That appears to be the case. I have the same bridge set-up except with ST-50
Wind. When I was designing it, I became concerned about data sentence
conflicts with Seatalk, converted Seatalk and NMEA all running on the same
buss. And, like you, I was curious about whether it was possible to use GPS
SOG rather than boat speed. One chap who seemed to know a lot about this
indicated that in case of conflict, Seatalk prevails. So it seems that if
boat speed is available in Seatalk, your instrument will ignore other data.
Additionally, as far as I know, Seatalk format speed is the required input
for that instrument. So short of some programming to fake it (convert NMEA
to Seatalk), I don't think there's a workaround. I wasn't willing to go that
route, and it had the shortcomings descibed in other posts.
Len


Unfortunately, I think I've received my answer from the other posters in
here - I need to go outside the Raymarine stable to solve the problem by
changing boat speed over ground into boat speed over water :-( I should
have bought a different NMEA - Seatalk bridge that could do the data
manipulation for me instead of the E85001.
..