Right. I'm looking for the keel killing rock sticking up from the bottom in front of the boat. I'm planning to also install a DST800 so that will give depth as well.
If you think of the sound signal as a 60 degree cone coming from a transducer aimed straight down, thirty degrees of the cone is fore and aft of perpendicular to the bottom. Even using a 0 degree transducer on a 17 degree deadrise, 13 degrees of the cone will be on the aft side of perpendicular. Airmar thought it was not a good idea. Their concern was that the signal return angle would not be aimed at the boat. Maybe so unless it was that keel killing rock. I dunno.