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Over the Cold War the many navies of the world moved away from significant;y armouring their ships besides splinter protection for some vital spaces, with a few exceptions. What prompted the shift from armouring the ships even though the weapons of that time were about as powerful on average, both penetratively and explosively, as the shells of the Second World War?


Note: I am asking about warships in general, not just battleships. At the end for WWII ships from carriers down to light cruisers had armour. And for what I know, some warships nowadays still have some armour but not enough to defeat any missile.

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@Mustang That is false, torpedo bulges had been developed as a form of armour to counter underwater attacks and were relatively successful. Furthermore, the armour plate on battleships descends below the water line, see: en.wikipedia.org/wiki/Belt_armor. – BOB Mar 8 at 13:43
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Lemme fix it. Battleship armor only covered areas that older weapons were likely to hit. Modern weapons can hit anywhere on the ship, so armor doesn't help as much. – D J Sims Mar 8 at 13:48
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Belt armour was phased out because torpedoes evolved from impact to cavity detonations - rather than detonating on the hull of the ship, they detonated below the ship, causing a cavity to form into which the ship "fell", usually breaking its unsupported back in the process. Armour was useless against this, so the weight allowance was used elsewhere on the ship. – Moo Mar 8 at 15:34
    
@Moo There were also torpedoes which broke the bottom of the ship by increasing the pressure on the hull. – SMS von der Tann Mar 8 at 15:43
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@Moo I was completely ignorant about the workings of torpedos and just how devastating the shock of an underkeel explosion is. Just wow. Article comparing it to "traditional" side puncture, video of Australian Torpedo test – Nathan Cooper Mar 8 at 23:52

Analysts concluded that battleships were killed by carrier aircraft, not by other battleships. This conclusion might have been influenced by several factors:

  • The US became the clear naval winner when the industrial potential was mobilizied. The US had lost her battleships early on, fighting with her surviving carriers instead. The Japanese and German battleships never had a real chance against those numbers of warships, regardless of type.
  • Carriers are easier to build than battleships. Big guns and big armor plates require very specialized factories.
  • Carrier air groups were useful in all stages of an invasion, not just for the initial bombardment.

And as Jon points out, battleships didn't have the same armor over all faces. Early battleships just armored their waterlines and their guns. High angle fire made deck armor necessary and torpedoes made underwater side armor necessary. There was no comparable protection for the underwater underside.

Torpedoes come not just from submarines but also from torpedo bombers.

  • The Bismarck was cripped by a torpedo from a Swordfish.
  • The Yamato was sunk by torpedoes, while bombs just reduced her AA and secondary artillery.
  • Bombs were involved with crippling Musashi, but so were torpedoes.
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The answer to the anti-torpedo bulge was the deep-running torpedo using a proximity fuse to detonate under the enemy ship, using hydrodynamic forces to break the ship's hull, something no realistic armor could protect against. – DevSolar Mar 8 at 16:19
    
DevSolar, answer was a multi-hull ship. – Stuart Allan Mar 17 at 21:18
    
Underwater side armor is used for the old warship which TDS is not wide enough. – Him Mar 18 at 3:23

An Iowa-class battleship's 16" guns could pitch 1.2 tonnes of high explosive with excellent accuracy, for unguided projectiles, out to 30+ km. A Harpoon packs "only" a 200 kg warhead, but with even greater precision, to a range of 120 km. At first glance, those advantages could well balance out.

But here's the kicker: the battleships didn't use their guns, at least not against each other. Sure, there were a few battleship-to-battleship clashes (Wikipedia lists nine) but far more often, battleships were used for shore fire support, and were attacked by bombers, not guns. So why add expensive and heavy armor when you probably won't need it, and it probably won't help much if you do get hit?

Note that cruise missiles can come in low or use top attack, so you'd need armor all over, not just at the waterline, to try to counter them. Gun-era ships focused their armor where it would best prevent the outright loss of the ship (i.e., the waterline), without adding an outrageous amount of mass. Armor belts were typically twice as thick as the deck for WWII battleships; earlier dreadnought-type ships were skewed 3:1 or 4:1. They were not concerned about air attacks, just plunging fire: at long range, shells can come down at a step angle, albeit more often into the water than the ship. As outlined, the heavy beltline armor built for close-range toe-to-toe combat with other battleships rarely served in such a role.

Venturing into speculation, I expect that modern electronic fire control systems are more delicate than the old analog calculators, so they may be vulnerable to shock damage even if their ship was sufficiently armored as to avoid taking on water. Similarly, fragile radars and radios can't really be armored, and without them a ship is helpless in over-the-horizon combat.

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I have a little bit of experience designing USN shipboard systems to mil specs. They have to be capable of surviving something ungodly like 100G's of shock and vibration. I'm not sure the human body can survive that. I always pictured a perfectly functioning ship steaming in a big circle in the ocean because all the crew died. – T.E.D. Mar 8 at 14:17
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@T.E.D. Shock/vibration Gs and sustained Gs are different, though. Wikipedia notes "A hard slap on the face may briefly impose hundreds of g locally but not produce any real damage; a constant 16 g for a minute, however, may be deadly." en.wikipedia.org/wiki/G-force#Human_tolerance_of_g-force – ceejayoz Mar 8 at 14:58
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@ceejayoz - Always suspected that, but never figured out where to look it up. Thank you! Not real sure how long the mil specs called for the G's to be sustained (and its vibration, not sustained linear G's like you'd get in an aircraft), but these days that's probably online somewhere too... – T.E.D. Mar 8 at 15:18
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@JonCuster - muzzle velocity of a mark 7 16"/50 caliber gun when firing the mark 8 APC (Armor Piercing - Capped) shell was 2,500 ft/sec, which is over twice the speed of sound (1,126 ft/sec). Shipboard armor was dispensed with because it was A) ineffective against repeated strikes from torpedoes and bombs, B) heavy, and C) expensive. Battleships became obsolete because aircraft launched from carriers had greater range than did the battleship's large guns. – Bob Jarvis Mar 8 at 15:59
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This shows a huge lack of knowledge about this and is a bad answer. For example "Note that cruise missiles can come in low or use top attack, so you'd need armor all over, not just at the waterline." is stupid. Shells were not like cannons fired in a straight line, 1880s-1950s ships were armored all over for bombs, torpedoes, and plunging shells. – Stuart Allan Mar 8 at 18:50

Battleships were actually obsolete by the onset of WWII, the world's navies just had not realized that yet.

Modern technology could deliver killing blows to the heaviest armor from a hundred miles away. Various encounters, like Pearl Harbor, the Battle of Midway and the hunt for the Bismark, made that clear.

Take the USS Arizona, blown up at Pearl Harbor by what was essentially a 16-inch shell turned into a 800kg bomb, carried into the defensive fire by an expendable resource (a.k.a. dive bomber)...

So why bother with slowing down your vessels, reducing their operational range, for a "protection" that was marginally useful against something that was on its way out (ship guns), and was useless against what was used now (bombs, and later, missiles)?

If an enemy air threat made it through SAM and point defense fire, no reasonable amount of armor protection would save the ship.

So, trade the armor for mobility, operational range, and defensive armament. (Stealth came into the picture as well, later on.) The result of this development was the modern missile cruiser.

A similar development took place on land as well. Modern AP ordinance made heavy armor useless, so heavy tanks were abandoned for tanks like the German Leopard 1, forsaking armor for speed, agility, and firepower.

While composite armor made heavily-armored MBT's a viable option again (in part because they have some kind of choice which way they will be facing the enemy), the song had ended for heavy armor at sea. You are too much of a target, and too easily sunk, regardless of the thickness of your belt or barbette armor.

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Not quite obsolete: they were excellent for shore bombardment; still are, arguably. But they were not really useful for their assumed mission of ship-to-ship combat alá Trafalgar or Jutland, true. – user4139 Mar 8 at 16:21
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@JonofAllTrades: AFAIK the only battleships that did significant shore bombardment were those of the US fleet, and they enjoyed total air supremacy plus the absence of counterbattery when they did. You don't need armor for that. The US used their battleships in that role because they had them. They wouldn't have commissioned them for that role. – DevSolar Mar 8 at 16:29
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"Commissioned" is the wrong word, I just realized. "Built" is what I wanted to say. – DevSolar Mar 8 at 16:35
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Actually HMS Hood isn't a good example, since it was a battle-cruiser not a battleship. The British battle-cruisers intentionally sacrificed some armor protection to allow them more speed. – Steve Bird Mar 8 at 17:11
    
@SteveBird: Actually I just found that the whole "plunging fire" theory is no longer the main one about why the Hood blew up. Removing mention of that unfortunate ship; though the question was not only about battleships, and the Hood had significant armor protection. – DevSolar Mar 8 at 17:18

Missile and aviation advances during WWII changed the threat models used for naval ships along with their primary use (as others have commented, battle ships were largely retired for aircraft carriers). As such technology was made to directly counter these threats while maximizing the effectiveness of the ships. It may be a questionable focus, given the consequences the US military faced from lack of armor on vehicles in Iraq and Afghanistan and the damage done to the USS Cole from a similar paradigm of asymmetric attack strategy.

The advent of counter-missile systems like the Phalanx CIWS presumed or hoped for at least non-direct impacts to ships. Introduced in 1959 and with continued development this addressed the dangers of missiles with far less impact to the mobility (range, speed, turn rate etc.) of ships. In the same vein anti-air systems evolved to counter threats which had proven their effectiveness throughout WWII. Jamming systems were developed and continue to develop in tandem with more overt threat mitigation systems.

The introduction of carrier battle groups further enabled the specialization of threat mitigations that were likely viewed as more effective than armor in terms of mitigation potential, cost, and effect on mission capabilities. The carrier itself has anti-aircraft capabilities with fighter planes, and the fleet typically includes submarine hunting ships and helicopters.

The role of the US navy to secure supply routes during the cold war required a relatively small force to patrol a massive area. That trend has continued to present with naval forces deployed for peace keeping, emergency humanitarian aid, and anti-piracy missions requiring significant range and speed to adequately perform the assigned missions. The increasing threat of asymmetrical warfare, which started with proxy wars during the cold war has also continued. Whether a reduction in overall armor strategies is the right approach remains to be seem, but appears to be around until armor can be developed that doesn't significantly detract from the increasingly mobility based mission requirements.

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Immediately after WWII, Germany, Japan and Italy lost their navies, while Soviet navy was not a serious force to take into account. The armored ships remained in the navies of those who won WWII. (US maintained and deployed battleships until the end of 20s century, but their armor of course played no role). When the next naval arms race began (with Soviet Union, in the late 60th) the weapons were already very different from WWII weapons: guided missiles (and guided torpedoes). No armor can protect you from guided missiles. Especially from the nuclear ones. And all countries who had substantial navies in the 1960s were thinking of a nuclear war.

EDIT. But the most important consideration is what your potential enemy does. After WWII, the US concluded that aircraft carriers and submarines were the desirable ships, and kept some old battleships, just for the case. Soviet Union never built a battleship, but they had some foreign-built. They were working on how to destroy the US carriers and submarines, and decided that building battleships is not a solution. And so on.

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Hmm, doing a little poking around, it seems that a number of anti-ship missiles (Exocet MM39 for example) can only penetrate about 5 inches max of armour. – SMS von der Tann Mar 8 at 21:17
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Yes, of course. The missiles are designed to hit the modern, existing ships:-) The point is that it is not much harder to make the missiles that will destroy armor of any thickness. But the existing missiles are optimized for existing ships. – Alex Mar 8 at 21:49
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Exocet is a comparatively small missile. Some of the Soviet ASM's were considerably larger, and surface to surface missiles like the SS-N-3 could take out a frigate with one hit. Made Exocet look like a firecracker. – KorvinStarmast Mar 10 at 19:22

The short answer is that the speed penalty that armor inflicted was becoming too great.

The USN (for example) didn't abandon armor completely: it changed form.

What all navies addressed was that the kinetic energy of incoming projectiles and missiles created too great a penalty in weight to be resisted by armor.

Other means were chosen to include a variety of missile and gatling gun systems. Stop the projectile before it hits you. (Aegis, Sea Sparrow, the SM family of missiles, CWIS, Rolling Airframe Missiles ...). Use of air assets and cruise missiles were intended to keep other surface combatants out of ship's guns range. (From carrier aircraft to armed helicopters like the RN's Lynx).

Most navies stopped putting 8", 12", 14", and 16" guns on their warships, as the structure to support them also inflicted a weight and performance penalty. 76mm to 155 mm naval artillery (depending upon class and Navy) became the norm since the heavy hitting was being done now with missiles and aircraft. (The US standard was the 5"/54, or the Otto Malera 76mm ... but I digress from Armor).

Some new ideas on armor borrowed from the ideas that land armored forces initiated with reactive armor.

One example is the 5 Ticonderoga class guided missile cruisers (Ticonderoga, Yorktown, Vincennes, Valley Forge, Thomas S Gates) and the vertical launch (from the Bunker Hill onward) CG's. They added a layer of armor to the vulnerable superstructure (on the O-3 and O-4 levels) in the form of Kevlar. This was explained to me as defense against small arms and small caliber "incoming" but it was not expected to handle a Air-to-Surface cruise missile like a Kitchen or a Kelp.

I served on both the Yorktown and the Bunker Hill. (80's and 90's).

Another example of responding to the threat over time was in the Arleigh Burke DDG class, which returned to more steel in it construction: Perry, Spruance, and Ticonderoga class combatants had been assessed as perhaps too lightly armored compared to the advancing threat. The Burkes were a bit "tougher" in a case of the wheel turning back around.

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Was the steel for armor or for fire protection? The damage suffered in the Falklands war caused navies to reconsider some lightweight construction practices. – o.m. Mar 10 at 6:28
    
From what I recall, a bit of both. Aluminum is susceptible to cracks that steel is not, and the fire resistance of steel versus aluminum was another reason to go with steel. (I recall the Bunker Hill and its sister ships having a class problem with cracks in the forward superstructure). The USNI Proceedings articles that covered Burke as it was coming to the fleet are long since gone from my possession, there is a brief treatment of this in the wikipedia entry: not just Falklands, but incidents like the fire on the Belknap informed the design choices. – KorvinStarmast Mar 10 at 11:57

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