De-Capping Layer

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Garyt
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De-Capping Layer

Postby Garyt » Wed Dec 10, 2014 10:12 pm

I'm curious what info we have out there about the de-capping layer of the New Jersey class for instance, but my understanding is the Littorrio class had one as well.

From what I understand, this was not designed as a de-capping layer in either of the two vessels. For the New Jersey class, it was a desire to have the armored belt on the interior of the vessel.

As far as it's successes, we have little to go by as their was little real life testing. The one real "incident" I believe is when the Kirishima hit the South Dakota's Barbette. The issue I have here is that a round barbette is not very easy to penetrate anyway, I'm not sure how many degrees of obliquity this gives but penetrating the round barbette from the South Dakota would be tough with a 14" projectile, with or without any de-capping layer.

Also, in Okun's later calculations I think a 1.5" decapping layer would struggle to decap a 16" shell.

Anyone have any more in depth info on this? Is there an original source other than Okun who has done research/calculations on the effects of a de-capping layer?

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Re: De-Capping Layer

Postby aurora » Thu Dec 11, 2014 10:55 am

According to Conway's the Littorios were designed to defeat 15" AP from 17,500 yards and beyond. However the Littorio scheme should have been, theoretically, more effective at decapping AP shells (and minimizing their effectiveness) than ships with armour of similar thickness.
Also from Conway's:
"The side armour was sloped at 11 degrees and layered from outboard as follows: 70mm hard faced steel to decap AP shells/ 250mm gap/ 280mm KC (Krupp Cemented) armoured belt.
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Re: De-Capping Layer

Postby Dave Saxton » Thu Dec 11, 2014 2:54 pm

Extensive testing of de-capping plates revealed that it takes a thickness of armour grade material about 20% the diameter of the shell when striking near the normal. Thus a 16" shell would require a thickness of about 80mm. The STS shell plating on the Iowa class is not near thick enough.

Nevertheless, it changes with the striking angle. For example, deck hits come in at such an angle (even at extreme battle ranges the striking angle is at least 50* from the normal) that 50mm is thick enough to insure de-capping of battleship caliber shells in the case of deck hits. Less than 50mm and/or the use of non-armour grade materials and de-capping can not be insured in the case of deck hits. In the case of the Kirishima 14" hit on South Dakota's Number 3 barbet, the shell struck the upper 38mm armoured deck at acute obliqity and so it was de-capped before striking the barbet.

The theory behind de-capping, in the case of striking vertical armour, is that a de-capped shell will shatter when it strikes face hardened armour -if it has enough striking speed. The trick is to have a main belt thick enough to reject penetration of shells striking at speeds below the shatter velocity. Thus shells with out caps that strike with enough energy to penetrate will be shattered, and those that are going slow enough not to shatter can not penetrate. The hit on South Dakota's barbet proved the theory. The shell broke up on impact with the barbet giving it a good dent.* This would also be the case of barbet hits in Bismarck during its final battle, striking below the upper deck, but above the main armoured deck.

The Germans found that a rather large interspace was required. This large interspace requirement created a problem for the Italians, because of flooding between the layers and the large amount of hull volume required. The Italians used a small interspace filled with concrete. They claim that de-capping was insured because of the force exterted on the shell not yet clear of the outer plate before striking the main belt, but others are not convinced. Nonetheless, they had test results that indicated that the Littorio belt system could defeat 15" shells down to a range of 18km at broadside on target angles.

In the case of deck hits, de-capping can degrade the penetration performance of the shell if the main armour is of sufficient tensile strength. The main armoured deck needs to have a tensile strength of at least 80kg/mm^. This will require a hardeness of greater than 250 brinel in the case of nickel/ chrome homogenous armour. The hardness will not need to be as great in the case of Chrom/moly homogenous armour. Other factors are the significant reduction of mass of the shell by de-capping. The reduction of kenetic energy is the main factor in inducing yaw as well.

Another factor is how easy the shell is to de-cap. American naval AP shells were very easy to de-cap because of the more rounded shape of the shell head the cap was fitted over. This could lead to skewed test results of required de-capping thickness compared to other shells that did not have such round head shapes.

* According to several accounts the shock of the non-penetrating hit knocked the main battery out anyway.
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Re: De-Capping Layer

Postby Thorsten Wahl » Thu Dec 11, 2014 5:23 pm

Anyone have any more in depth info on this? Is there an original source other than Okun who has done research/calculations on the effects of a de-capping layer?


Nathan Okun had mainly taken its information on decapping from reports of watertown arsenal. Nevertheless he did not mention any primary source he uses.
from that information he concluded the 38 mm waterline protection(of SD and Iowa classes) could be sufficent to decap naval shells so his conclusion may be true for japanese and american shells.

The tests were made with relatively small army caliber weapons(57mm/75mm and smaller) and showed that capped american shots were decapped by approximately 8% caliber thick fore plates. Naval shells wer not subject of american tests.

Google search for "Hurlich spaced armor" look for the references in this document

The only nations that made comprehensive tests on decapping prior to WW2 were Germany and Italy.
The german recommandations for required thickness in the direction of flight from these tests were approximately 20% shell caliber - better 100 mm for 38 cm shells. Additional some distance between for plat and main armor was required.

Image

The mentioned Hurlich document from 1950 is also confirmation for the german finding 15 years earlier; certain arrangements of spaced plates may offer up to 30% better protection, when compared to single plates( not under all circumstances)
the protective value of the german 50 + 80 mm arrangement when attacked by artillery type projectiles, is at least equal to approximately 150 mm single plate, at the cost of a area of complete destruction of the area above the main armor deck.
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Re: De-Capping Layer

Postby Garyt » Thu Dec 11, 2014 8:24 pm

Interesting info, thank you for the help gentlemen.

Of course feel free to post any additional info! :D

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Re: De-Capping Layer

Postby Garyt » Fri Dec 12, 2014 1:47 am

From what I gather, the 1/5" de-capping layer of the Iowa class would have a hard time decapping anything above 8", and It's belt should be proof against 8" anyway. Does this seem about right, other than issues with obliquity? And to decap 16" shells, you need .63 meters between the layers?

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Re: De-Capping Layer

Postby tommy303 » Fri Dec 12, 2014 5:35 pm

While it is often referred to as a decapping layer or strake, the main function seems to have been to protect the spaces between the side plating and the internal, angled armour belt from splinter damage or hits by light nose fuzed shells, as with the belt now internal, these outer areas were no longer protected.

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Re: De-Capping Layer

Postby Garyt » Fri Dec 12, 2014 7:24 pm

While it is often referred to as a decapping layer or strake, the main function seems to have been to protect the spaces between the side plating and the internal, angled armour belt from splinter damage or hits by light nose fuzed shells, as with the belt now internal, these outer areas were no longer protected.


Thanks Tommy303. I knew it was not designed as a de-capping layer, but I had forgotten the actual reason for the outer layer.

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Re: De-Capping Layer

Postby Garyt » Sat Dec 13, 2014 1:20 am

Tommy 303, could not a hit on the STS layer by a nose fused common shell (5" or greater) cause some progressive flooding, as the armor belt is now inside of the STS layer?

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Re: De-Capping Layer

Postby tommy303 » Mon Dec 15, 2014 7:30 pm

As a rule, 1.65-in STS plate would probably provide protection against direct hits from 5-in nose fuzed HE shells and possibly up to 6-in nose fuzed shells, although anything above that will likely go on through or burst in the plate. I would expect denting or dishing of the plate, but the HE and splinter effect would probably be outside of the plate. On the other hand, shells which do rupture the plate will only cause flooding if they are above the waterline. Hits below the normal waterline with HE shell would be difficult as the shell will burst on contact with the water and not directly strike the ship. Any holes or cracks in the plate above the waterline can still cause progressive flooding if they become submerged, as in the case of a ship which has developed a list.

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Re: De-Capping Layer

Postby Garyt » Mon Dec 15, 2014 10:52 pm

Any holes or cracks in the plate above the waterline can still cause progressive flooding if they become submerged, as in the case of a ship which has developed a list.


That is what I though, one of the deficiencies in the Iowa class armor scheme. The outer layer from what I can see was not really effective. A decapping layer that generally does not decap anything greater than an 8". From what I am gather it could decap larger shells if the angle of descent is big enough.

In addition to this, shells penetrating the 1.5" layer can cause minor progressive flooding.

Does not seem like a overly effective armor scheme.

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Re: De-Capping Layer

Postby Steve Crandell » Tue Dec 16, 2014 3:58 am

Garyt wrote:
Any holes or cracks in the plate above the waterline can still cause progressive flooding if they become submerged, as in the case of a ship which has developed a list.


That is what I though, one of the deficiencies in the Iowa class armor scheme. The outer layer from what I can see was not really effective. A decapping layer that generally does not decap anything greater than an 8". From what I am gather it could decap larger shells if the angle of descent is big enough.

In addition to this, shells penetrating the 1.5" layer can cause minor progressive flooding.

Does not seem like a overly effective armor scheme.


The outer layers of the TDS are liquid filled in the Iowas. What exactly is going to be flooded?

I do realize that made the TDS less effective against torpedoes.

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Re: De-Capping Layer

Postby Garyt » Tue Dec 16, 2014 4:55 am

The outer layers of the TDS are liquid filled in the Iowas. What exactly is going to be flooded?


I'm not sure of the arrangement of the 2 layers of void and 2 layers of liquid, but even if the outer layer is liquid it is still a hull breech, hardly what one is wanting.

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Re: De-Capping Layer

Postby Thorsten Wahl » Tue Dec 16, 2014 8:12 pm

tommy303 wrote:As a rule, 1.65-in STS plate would probably provide protection against direct hits from 5-in nose fuzed HE shells and possibly up to 6-in nose fuzed shells, although anything above that will likely go on through or burst in the plate. I would expect denting or dishing of the plate, but the HE and splinter effect would probably be outside of the plate. On the other hand, shells which do rupture the plate will only cause flooding if they are above the waterline. Hits below the normal waterline with HE shell would be difficult as the shell will burst on contact with the water and not directly strike the ship. Any holes or cracks in the plate above the waterline can still cause progressive flooding if they become submerged, as in the case of a ship which has developed a list.


Special Treated Steel (STS) Shell Plating of BB 61 class : 1,5 inch STS Armor between the Second and Third Decks from frame 50 to 171-3/4

this thickness is suitable to keep most splinters from near HE-misses out of the hull
Splitterwirkung.png
Splitterwirkung.png (157.95 KiB) Viewed 1024 times


Direct hits with HE projectiles against this thicknesss however produce holes (from german non systematic trials - values has to be undestand as approximations)
15 cm HE hole diameter ~40 cm (against 40 mm armor plate)
20 cm HE hole diameter ~100 cm (against 40 mm armor plate)
28 cm HE hole diameter ~170 cm (against 50 mm armor plate)
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Re: De-Capping Layer

Postby Garyt » Tue Dec 16, 2014 8:23 pm

Thorsten - I was thinking of a direct hit.

IIRC, Nathan Okun, stated nose-fuzed projectile would penetrate 0.2 calibers of homogeneous ductile plate or 0.3 calibers of a face-hardened plate.

So 1.5" of face hardened plate takes a 5" shell to penetrate. Was the STS Belt face hardened? If not, then it takes a 7.5" HE shell to penetrate.


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