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Take the situation of a modern carrier group (Aircraft carrier, etc.) anchored just off shore (far enough that they initially couldn't run aground) with all the crews abandoning ship for land (in a hurry). The ships then stay there for many years with only the odd adventurer/scavenger visiting.

  1. How long would the ships survive with no occupants or maintenance?
  2. How long would it take for them to break down (rust away)?
  3. When would they likely sink completely?
  4. What condition would they be after 10 years? 25 years? 50 years? 100 years?

Assume they were anchored off shore in an area without seasonal hurricanes.

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  • $\begingroup$ Are we talking WWII-era, modern, or post-modern vessels? $\endgroup$ – Frostfyre Apr 21 '15 at 17:44
  • $\begingroup$ Check out Life After People. I think they did an episode on mega constructions (and I remember seeing an aircraft carrier featured in on of the episodes.) It lasted around 100 - 150 years, but by around 20-50 it was inoperable due to plants and undersea life settling on it. $\endgroup$ – JDSweetBeat Apr 21 '15 at 17:49
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    $\begingroup$ The answer to this will partially depend on where it is. Meaning where in the ocean and how far from shore, this will dictate what currents are present and may end up running it aground or taking it out to sea, either of which could shorten its operability drastically. $\endgroup$ – James♦ Apr 21 '15 at 17:55
  • $\begingroup$ @James Since the OP specified the absence of seasonal hurricanes, I assumed the group would be anchored in a cold clime, most likely Scandinavia, Russia, or Canada. $\endgroup$ – Frostfyre Apr 21 '15 at 17:59
  • $\begingroup$ What was the cause of the apocalypse? Apocalyptic phenomena often have an impact on the seas. $\endgroup$ – Frostfyre Apr 21 '15 at 18:06
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Why doesn't anybody ever ask the sailor?

In your case, the anchor isn't very likely to be down. Anchoring is a purposeful operation. Abandon ship is a different one. They'd have to have been at anchor first. Also depending on the nature of the emergency, they might have attempted to scuttle the ship, a process of self destruction which leaves classified material destroyed and the ship 100% unusable - every critical system would be hopelessly damaged, usually right around the special, hard to find parts.

I can say without a doubt in my mind that the hull can last for countless generations, especially if the ship isn't moving under its own power. The reason hulls have to be maintained is because you are pushing a piece of painted steel through the water at (nautically speaking) high speeds. This is an incredibly erosive force. Sitting (maybe at anchor) for centuries is nothing compared to that, even if the thing is getting hammered by hurricanes.

The barnacles would be obscene and it would need a serious amount of work to restore its original performance at speed, but it wouldn't rust through just sitting there. Even after the paint finally gave way and rust eventually set into the (rather high quality) steel, rust doesn't rust once it rusted. Something would have to physically remove the outer layer of rust to expose new metal to the sea. I have no doubt that the ship itself will be ran aground by prevailing currents before the hull simply springs a leak from sitting too long.

Now, a carrier group has lots of ships in it. If every single sailor in every single ship bailed out, and none of them were properly set at anchor (I'm telling you, this takes planning), the thing that's going to sink these ships is them bumping into each other. Eventually they'll bob up and down against each other's hull a few times, and eventually the damage will be severe enough to make the ships take on water - and then down they go. This scenario wouldn't take that long - unless they drift apart, this could happen in a few weeks to a few months depending on conditions.

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  • $\begingroup$ The ships are anchored... $\endgroup$ – Steve Ives Apr 22 '15 at 9:54
  • $\begingroup$ He did ask, you just took a while to answer. :) the ships being anchored was part of the question, we just don't know why. $\endgroup$ – AndyD273 Apr 22 '15 at 12:21
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    $\begingroup$ @AndyD273, sure - I'm just providing a bonus reality check. There aren't a lot of ways to find this stuff out, and one of them takes years and isn't much fun. $\endgroup$ – Sean Boddy Apr 22 '15 at 15:43
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There was a Life After People episode about ships. Most of the navy ships would still be floating after a couple hundred years (depending on hull thickness, weather, hull imperfections, if the interior hatches are open or closed to allow water to flow freely). The big problem is when the anchor chains fall apart after a few decades allowing the ships to either float away or run aground.

The LAP wiki has some of the information, though of course not all of it is scientifically verified I'm sure. I'd use them as a rough estimate more than anything.

The rusting away part depends on the depth of where they are when they eventually sink. Say the anchor chains rust away and the wind/currents runs them aground. Eventually they take on to much water and sink, but since they're in shallow water they stay mostly upright and mostly above water. After a couple more centuries they've lost enough structural integrity that they begin to collapse under their own weight. The parts underwater take the longest to disappear.
The aircraft carrier is the last to go.
They are now active marine habitats/artificial reefs.

TL;DR: A really long time, but how long depends on a lot of stuff we don't know.

Edit: Post apocalypse they would probably become refuges for wandering people, which could extend their lives a bit as people would do maintenance and seal leaky compartments, or could shorten their lives a bit if those people come under attack.

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    $\begingroup$ Rivet City, anybody? $\endgroup$ – nhgrif Apr 21 '15 at 22:31
  • $\begingroup$ Welcome to the Enclave. $\endgroup$ – Mazura Apr 22 '15 at 1:42
  • $\begingroup$ After they sink, what happens depends on whether ocean levels rise or fall. You may get a small island or a small mountain. $\endgroup$ – Raphael Apr 22 '15 at 8:55
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AndyD beat me to it with the anchor chain...

I have no specific knowledge about marine, but I assume the anchor chain would be the first to break. After that, the ships are likely to run ashore at the nearest coast, where they probably will be taken apart by scavengers.

If your ships are from military origins, the owning country may think about blowing them up, that the weapons and munition won't go in wrong hands.

For reference about the condition of the ships in your timeline, you could look up photos of ship-graveyards. Wikipedia has a nice list for more search.

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    $\begingroup$ About blowing them up, and allowing for the post-apocalypse tag, there might not be nations left to blow them up, and the stuff on board would be way to valuable anyway. Good post and welcome to world building! $\endgroup$ – AndyD273 Apr 21 '15 at 18:32

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