Shipbucket

With the new fuel cells and AIP is it possible to build a non-nuke sub in the 7,000+ ton range? I'm interested not only in longer endurance, bigger may not mean longer endurance, but greater weapon loads.

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Eschew obfuscation

Possible... well, anything is POSSIBLE. The Japanese build the I-400 class of submarine, which displaced 6,560 tons submerged, in the 1940's.

Of course, we all know the failings of the class; many of these can be overcome by the modern refinements that you have mentioned. Unfortunately, the largest problem- that of sheer size versus propulsive power- will remain regardless of those developments. The bottom line: a submarine will use more power as its size increases, and diesel/electric submarines simply have less of it. You could probably get a great extension of the time the design could spend at sea overall, but when compared to a nuclear submarine- or even several smaller and similarly equipped diesel/electric boats, I would imagine- the actual time submerged wouldn't be increased that much due to the larger power demands.

Yes you can. Such a sub might even has incredibly high endurance just from the fuel load, and might even be able to run the diesels at full load and high speed while snorkeling.

To be honest though, I'm not sure that being twice the size of a Sōryū is going to buy you all that much.

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...and the Swedes have very convincingly demonstrated that, even in the vast expanses of the Pacific, size doesn't necessarily matter, as their "loan" to the USN of HMS Gotland proved. This submarine, though, is fitted with the indigenous Sterling machinery which sometimes is labelled as the "poor man's nuclear engine" (though the Swedes are by no means poor...)

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My Avatar:Петр Алексеевич Безобразов (Petr Alekseevich Bezobrazov), Вице-адмирал , царская ВМФ России(1845-1906) - I sign my drawings as Ari Saarinen

Well, not quite.
The Swedes had a major advantage in those exercise in that they new where the task-force was going to be and always had plenty of time to preposition themselves.
Lack of sustained underwater speed is the Achilles Heel of diesel subs, even if they have AIP. In open water even if they detect a target, chances are they won't have the submerged speed to intercept it. This isn't such a big problem in coastal regions, which is why they excel there, but in the open reaches of the Pacific? Not so much.

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“Close” only counts with horseshoes, hand grenades, and tactical nuclear weapons.
That which does not kill me has made a grave tactical error

Worklist

Source Materiel is always welcome.

Well the Walrus types are official the best non Nuke, diesel submarines.

That's simply untrue, unless you consider the sole use of a submarine to be a mobile minefield. In this mission a good diesel boat is just as good as a nuclear submarine, perhaps better. However, the ocean is a magnificently large place, and diesel submarines have very limited strategic mobility compared to a nuclear submarine.

How much you care about strategic mobility is related to the mission of the submarine in the first place, and the region in which you intend to use it. The Japanese are constitutionally limited to operations near their home waters, and so large diesel boats make considerable sense for them. Even more so the Swedes, who will always be operating in the confines of the Baltic. The math is very different for the USN, the Royal Navy, and for the Russians, who can reasonably expect to be conducting extended missions several thousand miles from home, and needing to rapidly transit from one place to another.

So really, the Aussies should really be looking at nuclear power when replacing the Collins class, given the large distances they also need to transit.

I'd say very much so*. Right now the Australians can't circumnavigate without refueling, and this makes war planning mightily complex.

*However it comes down to what the Australians want for defense priorities (doesn't it always?).

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As near as I can tell, the Aussies have gone with and will continue to go with conventional power for exclusively political reasons. They would absolutely be better served with nuclear power.