My unit is as Rob describes, for "shooting through the hull" rather than "mounting through the hull" in a special tube.
It is bedded on epoxy [Glued inside the hull] works fine up to 20 knots and readable above that just like Martins.
Best way to test the right spot, or the transducer I know of is;
Take the boat to a bit of deep water, then compare the display from where the transducer is normally mounted to the display after it has been moved to a temporary position when it can be placed directly into the sea.
Not easy to do as any speed, unless it can be temporarly mounted to the transom or attached to a pole.
Another method may be to take your unit onto a boat with an identical set, go to sea and compare the boats set display, with yours after swapping the units while the boat is in the same area.
Good luck in finding the problem, especially as this weather is far from perfect for sea trials
Charlie