Mark,
Noticed a lot of folk have cked. your post, just as I have, but am a bit confused. What kind of boat? And a keel...what kind of keel? Normal transducer install for best performance is thru-hull, AWAY from the keel, but close as possible to the deepest part of the bilge at the stern. And making sure the sender itself views clean water, even at high speed, without any disturbance/interference from forward livewell/washdown clamshell pickups, etc.
You will need an angled mount internally so that the 'ducer shoots straight down.
I've had satisfactory success transom mounting, tweaking the face of the sender upward so doesn't pick up air.
Best,
G3