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I recommend placing it on the down side of the propeller stroke. You try to place it as parallel to the water without submersing the unit. You only want the bottom third or so to be bellow the bottom of the boat. With the angle of the boats bottom you kinda split the difference for submersing and parallel. Also tilt the back of the unit down a little so it reads what is under the entire boat and not just the transom. Eye ball an imaginary line if you are looking at the bottom of the unit it intersects the boat 2/3 up. As far away from the prop as reasonable/possible and still stay submerged.