How do sunken ships like the titanic not get crushed under the pressure?

Question

I assume the metal and materials the boats are made of are strong but surely not to withstand the pressure of being 12,000 feet underwater? At the end of the day it’s not like the engineers had to consider holding up to that much pressure in their design right?

In: 495

Answer 1

You need to think a bit about what “crushing under pressure” is, and why it happens.

The implication is that there’s some watertight structure maintaining sea level air pressure inside, with the external water pressure outside squeezing it. If the structure isn’t strong enough, it gets compressed, until either

1. the structure gains more strength due to deformation and now touching itself more internally,
2. the air pressure inside increases due to the compression and can support the water pressure, or
3. the structure loses its watertightness, water floods in, and *equalises the pressure* so there’s no more crushing

1 is what happens if you’re trying to squish, say, a carboard box. It folds in on itself, ridges etc form internally touching the other side, and it gets harder to squish.

2 is what happens if you blow up a balloon and then take it underwater – the water pressure will cause it to shrink, maintaining the balance of air and water pressure at increasing depths. (this is also happening to your lungs at the same time if you’re swimming down with the balloon!)

3 is what happens in most cases with rigid materials if you go deep enough, eg a shipwreck. There’s no air left anywhere in the ship once all air pockets have been crushed and broken by the pressure, so everything has water surrounding it so there’s no bulk crushing force any more. Sure each individual steel beam or whatever is being squished by water pressure, but solid metal is pretty strong stuff so the difference is barely measurable (I haven’t looked up the numbers, but probably a 1 meter steel beam is like 1 micrometer or something shorter at that depth)

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And finally…

>but surely

It’s almost always a bad idea to preface your (quite possibly incorrect) suppositions with this