My old lowrance system is giving me some trouble. I believe a bad power cable, but a buddy gave me an offer I couldn't refuse on his old 1198c SI w/ 360. Not really in a position to spend a lot of money right now on a brand new FF system so I was excited to get this system that he isn't using. The bigger screen and the 360 what got me. It comes with the XHS 9 HDSI 180 T transducer. Ive talked to David from shut up and fish and I agree with him that I do not need to run a separate 2D transducer cause the XHS 9 HDSI 180 T is capable of doing SI, DI and 2D. My question is, what are the advantages of running a separate 2D transducer on this system? If I run just the single XHS 9 HDSI 180 T, will the 2D sonar give me an accurate depth reading while the boat is moving at high speed?

Probably not. To get high speed readings on most boats it needs a transducer mounted in the hull. You would also need a splitter cable to run the second transducer. I have had boats that would read up to 30mph but my latest one loses contact pretty fast from transom. I just use the gps and mapping to know my depth on plane.

The HDSI transducer does Side Imaging, Temp. and 2D sonar. It will give you on-plane depth readings if installed so it can.
Some hulls do better with a shoot-thru 2D transducer especially at "high speed", just depends the hull type and how high the "high speed" is.
A proper install takes onto consideration the hull type and speed capability.
The 1198c SI Combo does not have Down Imaging and the transducer does not have a DI piezo. It has composite DI Views created by blending the two SI pulses

Interesting. Thanks for sharing. Did not know about the DI part. So I guess there is no real advantage to running a separate 2d transducer? I'll stick with the one I got then. Just kept seeing YouTube videos of humminbird 1198 running both transducer but never explained why.