Is there some sort of limit or minimum height/width that defines a tunnel?The outboard "rigger" in the pics would not be legal in IMPBA or NAMBA simply because it has no tunnel. It would have to be decked over to form a tunnel and then well, out goes the reason it was designed in the first place. Might make a cool OB hydro. Don't forget also that one of the ways a tunnel gets its "design parameter" speed is by trapping air.
what comes to mind is the Byrd Hornet, which I believe is a IMPBA legal tunnel.
perhaps subclassifications for tunnelhulls could be similar to that of hydros-
having outrigger hydros, and a scale/sport "tunneled" hydro class.