There are two things you have to allow for.
First, is if you want 2D sonar and Depth when on plane, the bottom of the transducer has to be at or just below the bottom of the hull. It don't work without water contact.
Second, is the two Side Scan crystals have to be able to "see" to the left and right so you need side clearance for the Scan pulses.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, 360, RC-1