Theoretical warship: Evaluate

[phongn]

The problem is its sheeer size, even a dozen torps wouldn't put enough water in it to sink it, you'd need a few dozen, possibly even hundreds of torps.

Modern torpedos do not slam into the hull to let water in. They detonate under the keel to break it, which has the added side effect of letting water in.

[Sea Skimmer]

The problem is its sheeer size, even a dozen torps wouldn't put enough water in it to sink it, you'd need a few dozen, possibly even hundreds of torps.

Actually, no, you would easily sink it with an Oscars torpedo payload.

[Darth Wong]

(sigh) OK, let's do the math on a mile-long steel ship, OK?

Let's take Manji's dimensions: 1600m long, 274m wide, 137m deep. If we assume its underwater shape to be a triangular prism, we get roughly 30 million cubic metres of water, and 1 cubic metre of water is roughly 1 ton, so we get 30 million tons of water. Therefore, in order to float, this beast must weigh in at 30 million tons (let's just disregard his asinine 4 million ton figure).

Now, let's disregard all above-water structures and simply look at a triangular prism of the aforementioned dimensions. Each side of the triangle would be roughly 310,000 square metres, so the total surface area is 620,000 square metres. If we assume 50% of the vessel's mass is its hull and 50% is internal structural members, we have roughly 3.8 million cubic metres of steel to work with, half of which is the hull. This means that the wall thickness will be a hefty 3 metres, which should be enough to stop most conventional weapons.

We have enough steel left over for 34 solid cross-beams of similar width (this is an extremely simplified analysis; it's not as if I'm seriously trying to build this thing), which would imply one cross-beam every 50 metres or so. This, in turn, means that a 50 metre wide piece of steel must withstand ocean pressure.

The average water pressure from surface to 137m depth is roughly 670 kPA, so each 50m long segment would be taking roughly 6.5 GN of force (normal vector to surface). The maximum bending moment in the middle of each segment would be roughly 4E10 Nm. The associated stress is Mc/I, where c is half the thickness (1.5m) and I is moment of inertia (112.5 m^4), which works out to roughly 530 MPa.

Hmmm, slight problem: this is twice the yield stress of normal structural steel, and while there are alloys which can meet this criterion, they carry significant penalties in terms of formability, weldability, cost, ductility, etc. Worse yet, most of them only attain these characteristics after heat treatment, and you can't put a mile-long ship into a heat treatment oven. The welding process will leave heat-affected zones in which these theoretical stress limits are mere fantasy, and the ship will sink.

In short, while this analysis was very superficial (again, it's not as I've actually designed this thing yet), nobody needs a nuke to sink this thing. If it can be built at all, the water pressure load alone will be enough to buckle its hull, and if it doesn't by some miracle, then the slightest impact from any heavy conventional-explosive torpedo should easily push it over the edge, since it doesn't have any extra strength "left over" after resisting water pressure.

[EDIT: I neglected to mention that many conventional weapons can punch through 3m thick steel, and that in order for it to withstand hull breaches through compartmentalization, those compartments must be strong enough to withstand water pressure, which in turn means that a correspondingly large amount of metal must be allocated to their structures and taken away from the hull]

[weemadando]

I hope you realise that WAVE ACTION. Just regular WAVE ACTION would destroy this vessel unless you made it with multiple hulls, hinged hulls and flight decks etc etc etc.

[Sea Skimmer]

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[EDIT: I neglected to mention that many conventional weapons can punch through 3m thick steel, and that in order for it to withstand hull breaches through compartmentalization, those compartments must be strong enough to withstand water pressure, which in turn means that a correspondingly large amount of metal must be allocated to their structures and taken away from the hull]

Hull breaches and flooding aren’t the biggest problem. The keel snapping when 10,000 pounds of explosive displace the water under several hundred meters of its length is going to be the problem. Shock damage is also going to be real bad, since the ships size and depth will serve to focus the blast under the hull.