So I was talking to a friend of mine who has taken a deep dive into World of Warships. As a result he has developed an fascination in warships from HMS Dreadnought to the USS Midway and everything in between.
So I was sitting down with him and we were talking when he mentioned that armour for British Battleship armour during WWII was some 25% better than it's American armour counterparts for the same thickness and mass. This applies only to British vessels from the King George V Class (1940) onward.
This seems like an extraordinary claim. Is there any truth to it?
Both the British and Americans used, at this time, what is called Cemented Armour, or face-hardened steel, to protect their battleships. This has a very hard, but thin surface layer, backed by a layer of hard steel, in turn backed by a layer of soft steel. The hard layers are intended to break up an armour piercing shell, while the softer layer was designed to transmit the shock, and prevent 'spalling' - the hard armour was brittle, and prone to being fractured and knocked around by impact, so the soft layer kept it in place and caught splinters. While there were some differences in the composition of British and American armour steels, the most important difference was in the thickness of these layers.
The main face-hardened steel used by the US Navy was called Class A Armor. This had a much thicker, harder face layer than those used by its contemporaries. This made it much more effective when the plates were thin, as on cruisers. However, when the plates were thick, as on a battleship, scaling effects reduced the effectiveness greatly. To compensate for this, homogenous armour (steels that were of equal hardness throughout) was used for the thickest plates on American battleships. Meanwhile, the British used their own formulation of Cemented Armour. This had a thinner surface layer, which was also slightly less hard, but the hard layer behind it was thicker. This increased the maximum plate toughness, and greatly reduced scaling effects, when used in plates of battleship thickness (10" or more). This made it the most effective battleship armour steel of the war. The Royal Navy only used Cemented Armour on battleships and battlecruisers, using homogenous armour on its cruisers. In this paper, Nathan Okun compares the various armour compositions and layouts of the battleships in use during WWII, and how effective they were against the 15in gun of Bismarck. The South Dakotas were protected against this gun at ranges above 20,000 yards. If the American steel was replaced with British, he calculates that this range would drop to 15,000 yards. While British steels were superior, it should however, be noted that American ships used more effective layouts for their armour, meaning they were ultimately better protected.