Some questions regarding Bismarck's armor scheme

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Some questions regarding Bismarck's armor scheme

Post by Roll » Thu May 02, 2019 1:20 pm

Hi, I'm a newcomer whose knowledge about BB Bismarck and naval combat in general is limited to online hearsays and a few books I read many, many years ago.
It just randomly and suddenly came to my mind that maybe I could get some answers in this forum and sate the curiosity set aside in my head a long time ago as unimportant then.

Back then, in a "lite" mil enthusiast forum that I was lurking there was a following assertion regarding the ship:
"Due to the R&D disruption since the end of WWI, Bismarck was built with a rather archaic armor scheme that resembles that of WWI dreadnoughts, which meant an unnecessary weight gain due to the inefficient armor distribution compared to All-or-Nothing model. As a result, Bismarck is outmatched by any AoN ship of similar weight class (i.e. Iowas) and sported a weakness to long range fires which is critical in light of ever increasing combat ranges."
To this, there was this rebuttal:
"Bismarck's armor scheme resembles that of old dreadnoughts not because Nazi Germany lacked ideas but because it was tailored to fulfill a specific task of commerce raiding in North Sea and Atlantic, which required citadel protection against close range battleship guns and non-vitals protection against cruiser guns, all the while being able to make 30 knots. Thus, the armor scheme is not a defect but a necessary feature."
Like most online forums do there was no consensus and everyone went their own way.

I did some searches in this forum to try and update my understanding of the ship but it brought more questions than answers.
So if you wouldn't mind, here goes, in bulletpoint form:

1. It seems Germans were aware of the increasing need to protect against long range fires and aerial bombs. If so, why did they opt to use the "conventional" layout of sloped citadel wall behind the main armor belt, which is, to the best of my knowledge, optimized for close range combat?

2. What is the purpose of the 145mm armor belt above the main belt? My impression from the forum searches was that it is intended to induce yaw against incoming projectiles before they strike citadel roof - is this true and how does it work? I thought it was intended for protection against cruiser guns but it seems that is not the case.

3. Why were the barbettes armored with "only" 220mm armor? It seems underprotected compared to contemporary BBs of other nations.

4. Would it have been better for Kriegsmarine if it built Bismarck as an AoN armor scheme ship with comparable performance to the historical one and save the resources? After all, Royal Navy operated in the same theater and it still built AoN battleships that were certainly not lacking in performance.

Thanks in advance.

Oh and anyone know of a good way to get a most recent Garzke & Dulin Bismarck book? Google is surprisingly not helpful.

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Re: Some questions regarding Bismarck's armor scheme

Post by Thorsten Wahl » Fri May 03, 2019 11:10 am

1) the heavy belt + sloped armor deck combination is "optimised for all combat distances". Additionally the torpedobulkhead acts as debris catcher
depending on AOF any shell has to penetrate at least about 550 mm armor grade steel to get into the citadel area.
the protective effect is more pronounced the flatter the trajectory.
The heavy belt completely protects most of the waterline area against smaller weapons(destroyers and cruisers) and HE Shells of all types even hits
of shallow running torpedos ar not likely to impair the watertightness of the ship.

2) the 145 mm vertical citadel armor and the 50 mm upper deck are protecting the non vital areas of the second and third deck spaces and the
80/100 mm Panzerdeck against explosive effect of direct hits of all lighter weapons, heavy HE bombs.
the 50 mm thickness thickness ensures that all heavy armour piercing ordnanc is fuzed early on. Until an AOF of about 23 degrees almost all heavy ordnance is expected detonate above the Panzerdeck du to the fuze arming times.
At larger AOFs additional protective effects came into play, due to decapping and induced yaw of the incoming shell. In thes cases the distance
between the first plate and the main armor deck is of essential importance. Normally one can expect that two separated plates had a reduced
protective effect compared to a single plate of same total thickness. But in this special case the common is wrong. In reality the protective
effect of the divided 50 mm + 80 mm horizontal protection appears to have a protective effect as a single 150mm armor plate, wich is
confirmed by british post war ballistic test.
But 50 mm + 80 mm is only about one third of the deck area. a further third of the deck area is protected by 50 mm + 100 mm and the remaining
area is protected by 50 mm +110/120mm(slopes).

3) the 220 mm parts of the barbettes are additionall protected by the 50 mm whether deck / 145 mm upper belt.
any incoming armour piecing ordnance will be decapped before reaching the barbettes(see 2)) and likely break on impact on the 220 mm FH armor.

4) Bismarck class was built with considerable redundacy of vital systems based on the catalog of requirements of the military leadership.
pumping capacity of all pumps was about 16000 tons of water per hour
electrical power was two fold the required power.
firecontrol facilities had also two fold capacity.
exceptional compartmentalisation.
this redundancy cost weight and space. British estimate about 10,000 tons overall.

And maybe the redundancy becomes somewhat or partly "useless" if engaged as single ship by an superior force.

---------------------------------------------------------------------------------
with regards to "spaced armor" have a look at
dtic. mil

Accession Number : ADA954865
Title : Spaced Armor
Descriptive Note : Experimental rept
Corporate Author : WATERTOWN ARSENAL LABS MA
Personal Author(s) : Hurlich, A
Full Text : https://apps.dtic.mil/dtic/tr/fulltext/u2/a954865.pdf
Report Date : 20 Nov 1950
Pagination or Media Count : 24
Abstract : It is the purpose of this paper to explore the utility of spaced armor in greatly increasing protection with only a slight increase in the weight of armor, to consider the types of attack which may be effectively coped with by spaced armor, and also to describe the limitations of spaced armor. Means of improving the effectiveness of armor-piercing projectiles against spaced armor will also be considered. 'Spaced armor' refers to a structure consisting of a moderately thin plate in front of, and separated by a space from, a considerably thicker armor plate which constitutes the main armor of the vehicle under consideration. The thin front plate, called the 'skirting plate,' faces the attack, and represents approximately 10% or less of the total weight of the armor. A more complex spaced armor arrangement may comprise a number of skirting plate

.. because it was tailored to fulfill a specific task of commerce raiding in North Sea...
the german battleships were not intended to be used primarily in confined waters i.E. North Sea, Baltic Sea
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Re: Some questions regarding Bismarck's armor scheme

Post by Roll » Fri May 03, 2019 2:49 pm

Thorsten, thanks for responding. I didn't know the system redundancy costed as much as 10,000 tons. Do you know such values for other contemporary battleships as well? I am curious how much "weight spending" BS has made on this matter compared to others.

Also, I am still not clear on the armor issue, especially the 145mm belt.

1. If protection against cruiser caliber guns and decapping large caliber guns are two main roles for this armor, isn't it sufficient to have a thinner belt? Especially if the decapping is the main role,
http://www.navweaps.com/index_tech/tech-045.php
this simple formula by Nathan Okun (no doubt you've seen this before, but still) suggests that something as thin as 36mm decapping plate that was IIRC used by USN ships, or 70mm used by Littorio is good enough. What is the reason behind choosing the number 145mm and incurring the extra weight/steel resource for it?

2. Does the 145mm belt cause yaw to the incoming projectiles as well? I doubt it'd cause yaw seeing that the belt is standing more or less vertical but maybe I'm under wrong impression.

3. Why is this 145mm belt or armor belt of comparable thickness not present on Scharnhorst class? 145mm belt is a substantial investment of weight and steel with possible negative side effect of fusing incoming projectiles to explode inside the ship instead of letting them exit the other side of the ship without fusing. I'd like to know the main concerns behind this design decision.

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Re: Some questions regarding Bismarck's armor scheme

Post by Dave Saxton » Fri May 03, 2019 2:59 pm

Thorsten has covered the important technical points, just some comments on some of the questions based on primary documents reading:

The Papers of Kurt Assman: The OKM specifications issued demanded an Immunity Zone of at least of 20,000 meters to 30,000 meters battle range.

The comments in the introduction of ADM213/951 by British engineers mention that the Germans never really suffered an R&D hiatus because of continued R&D practiced by the private sector, especially Krupp. They did not consider German progress to be lagging.

Ironically, other nations had an hiatus as demanded by the terms of the Washington Treaty. This is made clear in the USN BurOrd book about their own history.

Various deck protection schemes, including their own, were tested against bombs by the Luftwaffe Technical Academy. Luftwaffe research indicated that an armoured grade upper deck helped protect the main armoured deck from bombs by consuming a portion of their energy.

According to the 1943 lecture given by Hoyer, the Germans knew about and understood various alternative protection schemes. Hoyer points out that the scarp triangle scheme was fundamentally different from earlier approaches by virtue of the heavy scarp, and that the heavy scarp was the essential component.

Bericht 166: The Germans had determined that traditional belt systems were inadequate against modern battleship guns at likely battle ranges because significant armoured weight fraction now needed to be committed to deck protection. The scarp triangle scheme and the spaced array decks provided a solution.
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Re: Some questions regarding Bismarck's armor scheme

Post by Roll » Fri May 03, 2019 4:02 pm

Dave, thanks for the answers.

So it seems the Germans surmised that after the thicker deck armor claims its share main belt armor cannot be made thick enough to offer good enough protection. Correct?
Did other navies arrive at different conclusion or did they arrive at same conclusion but did not think the scarp as worthwhile?
I know that treaty battleships were struggling under weight constraints but still, some ships with very thick belt like KGV did not opt to distribute its 373mm thick belt into thinner belt + scarp setup as seen in German battleships and some other European ones as well albeit with thinner and less angled scarp.

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Re: Some questions regarding Bismarck's armor scheme

Post by RobertsonN » Fri May 03, 2019 6:43 pm

One point about the overall protection scheme used by the Germans is that it reduced the chances of hits penetrating the citadel by effectively reducing the target area. By such means the chances of damaging hits at ranges where the ship's main armor could be penetrated could be reduced. While a ship might have a good immune zone when it was new this IZ was likely to shrink with age, particularly as better armor piercing projectiles were introduced.

At close range most shells penetrating the belt will pass over the lower armor deck or detonate above it by fuze action.

At longer ranges it is the main armored deck that is endangered. In armor layouts which take the form of an upside down box any penetration of the MAD will potentially cause serious damage to the machinery or even lead to loss of the ship if splinters enter a magazine. A partial exception to this rule is formed by the British ships after the British enclosed their magazines in 1.5 in armor. However, this added protection was localized and limited to the magazines. In the German designs a penetration through the main belt/sloped armor deck/anti-torpedo bulkhead is virtually excluded. On the far side of the ship a shell is most unlikely to penetrate the reverse slope of the armor deck and were it to do so and detonate before exiting the ship there is still the anti-torpedo bulkhead to limit splinter penetration.

Therefore, whereas the complete armor deck formed the target in most designs, in German ones the relevant target area is the flat part of the MAD between the torpedo bulkheads where not shielded by the main barbettes. In four turret ships the barbettes shelter a larger part of the MAD on the far side of the deck than in three or two turret ships. Leaving out the barbette screening effect in Bismarck the deck target area was reduced by about 30% compared with most other armor layouts. The main disadvantage of the German layout was the larger volume of hull above the MAD that was endangered by AP projectiles.

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Re: Some questions regarding Bismarck's armor scheme

Post by RobertsonN » Fri May 03, 2019 7:33 pm

The point about the 220 mm thick barbette armor on Bismarck. This is indeed low for the period in question, although well up on WW1 ships. This armor was KC. No shell could strike it without having first gone through the upper belt or the upper deck. In both cases the shell would be decapped. On hitting the cemented barbette armor the shell would almost always shatter. Gercke mentions that the Germans carried out tests that showed that decapped AP shells would always shatter if they hit KC armor off-normal. At normal they shattered if they struck a half-calibre plate or thicker. So for a 220 mm KC plate, a shell of higher than 440 mm calibre was required to penetrate whole even at normal impact. Of course, even the fragments of a shattered shell can penetrate thick armor if they are moving fast enough. Even so, a plate will act as if it were effectively 30% thicker against fragments than against an intact projectile. In this case, this raises the effective thickness to around 280 mm. There is also the question of whether the charge in the shell burns/explodes/detonates. I think it is most likely to do so at the moment of shatter, i.e. when the lower part of what remains of the shell is still some distance outside the plate, irrespective of whether fragments penetrate/do not penetrate the armor. So, as far as I can see, because of the decapping effect, the 220 mm area of the barbette offers better protection than the 340 mm area above it because the latter can be penetrated by intact shells at closer distances and favourable obliquities,

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Re: Some questions regarding Bismarck's armor scheme

Post by RobertsonN » Fri May 03, 2019 7:56 pm

The question about AON battleships in the RN. I think no one would dispute that Nelson and Rodney were armored on the AON principle. However, in the KGVs there was at least as much armor before and aft of the citadel as in traditional British designs. The KGVs also raised the armor deck by one deck level entailing a very deep main belt. Later, they had much splinter-proof armor added to the sides and roofs of the magazines when doubts arose over the effectiveness of the armor box, much as was done on a lesser scale to ships in the First War. The Vanguard further evolved to incorporate an additional light belt at the ends (218 tons) over the thicker armor there and extensive splinter-proof protection to fire controls, leads and bridges (141 tons) outside the main protection. It also featured a good deal of weight allocated to medium thickness armor to the casements of the secondary armament (443 tons according to Raven and Roberts) as well as 626 tons applied to splinter protection of the cordite handling rooms (according to Raven and Roberts). So, it seems to me that the British progressively abandoned the AON concept from the 1930s onwards

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Re: Some questions regarding Bismarck's armor scheme

Post by alecsandros » Fri May 03, 2019 8:05 pm

Bismarck's armor scheme was naturally more resistant to intact perforations then contemporary competitor designs, because it incorporated substantially more amor grade steel in the ship. Estimates of Bismarck's armor weight range between 17500 to 18500 tons, and neither figure included some components (such as turret face plates). This is at least 10% more then all contemporaries (Yamato and Iowa classes were not in service at the time) , by any means of measuring designed armor weight.

My amateurish opinion is that the Bismarck design was optimized against cumulative damage from multiple types of weapons, covering a relatively large percentage of hull volume, which was afforded more or less the same level of protection. Contemporary battleship designs , in service in May 1941, were more oriented against localized damage from multiple types of weapons (ex - exceptional protection of the main magazines), with different level of protection afforded to the other ship systems.

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Re: Some questions regarding Bismarck's armor scheme

Post by Mostlyharmless » Fri May 03, 2019 8:26 pm

The USN started strengthening the weather deck using 60 lb STS in North Carolina, South Dakota and Iowa and 60 plus 30 lb STS in the Montana design. This is normally called the bomb deck but I think that Dave once found a letter referring to this as a yaw deck. However, the outer hull above the armour belt remains 60 lb in Montana, so the outer section of the main deck had to be thicker, 6.1 inches on 50 lb, than the central 5.8 inch on 50 lb section.

Designs with much thicker decks to resist bombs soon appeared such as H41 and a May 1942 proposal for Montana. In the German case, I have seen it suggested that the hits on Scharnhorst at La Pallice were influential.

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Re: Some questions regarding Bismarck's armor scheme

Post by Dave Saxton » Sat May 04, 2019 3:09 pm

Roll wrote:
Fri May 03, 2019 4:02 pm
Dave, thanks for the answers.

So it seems the Germans surmised that after the thicker deck armor claims its share main belt armor cannot be made thick enough to offer good enough protection. Correct?
Did other navies arrive at different conclusion or did they arrive at same conclusion but did not think the scarp as worthwhile?
I know that treaty battleships were struggling under weight constraints but still, some ships with very thick belt like KGV did not opt to distribute its 373mm thick belt into thinner belt + scarp setup as seen in German battleships and some other European ones as well albeit with thinner and less angled scarp.

The evolution of the H class design is instructive. In the H class the thickness of the main belt was reduced and it extended deeper below the waterline, while the thickness of the heavy scarp was increased. The overall protection the system provided was significantly improved. The Scharnhorst and Bismarck classes would of had been better with this slight modification as well. Hoyer points out that if the scarp is heavy enough, it excludes penetration of the belt system at all ranges, because the necessary velocity to penetrate the system becomes so great that no incoming projectile can withstand the impact against the face hardened armour belt without breaking.

No practical thickness of a conventional main belt design, even sloped, can match that. Furthermore, the point of diminishing returns in terms of thickness of belt armour vs weight and vs quality is only about 12.7-inches.

I don't find the German approach to be especially weight inefficient in terms of armour weight. They considered a 70% protective length as necessary. Therefore, design features to shorten the citadel length as used in other designs are not that relevant. The modifications in other designs to raise the height of their citadels above the waterline off set the Bismarck's upper belt application as a comparative weight inefficiency. The German panzer deck and heavy scarp arrangement means that those armour weights provide both horizontal and vertical protection. The Germans are not wasting weight on backing plates, splinter decks, or excessive framing. They are using armour grade steel where it is doing some good, and using construction grade steel where using armour grade steel does not matter.
2. Does the 145mm belt cause yaw to the incoming projectiles as well?
Why wouldn't it?
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Re: Some questions regarding Bismarck's armor scheme

Post by Dave Saxton » Sat May 04, 2019 3:19 pm

When we consider the types of surface warfare actually fought by battleships during WW2 the Bismarck's armour scheme looks to be a pretty good design.
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Re: Some questions regarding Bismarck's armor scheme

Post by Roll » Sun May 05, 2019 9:03 am

Dave Saxton wrote:
Sat May 04, 2019 3:09 pm

2. Does the 145mm belt cause yaw to the incoming projectiles as well?
Why wouldn't it?
I was asking because, at least from what I know from tank armor designs, yaw is not guaranteed for a projectile that passes through any armor plate.
Depending on factors (angle of hit, plate thickness, projectile construction, etc.), the penetrating projectile may just deflect to different vector instead of yawing. I was curious about the penetration scenarios that would cause yaw to battleship caliber projectiles passing through that 145mm plate and whether the Germans calculated and intended such yawing by deliberately setting the plate thickness to 145mm and no other number.

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Re: Some questions regarding Bismarck's armor scheme

Post by RobertsonN » Sun May 05, 2019 4:16 pm

The mention of the paper by Hoyer has drawn my attention to something I had overlooked before. Namely, I had thought that the curves Vg in Figs 12 and 13 were curves of terminal velocity (from its shape) rather than of the velocity at which the shell fractures on impact. Also generally in GKdos 100 at 40 dez it is stated 'no intact penetration. At this obliquity the shell is broken on penetrating the KC plate. However, in Figs 12 and 13 the shell breaks even at low obliquity. It should not do so against the belt so near normal impact. Perhaps breakage was on the scarp. Certainly, the Wh penetration curves in Gkdos 100 state that this armor type did not break these shells. However, the curves given are for perforation, which required a high velocity at high obliquity. Perhaps shells that struck the same armor at lower velocity did break (and did not penetrate).

I have only the English translation of this paper. While it probably required a lot of work it is not very good in places and seeing the original German would perhaps clear some misunderstandings. Hoyer's use of the 'square root of the sum of velocities' squared' formula is only correct more or less where all plates are of homogeneous type. Where KC is involved a shell needs to eject a plug, which has to be ejected with at least the same velocity as the shell exits the plate. The kinetic energy absorbed by the plug reduces the exit velocity of the shell in this case. So what Hoyer says is only approximately true for the lower zone limit if the shell exits the plate at a low velocity, which is generally not the case. Hoyer mentions, apart from this, the effects of decapping, deflection on penetrating a plate and yaw. Deflection (turning to the normal on penetrating a plate) has the effect of lowering both the lower zone and upper zone limits. Incorporating deflection would move the curves for 0 dez, 2 dez and 3 dez upwards, thus reducing the lower zone limit and increasing the width of the zone marked as 'Schutzzone' (protection zone) in Figs 12 and 13. The Gercke multiplate formula has factors for the loss of shell weight due to decapping and for the change in head shape due to decapping.

Hoyer evidently uses the same calculation method as that applied by the authors of Gkdos 100. Even this though shows the marked superiority of the German scarp reinforced layout for the lower edge of the protection zone as obliquity increased. For example, for Bismarck v Royal Sovereign the British ship could penetrate the side + scarp of the German ship out to 21000 m 'quer' (abeam). But at 20 dez the inner zone of the protection zone extended down to 10000 m and at 4 dez there was no intact penetration. The German ship could penetrate the Royal Sovereign's side system out 26200 m abeam, falling at 2 dez to 22000 m and at 4 dez to 6500 m. In a realistic battle situation with varying obliquities the risk to the German ship of penetration of the vitals through the belt was appreciably less than in Royal Sovereign.

If you apply the multiplate Gercke formula and also include the effects of plug ejection in the calculation then you find for a 40.6 cm APC attacking a 300 mm KC n/A belt and a 120 mm scarp at point blank range (R = 0 m) that the shell narrowly fails to penetrate. In this case there is no deflection on penetrating the belt. This narrow failure to penetrate also remains the case as range increases to above 10000 m (albeit here deflection, which in this case increases the velocity needed to penetrate the scarp, is neglected). This kind of behaviour is also shown by the curves for Vg and 300 + 210 in Fig. 13 of Hoyer running quite close to each other over a long range interval. By way of contrast a 40.6 cm APC narrowly penetrates a 320 mm KC n/A belt and a 110 mm scarp at R = 0 m

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Re: Some questions regarding Bismarck's armor scheme

Post by Roll » Sun May 05, 2019 7:10 pm

Dave Saxton wrote:
Sat May 04, 2019 3:19 pm
When we consider the types of surface warfare actually fought by battleships during WW2 the Bismarck's armour scheme looks to be a pretty good design.
Yes it seems to be that way.
But you see, AoN scheme is relatively easier to understand; either the citadel box holds or it fails.
Bismarck's armor scheme, while not necessarily superior or inferior to AoN system it is more complex, always involving two plates of varying thickness and obliquity for all penetration scenarios except for double deck pens. Hence why all the questions.

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