The installation instructions show for a V bottom to have 1/2 of the transducer thickness below the hull on the lowest side. That means the other side will mostly be below the hull bottom.
Since it is Side Imaging, it has to be able to "see" to the sides. It can do that best being low as possible on the transom (near the keel). Humminbird's SI covers from the water's surface to past vertical under the transducer on both sides. It will "see" the engine mount a lot more were it is than if it was closer to the keel.
May have to put a spacer between the mount and transom to get clearance past the drain.
I would suggest the Transducer Shield and Saver Spring Back Bracket for that.
The transducer is the Compact SI that comes with the Helix 5, 7, and 9 Side Imaging units as well as the 899ci HD SI Combo and all the smaller number series SI units.
The unit will product sharper images with the HDSI transducer.
Have to trim out the outboard when scanning at idle speed to get clearance for the left side SI pulse.
That transducer comes with a rooster tail plug for the mount. Looks like it is installed, can't really tell.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, SOLIX 12 SI, 360, RC-1