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One option is to install masts and use wind, the Thomas W. Lawson was a ship 7 masted steel schooner that could carry 58,000 barrels of paraffin oil. Your cargo ship is bigger, but could probably just have more masts. It could make for a very interesting looking artist rendering.

Whale oil.

If civilizations have fallen the whales might be doing alright. Round them up and render the oil in the old school way.

Or if that seems too mean, the first steam ships used to run on coal. Digging that up is not especially high tech and you could switch up your ship to use a coal-fired boiler.

Cargo ships are designed to run on bunker fuel. Bunker fuel is basically what's left over after you're done refining all the interesting bits out of crude oil, and is highly variable in quality. Because of this, marine diesels are quite versatile in what they can run on.

In a not-quite-post-apocalyptic Earth that has a use for something as large as a small cargo ship, there's not really any conversion needed. Just dump whatever flammable liquids you can find in the tanks and go on your way.

(The fuel doesn't even need to be liquid at room temperatures. Bunker fuel is so viscous that the tanker's fuel system will include provisions to heat the tanks and fuel lines.)

Multi-factorial approach:

• sails: you'd just need a lot of them


• paddle wheel: rig up some big paddle wheels to be turned by...


• coal powered engine: there's still loads of coal and in the Post-Pockyclypse


• ox turned engines: just like old fashioned mill stones


• oars: yep --- power to the people!
  

I think we need to be cognizant of several things for this scenario:

1. You've already ruled out petroleum and nuclear as viable energy sources


2. In ruling out petroleum, you're basically ruling out nearly everything else in the entire world: no petrol means no cranes to build or fix wind turbines; no plastics to make or repair solar panels; no fuels to transport goods or people; also no fuel for heating, cooking, petroleum based power plants, etc.


3. This means your big ship and her crew will need to be much craftier and much less reliant on a single fuel power plant.

That said, the largest sailing ship ever, as every true sailor knows is the Irish Rover:

With 23 masts, she could outrun the wind and outmanoeuvre the White Whale; and her carrying capacity was every bit as incredible. She hauled all the bricks needed to build the city hall of New York, and in addition, could easily handle a million bales of old billy goats' tails, two million buckets of stones, three million sides of old blind horses hides, four million packets of bones, five million hogs, six million dogs, seven million barrels of porter and eight million bags of the best Sligo rags. She could remain at sea for seven years and pilot herself through any fog. Rocks and shoals were no problem for the Rover: she was designed to pull off a Poseidon manoeuvre and roll nine times around before she would sink to the bottom of the wine dark.

Since sailing ships of this class are narrower than a modern cargo ship, I'd suggest running three rows of masts: along the center and also along either side.

Your cargo vessel is tall enough above the water line to handle two galleries of gigantic beast and slave driven oars. She'd be a bireme, with eighty oars in each gallery, for a total of 320 oars.

So no worries! Let the winds drive her while they're blowing; and when they're down, put the men to work! In a pinch, fire up the (already firing) boilers to drive her through anything the other means can't!

In a post apocalyptic world there is no reliable supply chain.

Ergo hedge your bets.

• Sail


• Wind Turbine


• Solar


• Steam


• Human

Sail is pretty obvious, install some masts and use the wind to directly impart some motion.

Wind Turbine and Solar provide electricity which can be stored in batteries. The battery could be a water tank which releases hydro power, or something more advanced such as chemical or open air. Either way this energy source has a reliable supply chain, and regenerates even when at sea. Electric engines are also relatively low maintenance.

Steam. Unfortunately a very large vessel requires very large amounts of energy to make it move. As the supply chains are chaotic you won't know in advance what fuels would be available in port, and even how much could be obtained. In short this ship needs to run on everything: coal, oil, diesel, wood, charcoal, plant material, fat, etc... The simplest way to work with such a range of fuel sources is to burn it to boil water to make steam to drive a steam engine. The steam engine could directly propel, or indirectly propel via an electric system/electric motor.

Lastly, no one likes the idea of being stuck in a large ship at the mercy of wind, sun, and no fuel. The final modification would be to introduce a generator that runs on human labour. It will take a lot of effort, it won't be nice, but its better than drifting. Couple that with some sort of energy storage system to build up the energy needed to operate the engines at capacity for a reasonable amount of time. Perhaps that could be coupled with the energy storage system, like a water tank + hydro generator. The human element then would be a manual pump.

Finally the unspoken problem is that a 28000 ton ship needs lots of energy to make it move. A 27000 ton ship needs lots of energy to make it move. A 27000 ton ship needs lots less than a 28000 ton ship. So what can go? As a side-effect what you remove can be sold to help pay for the ships modifications, or be used as the raw materials to remake the ship (so as to not increase the weight after modification).

I imagine a savvy Captain would:

• invest in an Electric/Steam engine plant


• a few masts with a mix of sails and/or vertical wind-turbines on each mast.


• line the sails, and parts of the deck with solar cells


• invest in an electric energy storage system, even something as simple as a water tank/pump/hydro generator to provide some engine power/general habitation power.


• install a human powered electric generator/engine, perhaps a hand pump to move water into the water tower/hydro generator, or some other human powered motor system.


• reduce the ships weight as much as possible

Swap out the engines for electric motors, wire solar panels / windmills to batteries. Should be doable by an electrician and a mechanic, I just don't know what kind of capacity it will be needed.

Many answers either don't give you enough energy (oars), require advanced manufacturing (big electric engines) or both (solar).

You could go for biofuel or coal-based fuels, given that those engines run on nasty bunker fuel to start with, but this would require massive quantities of fuel, which is unlikely to be available. Keep it around as secondary propulsion, but don't rely too much on it.

Solar isn't an option (not enough energy in the first place), but wind is. Using sails on such a behemoth is going to be a challenge. And you are going to be limited in your travel direction. Still, it may be a good idea when conditions are favorable.

If you want more flexibility with wind (and go more greenpunk), use wind generators as well, especially airborne ones (assuming the tech matured enough before the apocalypse): they are lighter, can potentially reach higher-altitude winds and are easier to retract in case of storm. You can go against the wind with those, in particular.

But wait, didn't I say that big electric engines were a problem? Well, if you managed to salvage some of those pre-apocalypse ones, you could still be good. However, let's assume you couldn't. Then, go for direct mechanical transmission. Your flying wind generators don't carry big turbines. Instead, they are kites that go up and down, periodically pulling on their line, which then directly (through a gearbox) make the propeller(s) move. Don't get me wrong, that's going to be a complex, maintenance-heavy, potentially breakage-prone (plot! drama!) piece of hardware.

Note that when conditions are favorable and you don't want to wear your hardware, you can put flying sails on your kites instead.

The kites may still have small turbines for onboard electricity, and for powering the kite control surfaces (or even reversing the turbines as propellers to gain altitude). You may also want some batteries for that. If you need only electricity - say, you're at port and want to sell on the energy grid - plug them on generators (or mount turbines on them) instead: many small, fast-rotating generators are easier to make than few giant, slow-rotating ones. Which is, in fact, why you'll see small flying windgens, an possibly solar-thermal, instead of traditional giant windgens on land.

You may want to store energy for dead wind days. If you can, cram a small power-to-gas station on the ship using, say, electrolysis and Sabatier reaction to produce methane on windy days. Note that you'll have to find a way to use methane on your engines.

Now, you tell me, why not have bigger, more efficient power-to-gas stations on land and simply use them to refuel your ship as before? Because it's cool. And because you may not want to count on external infrastructure, but mostly because it's cool.