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You don’t need that - if you’re doing short routes you can push enough power into batteries during docking for the next voyage.

Example (7 minute charge time):

https://global.abb/group/en/technology/did-you-know/maid-of-...

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The maintenance of these power points would be astronomical, even if the basic theory could work, which I'm not 100% sure of. You'd have a single high-voltage cable, a pickup pantograph that wraps around the cable acting like a transformer that wheels along the cable, and AC current in the cable with the water itself being the ground reference. I can't imagine any other way it could work.
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I had always imagined a system of autonomous submarines that function as tugboats along popular routes and then container ships would just function as barges that are hauled by these autonomous submarines. These submarines recharge their batteries automatically by docking to underwater charging stations.
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sea water is too corrosive and too conductive. You could make a single overhead cable and use sea water as your return work. However it wouldn't be practical since ships multiple stories tall need to pass under the wire (and draw a lot of power), while also tiny fishing boards that couldn't even get a pole that high.
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The same problem with height would also be an issue for the underwater thing. Quick googling suggests a very large container ship might have a draft of 16-17m when fully loaded. Probably you want the catenary at 20m depth then. How does a small get their pantograph (or whatever you'd call it) down to that depth, and then move forwards without the pantograph sticking 20m down creating a huge amount of drag. Not to mention how do you make the connection between the pantograph and the catenary in the (conductive) sea water?
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Make the cable AC and the "pantograph" a (water tight) transformer that glides around the cable. I can't see any other way - DC is not going to work.
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