Transatlantic and transpacific container shipping is still going to be among the last sectors of the economy to be electrified.
It's probably more practical to build modern sailing ships than to try to electrify bulk overseas cargo shipping. Or maybe a hybrid approach where the sails are also lightweight efficient hardy cheap solar cells that turn electric motors to add a few knots to the speed during the day.
But even then you need massive energy storage to make sure you have power to maneuver in a storm.
Back in the heyday of the battleship, replacing the engine was one of the things it wasn't worth doing. It's so hard to do, you might as well just build a new ship. I have no idea if this is still true, or not. Construction techniques have come a long way. Today, there are cruise liners that have all of their propellers in multiple swiveling pods containing electric motors. (They generally have generators run by diesel or steam turbine.)
Seems to me, especially with electric engine pods, ships can be designed to allow the replacement of the engine/generator.
Shipping companies have had decades of success fighting against legally-mandated retrofits - double-hulled tankers, trivial safety features for passenger ferries... I don't think your idea of replaceable engines is going to get any traction.
The debate back in the 30s was if diesel could take the place of steam turbines but they ended up weight prohibitive and some maintenance would be similar to the work required to just swap engine types
On a steam ship, which had a much less monolithic power plant, replacing an engine or a boiler was feasible. Replacing reduction gears usually was not.
Moving through the water is very efficient - if you go slowly enough. If ships travel slower than the hull displacement speed, they could travel on electric motors and solar power today.
The challenge is convincing global markets that slow shipping is worthwhile, when everything is moving in the opposite direction (faster and faster).
Check your math here... it doesn't work out. Most large ships DO travel at less than displacement speed, going faster would take more energy than their engines produce.
Even covering an entire ship's deck with present solar panel technology, the ship couldn't run on the panels... they would have to charge a battery bank, then when full the ship sails a certain distance, stops, recharges, and repeats.
The only practical means of powering modern ocean going vessels given their range and size are fossil fuels or nuclear reactors.
If you can get by with sails for some things, that's great, but unless someone starts building massive numbers of sailing vessels, there's not going to be enough of them to replace motorized shipping, and even then some routes and cargoes won't be able to ship that way. Some cargoes are perishable, for instance.
Any plan anyone comes up with for fighting climate change will fail if it requires a significant portion of humanity to change its behavior all at once.
Idly musing on what it level of nonsense it might actually take with current tech, would be the best description. The panel would have to fold up and detach from the container ships, becoming a separate vessel, for rough weather, ports and canals.
I did say a very big multihull.
edit - suspending a power rail underwater for electric container ships to link to, powered by a corridor of floating panels, is another potential approach.
Use fission plants ashore to create synthetic liquid hydrocarbon fuel for ships.
Fusion (if it ever works) will be too large and complex to use in merchant vessels.
Climate change makes a big difference in the equation, though. Marine nuclear power is one of the best understood, safest areas of nuclear use that exists, even using the old technologies.
The idea of using "modern sailing vessels" for shipping in the amounts that are presently moved from continent to continent is just plain nuts. Unless most of the world is willing to go back a hundred years in technology, we're going to need to use motor vessels going forward.
About the only way to eliminate all carbon emissions from large ships is to make them reactor powered. Batteries will work for short trips, but if you combine the need for long range and high power in shipping of large cargoes, the only way to power ocean going vessels in any practical sense are internal combustion engines running on fossil fuels or steam generating reactors powering turbines.
That's why the navies of the world run on one or the other.
In short, nuclear powered merchant ships are never going to happen regardless of the potential advantages.
> the only way the US Navy succeeded was by using a huge amount of resources under tight top-down control.
Everything in the military is done with tight top down control for the most part. That's not why it succeeded, though. The military is the only organization that did it since it's the only organization for which nuclear wasn't constrained by the need for profit.
There was a demonstration nuclear cargo vessel built that operated for four years, but they weren't mass produced and so weren't economically viable when oil was cheap.
However, if you combine the need for lowered emissions with the higher modern price of oil, larger scale production of nuclear civilian vessels begins to make some economic sense.
This is aided also by the fact that modern reactors could be designed to be lower cost to operate.
Security wise, nuclear powered vessels would be less vulnerable than oil powered vessels presently are because they'd be at least as fast and would need refueling less often.
They would not be a more attractive target for terrorists than other ships despite nuclear propulsion, because the reactors can be designed to be tamper proof, meaning the only thing a terrorist in possession of a nuclear ship can do is sink it with little resulting fear or hardship for the country they are trying to affect. Water blocks radiation very effectively, and ships with sunken nuclear reactors don't spread radiation around, they contain it.
Having one of these ships sink is not as terrible as you might think. Water is an excellent moderator and these very heavy elements will tend to stay on the ocean floor. There's a reason we store nuclear waste in big swimming pools.
The ITER core currently weighs about 5100 tons. I'd expect refined fusion systems to be able to come down in weight. Even if all the supporting infrastructure weighed more than the combination of bunker oil and diesel engine, it'd have to be a lot more to negate the advantages.
As far as too complex, that really depends on the commercialization potential of fusion and how complex final designs are.
Whether the liquid hydrocarbon fuel is synthetic from renewables or natural (dug out of the ground), burning it will still give off greenhouse gases.
Synthesis is only carbon-neutral if the synthesis method involves capturing those greenhouse gases again.
It probably boils down to energy density, plain and simple.
A windmill would help both push the ship and generate power to the batteries.
The scale here is enormous, and just for a 4km journey
All this to say, these ships do create super high emissions of Sulfur Oxides, but their CO2/equivalents emissions are quite low (the lowest per kg*km of any means of transport!  ). Worth addressing, but not nearly the equivalent of 50 million cars in the "emissions" that the common dialogue refers to.
And yes it's very interesting to look at transport modes by hp per ton. You find that ships, trains, and trucks requirements are modest compared to passenger cars.