Well, as promised, here's where my transducer is mounted.... it is quite a large lump, and the last transducer I had with a 160 Blue was much smaller and worked at all speeds.
This thing also pops up from time to time in choppy water. I had to mount it right on the chine, or otherwise half of it would be above the back of the hull, and the other half would be quite deep since the V is quite sharp.
I think I will leave it here unless anyone can come up with any suggestions around that sort of area of the back of the boat (not inside). I am happy that it works upto 15knots, and then again at over 20 knots most of the time.