Armor

For a discussion of armored fighting vehicles, see the article on Tanks.

During the Pacific War, most warships larger than destroyers had some degree of armor protection against bombs and gunfire, and most destroyers had some degree of splinter protection. The purpose of armor was to prevent enemy fire from penetrating to the magazines, machinery spaces, or other vital spaces of the warship so that the warship could survive a number of hits and continue fighting. Splinter protection sought to contain the damage from penetrating hits rather than to prevent such hits.

Armor defeated projectiles in one of three ways. An incoming shell might not have sufficient energy to penetrate the armor, in which case there was little damage. A shell with high energy might shatter against the armor or be so damaged that it would not detonate even if it penetrated. In the former case, the damage was slight, while in the latter case it was still much reduced. Finally, a shell coming in at a shallow angle might be deflected by the armor and do little damage.

The Armored Citadel

Prior to the First World War, naval architects tried to provide some degree of armor protection to most parts of a warship. However, starting with the battleships of the Nevada class, this total protection philosophy gave way to an "all or nothing" philosophy in which those portions of a warship that could not be given heavy armor protection were given no protection at all. The portion of the ship protected by heavy armor typically extended from the forward turret to the aft turret and included the magazine and machinery spaces. This portion of the ship was known as the citadel. Most of the bow and stern of the ship and the upper decks were given nothing more than splinter protection.

At a minimum, the citadel consisted of a belt of armor on each side of the ship covering the area close to the waterline. This belt protected the magazines and machinery spaces, which were usually located at or below the waterline. Small belts were fashioned from single plates, while larger belts could be fashioned from multiple plates locked together to avoid weakness at seams. Though substantial over its entire length and width, the belt was not necessarily uniform. It tended to be somewhat thicker where it was protecting magazine spaces, the most vulnerable part of a warship, and to taper off in thickness below the waterline. There was considerable variation in how much protection was provided below the waterline, ranging from minimal protection to the very deep belt on the King George V-class battleships.

During the period leading up to the Second World War, naval architects began experimenting with armor belt designs in which the armor was sloped away from the vertical. This increased the effectiveness of the armor by presenting a greater thickness along the likely trajectory of incoming shells. However, an inclined belt was more difficult to arrange than a vertical belt, and only a few of the largest classes of battleships adopted such systems, among them the Yamatos and the Iowas.

At long range, shells could come in at a steeper angle (plunging fire) and penetrate the vital spaces from above the upper edge of the belt. Thus, the belt on larger warships was supplemented with horizontal protection in the form of an armored deck that merged with the belt at its upper edge. Because even plunging fire came in at a relatively shallow angle, the armored deck was usually much thinner than the belt. This also saved considerable weight since the area of the armored deck was large. As a rule of thumb, the maximum thickness of the armored deck was about a third of the maximum thickness of the belt. Of necessity, the armored deck had openings for the boiler uptakes and hatches for access, but these constituted a small percentage of the area and only a lucky hit would penetrate through these openings.

The better armor systems included armored bulkheads closing off the ends of the belts. These were typically equal in thickness to the belts, extended to the level of the armored deck, and were joined to it. Thus, the vital spaces were protected by an armored box that offered protection from shells coming in from almost any direction and range.

Some warships had lightly armored boiler uptakes or armored boxes around the magazines and steering spaces.

Turrets and Conning Tower

In addition to the armored citadel, most large warships had armor protection for the turrets containing the main guns. This protection was usually comparable to the belt protection on the front face of the turret and somewhat thinner on the sides. The rear armor was occasionally quite heavy, not so much for protection as to balance the turret. The roof of the turret was lightly armored to provide horizontal protection analogous to the armored deck. The barbette (the cylindrical structure on which the turret sat, which usually extended down to the magazine) was generally also armored, with the armor typically thicker to the sides than fore and aft.

The gun ports on turret faces were a source of weakness, so that a greater thickness of armor was required to give the same protection. The turret face was usually sloped well back to increase the likelihood of a shell ricocheting upwards off the face.

Larger warships typically had a heavily armored conning tower from which the command crew could direct the activities of the ship. This armored conning tower became controversial because it severely restricted visibility, an important consideration in a sea fight. Observations could only be made through narrow slits. Some degree of armor protection (rarely more than heavy splinter protection) was also provided for range finders and other vital fire control elements.

Bomb Protection

With the advent of aircraft capable of attacking a warship with bombs, there came increased emphasis on horizontal protection. The obvious solution, increased thickness of the armored deck, carried a high cost in weight, but most powers accepted the inevitable and increased the thickness of the armored deck. Some warships were fitted with a bomb deck designed to activate the fuse of an armor-piercing bomb and cause it to detonate between the bomb deck and the main armor deck, where it would do less damage. Horizontal protection was also extended to cover the steering spaces of many warships, which were more vulnerable to bomb damage than to shellfire because of their location deep in the ship.

Ultimately, it was the development of aircraft carrying ever larger and heavier bombs that doomed the battleship, because it became prohibitively expensive to equip battleships with sufficiently heavy deck armor. The race was lost.

Armor Plate

The protection provided by an armor system is obviously dependent on the quality of the armor plate itself. One form of armor plate began as high-quality medium steel, often nickel-chromium steel, rolled to the desired thickness. One face of the plate was heated in the presence of carbon or sometimes nitrogen, which diffused into the steel, and then the plate was quickly cooled by quenching in water or oil. This greatly hardened the outer 30% to 50% of the thickness of the plate. The process was carefully controlled so that the remainder of the plate remained softer but much less brittle. Thus, a very hard face was supported by a very tough back layer. This so-called cemented armor was appropriate when the projectile was expected to hit at a steep angle. The face hardening was actually counterproductive for hits at a shallow angle or if the hardened face was actually penetrated. Where this was likely to occur, homogeneous armor was a better choice.

Splinter protection took the form of relatively thin plates of homogeneous armor otherwise similar to the mild steel structural plates normally used in ships. These were not expected to stop armor-piercing shells, but were resistant to shell fragments and thereby limited the damage caused by a penetrating hit.

In the U.S. Navy, cemented armor was known as Class A armor while thick homogeneous armor was known as Class B armor. Thinner homogeneous armor plate (under 4" or 102mm) intended for splinter protection was known as Special Treatment Steel or STS and was used extensively as structural steel in warships. Half an inch (6mm) of STS could stop a 0.5" (12.7mm) bullet. All were nickel-chrome alloy. Small armored housings, such as those used for range finders, that were cast as a unit out of high-carbon steel were known as cast armor. Cast armor could be homogeneous or face-hardened.

The British equivalent of Class A armor was CA (Cemented Armor), which was used only as heavy armor on battleships or battle cruisers or as deck armor on aircraft carriers. It was possibly the best armor in the world when used in thicknesses greater than 10" (25 cm). The British equivalent of Class B armor was NCA (Non-Cemented Armor) and the British equivalent of STS was Dücol or Type D steel, a high-manganese, medium-carbon alloy used extensively as structural steel in British warships.

The Japanese equivalent of Class A armor was VH (Vickers Hardened), which was an obsolete version of CA. The Japanese economized on cemented armor because of its high cost, employing NVNC (new Vickers noncemented armor) even for some of the very thick armor on Yamato. NVNC was also known as nitsukeru kurōmu kō or "hardened chrome steel." HT (High Tensile) steel was used as an STS equivalent in older ships, while more modern classes used Type D steel. The Japanese also experimented with armor plate that substituted copper or molybdenum for scarce nickel. The former was known as CNC (copper-alloy noncemented) and was equivalent to NVNC in thicknesses up to 100mm. The molybdenum alloy, MNC, was used exclusively for the deck armor of Yamato.

The following table gives the relative quality of various forms of homogeneous armor:

Torpedo Protection

Torpedo protection (sometimes described as underwater protection, since it also protected against mines and near misses by bombs) was based on different principles than armor, though a heavy armor belt did provide considerable protection against shallow-running torpedoes. Torpedoes did their damage, not by penetrating and exploding inside the ship, but by producing a massive shock wave that tore apart the outer shell of the ship, producing fragments that penetrated the adjacent bulkheads and induced serious flooding.

The underwater protection system was located below the armor belt, and began with as thin an outer shell as possible, to reduce the number and size of fragments. Interior to the shell were a series of liquid-filled and void spaces, at least twelve feet (four meters) in thickness, that were designed to absorb much of the energy of a shock wave. The outermost layers were typically liquid spaces intended to slow and contain fragments and soften the shock. The inner layers were void spaces that gave the ship buoyancy under normal conditions and contained any flooding from fragments that penetrated through the inner bulkhead of the liquid layers. The inner bulkhead of the void layers, known as the holding bulkhead, provided a final layer of splinter protection and was typically composed of STS on larger U.S. warships or Type D steel on larger British or Japanese warships.

The most sophisticated underwater protection systems had multiple layers of liquid and void spaces, whose transverse bulkheads were staggered to prevent stresses from being transmitted directly to the holding bulkhead. The longitudinal bulkheads were designed with enough clearance to deform and rupture without contacting the holding bulkhead. Venting plates allowed the hot gases resulting from the explosion to vent into the air or into noncritical areas of the ship. The protection system thus could not prevent rupture of the shell, but it could limit flooding by preserving the holding bulkhead, which was sufficient to keep the ship in the fight. Everything outside the holding bulkhead was sacrificial, being designed to absorb the energy of the underwater explosion before it could penetrate the holding bulkhead.

Older warships whose original design lacked adequate underwater protection were sometimes retrofitted with torpedo blisters, which were underwater protection systems attached to the outside of lower armor belt and hull of the ship. These blisters also improved stability by adding significant void space to the beams of the ship.

Tank Armor

The United States deliberately chose to build tanks that were ideal for mass production and would fit into ship cargo holds and landing craft, rather than necessarily be a match for the best enemy tanks. One way of simplifying production was to make armor plating from cast iron, as was done with the Sherman M4 medium tank. Cast iron is very hard and resistant to sudden impact, a desirable quality in armor, but it is also quite brittle, which means that a solid hit is likely to shatter the plate and spray the interior of the tank with shrapnel. In the Pacific theater, the Sherman was up against Japanese tanks and antitank weapons that were inferior enough that the Sherman's armor protection was adequate. This was not the case in the European theater, where the German Panther and Tiger tanks completely outclassed the Sherman.

Personal Armor

U.S. airmen were equipped with flak suits, which consisted of overlapping plates of manganese steel sewn onto a heavy fabric vest. These reduced low-velocity missile wounds by about 60%.

Navy and ground personnel of most nations were equipped with helmets that afforded a measure of head protection. These could deflect a spent or glancing bullet or smaller shell fragments. They were typically made of medium steel rather than any kind of armor plate in order to simplify production.

References

Brown (2000)

Friedman (1985)
Garzke and Dullin (1985)
Lacroix and Wells (1997)

Okun (accessed 2003)

Roberts (1982)

Sumrall (1988)

The Pacific War Online Encyclopedia © 2007-2009 by Kent G. Budge. Index

Visit our discussion forums.

Comment on this article