A lot of ships don't go across oceans and can benefit from batteries. Think ferries, feeder ships, river transport. Those all dock in relatively predictable intervals.

As a crazy thought: On predictable routes in shallow enough water, implement an underwater power source similar in function (if not physically similar) to overhead electric bus/trolley power lines? The boats would move along their routes with a cable running down to to a connector that slides along the power source.

I'm imagining channels in a busy waterway or a regular ferry route. Of course it needs economy of scale, but at that point everyone saves on batteries, etc. Unless there are submarines, I don't know what would pass under the boat and cut the connection.

Maybe watercraft usage or routes aren't predictable enough? Maybe the sliding connector is too expensive underwater, especially in sea water.

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.

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?

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.

Just looking at a couple problems:

> 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,

Does the connection from the 'pantograph' to the boat need to be rigid? Use a cable? Maybe it's not for the smaller boats which can't handle a 20m (probably more like 35-40m for the angle and slack) cable, but also need less power and thus pollute less.

> and then move forwards without the pantograph sticking 20m down creating a huge amount of drag

Presumably, the catenary's route would be cleared of obstructions - maybe even on a frame elevated over the floor, maybe at relatively constant depth - or do I misunderstand?

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.

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.

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

Example:

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

I wonder how that expense compares to just building a catenary electric rail bridge and skipping the boat entirely.

Interesting. Generally, waterborne freight is far cheaper than rail. Also, at least sometimes you might need to transfer between modes - ship to rail then rail to ship. But with the underwater catenary solution now we're talking about new infrastructure (not to mention R&D, etc.).

As a sort of transitive analysis: Standard waterborne energy costs are included in the superior pricing of waterborne freight compared to rail. If the underwater catenary solution is competitive to standard waterborne energy costs, it beats rail easily. If the underwater catenary is not competitive, it has no value anyway.

It could still have value if we price in, as we should, the externality of GHG output. THe underwater catenary could reduce GHG output in busy waterways by powering boats that don't have batteries.

In addition to the practical challenges, this would also be highly vulnerable to sabotage.

I suspect the batteries end up cheap enough to make this not worthwhile. If not today, then certainly soon.

tesla's wardencliffe tower wanted to transmit power to the ionosphere. if it worked you could put a receiving tower on each cargo ship, and discharge through the water. goodbye sea life though, which is our problem since they produce a lot of oxygen and food for us.

i'm thinking of an undersea cable running on the sea floor with periodic vertical cables tied to floating platforms for cheapness, or oil rig type platforms if the cables are too heavy. then the boat only needs to charge its batteries there long enough to reach the next charging rig.

you would have at least two charging points for each direction of travel, that way boats travelling in opposite directions are not blocked.

> Question is can they get enough power to be useful. Marine diesel engines are very fuel efficient, fuel oil is very energy dense, and they need to run for a long time.

Washington State Ferries refit one of their largest ferries (190-car capacity) as a hybrid recently. They replaced two diesel engines with batteries, leaving the other two engines in place. This provides for significant fuel savings. When shore charging is implemented, fuel use will be reduced even more.

I imagine there are also uses where batteries fully replace engines. I've got a tiny boat for casual fishing, you tend not to use the engine that much, and electric propulsion is likely quieter and can probably modulate to slower speeds which could replace a separate electric trolling motor.

A bigger ferry than that has been fully electric in Norway for five years between Horten and Moss. Probably a shorter route than the Washington State one though.

https://www.dailyscandinavian.com/worlds-largest-electric-fe...

https://indre24.no/2021/03/01/verdens-storste-elektriske-fer...

Probably not bigger... it looks like M/V Wenatchee [1] is wider and likely taller, has a much higher passenger capacity, and travels faster (and I misremembered the vehicle capacity a bit). Some of the routes in the system are similar distance than this Norway route, but most are longer, including the one that usually gets the hybrid ferry.

Also, there's a big chicken and egg problem. Can't have electric ferries without shore charging, can't justify building shore power unless there's boats to use it with. There's several ferry routes and individual ferries need to be able to serve multiple routes to allow for flexibility during maintenance, an electric ferry that can only serve one route is not super helpful.

[1] https://wsdot.com/ferries/vesselwatch/VesselDetail.aspx?vess...