When you go too fast for the transducer location, it looses water contact and records the dark lines. SLOW DOWN for scanning.
You will not get on-plane depth readings with the transducer in that position. The correct installation for your boat is a shoot-thru Dual Beam transducer--XP 9 20. That and the SI transducer connected to the display with the AS SIDB Y cable.
You also get distinct lines parallel with the SI center line from echos from the inside the jackplate. You will see those lines on SI and DI.
Humminbird Helix 12 CHIRP SI, 1198c SI, 1198c SI, ONIX 10 SI NT, Helix 7 CHIRP SI GPS G2, 360, RC-1