If you could build it there would be no limit on size. Just a limit on use.
The only time a larger crew would be needed would be docking, more line handler, that is if it could come into port.
building would also be a problem. The limit would be the ship yards.
I doubt if the engine room watch would have to be bigger. And the bridge watch would not need more people. And when maneuvering, it is done with the use of computers. There would be more equipment and surfaces so a few more crew members to do the maintenance, and a few more wipers and deck crew to clean and paint.
I think the practical limit is how big a ship you could build that wouldn’t break apart upon encountering rough seas. If you build a ship as strong as you can and still have it float, you eventually run into a size limit where the ship couldn’t stay in one piece in the worst conditions, like Cape Horn or a Pacific typhoon.
There is a physical limit set by the curvature of the earth. Beyond this limit the ends of the ship stick out into space, where they are limited by strength-of-material: there is a practical limit to the largest (longest and deepest) possible cantilever beam.
You can presuppose a “ship” that is a girdle for the earth, instead of being a ship, but I propose an axiom that a “ship” must be ship-shape.
Really? That sounds like it would be an administrative issue (i.e. they are too big/risky to be allowed) than a practical one. the English Channel is 20 miles wide at the narrowest point and at least 60 metres deep at the shallowest point of any through routes.
Maybe you meant The Solent? (the stretch of water between the Isle of Wight and the UK mainland?)
If you mark a straight line between two sea-level points separated by one mile, the midpoint of that line will be about two inches below sea level.
So the Shell Prelude, mentioned upthread as about 1600 feet long, would need to bow upward in the middle about 2/3 of an inch in order to float in dead-calm water without inducing any unusual bending stresses. Given that the ocean is never particularly calm - with wave heights far in excess of 2/3 inch - I’m guessing this is insignificant.
If you want to build a ship 8 miles long, the mid-span deviation is about 10.6 feet. I’m guessing you still don’t need to compensate because a ship eight miles long is going to be pretty flimsy and can flex to match the curvature of the ocean’s surface without inducing any terribly high stresses in the structure. The constant cyclic stresses of wave action are a far bigger threat to the integrity of the vessel, fatiguing the metal and constantly flaking off corrosion patina to leave bare metal vulnerable to new corrosion.
Or above sea level if the Earth is really concave, with the sky in the middle of it.
(But it isn’t, so don’t worry).
Yeah - I don’t think we’re actually capable of building anything that is rigid enough to worry about this.
At larger scales still, tidal variations probably come into play (that is, the curvature of the ocean is variable), but again, can probably be largely ignored due to the inherent flexibility of materials.
In theory we could, but the cross section of the ship would have to be extremely tall. The Emma Maersk is 98 feet from the deck to the bottom of the keel, and it’s 1302 feet long. For a ship 42,000 feet long (8 miles), you might imagine scaling the height of the structure accordingly, creating a ship that measures 3100 feet (0.6 miles) from deck to keel. Be prepared to spend some money improving deep-water port facilities: you’ll need to do some dredging unless your ship floats really high on the water, and either way you’ll have to install some truly fabulous cranes for loading/unloading cargo.
A naval military vessel isn’t a commercial craft. The differences in pay and mission alone make comparing the two a logical fallacy. Different mission, different crews. The only commonality is that they ply the oceans.
A larger would require more technician and sailor to care for what would be an entirely different propulsion system and to ensure that the myriad systems didn’t fail. While these techs could be flown out the ship if there were a problem, there are areas (specifically in the Pacific and the Indian oceans) were this would be prohibitively expensive, bordering on impossible.
There’s simply no commercially viable reason for a ship larger than a ULCC even if there more harbors which could accommodate them, which there not.
If we were going to go extremely long/wide, I don’t think we’d have much reason to also go tall/deep - a miles-square raft composed of supertanker-sized pontoons wouldn’t need to flex a whole lot anyway - the edges would, but the structure itself would suppress waves.
It’s difficult for me to regard a bunch of loosely-coupled hulls as a single ship. If the idea of making one big ship instead of 2+ small ships is that a single hull has less hydrodynamic drag than multiple hulls (on a per-cargo-ton basis) , then a raft of supertankers lashed together with cables won’t be any more efficient than if you decoupled them into fully independent vessels and just had them sail to their destination as a coordinated fleet. For the purposes of the OP, I think discussion ought to deal with a vessel contained entirely within one single, contiguous outer hull.
If you just bolt them together side by side, I agree, but if you have a rigid platform spanning supertanker pontoons spaced apart from one another, then it could still be considered a single vessel (just as a catamaran or trimaran is considered one thing, not many).
A half mile wide multi-hulled platform will have smaller displacement than a single solid half mile wide hull (and thus smaller carrying capacity*), but as water can pass between the hulls (rather than going all the way around) must surely make it easier to move.
*I’m not assuming maximum load capacity or practical use of any sort is the only goal here.
Notice that earlier in the sentence mentioning aircraft carriers I specifically said “For a completely hypothetical ship like the one posed in the OP, crew size certainly wouldn’t be a problem.” That means I was talking about “a completely hypothetical ship like the one posed in the OP.” New paragraph, new idea.
I wasn’t directly comparing naval and commercial vessels, I just used carriers of an example of how many people you can cram onto a ship. As for the rest of your post, I fail to see why “a completely new propulsion system” would be needed. The current basic “diesel engines turning propellers” or “diesel generators powering electric motors which turn propellers” systems scale up fine. Either use more or bigger engines, or use existing ones and accept diminished speed and maneuverablility. I’m not sure you realize A) how few people are currently needed to operate large ships and B) how little they get paid in relation to the operating expenses of the boat. The Emma Maersk, one of the largest ships in the world, operates with a crew of less than 20 people, most of whom are paid on the order of a few thousand dollars a month. Even if you quadrupled the size of the crew, the monthly payroll would be tiny compared to the other expenses, namely fuel. It’s also worth noting that such an extremely large ship wouldn’t even have to enter a harbor to be useful: A tanker could theoretically pump oil to tenders outside of a harbor to be delivered to port, or could hook up to an underwater pipeline offshore.
Regardless, the OP is talking about hypothetical ships, and clearly crew size is not going to limit how big a hypothetical boat can get.