Why not simply pull the ferry using the steel cables? In other words, put traction systems on both riversides and move the ferry using them. It even should be more efficient than a propeller-based propulsion system.
The German wikipedia entry has a small picture that shows how it works: https://de.wikipedia.org/wiki/Gierseilf%C3%A4hre
And the following technique seems to be even more convincing as the stream is never blocked from the cord: https://de.wikipedia.org/wiki/F%C3%A4hre#Rollf%C3%A4hre but probably more difficult for larger streams.
If the ferry is pulling itself along using the cable, the cable doesn’t really even have to be under tension and there is a lot less stress and forces on the system overall.
To answer GP's question: I don't know, but given that this was a retrofit of an existing diesel-powered ferry I could guess that this was the cheaper way of doing it.
- It would be difficult to synchronize the movement of the two motors sitting on either side of the river. A single motor whose output is local to the ferry’s impulse is easier to control.
- The ferry can likely move somewhat independently of the cable, and the inverse of its current design makes this more difficult.
- When comparing the two approaches, the one with fewer moving parts in the system is typically the one that is easier/cheaper to build and maintain.
- It might be more difficult to properly maintain the steel cables if they are stressed in this way vs. the current approach.
- There might have been infrastructure already in place that made this approach easier to implement, and parts for a slightly modified boat are going to be easier to find than parts for a proprietary “raft pulled by a steel cable” system.