Battleship Armour Matches Guns

Generally speaking, a battleship was expected to have armour to match that of its guns:  if it fired 16 inch shells, it's armour should be able to defeat incoming 16 inch shells.

This was a much harder requirement to meet for battleships that had been left over from world war one, or had been built between the two world wars.

The British learnt this lesson the hard way, when for example, HMS Hood was lost to the German Bismarck.

Unlike the American South Dakota class, HMS Hood did not feature 6 inch deck armour, instead only having a maximum of 3 inch deck armour.

As world war two progressed, it was to be seen that even 9 inch deck armour (as found in the IJN Yamato), was to be proved in-sufficient.