Michael — you’re correct, you need boat speed through the water to calculate TWA and TWS from the AWA and AWS you get from the masthead wind transducer. I’ve not seen it calculated using GPS SOG.
And you definitely need to be able to compare boat speed and heading with SOG/COG to calculate set and drift from current. So I’d say a knotmeter is a pretty important piece of gear on the boat. Necessary to doing dead reckoning, too — if all the other electronics go down, you can do a lot of navigating with magnetic compass and knotmeter. Fred Street -- Minneapolis S/V Oceanis (1979 C&C Landfall 38) -- Bayfield, WI > On Aug 11, 2015, at 12:58 PM, Michael Brown via CnC-List > <cnc-list@cnc-list.com> wrote: > > No idea with the newer i50/60/70 series, but the older Raymarine > wind instruments used speed from the knot meter ( wheel in the thru hull ) > for calculating TWS and TWA. There didn't seem to be an option anywhere > to use GPS speed. > > Fred - any update on that? > > I may at some point upgrade the instruments on Windburn. I have mainly > ST50, with a new SPX/5 and one i70. The new stuff is nice but how much does > one spend on a '77 C&C 30-1? > > Michael Brown > Windburn > C&C 30-1
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