The ship I work on has a 36" thruster prop and a 6" grate. You'd have a hard time getting through the grate to the thruster prop...
No doubt true, but I bet it would hurt. Sea chest grates are much smaller but divers have been sucked up against them, lost their FFM (w/o comms), and drown.
I have seen hull cleaners that don't think twice about getting between a dock and a 60' hull when the bumpers were only 12" in diameter. I am "guessing" that a 175' boat with a single bow thruster would be 18-24" in diameter.