That depends: do you want to use the 798 unit while running the trolling motor or when running the main motor?
For a first time Si unit user it may be better to start with the unit console mounted and the transducer transom mounted. Post several pictures of what the transom area of your boat looks like and we can make transducer installation suggestions. More than likely you will have to mount the Compact Side Imaging (CSi) transducer off to one side of the tunnel and may have to trim the motor up to get unblocked Si sonar to each side.
You may also consider installing a second (DualBeam) transducer inside-the-hull so that you can mount the CSi transducer higher up to keep it from damage. You would need the XP-9-20 inside-the-hull DualBeam transducer and AS-Si-DB-Y cable for this and would need to know that the hull of your boat is less than 1-inch thick and is solid fiberglass (no wood, foam or other type of core where the transducer is epoxied in place).