BISMARCK turret protection reconsidered.
Following a hint of Herr Nielsson that the turret faceplate appears to be the only element protected by face hardened armour of the revolving structure of the turret (the similarely face hardened KC/n.A. plates of the barbettes are fixed to the ship´s structure), I once more looked into GKdos-100 curves kindly provided to me by Thorsten Wahl to approach the rational behind the turret protection. I´d take opportunity to thank them providing me valuable information.
Previously, I took it as granted that the slope plates are made of KC/n.A. and not homogenious, full armour grade materials (Wh./n.A.), though altough this notion maybe found in various secondary sources, this is not reflected by support frm primary sources. Parts of these questions have been dealt with in Nathan Okun´s influential article "ARMOR PROTECTION OF THE BATTLESHIP KM BISMARCK" (9/6/1991) to be found here:http://www.combinedfleet.com/okun_biz.htm
However, several aspects leave a lot to be desired.
"The rotating turrets are also slightly lighter in armor than the average foreign designs with 14.17" (360 mm) KC n/A near-vertical faces (port plates), 8.66" (220 mm) KC n/A vertical sides, and 12.6" (320 mm) KC n/A backs (this thicker back is due to it being used as a counterweight for the gun barrels to balance the turret). Most foreign designs used heavier faces except for the British HMS KING GEORGE V Class which used smaller 14" (356 mm) guns and only 12.75" (324 mm) CA faces (the lower scaling effects of thin-faced British CA partially offset this slightly low face thickness).
Turret roofs on the BISMARCK were of the unique standard German WWI pattern, being "faceted," with a central flat rectangular 7" (180 mm) Wh region raised up above the upper edge of the turret sides and connected to the sides using prism-shaped 7" Wh sloped plates on the sides and front. This raised the roof of the turrets a couple of feet without having to increase the height of the heavy side and face armor and thus this design lowered the weight of the turrets. Most foreign contemporaries had turret roofs of that thickness or greater that were flat or slightly sloped to the sides or fore-to-back. However, the BISMARCK's steeply sloped roof edge plates were much less oblique than the flat central roof and were not increased in thickness to compensate. To me, that makes them weak spots in the turret; they should have been thickened to at least 9" (229 mm) or, if not, at least the front sloped plate should have been sloped at a much shallower angle, as it was the one most likely to be hit by the enemy that the turret was shooting at. "
-Nathan Okun (1991), see above for reference
A number of updates appear to be necessary in order to bring that part of the article to the present state of knowledge:
[A] the central flat portion of the turret is not 180mm Wh but only 130mm Wh
[B] the degree of the sloped part of the turret is determinable to 66 to 68 deg (depending on the detailed plan from which the measurement is taken, I use therefore 67 deg herewith as a working prelimiary)
[C] the 360mm thick frontal plate is leaned back 8declined by between 8 an 10 deg (again, depending on sources, using 9 degs herewith)
[D] there is no evidence to suggest that the thick side- or back plates of the turret are similarely made from face hardened KC derivates as the turret face.
It does also require an analysis in order to allow for a conclusion that there is a weak spot with regard to which thickness which is required to allow protection from BISMARCK´s own guns at certain distances.
I. Barbette and turret faces:
Thus, the Elements of face hardened armour (KC/n.A.) appear to be the ~9 deg declined 360mm turret faceplate and the 340mm vertical but rounded barbette plates, while homogenious armour appears to cover other areas of the turret such as the 180mm thick frontal and back slopes (67 deg declined), flat turret roof (130mm) and turret back.
In order to determine the protection of these areas, these surfaces are to be compared against the 38cm L4.4 800kg APC fired from the SK C/34e housing in these turrets. I am aware that this is a very artificial comparison. The penetration curves and ballistic data are taken from "Unterlagen zur Bestimmung der Hauptkampfentfernung und Geschoßwahl, Heft h" with reference to individual penetration charts for homogenious armour (drawing 5 AKB 9259), face hardened armour (5 AKB 9214) and ballistic curves (5 AKB 1861a).
The individual points for striking velocity and impact obliquity (adjusted for angle of fall and decline of the plate) can be plotted against the penetration curves for face hardened armour. One has to keep in mind that turrets are exposed and even a partial penetration can have catastrophic effects due to driven in hot fragments of the projectile or plate, even without high order detonation. I therefore feel justified to use the (black) penetration curves for "grenz" penetration, which is sometimes incorrectly called "broken". The definition for "grenz" is dependent entirely on the complete depletion of the striking energy during the penetration process, thus the whole projectile is expected to pass the plate but with no remaining residual velocity either intact or broken. One has to reckon that in conditions of high or medium obliquity, this would likely cause the projectile to break but it´s not necessary for the projectile to be ineffective, it´s only necessary to deplete it´s energy to attain this definition. Thus, in terms of US terminology, this definition touches "partial penetration", "NBL" and "EEF", altogether but is closer to the US Naval Ballistic Limit than any else.
From the penetration curves in GKdos-100 it appears that the slightly declined turret face is vulnerable to BISMARCK´s own guns out to 300hm / 32,820 yard to penetration in "grenz" conditon. This is a very long range by any standarts but one may easily see that even a 18.1 inches or 460mm KC/n.A. plate -installed the same way as in BISMARCK´s turrets- would not have provided ballistic protection against the 380mm L4.4 APC at likely battle range of 200hm (21,880 yard). In order to provide ballistic protection at 150hm a plate similarely thick as SHINANO´s famous turret faceplate would be required and the weight for it -together with the balancing turret back plate may have been prohibitive in this case from the point of view of topweight and stability considerations.
The barbette plates, albeit vertical, allow protection from 270hm and larger (roughly 30,000 yard) for centre barbette impacts (no horizontal angle involved). Again, this appears to be a very large distance with very little immunity provided.
II. Horizontal armour protection of the turret.
Homogenious armour is -unlike face hardened armour- in general not able to damage the L4.4 Psgr APC projectile, at least for the range of thicknesses (to 200mm plates) referred to in the penetration graphs. Thus, a penetration at "grenz" is always also an effective penetration with the projectile capable of penetrating high order behind the plate. In this case, the "grenz" curves are identic to the "US effective ballistic limit". It should also be noted that, because the projectile impacts with windscreen and armour piercing cap attached, the GKDos-100 penetration curves are valid. They also assume a fully intact projectile impact with all nose coverings and caps in place.
The flat roof plates (130mm Wh) and the sloped parts (180mm Wh) have been plotted against the penetration curves for homogenious armour and the speific impact conditions caused by striking velocity, angle of fall and decline of plate:
As can be seen in this plot, the sloped parts of the turret are not -as has been claimed- vulnerable at short or medium distances to penetration. It requires a minimum distance of 230 to 240hm (roughly 26,000 yard) to allow a penetration according to GKdos-100 curves. The flat 130mm Wh roof is vulnerable to penetration only at extreme range, ca. 300hm, which is coincidently the same range which we have seen where the penetration through the face hardened turret front ceases to be possible.
It appears that the statement that there "is no classical immune zone for BISMARCK´s turrets" still holds true in light of the faces vulnerability not ceasing before ~33,000 yard, while the vulnerability of the flat turret roof appears right there and that of the 180mm turret slopes already earlier at 26,000 yard.
Still, not every plate of a turret is vulnerable to ballistic penetration at any range. Hits impacting the turret slope or roof plate at, say 25,000 yard likely will not get through while impacts on the barbette or turret face at extreme range equally have little chance to be successful.
The possible impact trajectories are plotted below.