As battleship construction resumed in the late 1930s, designers were faced with an array of challenges that had only been on the drawing board when battleships were last built. These significantly changed the nature of the protection game, although they resembled the All-or-Nothing schemes of the last WWI-era battleships. Iowa's armor scheme was developed to meet these challenges, and examining it will show how they were met.
Iowa’s belt is 12.1” of US Class A armor1 on 30 lb STS,2 sloped at 19 degrees inboard.3 This slope makes it lighter than a vertical belt would be for the same immune zone,4 but requires the belt to be mounted inside the hull. This means that the damaged waterline of the ship is somewhat narrower than the undamaged waterline, compromising damaged stability. This tradeoff was forced by the treaty limits and the need to get through the Panama Canal, and the next class, the never-built Montanas, would have reverted to an external belt. To enhance the belt, the outer hull plating near the belt was 60 lb STS, an amount that should be sufficient to remove the AP caps from any shells, increasing the resistance of the belt by 30% or more and protecting the waterline from splinter damage.
The lower strake of the Iowa’s armor is made of Class B armor and tapers from the full 12.1" down to 1.625” as it runs almost to the top of the double bottom. This was intended to counter underwater shell hits, which had become an increasing concern as battle ranges rose. Steeply-falling shells are capable of reaching deeper down the side of the ship before being brought to a stop by water resistance. In practice, very few hits of this type occurred on US warships, despite the Japanese optimizing their shells for this kind of hit. The lower bulkhead strake also forms the third bulkhead of the torpedo defense system. The transverse armored bulkheads that bound the forward and aft ends of the citadel taper from 11.3” to 8.5” on Iowa and New Jersey. These bulkheads were a known weak point in the design, and when the treaties lapsed at the start of WWII, the USN wanted to correct it. The armor for the first two ships had already been ordered, and replacing it would have imposed an unacceptable delay. On Missouri and Wisconsin, these bulkheads are 14.5".
The biggest new threat designers had to confront was the armor-piercing bomb. While pre-war predictions about the effectiveness of level bombing turned out to be grossly wrong, dive bombing was an important means of attacking ships. This actually made the task of protecting the deck less important, as the terminal velocity of an AP bomb was about 300 m/s, as opposed to the 150 m/s limits of a dive bomber.5
Iowa's deck protection is in three main layers. The main deck is 60 lb STS, and is called the bomb deck. The idea is to explode light-case HE bombs high in the ship, and start fuse action in AP bombs, hopefully detonating them before they reach the main armored deck.7 The 2nd deck is the main armor deck, 4.75” of Class B armor on 50 lb STS, intended to keep out shells and bombs. Directly below it is an mesh of empty compartments two or three feet deep, with a splinter deck of 25 lb STS closing them off below to catch any fragments knocked off the main armor deck.8
The whole system was very similar to that of the South Dakota, with an immune zone of 20,400 to 26,700 yards against the 2700 lb shell from a 16”/45 gun and a zone of 17,600 to 31,200 yards against the same gun with a 2,240 lb shell. Against her own 16"/50 gun, these values are 23,600-27,400 yards and 21,700-32,100 yards respectively. Note that the outer edges of the immune zones are slightly greater for the 16"/50 guns, due to the shallower angle of fall. Reduced charges were proposed to solve this, but never fielded.
The barbettes are 17.3” to 11.6” of Class A above the armored deck, with the thinner armor is used when the barbette structure is screened by other parts of the ship. Below the armored deck, the barbettes are 3". The sides of the conning tower, essentially an armored vault for the admiral, captain, and gunnery officer, are 17.3" of Class B. The door alone weights over 2 tons.9 The turret faceplates are 17.0” of Class B on a 2.7” backing. The conning tower and turrets all have 7.25” roof plates. The rest of the turret armor is Class A, with 12” back plates and 9.5” sides.
Aft, the armored deck drops down to the 3rd deck and runs aft to protect the steering gear and the connections to said gear. This is 6.2" of Class B over the gear itself, and 5.6" over the steering gear leads. The communications tube to the conning tower is 16" across inside, and 16" thick. The secondary battery has 2.5" of STS, and the directors and director tubes vary between 1.5" and 2.5" of STS.
Battleship armor is a fantastically complicated subject, and I haven't been able to cover everything here.10 It's not as easy to sum up as gunpower, but understanding the battleship's defining feature gives a much better view into the conditions the ships were expected to face.
1 There's an interesting story behind this armor. British and German armor manufacturers achieved about a 25% improvement over the early-20s armor for their battleships, and the US didn't. This was not due to inferior US metallurgy, but due to a quirk of US armor testing. The US standard was centered around making sure that the shell didn't penetrate the plate intact and in condition to detonate, so as US shells got stronger, the hardened layer got thicker. This meant that the shell was more likely to penetrate in a damaged state, instead of being kept out. Nobody else tested this way (or made shells as strong) so they didn't have the same problem. ⇑
2 Special-treatment steel, essentially a homogeneous armor steel. Distinguished from Class B armor because it was provided by the Bureau of Construction & Repair instead of the Bureau of Ordnance. Usually thin, and used extensively above the waterline for structural purposes. ⇑
3 Bottom-in, the opposite of the way armor is laid out in a tank. ⇑
4 As pointed out earlier, the advantage of sloped armor is explained incorrectly in most literature. The sloped belt was no lighter than a 12.8" vertical belt would have been, although it would have been about 3% more effective at a 0 degree angle of fall due to deflection effects. However, this is not the main effect. As the angle of fall increases, the belt grows apparently thicker more quickly than a vertical belt would. At 40 degrees, the shell is going to end up passing through 40% more metal on the sloped belt than on a 12.8" vertical belt. There will be some different in deflection, but not that much. ⇑
5 The German development of the guided bomb during the war removed this limitation, and such weapons were arguably the most important factor in the final demise of the battleship. ⇑
6 Kentucky was an Iowa class ship, cancelled while under construction. ⇑
7 This provides protection to a great deal of living space and workshops on the 2nd deck. Interestingly, this can be thought of as a return to the old armor schemes where sections of the ship were armored against light weapons. ⇑
8 This arrangement is somewhat compromised compared to a single heavy deck. To a first approximation, the main deck will be equal to 4.75" plus 0.7*1.25" or 5.625". The other plates will contribute about half their thickness to the total equivalent horizontal protection. ⇑
9 I always enjoyed pointing out to guests that this was probably more than their car weighed. ⇑
10 Much like the battleship designer, my task is to cover the important things and leave the rest bare. ⇑