LG,
A perfectly vertical bridge piling or a pole should display a perfectly horizontal shadow in the Si sonar.
If the back end Si transducer was angled downwards (pointing forwards), than the shadow will be skewed towards the top of the display because the bottom part of the Si sonar beam will contact the piling first and the resulting shadow will angle towards the top of the display (new Si sonar data).
If the back end Si transducer was angled upwards (pointing backwards), than the Opposite will happen and the shadow from the piling will be angled towards bottom of the Si display window.
Attached is a file I did on Si transducer twist (forgot I had this).