Backing plates under armour?

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Mostlyharmless
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Backing plates under armour?

Postby Mostlyharmless » Fri Feb 07, 2014 7:00 pm

Were there backing plates, presumably of ST 52, underneath the armoured decks of Bismarck and Tirpitz? Most discussions do not mention any but several such as viewtopic.php?f=1&t=2908&hilit=backing+armor+armour+deck&start=15#p29652 explicitly only list armour. However, using Google image search, I found at least one image on the web at http://img42.imageshack.us/img42/8590/bismarckt.jpg which suggested such backing.

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. For example, South Dakota or Iowa had a 1.5 inch weather deck (37 mm), a 4.75 inch on 1.25 inch main deck (152 mm), a splinter deck (16 mm perhaps 5/8 in.) directly below the main deck and finally a 13 mm (presumably 0.5 in.) third deck, for a total of 218 mm. Richelieu has a 26 mm weather deck, a 150 mm main deck and a 40 mm third deck for a total of 216 mm. Of course Richelieu has a 170 mm main deck over the main magazines.

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. Even if we used the weight of the 145 mm upper belt, we could only add about 25 mm of extra deck. Of course the area of the deck is much greater in Bismarck and we do also have to add the 110 mm scarp. However, Bismarck is also a larger and heavier ship (at least compared to Richelieu and South Dakota). Thus I would be quite happy to find that there was an extra 30 mm of backing steel.

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Re: Backing plates under armour?

Postby alecsandros » Fri Feb 07, 2014 7:05 pm

Bismarck main armor deck was 95mm thick above magazines; TIrpitz 100mm. Some indication suggest the weather decks of both German battleships were 80mm thick around secondary barbettes, and possibly also around main barbettes [thus offering further protection for the magazines in case of free-falling bombs]

IIRC, there were several 6mm thick St52 decks, but I don't know for sure.

As I understand it, the weather deck had no backing...

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Dave Saxton
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Re: Backing plates under armour?

Postby Dave Saxton » Fri Feb 07, 2014 11:49 pm

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.
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José M. Rico
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Re: Backing plates under armour?

Postby José M. Rico » Sat Feb 08, 2014 2:22 am

Mostlyharmless wrote:using Google image search, I found at least one image on the web at http://img42.imageshack.us/img42/8590/bismarckt.jpg which suggested such backing.

That midship cross section is wrong. The decks of the Bismarck Class didn't have a St52 plate underneath. The 45mm torpedo bulkhead didn't have a backing plate either.
Only the vertical armored belt had a 16-25mm thick plate behind the 50mm layer of teakwood.

alecsandros wrote:IIRC, there were several 6mm thick St52 decks, but I don't know for sure.

These were the battery deck and the small deck directly over the slopes, known as the Zwischendeck.

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Re: Backing plates under armour?

Postby Mostlyharmless » Sun Feb 09, 2014 10:59 pm

It took me some effort to find again the page where Google had found the dubious image. It was on a forum associated with a game called Pacific Storm (http://www.pacificstorm.net). The Wikipedia article on the game said “This game has been criticized by many gamers due to some historical inaccuracies” http://en.wikipedia.org/wiki/Pacific_Storm so I shouldn't have been surprised.

The much lower weight of horizontal or near horizontal steel per metre of Bismarck's citadel allows Bismarck to have a much longer citadel (171 m.) than Richelieu (131 m.) or especially South Dakota (113 m.) (using the lengths from viewtopic.php?f=1&t=2908&hilit=backing+armor+armour+deck&start=15#p29652). In fact, Bismarck has a longer citadel than either Yamato or Iowa.

I am guessing that the designers were very happy to be able to ensure that Bismarck would not suffer the fate of Lutzow at Jutland and hadn't imagined the threat of a Fritz-X type bomb. Perhaps an interesting what if is how Bismarck's design would have turned out if Lutzow had survived but the torpedo that hit Seydlitz had disabled her rudder and led to her loss.

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Re: Backing plates under armour?

Postby alecsandros » Mon Feb 10, 2014 6:13 am

Nontheless, it is important to note that not all steel was armor-grade steel, and not all armor-grade steel behaved the same.


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