Chewing gum
The factory installed depth gauge on my 1983 H34 registers flaky depth readings sometimes. While investigating the problem, I bought the cheapest fish finder at Boatersworld. Figured it'd be good to have a backup unit anyway.To experiment where to put it, I used chewing gum to stick the transponder to the inside of the hull aft of the engine. It has worked without interruption almost a year now. Haven't yet got around to replacing the gum with more permanent material, may never do.