How big can a ship get

I am not talking about the largest currently. I mean how large can they get theoretically using materials and techniques currently used or feasibly predicted to be useable.

By largest, I mean largest by tonnage, although I would be fascinated to learn how big a ship can get by length, beam etc, as well as internal volume.

If you want your ship to easily travel between the Atlantic and Pacific then it has to fit through the Panama canal. This limits it to 950 feet long, 106 feet wide, and a maximum draft of 39.5 feet, and it can’t be more than 190 feet from the waterline to the highest point on the ship (numbers according to Wikipedia). This is the limit for a lot of ships these days.

If you just want something that floats, I don’t see where there’s really much of a limit at all theoretically. Just imagine bolting aircraft carriers together. There’s no reason you can’t make larger and larger groups of huge aircraft carriers all attached to each other, other than practical reasons like cost, being able to fit into harbors to dock, fuel costs, crew logistics, and that sort of thing.

I mean actual ships which float and sail under their own power.

I did one one the other day that I couldn’t flush away.

Oh sorry. That’s probably thread shipping.

Tonnage” is in fact the measure of the volume of a ship. What you probably mean is displacement.

How would you define a single ship as opposed to several ships linked together (perhaps using the individual ships as pontoons carrying a larger structure)?

There’s probably no size limit. The limits would primarily be:

[li]Practicality - What would be the use of too large a ship? Most harbors are relatively shallow and you’d have construct specialized equipment just to load and unload it.[/li][li]Control - it would be almost impossible to control a vessel beyond a certain size without a complex computerized system and a relatively large crew. That would add to operational costs and decrease the usefulness of the ship.[/li][/ol]

You could theoretically construct a ship a mile in length. However, with no harbor able to accommodate it and its prodigious fuel requirements, it would be costly and impractical.

Here’s a ship claimed to be the current champion - the Shell Prelude.
Length = 488 m (1601 ft)
Beam = 74 m (243 ft)
Fully loaded weight = 600,000 tonnes

Emma Maersk is bigger than that, but can get through the Suez Canal.

Existing large oil tankers operate with impressively small crews.

Fuel requirements per unit of cargo are smaller for large ships - which is one of the principal reasons they are built.

I understand the problems wrt the harbours, IIRC the larger ships in the world cannot even sail the English Channel, never mind the Suez/Panama etc. But, I am asking for the largest ship that can be made (and The Stafford Cripps, thanks for the correction, I was aware, but mixed them up), with current and realistically imaginable techniques.

My own thought has been after a certain displacement or length or tonnage the material will be unable to keep her sea worthy.

I mean is a Million Tonne displacement ship possible? 2 million? Ten Million?

Again, those ships are of a “reasonable” size. The question presumes a vessel far larger than a current ULCC (Ultra Large Crude Carrier) the largest of existing ships. Larger than that you would be talking a vessel that might require several normal crews to operate it safely.

Supertankers don’t have proportionately larger engines. There are training facilities to teach people to handle such a massive vessel without much power. They put the students in mini-supertankers, long canoes with little electric motors at the back. All steering and power decisions have to be made long in advance.

Well, there have been icebergs reported with a length of a couple of hundred miles. Given that icebergs float (and are thus technically analogous to boats) and that ice is structurally a lot weaker than steel, I’d wager that there’s no particular upper limit. Your limitations would be engineering issues. Such as if you actually wanted your boat to go anywhere, you’d need very massive engines. And you’d probably run into some mega-structure issues where you’d have to put in expansion joints just to keep the thing flexible enough to keep from disintegrating on you.

Money and materials is the limiting factor. At this day and age, a lot of simulation will handle the majority of the real-world factors that would have an impact on size and can be handled. Go back 30 years and that would not be so.

Only other thing I can think of would be the process and logistics of building a large ship. Usually they are built on land in a dry-dock that is flooded with water and then its floated. So, if you were to build an iceberg the size of B-15 you would need a dry dock and supporting infrastructure that could handle such beast. That’s a lot of materials and people.

In theory, I guess one could commandeer an iceberg like B-15, strap on some form of propulsion system, and build a little city on it. But, would that be a ship?

It can be as big as the entire observable universe

For comparison, the original U.S.S. Enterprise – NCC-1701, Captain Kirk’s starship – masses 600,000 tonnes and is 289 m long.

(Source: Constitution class | Memory Alpha | Fandom )

I don’t think crew size alone is going to impose a size limit even if we’re talking about commercially viable vessels. Payroll just isn’t a significant expense for large freighters, and any increase in personnel-related costs would be dwarfed by the economies of scale that a well-designed super-large ship would provide.

For a completely hypothetical ship like the one posed in the OP, crew size certainly wouldn’t be a problem - we have no trouble cramming thousands of sailors into aircraft carriers that are “only” ~1,000’ long.

Not quite the same but a 1000 meter floating runway was constructed in Tokyo bay and used to test for any effect on take-offs/landings and marine life - everything worked fine. There are plans out there for up to 4000 meter runways/airports floating about.