Submarine Amour

[MarcA]

I notice all subs have 0 armour everywhere. Don't the pressure hulls of subs have substantial thickness just to survive the effects of pressure at depth? Wouldn't this give all subs some armour factor.

(I must say I have no idea how thick a sub hull is)

[wild_Willie2]

Subs are build to endure water pressure, NOT to prevent shells from penetrating.
Although its diving body is made of steel, this is not the hardend steel used in armour. 
 
ANY penetrating shell in it's primary dive body will render a sub virtually useless. When the primary divebody has been compromised it will be unable to withstand any degree of water pressure without catastrophic failure. 

[Speedysteve]

100% concur with WW.

[bbbf]

What, you don't want to crash dive in a sieve?

Wimps!

[wild_Willie2]

Well it WOULD improve your crashdive time average.
Like they say, the fastest laptime for an engine is the round before it goes boom...

[MarcA]

ORIGINAL: wild_Willie2

Subs are build to endure water pressure, NOT to prevent shells from penetrating.
Although its diving body is made of steel, this is not the hardend steel used in armour.

ANY penetrating shell in it's primary dive body will render a sub virtually useless. When the primary divebody has been compromised it will be unable to withstand any degree of water pressure without catastrophic failure.

Thanks Willie

I thought it would be something like that

[Feinder]

FWIW, the "durablity" of a sub is not it's submerged disp (as suface ships it is a ratio of displacement), but rather it's dive depth divided by 10 (roughly)...

-F-

[Tiornu]

Submarine love? I don't think that's legal in this state.
The British felt that you needed a 4in gun to give a submarine hull a good thumping. It may be the long American 3in gun was adequate.

[rokohn]

With submarine steel, ductility (ability to bend and absorb a shock) is more important than the hardness (ability to resist penetration).  Hardness and ductility are mutually exclusive properties in steel.
 
Armor plating for the side belts on BBs and CAs is actually a compound material.  The part of the plate facing out has been impregnated with carbon (the cementing process) leaving a surface with a very high hardness and strength but no ductility (think of glass).  The back of the plate is left alone, leaving a backing of high ductility but lower strength.  The plating will break up the smaller incoming shells on the hard surface.  But any shell powerful enough to get past the hard surface of the plate, will be slowed down by the back half as the plate absorbs the impact of the shell. (Actually 80-90% of the plate thickness is ductile).  If possible, the shell will be slowed enough so that it will not bust through the plate intact and will explode while in the plate, minimizing damage.
 
With Battleships, there is enough room for defense in depth and there will be additional armor plating several feet away from the first (primary) armor plate.  This is to stop the remains of the bursting shell, or to slow the shell again if it breaks through the primary plate.  Remember that the timer in the shell is counting down to the time it will explode, (we are talking milliseconds here) therefore, by slowing the shell  down you will increase the chance that the shell will explode before it gets into the "meat" of the ship.  A feature of the internal primary plate of the Iowa's, is that the hull plating will strip away the windscreen of the shell and start the timer before the shell ever hits the primary armor plate.
 
The armor plating used on decks will not have the hardened (cemented) surface.  Bombs are not armor piercing the same way shells are, (which allows for a much greater bursting charge in a bomb), so the armor deck is designed to slow the bomb down (or stop completely) as it goes through the ship.
 
In the case of the submarine, the hull must bend and contract as the pressure increases requiring a material of good ductility.  The strength of the plating is increased by alloy additions, as opposed to heat-treating for high hardness.
 
As a submarine skipper, I would not be worried about a belt of 50 cal. or drum of 20mm rounds, but a good dose of 40mm would give me pause to worry.  (I will ignore range considerations for the nonce)

[Tiornu]

None of the Japanese cruisers (unless we go back to armored cruisers!) had face-hardened armor. No American cruisers had it before the Brooklyn class, some units of which did while others didn't. Wichita and all subsequent cruisers (except for the Atlanta types) had face-hardened armor.
The outer hull plating of the Iowa and SoDak classes is not very useful for decapping. The excitement that this idea caused a few years ago has been muffled by subsequent research. But it's not that important--most Japanese shells that hit the Iowa belt would be exceeding their proofing limits anyway.
An oddity: most battleship turrets had face-hardened faceplates and homogenous roofs, but the Americans used homogenous faceplates for their modern ships and the Japanese used face-hardened roofs.