I watched that video. He has it mounted to where it can only be used at idle speeds. It is out of the water when on plane. He has a second transducer in the hull and is using a Y cable so both will work.
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I think Garmin does offer a Y cable and shoot thru xducer option for glass hulls ...
Or you'll have to mount the transom xducer on the lowest step to actually be in the water on plane (as the video posted above indicates) ...