That’s a cracking explanation from expilot, I think that needs to be ‘stored’ somewhere safely and rolled out whenever ‘that bridge’ needs explaining.
Many years ago we hired Shimmering Light when it was a brand new boat. We had hired a forward drive from HW on the basis we wanted to get through the bridge. The airdraft needed was stated at 7ft, being a new boat the pilot was unsure of the clearance. He tried several times, gauging the possible clearance under the bridge, but to no avail. All credit to him, he really did try for us.