Good results with thru hull and inside hull
My Cat 30 has had both a tranducer attached inside the hull and a thru hull type. We sail salt water exclusively and both types have worked equally well. I have a Garmin GPS fish finder giving me depth and GPS position. It came with a transom mount "puck" transducer. A friend advised attaching it to the inside of the hull in a pile of silicone. As long as air is not trapped between the transducer and hull, it gives depth measures to 50 fathoms (in salt water). Water in Puget Sound and adjoining waters can reach 200 fa, but I don't care. Her keel is barely 1 fa. I'm most concerned about water less than 5 fathoms and changing depth quickly. During my last haulout, the boat yard installed a bronze thru hull transducer in the same position as the original factory transducer; port side, aft the keel. The thru hull installation has the same performance as the transducer in silicone. When the sounder reads 0 from the keel, it is accurate for both transducers.
