The line to the left of the center line is most likely something on your transom to the left of the xducer that the pulse is reflecting off of...jackplate, livewell screen, motor...etc...

Set the SI to about 10ft...left side only and it will give you a good idea how far the reflective obstruction is from the xducer....
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A couple of tips about scanning docks, pilings, vertical trees... (and under docks)...

*Slow the boat down to less than 1mph to allow more pings to reflect off the fish...
*Speed the chart up to 2 or 3 to "stretch" those fish echoes down the screen....
*Use the 455kHz because it covers all the way to the water's surface...

Rickie