hull material on 39
On my 88 catalina 30, the hull was solid fiberglass. I placed my depth dransducer under the galley sink on the inside of the hull. I put a large glob of silicone on the inside of the hull set the transducer in the silicone. I had done this as a temporary measure because the one that went through the hull had gone bad. The new one worked perfect the inside and actually better than the old one. It is still that way on the boat after 8 years and working great. Just make sure there are no air bubbles in the silicone and if you have a wing keel, do not place above the wings.