You have to sail right down the middle, usually.
I am using old legacy boat. The smallest. I have been able to get through most of the bridges now by piloting myself. I can sail all the way up the river to Minoc and beyond.
The one bridge I was getting stuck on, I found I can spin the boat and go through backwards, but not forwards.
If the mast hits an archway piece, it gets stuck, but not on the bridge proper.
ZO