I received a quick reply from Germany. It seems that the ship in question was not a ship at all but a swimming mill. After a bit of research I was able to determine what a swimming mill was used to grind grain into flour for baking. The mill would be towed to the part of the river where the fast current would turn the paddle wheel which in turn turned the grindstones. In the mid-twentieth century there were so many swimming mills on the rivers that they interfered with commercial traffic. Hence, the demise of the swimming mill. Very few exist today as relics from the past.
We also enjoyed the cathedral, the 12th century bridge, and the oldest sausage house in Germany, in Regensburg. But we thought twenty-nine Euros for six small sausages, sauerkraut, and beer times two was a bit much.
notes: photos in the filmstrip above left to right: Regensburg, Viking Cruise Program Director, tugboat Ersekcsnad, Tony sketching on the tugboat. (Chantal, the cruise director has nothing to do with the post, her photo just adds color.)