Toledo Basser BCB are right about needing a second ducer. One follow up note is the selector switch is only needed if you choose to glass in the quadrabeam ducer. If you choose to use the dual beam ducer, you wont need the selector switch. With the dual beam ducer, the unit will automatically cut you off the transom mount and switch you to the internal. From what ive heard, most people will choose the dualbeam because you dont have to mess with a switch.