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There's this organisation with an underwater naval base on Île Pott masquerading as a third rate hotel resort. Said base features several underwater hangars housing a considerable fleet comprised of several cruisers, destroyers, frigates, amphibious assault ships, submarines, patrol ships, corvettes, hospital ships, littoral combat ships, research vessels and aircraft carriers. All of these ships are designed specifically to completely submerge themselves underwater for long periods of time when travelling, in order to greatly reduce the risk of being detected and resurface upon reaching their destination.

One such vessel is the Kharybdis-class aircraft carrier, which can house about 50 manned aircraft, including fixed wing planes and helicopters on it's flight deck. Due to being incompetent with mathematics, I haven't figured out how big or long this thing is going to be. But, what I do know is that this ship will need a great deal of internal space to store these vehicles inside, some sort of mechanism to help it sink or float, and a hydrodynamic design that allows it to move efficiently on top and under the water.

How practical would such a vehicle be?

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    $\begingroup$ A hanger is either a hangman or a device from which something usu. clothes, is suspended. Aircraft are stored in a hangar; the word comes (through French) from Germanic *haim-gard; the English form, if it had been inherited would have been home-yard. $\endgroup$ – AlexP Aug 26 '18 at 11:30
  • $\begingroup$ Not very; remember Archimedes' principle $\endgroup$ – nzaman Aug 26 '18 at 11:54
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    $\begingroup$ It would be the biggest waste of money in the history of the planet. But it's a classic idea still. People could see it as a homage to the old timy spy movies. You don't need to establish things that are already established. Evil guys have evil underwater vehicles and convoluted impractical lairs, that's almost mandatory $\endgroup$ – Raditz_35 Aug 26 '18 at 12:41
  • $\begingroup$ 5 answers and not a single up vote? Come on! If it is good enough to spend your time answering it is good enough to up vote. $\endgroup$ – Willk Aug 26 '18 at 15:35
  • $\begingroup$ Does the carrier only need to be submerged while traveling, surfacing to launch the planes? Or do you need a way to do even the launches from underwater? $\endgroup$ – abarnert Sep 10 '18 at 17:10
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If you do it right, it is extremely expensive

The only way to do this that would minimize detection underwater and provide reasonable speed underwater, is to build a HUGE submarine that happens to be an aircraft carrier, not an "aircraft carrier that can submerge". While modern submarines can dive to thousands of feet below the surface, even diving just a short distance has big technical concerns:

  • You need to be able to seal around everything with watertight doors. Not just watertight doors between compartments below deck, as is the case with aircraft carriers and other surface ships, but around the entire flight deck. That is a huge expense for the doors and a tremendous waste of space because a lot of space above the flight deck and under the doors (== ceiling when closed) will be just air space. That is different from a normal submarine where there are just a few very small openings (hatches for entry/exit, torpedo & missile tubes, etc.). In a normal submarine, in order to keep the size of the boat (subs are boats rather than ships, don't ask me why) as small as possible, every cubic foot is filled with supplies, bunks, equipment, etc. and this would be the opposite.
  • Pressure differential is a big problem. Roughly every 33 feet of depth adds another atmosphere of pressure. A normal submarine, by minimizing openings and using a very strong hull, can manage this quite well. But an aircraft carrier would be huge. Hard to tell exactly (I'm sure the information is out there) but based on some rough numbers, let's say a decent size submarine has a main hull 40 feet in diameter (I know it may not be circular, but that is the most efficient shape and close enough for this calculation). That would be just over 1 atmosphere difference between the top and the bottom. An aircraft carrier would easily double that, probably even more, starting with 40 feet (or more) below the flight deck and the doors above the flight deck would have to be high enough to cover all the planes and equipment on deck. With a circular (or even anything close to it) cross section, this would be much large as the flight deck needs to be quite wide to handle takeoffs, landings and aircraft movements. Plus these doors would have to be inherently strong without lots of support pillars as those would get in the way of the flight deck.
  • Even with a proper smooth, curved outer section, traveling when submerged without detection would be really tough for such a large ship. As everything scales up, the potential for noise from engines, propellers, and even from noise inside the ship (working on preparing aircraft for battle while under way) is a big problem.

And if you don't do it right...

You could make a minimal solution provided:

  • You have sufficient hangar space to store everything in sealed compartments below the flight deck.
  • The flight deck is totally cleared when submerged, so no need to put a big door over the top. Of course, your catapults and other equipment would need to be able to handle the water pressure.
  • You travel just below the surface so that the bottom of your ship has only a couple of atmospheres of pressure.
  • Your enemy doesn't have sonar because with sonar it would be trivial to detect this big messy thing traveling underwater.
  • You don't care about energy efficiency because moving this big messy thing underwater will not do nearly as well as a properly designed submarine. Think about the air resistance if you stick a regular house on a trailer and pull it along, compared to an Airstream.
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  • $\begingroup$ It might help reduce some of the design issues if all of your vehicles are VTOLs, eliminating the need for a runway. en.wikipedia.org/wiki/VTOL $\endgroup$ – CaM Aug 27 '18 at 18:13
  • $\begingroup$ Reduce but far from eliminate. Look at a helicopter carrier - even they are big and bulky. $\endgroup$ – manassehkatz-Reinstate Monica Aug 27 '18 at 18:28
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    $\begingroup$ @manassehkatz In your defense, I actually don't know of any submarine able to submerge more than a few meters that doesn't have a perfectly circular cross section for the pressure hull. $\endgroup$ – Morris The Cat Aug 28 '18 at 22:44
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The basic problem with Things that are ALSO other Things is that while they're Super Cool, whatever Other Thing you're trying to make them do always makes them LESS effective at the first thing.

e.g. Forcing your Aircraft Carrier to ALSO be a submarine just makes it a less effective Aircraft Carrier. Every bit of space allocated to making the thing watertight and allowing it to control itself underwater is space you AREN'T using for more aicraft, fuel for said aircraft, and weapons for said aircraft.

You can't HIDE something that big effectively, submersible or not. You certainly can't hide it WHILE it's launching and recovering an airstrike, which means that even IF you could hide from detection while you're moving around, by the time the aircraft come BACK from whatever you sent them to attack, your submersible aircraft carrier has been caught on the surface by a couple hundred cruise missiles and annihilated.

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The japanese actually built aircraft carrier submarines, the problem was even though they were the largest submarines at the time they could only carry three tiny purpose built aircraft. And they had to be disassembled before diving to fit through the airlock.

Submarines are small and cylindrical because they have to be much stronger than normal ships to withstand the forces of diving and surfacing. Aircraft carriers on the other hand have to be huge because aircraft and their support systems take up a lot of space. Aircraft carriers are built like upside down triangles to get as much deck space as possible.

The other factor is silence, subs require amazing precise machinery far more precise than normal ships equipment these machines take up more space than a normal one of the same capacity, they also need many various forms of sound dampening which also take up more space, so you end up with less space proportionally in subs.

Additionally larger submarines actually need to have more ballast proportionally as they displace more volume proportionally (square cube law) and thus you lose even more space to counteract buoyancy, Aircraft would really add to this since they are mostly empty space and lightweight materials. So again the bigger you make the sub the less space proportionally you have to spare.

Lastly the bigger a submarine the slower it is, a normal ship only has drag of a small portion of the ship while a submarine has it everywhere so subs get slower faster than ships as you make them bigger.

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Such a big ship will be so loud while traveling underwater that you might as well just have it above water. Plus a bunch of other logistics issues, I would call it not the most likely vessel unless you wanted to include style points in your criteria. On the other hand, it's actually possible to make, so given a slightly futuristic scenario you could definitely do it. Just be aware that you can't avoid ANY sub detection systems.

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  • $\begingroup$ What makes it loud underwater? $\endgroup$ – Trevor Aug 26 '18 at 13:16
  • $\begingroup$ @Trevor D Sound propagates through water a lot farther in water than on land. Its easy to hear even small sounds in the water. Ships on the surface still make noise, but it isn't a fatal flaw like it is in a stealth submarine. $\endgroup$ – Clay Deitas Aug 26 '18 at 18:16
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Such a gigantic craft under water would require very slow movement. So slow they I'm not even sure it would beat the current in the areas where it needs to go.

It would also need under water hangars. Probably accessible from the bottom of the ship. This creates a bit of a design issue as now your craft that swim in, have to be hoisted to the top of the craft to be launched again. And this has to be done quietly.

It is doable, it's not practical though. These big ships never see battle, so having one that can sneak around isn't really helpful. There's no need to sneak around your own fleet. It's too valuable to lose to a single torpedo. And that's all your ship is, a giant submerged wall with a great big kill me sign hanging from it.

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