Is the transom mount transducer from the 798 HD Si the same size as the trolling motor transducer for the 858 c HD Di XTM-9-WIDE-DI-20-T, as I have a transducer shield for the 798 and was wonder if I have to buy a new shield or if they are the same size. Also what is the difference between the XTM-9-WIDE-DI-20-T and the XTM 9 DI 25 T. Thanks for the help.