Minimum acceptable clearance when passing under fixed objects such as bridges

What is the normal or minimum acceptable clearance when passing under fixed objects such as bridges? Does this clearance determine by the master, operator or the shipowner?

Air Draft and stage of the tide

Very dependent on the circumstances but I have been on vessels where we literally had 6" to clear and made it. We had a mate at the highest point calling down the approaching clearance as the vessel got closer to the bridge (the twin bridges in Houma actually). Personally, if I were the master I would not do it if the vessel’s airdraft was within 95% of the total clearance (ie. a 5% margin). Sometime though you just don’t get such luxuries.

I had an old fleet supervisor who’s motto was “lose the mast, find a job”…

Totally subjective and depends not just on the actual clearance mid-span, but also how wide is the area of max clearance, how much current is pushing me around, and how well do I know the vessel.
Ultimately, as long as my heart rate stays below 100bpm, then we have plenty of room.

When in doubt: The faster you go, the more you squat.

Ports with have written policies on bridge clearances; if the air draft exceeds a certain height then a surveyor must come out and take an exact measurement. Even then, assuming the port signs off on the transit, it is ultimately up to the pilot and the master whether or not to proceed.