> There are practical limits to how big a ship can be.

The limits are way beyond what's at issue here. There are lots of ships which are already too big to go through the Suez canal.

Canals tend to limit dimensions based on:

* the canal width (limits beam)

* the canal curves (limits beam and length)

* the canal depth (limits draft)

* locks (limits length)

* bridges over the canal (limits height)

"Capesize" is the general category of ships which can't go through the Suez or Panama canals[0], and must therefore go around Cape Agulhas and / or Cape Horn.

[0] although with the deepening of the Suez canal it's now possible for many capesize to go through Suez, just not Panama.

>Apparently they're already about as long as steel can take, so they're getting wider instead.

How does a ship being longer cause stress on the steel?

Ships don't always float on mirror flat oceans. Waves wave around them which can create more buoyant force one the ends than at the middle. This makes the middle "sag down" and the structure of the ship has to be strong enough to take the associated tension and compression. It can also happen that there is more buoyant force at the middle forcing it up while the ends are somewhat less supported. (Also known as "hogging".)

The longer the ship is the stronger these forces can be.

Out on the ocean there are waves - if the ship is between two of them it acts like a bridge and the steel has to support the weight or snap in two.

Random thought: Why don’t we see things like ship trains? With many linked smaller hulls being pulled by a single tug. Or something to that effect. Seems like a solution to that phenomenon.

Steering would be painful. Trains work particularly great because of the tracks.

Also, drag on a hull is such that longer hulls tend to be more efficient.

Would you need to do much tricky steering outside of getting in/put of ports? At ports, even ships themselves are often pulled by tugs because of the difficulty of manoeuvring in such a small and busy space.

Wind and waves would be a problem as they'd push the train around and it'd be hard to control with a tug. I think it'd be less hydrodynamically efficient too which would mean higher fuel costs overall: assuming you could build a single large ship with the same capacity as the train then it'll have a lower surface area => lower drag.

I would think (i.e. pure conjecture) that it's about economies of scale. Making enough smaller ships to make trains will costs more steel. Making a train that fills up a similar area in the ocean reduces the available storage per shipment.

Follow this thought to "let's just make these trains as long as possible." You can imagine some wild consequences, both positive and negative.

River barges basically work this way, likely mainly due to tighter curves on rivers and inland canals.

We do, I have seen three or four smaller hulls. On river

At that point you might as well make multiple ships.

Surely it's a lot easier and less expensive to build an engineless, ship "trailer" than a self-propelled ship with an engine, fuel tanks, propellers, a generator etc?

The ships are already at the maximum efficiency size given their engines, so to pull a similar sized ship they’d have to add an engine to the first one, in which case why not add it to the second and have two ships?

Could they be pulled by multiple small tugs instead? (no idea if that's feasible on the open seas!)

Eventually, a long enough ship should be supported by 3 or more waves at the same time, assuming waves have some sort of maximum wavelength. (Disclaimer: no idea what I’m talking about)

But there's an obvious solution to that: more steel.

Still, a far more effective solution would be increase width and draught likewise to reach your target volume. And that's where we're back at ${canal}max classes (locks also impose a length limit but I'd assume that those are much easier to adapt than width/depth).

If Suez Canal authorities are serious about trying to prevent future Ever Given incidents (how serious they are likely depends on the exact outcome of the ongoing liability fights) they should keep some parts of the canal at the existing profile (locks perhaps?) to prevent suezmax 2.0 (or whatever their current version+1 is)

More steel/thicker/different steel is expensive, as is say some other material like carbon fiber composites .

We could make a much bigger , lighter efficient ship with carbon fiber entirely, however there is no economic incentive to spend 100x on the ship, fuel costs and economies of scale rtc don't help recover that capital cost

> More steel/thicker/different steel is expensive

Expensive would be OK, of course a bigger ship costs more to build than a small one, but you hope to make the investment back by carrying more cargo and additionally benefit from some economies of scale.

However, the biggest ships we have are at the limit of how thick we can join steel plates with current welding technologies. So there's some non-linearity here.

Yeah, it is ecosystem of changes, you need bigger docks , shipyards and canals in conjunction with the technology improvements.

The costs of all of it needs to be economically feasible even if the technology is there, I don't think welding is only reason we have been limited in the size

You can see the practical limit in the ships designed to go directly from China - Los Angeles - as it's just open sea.

Those ships still have to enter the harbor and then have to be unloaded. The Benjamin Franklin has a 52 foot draft and stacks containers 10 high [1]. That means deeper dredging and bigger cranes. She called on LA, Oakland and Seattle but now transits from Asia to Europe via the Suez Canal.

Who pays for this dredging and these cranes? The localities. It's difficult to defray these costs with shipping fees. American President Lines pulled their headquarters from Oakland to Phoenix when Oakland did that. [2]

[1] https://en.wikipedia.org/wiki/CMA_CGM_Benjamin_Franklin

[2] https://www.prlog.org/10551489-apl-phoenix-headquarters-relo...

No doubt those waters are ideal given the high traffic and limited geographic barriers, even these however ships would still need to dock in LA.

I am not saying the limit doesn't play a major role, I am saying if it's was the main bottleneck there would be a lot of r&d in solving it, solving it would certainly help, but the economics and other limitations mean solving welding won't mean a revolutionary change to ship sizes

Leverage, concisely.

why does that now make think of spherical cows? :-)