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Situation:

  • modern tech equivalent
  • sparsely populated planet
  • very limited amount of fossil fuels, but sufficient amount of nuclear (plus willingness to use it)

Consequences:

  • designing single nuclear reactor for maritime propulsion (to cut costs)
  • building a single shipyard with single dock, as its output would cover needs of the whole planet (to cut costs)
  • basing draft limit of all main sea ports exactly on what this shipyard produces

OK, so in practice that would lead to creation of dominating main class of sea going ships, as making something bigger would cause nightmare for the shipyard and most ports, while making it much smaller would seem pointless as one would have to pay for the reactor anyway, and get a container caring... speedboat.

Nevertheless, there are simply different needs for container or bulk carrier for ore. On the other hand, there were also cases of ignoring such nuances and making a single class for different class of cargo (ex. liberty ships) So how to realistically balance it out? (If already there are a few factors to push towards roughly same size, then push it even further and take advantage of effectively serial production? Make the dry dock and port slightly bigger than generally needed to stay around this sweet spot, while making slightly bigger bulk carriers and slimmer container ships?)

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    $\begingroup$ Are you required to run all of your ships off of just one of these standard power plants? For instance I believe most modern nuclear carriers have two power plants, and the USS Enterprise for reasons of its own has eight. If your navy absolutely must standardize power, angling for a smaller size and using multiples on larger ships would give you more flexibility. $\endgroup$
    – Cadence
    Sep 28, 2019 at 8:46
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    $\begingroup$ "Basing draft limit of all main sea ports exactly on what this shipyard produces": this is bass-ackwards. The draft limit of a port is whatever it is. Increasing it is possible, but definitely a complex and expensive project. See for example the Eurogeul to understand what this implies. In real life it is the shipyards which make ships suitable for various ports, it is not the ports which metamorphose to accomodate a given class of ships. (And I don't see how one single type of ships can accomodate both the Baltic and the Pacific.) $\endgroup$
    – AlexP
    Sep 28, 2019 at 10:47
  • $\begingroup$ @Cadence This approach is right for military, where no-one whines about excessive costs, while some redundancy in case of damage is paramount. In RL the only 4 built nuclear merchant ships, had only one reactor each. $\endgroup$
    – Shadow1024
    Sep 28, 2019 at 11:28
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    $\begingroup$ @Shadow1024 They can either complain about the excess cost of using two small reactors instead of one big one, or they can complain about the excess cost of using two small ships instead of one big one, or they can complain about the excess cost of using a big ship to do a small one's job. Or they can learn to build more than one size reactor. $\endgroup$
    – Cadence
    Sep 28, 2019 at 12:20
  • $\begingroup$ Modern shipping size is generally limited by key infrastructure (canals, locks, ports, bridges, cranes) instead of power available. Identify your key infrastructure constraints, the biggest commodity shipped, size your ideal biggest-commodity ship accordingly, determine the power needs of that ship type, and there's your optimal reactor output for most large vessels. Then work your way back down the tree for the best sizes of other-commodity ships using that power output. Of course, you're introducing a single-point-of-failure common to all ships. $\endgroup$
    – user535733
    Sep 28, 2019 at 13:17

3 Answers 3

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Make your ships modular. Make them barges.

barges

https://en.wikipedia.org/wiki/Barge

Your nuclear tugboats are all the same size and interchangeable. That makes repairs and maintenance easy. Like a tugboat moving river barges or a locomotive moving a string of train cars, the rest of the boat will depend on the cargo and use and will be assembled from modules according to need.

Modular boats will also be efficient because the tug is not tied up in port waiting. The boat can be broken down to modules once in port, and the tug can attach to new barges and leave on its next trip while the barges it just brought in are unloaded.

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    $\begingroup$ Ocean-going barges. Hmm... $\endgroup$
    – AlexP
    Sep 28, 2019 at 22:35
  • $\begingroup$ @AlexP Not exactly news. Remember the "portless garbage barge" from a few (okay, 33) years ago? That was an ocean-going barge, traveled all the way to Belize at one point (then back to New Jersey). $\endgroup$
    – Zeiss Ikon
    Feb 7, 2020 at 12:28
  • $\begingroup$ The issue I see with this is that separate hulls are less efficient than a single, really big one. $\endgroup$
    – Zeiss Ikon
    Feb 7, 2020 at 12:29
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One shipyard, but multiple dry docks.

Funnily enough, it is already the case that nuclear-fueled ships are built and serviced in only like 8 ports only around the globe.

However, "one size fits all" is the wrong approach! Make it 2 reactor sizes and 3 ship classes.

Small ships get 1 small reactor. Medium ships get either 2 small reactors or already the large one, depending on mission and energy needs. big ships get 1 large one (maybe 2 for max-sized large ones with military use).

2 reactor designs isn't much, just for the sake of improving it you'll have like 5 small ones and big ones after a decade or so anyway.

And regarding ship sizes: This depends on just HOW scarce oil and so on is (aka how desperately do you need to use a nuclear reactor on, lets say a 150m-length frigate class), because you MAY be able to skip the small size.

However: you can have infinite ship classes! Because all you are limiting is the maximum size (or the minimum to not have to fit into the smaller dock). You can still build any range of ships you want. (Well, within the limits of your giant dock)

The 2 reactor sizes I propose also have the advantage of @Willk 's Modularity in that you can get pretty much the exact energy budget you want while sill keeping development costs down.

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Two basic solutions here: first, you have one maximum hull that can have (at least) three superstructures: can be built as a container freighter, a bulk carrier, or a tanker. Might be a fourth, if these folks ever find a need to ship their limited fossil fuels in the form of liquefied gas, refrigerated and under pressure.

Second, the reactor design is small for the maximum hull. As with the American aircraft carrier Enterprise (the nuclear version), a large ship would have multiple reactors (Big E had eight as launched, of four different designs, though on its last refit before decommissioning that was switched to two identical ones, slightly larger). Generally, the biggest hull you can work with (post-Panamax, say) is the most efficient, both in terms of capital cost for carrying capacity and energy per ton over any given route. So make the (biggest) hull as big as possible -- but size the reactors so smaller ships, when needed, can use the same design. This helps safety (when they're all the same, no one makes mistakes by remembering the wrong version of a procedure) and cost (the more you make of the same thing, the less each one costs).

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