Pilings and other vertical structures are challenging with SI...

Like Gene mentioned boat speed is the most common culprit...due to the fact that fewer of the sonar pings are actually reflecting off the structure...before the boat passes completely past the piling...

This can be overcome with 2 possible scenarios..(depending on the unit)...
*Slow the boat down to allow more pings to hit the vertical pilings...
*Raise the Chart Speed..(if the unit is HB)...to stretch the pings across the display more...

Distance from the pilings vs SI beam angle also has a major effect on the amount of the structure that the sonar ping actually reflects off of...

Notice in the following depiction how the HB 800kHz 130° coverage only hits the bottom section of the bridge pilings...resulting in far less of the is sonar ping to be reflected back to the xducer..



This can be overcome somewhat (like brushnjigs mentioned above)...by using the wider HB 455kHz 180° SI sonar beam in combination with the above mentioned points of slower boat speed and/or higher Chart Speed...

Rickie