Garyt wrote:Most battleship and cruisers had 1 or 2 lighter armored "decks" before the main armored deck, did they not?
Not most, but it is correct that WW-I era incremental designs often had two or three thin decks (1-2-3") with protective plating (not really homogenous armour). In these designs the idea was that the upper decks and/or upper belts would initiate the fuse of the shell and the main armoured deck would prevent the resulting splinters from entering the vitals. The main armoured deck was not intended to "stop" projectiles fired from long range. It's a good idea in most cases out to about 20,000 yards battle range.
USN battleships always had two armoured decks though. The WW-I generation had a thin main armoured deck and a thinner splinter deck below that. With North Carolina (and then SD and Iowa) the relationship was flipped, with the thin armoured deck being the weather deck above the main armoured deck which is just one level below.
The British (and also Japanese with Yamato) application of all or nothing beginning with the Nelsons was to place as much as possible available deck armour tonnage into a thick main armoured deck. Thus the weather deck is not made of armour grade materials nor is it intended to de-cap, although at very acute striking angles it might.
The Germans of course designed a new two deck system with the maximal distance between the two decks (and a thin 6mm non ballistic battery deck between) so that yaw could become fully manifest and so that the panzer deck could also stand as part of the vertical protection, thus gaining superior vertical protection with out the additional weight cost. The upper armoured deck also doesn't weigh much more than a non-ballistic weather deck (which is usually the strength deck in structural terms) probably would, for more weight savings making up the horizontal protection.
The Italians used two thin (armour grade material) upper decks above their main armoured deck.