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Iowa-class battleship
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=== Armor === [[File:USS New Jersey armor citadel.jpg|thumb|220px|alt=A large gray wall, with an open door visible on the left. To the right, the door way into the ship is visible, revealing several inches of gray armor.|A view of the door and {{convert|17.3|in|mm|adj=on|0}} thick armored [[conning tower]] of the battleship ''New Jersey'']] Like all battleships, the ''Iowa''s carried heavy armor protection against shellfire and bombs with significant underwater protection against torpedoes. The ''Iowa''s' "[[All or nothing (armor)|all-or-nothing]]" armor scheme was largely modeled on that of the preceding ''South Dakota'' class, and designed to give a [[zone of immunity]] against fire from 16-inch/45-caliber guns between {{convert|18000|and|30000|yd|m mi}} away. The protection system consists of Class A face-hardened [[Krupp cemented armour|Krupp cemented]] (K.C.) armor and Class B homogeneous Krupp-type armor; furthermore, [[special treatment steel]] (STS), a high-tensile structural steel with armor properties comparable to Class B, was extensively used in the hull plating to increase protection.<ref>{{cite web |last= Okun |first= Nathan |date= 2017 |title= Table of Metallurgical Properties of Naval Armor and Construction Materials |website= NavWeaps.com |url= http://www.navweaps.com/index_nathan/metalprpsept2009.htm |access-date= 20 November 2020}}</ref> The citadel consisting of the magazines and engine rooms was protected by an STS outer hull plating {{convert|1.5|in}} thick and a Class A armor belt {{convert|12.1|in|0}} thick mounted on {{convert|0.875|in|adj=on}} STS backing plate; the armor belt is sloped at 19 degrees, equivalent to {{convert|17.3|in|mm|abbr=on|0}} of vertical class B armor at 19,000 yards. The armor belt extends to the triple bottom, where the Class B lower portion tapers to {{convert|1.62|in}}. The ends of the armored citadel are closed by {{convert|11.3|in|0|adj=on}} vertical Class A transverse bulkheads for ''Iowa'' and ''New Jersey''. The transverse bulkhead armor on ''Missouri'' and ''Wisconsin'' was increased to {{convert|14.5|in|0}}; this extra armor provided protection from raking fire directly ahead, which was considered more likely given the high speed of the ''Iowa''s. The deck armor consists of a {{convert|1.5|in|mm|adj=mid|-thick}} STS weather deck, a combined {{convert|6|in|mm|0|adj=mid|-thick}} Class B and STS main armor deck, and a {{convert|0.63|in|mm|adj=mid|-thick}} STS splinter deck. Over the magazines, the splinter deck is replaced by a {{convert|1|in|adj=on}} STS third deck that separates the magazine from the main armored deck.{{sfn|Friedman|1986|p=314}} The powder magazine rooms are separated from the turret platforms by a pair of 1.5-inch STS annular bulkheads under the barbettes for flashback protection.{{sfn|Friedman|1986|p=314}} The installation of armor on the ''Iowa''s also differed from those of earlier battleships in that the armor was installed while the ships were still "on the way" rather than after the ships had been launched.<ref>Stillwell, p. 15.{{Incomplete short citation|date=November 2020}}<!--Ambiguous, there are two Stillwells (1986, 1996) cited in article--></ref> The ''Iowa''s had heavily protected main battery turrets, with {{convert|19.5|in|0|adj=on}} Class B and STS face, {{convert|9.5|in|0|adj=on}} Class A sides, {{convert|12|in|0|adj=on}} Class A rear, and {{convert|7.25|in|0|adj=on}} Class B roof. The turret barbettes' armor is Class A with {{convert|17.3|in|0}} abeam and {{convert|11.6|in|0}} facing the centerline, extending down to the main armor deck. The conning tower armor is Class B with {{convert|17.3|in|0}} on all sides and {{convert|7.25|in|0}} on the roof. The secondary battery turrets and handling spaces were protected by {{convert|2.5|in|0}} of STS. The propulsion shafts and steering gear compartment behind the citadel had considerable protection, with {{convert|13.5|in|0|adj=on}} Class A side strake and {{convert|5.6-6.2|in|0|adj=on}} roof.{{sfn|Friedman|1986|p=314}}{{sfn|Garzke|Dulin|1995|pp=144β148}} The armor's immunity zone shrank considerably against guns equivalent to their own 16-inch/50-caliber guns armed with the Mk 8 armor-piercing shell due to the weapon's increased muzzle velocity and improved shell penetration; increasing the armor would have increased weight and reduced speed, a compromise that the General Board was not willing to make.{{sfn|Friedman|1986|p=314}} The ''Iowa''s' torpedo defense was based on the ''South Dakota''s' design, with modifications to address shortcomings discovered during caisson tests. The system is an internal "bulge" that consists of four longitudinal torpedo bulkheads behind the outer hull plating with a system depth of {{convert|17.9|ft|2}} to absorb the energy of a torpedo warhead. The extension of the armor belt to the triple bottom, where it tapers to a thickness of {{convert|1.62|in}}, serves as one of the torpedo bulkheads and was hoped to add to protection; the belt's lower edge was welded to the triple bottom structure and the joint was reinforced with buttstraps due to the slight knuckle causing a structural discontinuity. The torpedo bulkheads were designed to elastically deform to absorb energy and the two outer compartments were liquid loaded in order to disrupt the gas bubble and slow fragments. The outer hull was intended to detonate a torpedo, with the outer two liquid compartments absorbing the shock and slowing any splinters or debris while the lower armored belt and the empty compartment behind it absorb any remaining energy. However, the Navy discovered in caisson tests in 1939 that the initial design for this torpedo defense system was actually less effective than the previous design used on the ''North Carolina''s due to the rigidity of the lower armor belt causing the explosion to significantly displace the final holding bulkhead inwards despite remaining watertight. To mitigate the effects, the third deck and triple bottom structure behind the lower armor belt were reinforced and the placement of brackets was changed.{{sfn|Friedman|1986|p=285}}{{sfn|Garzke|Dulin|1995|p=93}} ''Iowa''s' system was also improved over the ''South Dakota''s' through closer spacing of the transverse bulkheads, greater thickness of the lower belt at the triple bottom joint, and increased total volume of the "bulge".{{sfn|Jurens|Morss|2016|pp=289β294}}<ref>{{cite journal|last1=Wright|first1=Christopher C.|title=Question 14/56: Concerning a notch in the hull at the top of the Main Deck level, port side, on BB-58|issn=0043-0374 |journal=Warship International |date=September 2020 |volume=LVII |issue=3 |pages=226β237}}</ref> The system was further modified for the last two ships of the class, ''Illinois'' and ''Kentucky'', by eliminating knuckles along certain bulkheads; this was estimated to improve the strength of the system by as much as 20%.{{sfn|Sumrall|1988|pp=132}} Based on costly lessons in the Pacific theater, concerns were raised about the ability of the armor on these battleships to withstand aerial bombing, particularly high-altitude bombing using armor-piercing bombs. Developments such as the [[Norden bombsight]] further fueled these concerns. While the design of the ''Iowa''s was too far along to adequately address this issue, experience in the Pacific theater eventually demonstrated that high-altitude unguided bombing was ineffective against maneuvering warships.{{sfnm |1a1=Garzke|1a2=Dulin|1y=1995|1p=141 |2a1=Lyon|2a2=Moore|2y=1978|2p=240}}
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