IOWA class battleship turrets varied in face thickness slightly, due to the manufacturer's limitations. As an example, the SOUTH DAKOTA Class battleship ALABAMA had solid 19" Class "B" armor turret faces, while the others had 18-19.5" (roughly) turret faces, in some cases made up of a thin back plate laminated (bolted flush with no gap anywhere) to a thick front plate. The result was slightly weaker than a solid plate, but in this design not by much. IOWA Class had similar variations in its Class "B" turret face armor from ship to ship.

The W.W.II U.S. Navy battleships from NORTH CAROLINA and WASHINGTON through MONTANA and here sisters were all equipped with Class "B" armor. This is homogeneous, ductile chromium-nickel steel. It is more or less KC armor without the cementing ("C") and without the deep face, both of which were used in most forms of face-hardened Class "A" armor.

It was originally used in the WASHINGTON Class ships due to problems with making the very thick (16" and up) Class "A" face-hardened armor for these ships after almost 20 years of never having made such armor. (The last was used in the COLORADO Class, it was not as thick, and it was an older, inferior grade material compared to what could be made in the late 1930's.)

The use was originally justified by the improved performance of U.S. Navy late 1930 AP ammo that was essentially impervious to the hard face of Class "A" armor at low obliquity (near right-angles) impact. Thus the hard face, which was expensive to make and which caused some plate weakening due to its brittleness, lost all of its previous advantages (breaking up the shell so that the shell had a more difficult time penetrating the armor) when used in the turret face that, by necessity, was always pointing directly at the enemy and thus could expect near right-angles impacts at all times.

Turret side armor and barbette armor would usually be hit at a more oblique angle, usually circa 30-45 degrees or more, though the very center of the barbette could be hit at near right angles. Using face-hardened armor to destroy the shell was more effective in these areas.

However, when extremely high oblique impacts occurred (55 degrees or more), as on turret roofs and decks, face-hardened armor, due to its brittleness, was a very poor choice since it could break and throw pieces into the region behind it even if the shell glanced off. DUNKERQUE's face-hardened turret roof, designed to maximize resistance to AP bombs from aircraft, was hit by HOOD and is a perfect example of this problem.

This was much less likely with soft homogeneous armor that could dent and tear slowly, easing the shell away and staying more intact, with few pieces torn off. It turned out that this reasoning was so true that even when Class "A" plates were later able to be made of maximum thickness, they decided to stick with Class "B" turret faces in battleships.

Not, however, in cruisers, which used Class "A" in the last few U.S. Navy late-1930's designs. Cruisers are frequently hit with uncapped Common shells and Class "A" armor was much better than Class "B" armor at low obliquity against them, so it was changed as soon as Class "A" armor was back in production circa 1937. This difference was demonstrated several times during W.W.II when fighting Japanese destroyers and cruisers, who used uncapped Type 91 (latest ships main armament) or older Common-type "AP" shells in their 5-8" guns. These hit turret faces of both kinds of armor in different U.S. cruisers. To my knowledge, no other country used Class "B" equivalent turret face armor in any regular warship, not counting monitor-type bombardment vessels, with guns larger than 8" designed after W.W.I.

The German TIRPITZ (and BISMARCK) armor design was, mostly, a beefed up version of that used in the W.W.I BADEN Class (as further modified in the never-completed W.W.I-designed successors to that class). The sloped 4.33" (110mm)--not 4" (101mm)--Wh was a somewhat over-hard cousin to U.S. Class "B" armor. This was angled at 68 degrees inward from the vertical, except near the ends of the ship where the angle was not quite as steep due to the pinching in of the hull. Positioned behind the main 12.6" (32cm) KC n/A armor main belt, this made side hits almost impossible to penetrate when they went through the main belt slightly below, at, or above the waterline. But, at the cost of throwing shells that penetrated the rather thin vertical outer 12.6" plate up into the upper hull, where it would tear that region of the ship apart from side to side behind the hit.

The U.S. 12.1" Class "A" belt in IOWA, while slightly thinner, was (1) tilted top over bottom at 19 degrees, giving it a much greater effective thickness to downward-falling shells at any range, even point-blank, and (2) it had an additional 1.5" of STS outer hull in front of it, which would cause some slowing of the shell before it hit the belt. Not as efficient as if it was laminated to the belt's back, but it had some effect none-the-less, as well as being thick enough to decap some, though not all, capped AP projectiles used by enemy battleships.

If it did decap the shell, the effect would be to add roughly 30% to the net belt armor thickness due to the shell nose shattering on the surface of the Class "A" belt armor, compared to that armor plate hit by a hard-capped projectile of otherwise identical design, which usually did not allow such nose damage. The most benefit was at right-angles impact with a gradual loss of benefit at higher obliquity's, dropping to zero at about 55 degrees obliquity, and actually improving the penetration ability of the shell by making ricochet more difficult at a higher obliquity.

Thus, though the U.S. also used thin belts, they did something to keep the damage outside of the ship's armored amidships area, while the TIRPITZ used its inside upper hull volume as a "damage sink" to protect the lower hull. Not good if the ship was torpedoed and needed that upper hull for reserve buoyancy!

This was made even worse by putting the TIRPITZ's armored deck almost exactly on the waterline. This was done on WW I-era battleships of many countries due to the short combat ranges of the time, which gave the guns high penetration ability against side armor, while the flat trajectories made lower hull hits very rare. It was beneficial to keep everything possible that was important below the waterline, since there was no reserve buoyancy protected against battleship-size shells above the waterline. Again, this is unlike IOWA, which had about 10 feet of protected hull below the armored deck that was above the waterline and that thus could be used to keep the ship afloat if it had its upper hull torn up above the armor and was torpedoed, too.

Bad design in TIRPITZ for a ship that was expected to have to fight against an enemy (Britain) that was always going to have numerical superiority, even if somewhat inferior in quality. Which, in the event, was really not true either, as what KGV and RODNEY did to BISMARCK demonstrated!

Thus, it is not how thick the armor was, but what kind of armor, how thick it was where, and was the armor designed to be hit in layers one behind the other (or not), that is important in judging the various protection schemes of battleships.

The turret face plate for the third (uncompleted) Japanese YAMATO Class battleship was exactly 66cm (26.01") of Vickers Hardened (VH) armor, a non-cemented (no thin superhard surface layer) form of "classic" WW I-type KC armor (that is, Krupp's original KC a/A version) using slightly improved manufacturing techniques and of very high quality control from plate to plate. This Japanese armor had very small manufacturing variations compared to most non-Japanese face-hardened plates.

The YAMATO might have had a slightly thinner face (though I do not know of any hard manufacturing data that supports this), since it had a slightly thinner VH belt according to the data I have, but not MUSASHI. She had the slightly thicker belt and, I assume, the same turret face thickness as SHINANO. One of these plates was tested by the U.S. Navy after W.W.II, so I know its characteristics and resistance quality exactly, and a broken piece of this test plate is on display at the Washington (D.C.) Navy Yard Navy Memorial Museum. You can measure it yourself to confirm my number.