I have mine mounted on the lower transom. The directions were pretty clear and easy to follow. It sad to make sure they were mounted level to the horizontal of the transom, 1/4" from the brackets lower edge to the edge of the transom. They are designed to take a pretty good smack and then they simply flip up and away. Simple to re-snap them into place. It's only happened once. And I was REALLY stuck. Lol
They do lose the ability to read the water above around 10mph. For that, I have my DI graph's transducer epoxied into the lowest, rear most, part of my mechanical box. Below the pumps. It works great and is always reading. The only reason I didn't do all of the graphs that way was first, I wanted the sonar on the TM to read what was below the front for drop shot. I also have SI and that one is at the console. It has to have a clear "view" of the sides... So it had to be on the transom. My DI for the front is also transom mounted, since the TM can only accommodate one transducer. I like the DI on the front too. So, yes, I now have four Helix 7's. Lol.
Eat. Sleep. Fish.