Japanese CA's and the Long Lance

[Garyt]

A question just came to mind that I do not see an answer for.

Is it the angle of obliquity of the decapping layer that is calculated? Or the angle of obliquity of the underlying belt?

I know the South Dakota's main belt was inclined at 19 degrees. Was the outer layer inclined as well? And if not, I think it would be the obliquity angle of the outermost belt, not that of the interior belt.

[Dave Saxton]

Most battleship and cruisers had 1 or 2 lighter armored "decks" before the main armored deck, did they not?

Not most, but it is correct that WW-I era incremental designs often had two or three thin decks (1-2-3") with protective plating (not really homogenous armour). In these designs the idea was that the upper decks and/or upper belts would initiate the fuse of the shell and the main armoured deck would prevent the resulting splinters from entering the vitals. The main armoured deck was not intended to "stop" projectiles fired from long range. It's a good idea in most cases out to about 20,000 yards battle range.

USN battleships always had two armoured decks though. The WW-I generation had a thin main armoured deck and a thinner splinter deck below that. With North Carolina (and then SD and Iowa) the relationship was flipped, with the thin armoured deck being the weather deck above the main armoured deck which is just one level below.

The British (and also Japanese with Yamato) application of all or nothing beginning with the Nelsons was to place as much as possible available deck armour tonnage into a thick main armoured deck. Thus the weather deck is not made of armour grade materials nor is it intended to de-cap, although at very acute striking angles it might.

The Germans of course designed a new two deck system with the maximal distance between the two decks (and a thin 6mm non ballistic battery deck between) so that yaw could become fully manifest and so that the panzer deck could also stand as part of the vertical protection, thus gaining superior vertical protection with out the additional weight cost. The upper armoured deck also doesn't weigh much more than a non-ballistic weather deck (which is usually the strength deck in structural terms) probably would, for more weight savings making up the horizontal protection.

The Italians used two thin (armour grade material) upper decks above their main armoured deck.

[Dave Saxton]

My point as that if one of the current "authorities" on the subject has had to change his "rules" if you would or laws as to how a de-capping layer performs, then it is still somewhat inexact.

When important German and Italian data on de-capping was received, Mr Okun, to his credit, revised his findings to reflect it. The Germans found that the de-capping plate needed to be armour grade material about 20% the diameter of the incoming shell to insure de-capping at striking angles closer to the normal. The Italians used a de-capping plate that was 70mm and sloped in front of their main armoured belt on the Littorios.

[Dave Saxton]

A question just came to mind that I do not see an answer for.

Is it the angle of obliquity of the decapping layer that is calculated? Or the angle of obliquity of the underlying belt?

I know the South Dakota's main belt was inclined at 19 degrees. Was the outer layer inclined as well? And if not, I think it would be the obliquity angle of the outermost belt, not that of the interior belt.

No, the outer shell on SD is not sloped except for where it may follow the contour of the hull. Your correct that attributing the outer shell as a de-capping layer is post ergo. Unless the striking angle is acutely oblique it won't act as a de-capping plate in the cases of large caliber shells.

[Garyt]

Interesting theories on deck armor. Seems to me at short to medium range it's likely to work to decap as you might be looking at 80-85 or so degree angles. As you get further out, it's less likely to decap as the plunging fire angle angles become close to a 45 degree or so.

Against bombs though it does not seem that it would be as effective - Except for perhaps setting off a HE bomb with a very short or no delay. With the layered decks you are then perhaps decreasing the value of the armor as it's easier to penetrate when it's 3 separate layers.

Any idea what thickness was thought needed to set off a bomb fuse?

[Dave Saxton]

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Against bombs though it does not seem that it would be as effective - Except for perhaps setting off a HE bomb with a very short or no delay. With the layered decks you are then perhaps decreasing the value of the armor as it's easier to penetrate when it's 3 separate layers.

Any idea what thickness was thought needed to set off a bomb fuse?

Probably about the same as shell. The BB59 gunnery manual suggests 1 1/2 -inches mild steel, IIRC.

The TAL (Technical Academy Luftwaffe) did extensive studies of battleship protection schemes vs bombs (documents of which actually survived the war). They found that an upper armoured deck reduced velocity significantly enough to increase the effectiveness thickness of the system by about 10%. Of course bombs usually strike at much less velocity than shells.

As you know the upper deck on the USN fast battleships is often called the bomb deck, but this may be somewhat post ergo.

[Garyt]

Well, your comments about de-capping makes me feel better about the issue

It just seems that de-capping is used so much by American BB fanboys to say why the South Dakota and New Jersey were so incredible of ships. Granted they were nice state of the art battleships, newer than most almost any other BB on the seas, and benefited from trial and error of previous vessels as well as newer technology, but lets be realistic about the outer armor layer, it was not a de-capping layer, merely something that came along with the decision for an internal main belt.

I'm surprised how many that you would expect to be more knowledgeable on the issue still believe the Dakota's and New Jersey's had effective de-capping outer armor. I'm not sure with many of these types if it's wishful thinking or they actually believe it.

[Garyt]

Just one more point on the S Dakota's and New Jersey's decision for an internal belt -

In addition to not working as a de-capping layer, the internal main belt posed a few problems. One was that piercing of the outer armor (which was comparatively easy to penetrating the main armored belt) could result in some progressive flooding, albeit minor, without having to penetrate the main belt.

It could also result in some hull deformity, slowing the ship down a bit, again without having to penetrate the main armored belt.

Lastly, I think I also read that repair of the main belt would be more difficult.