Who Me wrote:
I am trying to get True Wind from my ST40 Wind instrument.

I am using a Raymarine E85001 SeaTalk/NMEA converter box to take the
NMEA 0183 output from my Garmin GPSMAP175 GPS and send it to the ST40 Wind.

Connecting a PC to the RS232 terminals of the E85001 shows that both
instruments are talking to the E85001, but the ST40 Wind still does not
compute true wind. The GPS is sending NMEA sentences VTG and RMC which
both have boat speed information in them, so I don't understand why the
ST40 won't use it. I've tried choosing different NMEA output settings
on the GPS, and the sentences change slightly, but the ST40 still
ignores the boat speed info.

The ST40 computes true wind angle/speed when it knows speed thru water,
not when it knows speed over ground.

The solution is simple (but probably not what you want to hear): install
an STx0 speed/log instrument.

-- Kees