The Structural role of Main Armour Decks

Warship design and construction, terminology, navigation, hydrodynamics, stability, armor schemes, damage control, etc.
Mostlyharmless
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The Structural role of Main Armour Decks

Post by Mostlyharmless » Wed Jun 20, 2018 4:04 pm

Battleships were constructed as a girder of the keel, normally a double bottom, and the main armour deck (MAD). However, different navies constructed the MAD differently.

The USN made their MADs as laminates. Thus, Iowa has a 4.75" upper layer on a 1.25" lower layer of homogeneous armour. Friedman's U.S. Battleships has South Dakota with a 5" upper layer on a 1" lower layer and states that the lower layer was made 1.25" in Iowa as the ship was longer (the upper layer was thicker near its outer edge in both ships and the MAD may not even have been a laminate there in Iowa).

The RN methods were similar with King George V having an upper layer of 200 lb armour over the magazines and 160 lb armour over the machinery with a lower layer of 40 lb armour below.

Italy and Japan also used laminates but the lower layer seems to have been thinner and made of construction steel rather than armour. Thus, the MAD was 150 mm or 100 mm of armour on 12 mm in the Littorio Class and 190 mm (or even 200 mm) of armour on 10 mm of D steel in the Yamato Class.

However, Germany and France did not need to make their MADs as laminates.

Why did the warship designers adopt different methods? Was the armoured weather deck of the Bismarck Class the secret when combined with the 30 mm connection (continuing the 45 mm bulkhead)? If that is true, did Montana's designers miss a trick when proposing two laminates with a 60 lb on 30 lb bomb deck above a 5.8" on 50 lb main deck?

pgollin
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Re: The Structural role of Main Armour Decks

Post by pgollin » Mon Jun 25, 2018 2:56 pm

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Different navies counted armour as different structural entities, e.g. the USN used the main armour deck as a structural member in both compression and tension, whereas the RN only counted the main armour deck in compression.

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Dave Saxton
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Re: The Structural role of Main Armour Decks

Post by Dave Saxton » Tue Jun 26, 2018 3:46 pm

KM Chief Engineer Tobicke described that the German scarp triangle design performed both ballistic and structural functions. The panzer deck and the longitudinal armoured bulkheads essentially comprised a great H beam that the rest of the hull was built from.

Combining both ballistic and structural functions is obviously much more efficient design practice. Taking the armour thickness specs listed above for the Iowa class, for example, the effective thickness of the MAD is not 120mm + 38mm = 158mm. Rather it is the square root of 120mm (squared) + (38mm (squared) = 126mm. The Tirpitz gets about the same total horizontal protection (130mm over the machinery -150mm over the magazines) of the Iowa (140mm inboard with the yaw deck) with a lot less weight expenditure.
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Re: The Structural role of Main Armour Decks

Post by bigjimslade » Tue Feb 05, 2019 1:37 am

The U.S. Navy did not regard anything as "armor" unless it was at least 4 inches thick. While you are describing the deck armor as being 4.75" + 1.25" = 6", the designers considered this as 4.75" of armor. In almost every case, flat armor on the Iowas is attached to some structural backing. The major exception is the lower course of the side armor. The U.S. Navy was never concerned with "how does the thickness of the Iowa class armor compare with that of the Bismarck?"

If you look at U.S. Navy documents, the armor on the turret fronts is 17". There is also 2-1/2" of backing to support that armor. There is an inch of concrete between those two.

The Navy used STS of various thicknesses ad libitum throughout the ships.

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Re: The Structural role of Main Armour Decks

Post by paul.mercer » Tue Feb 05, 2019 5:49 pm

Gentlemen,
Please forgive my ignorance in this question but exactly what does the statement'The RN methods were similar with King George V having an upper layer of 200 lb armour over the magazines and 160 lb armour over the machinery with a lower layer of 40 lb armour below'. actually mean when all the others are measured in inches or centimeters?

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Re: The Structural role of Main Armour Decks

Post by paul.mercer » Tue Feb 05, 2019 5:52 pm

Sorry chaps i must have pressed Post twice, I think I have managed to delete it!

dunmunro
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Re: The Structural role of Main Armour Decks

Post by dunmunro » Wed Feb 06, 2019 12:28 am

paul.mercer wrote:
Tue Feb 05, 2019 5:49 pm
Gentlemen,
Please forgive my ignorance in this question but exactly what does the statement'The RN methods were similar with King George V having an upper layer of 200 lb armour over the magazines and 160 lb armour over the machinery with a lower layer of 40 lb armour below'. actually mean when all the others are measured in inches or centimeters?
40lb = ~1 inch.

KGV's MAD had 240lb over the magazines and 200lb over the machinery, with 20lb of D-steel under, IIRC. There was also a 60lb splinter deck directly over the shell rooms (~2 decks below the MAD).

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Re: The Structural role of Main Armour Decks

Post by paul.mercer » Wed Feb 06, 2019 10:40 am

Thanks Dunmunro
It seems the RN always like to do things differently!

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