Why did navies abandon armour?

It's not the abandonment of armor but the thickness. A Modern all steel destroyer with armor that thick would weigh 12,000-15,000 tons. And even if they did have armor the rise of modern anti ship missiles means even a battleship can be blown to bits. Armor has gave way to countermeasures; intercept/destroy the assaulter before it hits the ship. Armor was designed for battleships to survive hits from other battleships.

Never the less the future of armor is nano-composites, artificial materials and metal layers over ceramic or even in the future; Synthesized crystals. Moderate increases in durability without increasing overall weight.

The reason was that after the carrier battles of World War II, it was recognized that the greatest danger to ships was not "shells," but rather "bombs," and later, "missiles.

Battleships had played a limited role in the war (at least in the Pacific). Battles such as Coral Sea, Midway, Philippine Sea, and Leyte Gulf were primarily carrier actions. So modern fleets were built around 1) carriers, and 2) supporting vessels, especially cruisers, and later, anti-missile cruisers such as the Aegis, as well as submarines. Battleships were "mothballed" in the 1960s and 1970s as the other vessels (and missiles) got more sophisticated.

The requirements for these newer ships were not "armor," but rather, speed and maneuverability. In order to maximize these attributes, it was necessary to sacrifice all but the most essential armor.

The question, and most of the answers are based on a false premise -- it is not true that the U.S. Navy has abandoned the use of armor. During the 1960s and 1970s, speed, maneuverability, and the use of missile counter-measures, were hoped to be enough to protect ships like the Leahy (CG-16) and Belknap class guided missile cruisers (although originally built as "frigates" -- actually destroy leaders) and the Oliver Hazard Perry (FFG-7) Class frigates were made without armor and with aluminum superstructures. After the 1987 missile attack on the USS Stark (FFG-37), and the 1988 mine hit on the USS Samuel B. Roberts (FFG-58), led to incredibly hot fires fueled by the aluminum hulls (aluminum burns at relatively low temperatures compared to steel). Also, the Navy had a bad experience with fragmentation of a missile (one of our own, unfortunately), slicing through the Combat Information Center of the USS Worden (DLG-18, later CG-18), resulting in a mission kill.

With the construction of the Arleigh Burke (DDG-51) class of guided missile destroyers, in the late 1998s, and still under construction, the Navy made the ships more blast resistant, gave them more armor over vital sections (like the CIC)a steel superstructure, and made the ship more stealthy to protect it from radar guided missiles. The Arleigh Burke class also possesses superior capabilities to protect its sailors from radiation within the skin of the vessel during a nuclear attack assuming that Circle William protocols have been ordered.

The newest destroyer class, led by the recently commissioned USS Zumwalt (DDG 1000), in addition to the type of armor given the DDG-51 class, the Zumwalt class borrows an armor enhancement system from tanks, by installing vertically launched missile containers along the outer edges of the hull. If a missile hits one of the launchers, the missile inside will blow the force outboard. The Soviets similarly protected its Oscar class anti-carrier submarines, forcing an enemy to have its torpedos pass through missile launchers before reaching the inner hull.

As for other ships, the Nimitz class carriers have had their vital spaces armored with Kevlar since the Nimitz became the first of its class to join the fleet in 1987.

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.

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.

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

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

What all navies addressed was the fact that the kinetic energy of incoming projectiles and missiles created too great of a weight penalty in armor plating, so other means were needed to deal with them.

What were those other means? A variety of missile and Gatling gun systems. The idea is to "Stop the projectile before it hits you." Aegis, Sea Sparrow, the SM family of missiles, CWIS, Rolling Airframe Missiles(and now apparently laser and other directed energy weapons are being explored).

Defense in depth / layered defense was the operational concept.

Attacking by using aircraft and cruise missiles were intended to keep other surface combatants out of ship's guns range. "Aircraft" ranged from carrier-based aircraft to armed helicopters like the Royal Navy's Lynx.

Most navies stopped putting 8", 12", 14", and 16" guns on their warships, as the structure to support them 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 USN standard was the 5"/54, or the Otto Malera 76mm - but I digress from Armor.

Using new materials

Some new ideas on armor were borrowed from the ideas that land armored forces initiated with reactive armor. One example is the 5 original 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 the Soviet Kitchen or Kelp. Those missiles were to be defeated with missiles/guns as the situation demanded. (I served on both the Yorktown and the Bunker Hill - 80's and 90's).

Another example of changing materials (and adding a little armor sloping) was in the Arleigh Burke DDG class, which returned to more steel in its construction: Perry, Spruance, and Ticonderoga class combatants had been assessed as too lightly armored compared to the advancing threat. The Burkes were a bit "tougher" in a case of the wheel turning back around to using steel again.

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.

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.

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.