ddrenuf,
We would need to know which transducer you have or are planning on using on the trolling motor with your 798c Si unit. WayneP decided to go with the larger HDSi transducer that comes with the 898, 997, 998, 1197 units and the TM-HDsi bracket from Transducer Shied & Saver.
Which transducer you plan on using will dictate which bracket you will need to get:
XNT-9-Si-180-T (standard Compact Side Imaging transducer that comes with the 798 units) will need the TM-Csi
XTM-9-Si-180-T (trolling motor mount Compact Side Imaging transducer) does not need a bracket from TS&S.
Daniele,
Not everyone notices it but you should notice some added resolution to the Si sonar in the 798 unit when using the larger HDSi transducer.