Granted, that's not clear the way he said it. But since it's hard to imagine the boat wouldn't be capable of more speed under power, and since he next mentions that he would usually be going at 1 knot approaching a bridge, I suppose it's possible that he meant he would typically have been powering at 3-4 knots in an unfamiliar, close-in area.

I started to side with him a little bit more now that I understand they turned a corner and then only "suddenly" were able to see the bridge clearance boards. That wouldn't have given him much time to react. (Granted, they BOTH should have discussed air draft; but since the tow captain was the one with local knowledge, it's surprising he didn't ask since he would have known the route ahead of him.)