Alternative depth data
 

In common with many East coast sailors, I like my echo sounder to show depth
below the keel (mainly interested in whether I can get over this sandbank).
I have a lifting keel and every echo sounder as far as I know, on the market
will not show negative depths. E.G. My draft, keel down is 2.2m at this
point I want my echo sounder to say 0. If I raise my keel so that my draft
is 1.5m I want the echo sounder to say -0.7m. Ok I accept that echo
sounders can't do it so I have bought a Garmin 300c fish finder. which will
handle the situation perfectly..

That is the preamble.

My main instruments are Raymarine ST60, then nmea output goes to a laptop
using either Winchart Neptune software or SOB. What I want to do is suppress
the depth data from the Raymarine instruments (Only the depth, not wind GPS
info etc) and use the NMEA output from the Garmin fishfinder as my depth
data.
Is this possible?

Thanks

Phil