Since the transducer is likey an Airmar you can go to their site and figure out which one you are looking to buy. They have some tools to help with that (Pick you sounder manufacturer and follow the steps). Once you locate it you can download the user manual. In there they will tell you how thick a hull
you can shoot through. I seem to remember the one I used on the other boat was good up to 2 inches thick, but I may not be remembering that correctly.
You also have to be sure your hull
is not cored. You can use the transducer on a cored hull, but it is more work. You have to remove the inner skin and core
material, do some sealing with epoxy
and get it all back right, then you can install the transducer.
Also remember the hull isn't going to be as thick outwards as it will down close to the keel
, but you have to be sure you don't get so far outboard
that the transducers is above the heeled waterline.
If you can lay your hands on one to borrow you can put it in a bag of water
and light it up, move it around and pick a good spot.