Yeah, the tug can, theoretically, come out of the "notch". BUt as I understand it, the fit is so perfect that it creates a great deal of suction and plays all kind of hell with lines and hawsers and whatnot. When you are loading/discharging cargo, obviously the draft of the barge changes drtamatically, so there are two seperate "pin drafts" When the barge is light she rides about 20 feet above the tug, hence the imighty "air draft" of the wheel house.

(and thanks for always having such a keen interest in the technical stuff that nobody else seems to care much about...)