Take a straight edge and put it along the bottom of your boat running fore and aft. Check that there is no gap from the transom to a few feet forward. I'm wondering if you might have a little cupping in your hull. (Cupping is caused by too short a trailer not supporting the stern properly so over time the hull actually bends down a little because of the weight of the engine). I highly doubt it, but since you don't have any tabs, and only one transducer on one side it's about the only thing I can think of. 
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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