Actually Chum, if you look at the drawing in my last post under the Atlantic tug, you will see the Z-drive (Schottle) bottoms are aligned with the flat bottom of the hull. The bottom of the aft section of the hull is concave so the the Kort and propeller can be the largest diameter that would fit in there. I am adding a new drawing, zoomed in on the aft part of the boat.

The Eastern rivers (Mississippi, Missouri, Red, Arkansas, etc.) in the U.S. are subject to changing depth depending on the climate (rain, winter snow, etc.). Even dams holding or releasing water can have a dramatic effect. It seems like 9 feet is a good depth for barges and towboats. The US Army Corps of Engineers maintains minimum water depth through dredging where it can.

I'm not an expert on this mater but am interested in this. Never been on a towboat. There are a few of them here in central Florida that I have seen. Non of these are anything like what is on the Mississippi. They are the most efficient way of moving a massive amount of goods.

Lew