Hull thickness in bilge area for 23'Sceptre o/b

[Serenity]

I'm trying to determine the approximate thickness of my hull in the bilge area. I have a '78 Potter hull and I'm about to install a Airmar 1000 watt 50 kHz transducer for an in-hull installation and was told that the thickness of the fiberglass could mean a dB loss. It seems that for my setup the best would be about a 3/4" thickness. We mainly fish between 50 to 140 fathoms. Please let me hear your comments or advice also.