Mostlyharmless wrote:However, Bismarck is different. We have a 50 mm weather deck, a 20 mm deck and then an 80 mm main deck for a total of only 150 mm. ...
The battery deck on the Germa designs was 6mm ST-52. Not 20mm. This made it essentially not there in terms of post penetration ballistics. There was no backing plates on either armoured deck. Structural reasons are why some designs have them.
I started thinking about this by wondering if all WW2 battleships were very similar in the total weight of the decks between the weather deck and the machinery.
The British found that backing plates contributed next to nothing in terms of effective thickness, as they also discovered it would be better if there was space between the two plates instead of them being in direct contact. Therefore if they are not needed for structural concerns then there could be a great weight savings realized by using only single plates on each deck of the required thickness for ballistic concerns.
The Germans calculated that the effective thickness of multiple plates (including backing plates) was usually the square root of plate 1 squared plus plate 2 squared. On North Carolina for example, it would be the square root of 88mm squared plus 36mm squared = 95mm effective thickness for the main armoured deck. The total effective thickness would be that plus what the upper 38mm yaw deck could add to it depending on such factors as yaw. I don't think we need consider the 16mm below at all in terms of effective thickness. In North Carolina's case, if it was 100% the upper deck plus the effective thickness of the main deck, then it would be 133 mm effective, so the sqaure root rule does not always apply.
The Germans and later the British considered that Tirpitz's effective deck protection was 150mm or about 100% of the sum thickness of the upper deck plus the panzer deck so they could get the necessary effective thickness to meet the IZ requirement and trade off weight for other concerns such as armament...ect.