Depth Sounder questions
Best location is low and near the mid point of the boat. Maybe slightly forward but not too far or the readings won''t be true. Too far out board or fore or aft and the movement of the boat will scew your depth readings a lot as the boat pitches and heels.
Going back to the ST40? Heck I wouldn''t go back to the ST50<g>. I have a broken ST50 speed log. I would go for two transducers. One speed and one depth.