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.
Not really related but regarding energy usage: I was fascinated when I travelled in Rathen (Germany) with a ferry that uses just the power of a stream: https://en.wikipedia.org/wiki/Reaction_ferry
The cable is going to be submerged in the water and I think that you are underestimating the amount of force that it requires to drag the entirety of the cable through the entire length of the water.
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.
This (friction or drag) is the correct answer. You would need either huge tension to keep the cables off the sea floor (and additional complexity to relax/lower as needed) or an impossible amount of power (and tension at the shore) to pull the boat. Think about how an anchor works, the actual anchor is just the end point and some length of chain is also laid on the floor. Drag of the chain, not the anchor is what actually keeps you from floating away.
A very good question for which I haven't been able to find any official answers. Here are some thoughts:
- Wear of the cable at the shore ends. Just as abseiling reduces wear on rope compared to lowering, ferry mounted drive means limited dragging across what could be possibly very abrasive surfaces at each shore (sand, rocks, etc). Instead cables get lifted up and lowered down gently.
- A cable ferry pulls tension on one side allowing the other side to drop down. If you've ever navigated around a busy cable ferry you'll know that you don't wait for it to stop at either side, you just pass behind it based on current direction. To replicate this functionality you'd need motors for pulling from either side (which you've identified). In that light an electrical cable running to the centre is a quite efficient solution.
- The ferry could require power anyway for a range of functions like raising and lowering the drawbridge (I wonder what the proper ferry term for this is).
It pulls itself along the cables rather than using a propeller, yes, but the motor that's doing the pulling is on the ferry rather than on the shore.
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.
The point is that on the current system motors which are powered by the electric cable are on the ferry, so it uses propellers to move. The proposed alternative is to use steel cables (which are already present) without any electric ones to pull the ferry, such system should be more efficient power-wise and easier to service.
My guess is that there are several possible factors for this reason. Here’s some I thought up off the top off my head:
- 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.
But it already does have two steel cables spanning the river all the time. IIUC they get relaxed when ferry does not move and sink to the bottom to allow other ships to pass.
