorporate COG as an option,
which would make me happy. The rest of the capability is overkill for
me (adjustments for angle of heel error, heading error (compensate for
mast & sail induced air flow), a yaw sensor on the mast, etc.)