The Raymarine ST 60 Wind instruments can calculate the True
Wind Speed (TWS) and True Wind Angle (TWA) from the apparent wind data (AWS
& AWA) and the boat’s speed.
Unfortunately, they’ll only do this from the speed as measured by a
paddlewheel in the water. This measures the
boat's speed through the water and is itself an apparent measurement, as opposed to
the true Speed Over Ground (SOG) that can be obtained from a GPS receiver.
There is some disagreement among sailors as to whether the
true or apparent speed should be used for the true wind calculation.
Frankly, I think those using an apparent speed definition are from older
sources that haven’t fully incorporated the changes that GPS has made to
navigation. For my boat, I want to use
the SOG in the TWS calculation. I also
want to see the SOG displayed on my other Raymarine instruments that are designed
for the paddlewheel.
Opinions true vs. apparent aside, Take Two’s paddlewheel is not accurate and I’ve
been unable to calibrate it. I think the
problem may be because of water turbulence where it is mounted. Keeping the paddlewheel in the water all the
time gets it fouled with growth and swapping it in and out with a plug gets
water in an otherwise dry bilge.
The solution that works for me is a SeaTalk NMEA Bridge
from gadgetPool.de. Its primary purpose
is to translate sentences between a standard NMEA instrument network, and Raymarine’s
proprietary SeaTalk network. I don’t
really need it for this purpose, but it has a very nice feature to specifically
address the speed problem. When the option
is enabled, the bridge can translate the SOG sentence from the NMEA network into
the SeaTalk sentence from the paddlewheel.
This effectively tricks the Raymarine instruments into using the GPS
speed.
I’ve been using it for about a year now and it works well.
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