Welcome to the forum.
Since you're new, maybe nobody told you.
It costs a million dollars to join up here, and that will be equally distrubuted amongst the current members.
Cash only please.
Just joking.
This is what I've learned about it:
With a tranducer on the transom, it is limited to speed because 1) The boat tends to lift up when on plane, and 2) There will be lots of turbulance at the transom and air bubbles formed. Lots of air bubbles are created near the surface, which is where your transducer will be when you're on plane. Sonars don't like air bubbles. They need solid water to work at their best. Some people have had to move their transducers lower, but there is a limit to how low they can go because they could hit something. If you're transducer hits something, in all likelyhood, you won't get any readings at any speed.
Some people mount a shoot through the hull puck on the inside of the boat, and this gives them the depth reading at any speed (0 through hauling ass).
It depends on what kind of boat you have if this will work or not (aluminum, versus wood, versus fiberglass).
I think side-imaging is limited to about 10 miles per hour.
There have been many posts about the speed issue and transducer mounting. Use the forums Search feature to find them.
Good Luck.