I also had to replace my sonic speed from B&G. Stéphane from Pochon recomended this:
To replace the Sonic speed system we install the Ultra Sonic sensor by NKE (in housing),since 2 years on each Amel 55 we used this sensor and it works very fine.
This sensor is quite expensive therefore I followed Bill's recomendation and installed the Tinley NMEA paddlewheel. I have now SOG from the GPS on my Hydra network. Quite easy to install.
Wilson / SM 003 / La Rochelle