Capt Jeff,
Nice job and well documented.
The hull is cored up to the bulkhead between the V berths and the main cabin. There is a depression, in some of the boats just inside the cabin, aft bulkhead, where a bilge pump is installed. Some of the boats have partial core still under this area You can cut the inner layer of the bottom, and remove the core--and then re-inforce that part, with extra layers of glass and resin. I personally prefer epoxy, because it has better secondary bonding strength than Polyester. Some use vinyl ester which has strength between the epoxy and polyester.
If you want to test areas, put the transducer (a transom mount can be used) in a plastic baggie of water or mineral oil, and lay it on the bottom of the boat. If it is cored there will be no or very minimal transmission of the impulses, and you will not get a usable sonar image.
For non cored hulls, I have made a plywood and epoxy/fiberglass cloth box inside the hull, put the thru hull transducer into the lid, with a port to fill the box which is glassed to the hull. Then I fill the "bottomless box" with mineral oil. Had good echos thru 3/4" solid glass to over 600 feet.
