Folks...the mounting location has everything to do with your S I readings at higher speeds. For example, my S I transducer is mounted on my transom and is mounted about 1 1/2 inches up from the bottom of the boat bottom. Now, once you lower the S I transducer below the boats bottom line, you should be able to run at higher speeds and receive better signals.
Keep in mind, the S I transducers are very expensive and will cost several $$$ to replace if some type of object was to hit the ducer causing damage.
I get my readings at a low pace, 2 to 5 mph...once I am over that speed, my signal is weak. In my opinion, the S I units are not for high speed traveling trying to receive data...they are for searching for what you are after.
If you will look around on this forum, you will see that many folks use their units at a much slower speed and receive much better returns. Hope this helps.