Thanks Hoss, that makes sense about the turning cavitation. I've not tested it in calm water yet, each time I've been out its been 4-5ft minimum sea state. I guess the water movement just caused by crossing these waves would maybe be enough to upset it.
I think I get what you mean about the bracket. I need to figure a way to temporarily mount it with G clamps or similar until I get the position fine tuned. Moving it back from the hull makes some sense, but then closer to the prop too..?