Re: shoot thru the hull transducer
I installed one and it works perfectly.
Air is the enemy. Bubbles in the fiberglass, air core in the plywood or balsa, and foam core are the usual suspects. If any are present it won't work or will work erratically.
Buy it from someplace that will allow you to return it.
Hang it over the side of the boat and note the measurements. Fill the bilge with enough water to allow it to be submerged and weigh it down. Try it in several different positions. If the readings match those you took when it was over the side, you're golden. Dry the bilge, mix the two-part epoxy, and weigh it down for 24 hours.
If it doesn't work, return it and get an in-hull or transom mount.