Plunging fire

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paul.mercer
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Plunging fire

Post by paul.mercer » Wed Mar 09, 2011 8:22 pm

Gentlemen,
I need to pick your brains again.
Much has been said about the effect of a plunging shell on a ship as against a straightforward shot into the side.
If it is presumed that the average muzzle velocity of 14/15/16" shells is around 2500/3000 feet per second, what would the velocity be when it has reached the hight of its arc and is then presumably only falling under its own weight ?
The reason I ask is that Rodney was firing at a range of about 2-3 miles so would only be hitting her side and KGv moved off to try and get a plunging fire effect on her decks. Again, I presume a 16" shell fired at that close range would penetrate Bismarcks armour, so why try to hit from above?
I imagine that it was anticipatedthat most post 1st WW battleships would be firing at each other at greater distances than at Jutland and with larger and heavier shells so navies presumably would have increased their deck and side armour to compensate.
Given that Bismarck was considered to be one of the most heavily armoured ships of her time, would any plunging 14/15/16" shell actually go down inside her far enough to explode a magazine?

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Dave Saxton
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Re: Plunging fire

Post by Dave Saxton » Thu Mar 10, 2011 4:24 am

Plunging fire is kind of misnomer when talking naval gunfire. Even at extreme ranges of 30,000 meters (32,800 yards) or greater, the angle of fall of most 14"/15"/16" is only an average of ~35* with an average striking velocity of around 450-460 M/s. At more likely max battle ranges of around 25,000 meters, the angle of fall is typically less than 30*, in some cases much shallower than 30*. At these shallower angles of fall enccountered at less than 30km battle range, many shells will be detonated before they reach the panzer deck with the Bismarck design. Moreover, the typical deck armour penetration of battleship caliber shells is at most about 5-inches until the angle of fall exceeds 30*. Approx. 5-inches effective deck protection will in most cases suffice out to 30km battle range. At ranges where the angle of fall will become much more steeper, the amount of effective deck armour required rapidly becomes impractically thick. The Bismarck design was required to provide vitals protection out to 30,000 meters battle range vs 15" shell fire.

The chances are better for the attacker against decks than trying to get through the German side protection systems at short range though. The Bismarck class had a protection system with heavy scarps presenting an unfavorable attack angle. Defeating both the main belt and the scarp is virtually impossible, because the velocity required to defeat both the main belt plus the scarp will exceed the velocity that almost all conventional capped armour piercing shell can remain intact, once de-capped by the outboard armour.
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

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Re: Plunging fire

Post by RobertsonN » Thu Mar 10, 2011 9:37 am

While I believe that the the German system of protection was extremely effective at close range as you describe, there is historic evidence that the Germans themselves were more pessimistic. The official battle handling instructions for the Bismarck gave the inner edge of the immunity zone as 21000 m (at beam on) against the British 15 in (and this included the scarp since the barbettes were given as safe only over 25000 m). Complete side immunity was only effective for opponents at least 40 deg forward or aft of the beam.
On the Marinearchive site, item Gurtelpanzer Scharnhorst, contribution by Thorsten of 12 Nov. 2010, there is a figure of an "optimales Boschungsdreieck" taken apperently from a talk (Fig. 13) by Hoyer (1943) on the protection of an H class battleship. This shows that a 16 in shell could penetrate the 300 mm belt and 150 mm scarp out to about 32000 m. (In fact Hoyer gave 300 mm + 210 mm as necessary to defeat 16 in shells at all ranges.) This seems impossible (all the contributors on that discussion agree about this, but do not fully know why). The vertical penetration of the 16 in shell is given as 350 mm at 30000 m (higher than according to Nathan Okun). My understanding has always been that the official WWII German penetration figures were for KC armor of WW1 quality and not the improved KC n/A as used in the 2nd War, although Thorsten says it was 350 mm KC n/A.
A similar reason seems to lie behind the very low official armor penetration figures given for British guns; e.g., 11.7 in for the 15 in gun at 20000 yds (as mentioned above the Germans thought it was 13.7 in at 25000 m). The British figure was for armor of 25% greater resistance than that used in WW1 and perhaps by some others in WWII.
Coming back to Hoyer, it appears he assumed no deflection towards the normal on penetrating the belt (none is shown on the diagram), that is to say ignoring a factor that would have greatly increased the resistance of the scarp by making impact more oblique. He may have even assumed that the shell retained its cap, which is completely unrealistic. As to the shell breaking up on hitting the scarp or a richochet or scoop, these were also apparently not considered.
As to plunging fire, Hoyer gave a 50 mm upper deck and 150 mm panzer deck as necesaary for protection against a 16 in gun at 30000 m, whereas the actual H class had only a 100 mm panzer deck. Here, again he seemed to ignore performance degrading factors such an decapping and yaw induced on penetrating the upper deck.
I've been intrigued by this talk given by Hoyer for a while but this figure is the only part of it on the web that I am aware of.

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Dave Saxton
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Re: Plunging fire

Post by Dave Saxton » Thu Mar 10, 2011 2:50 pm

It could be something as simple as a typo. Nevertheless the terminology gets a bit complicated. There is intact penetration, penetration by a broken shell or shell fragments, and just making a hole in the armour. What was meant? Intact penetration in a fit state to burst can not occur above certain necessary velocities, or above certain necessary kenetic energy levels by de-capped shells, which is a fact alluded to by Hoyer throughout. Perfomance after de-capping will be better with shorter shells per caliber than with longer shells per caliber.

The differences in calculated single plate penetration by British shell and actual test range figures is the result of the relatively sharp head shape (not cap shape) of 1.4 caliber radius of British shell. This sharp head shape means that the shell requires less energy to penetrate when striking at or near the normal, but the relative performance falls off rapidily as the striking obliquity increases. German and American AP shell on the other hand had blunter head shapes. This improved the relative performance of the shell at more oblique striking angles at the expense of max penetration striking at or near the normal. The computor models don't take this fact of the British (and many others) into account. The Germans found in extensive testing that the best head shape for oblique performance was hemispherical but that a caliber radius for the main body head of 0.93 gave almost equal results to 0.50 caliber radius, and still allowed for good performance at or near the normal.

Thus the German shells performed well at all, including oblique, striking angles and could maintain remarkable penetration performance against the best quality armour materials at extended ranges even as the striking angle became more oblique. Indeed the naval L4.4 APC shell design was proofed at 30*-45*- and 60* and gave excellent oblique striking performance according ADM213/951 data.
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

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Re: Plunging fire

Post by Thorsten Wahl » Thu Mar 10, 2011 5:14 pm

While I believe that the the German system of protection was extremely effective at close range as you describe, there is historic evidence that the Germans themselves were more pessimistic.
They only communicate the calculated worst case scenario. They took only the available energy into account wich would be used for pentration under optimal circumstances . Any gimmicks like yaw, decapping, change of flight direction and so on were ignored to keep the calculation as easy as possible to understand.

Probably there is also a confidentiality effect.
The Unterlagen zur Bestimmung der ... were classified as "Geheime Kommandosache". So they were a possible target of enemy reconnaissance.
there is a principle of confidentiality "So viel wie nötig, so wenig wie möglich" (as much as necessary as little as possible) to keep a secret

Nevertheless there was more far reaching knowledge available.
The basics of this knowledge can be found in the "Unterlagen zur Bestimmung der Hauptkampfenetfernung ...Heft a Textband"
under
section II B Panzeranordnung -(armor arrangement)
and
section IV ...3 Die obere Durchschlagzone (upper limit of pentration)

a complete description of the stuff you can find (in german)at the german Wiki (Bismarck-Klasse (1939)
http://de.wikipedia.org/wiki/Bismarck-Klasse_%281939%29

I have translated a part of section II B Panzeranordnung, wich can be found somwhere in this forum (but lost track where)
Meine Herren, es kann ein siebenjähriger, es kann ein dreißigjähriger Krieg werden – und wehe dem, der zuerst die Lunte in das Pulverfaß schleudert!

RobertsonN
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Re: Plunging fire

Post by RobertsonN » Fri Mar 11, 2011 9:28 am

Thanks for comments. The figure of 25000 m for the barbettes (which were 340 mm thick) is in agreement with the 100/40 gKdos (on another Bismarck site), which gives vertical penetration as 335 mm at 25000 m. The figure given for 21000 m was 392 mm. Both are consistent with data (the NL) on the NavWeapons site. The diagram 13 'optimal armor triangle' does not say what kind of penetration but it cannot have been in a condition fit to burst.
I read through some of the material by Gercke again. The figures he gives in relation to head shape trials (at 45 deg incidence) show that shells with a crh of 1.3 required at least 24% more kinetic energy to penetrate than shells with a crh of 0.9. So the performance of British shells was likely well down on some of their contemporaries for shells striking far from the normal.
Gercke also says the Germans were unable to really solve the problem of damage to the base of shells (and therefore to fuzes) caused by the very great transverse forces during the initial turn away from the normal at oblique impact. Others probably had similar problems.
He says shell length was limited to 4.7 times calibre to prevent shells from being broken by the same transverse forces during the turn away from the normal.
The point about armor penetration computer programmes not taking into account head shape is interesting. This means the various formulas used probably applied only to particular families of shells (with similar head shapes). Adjustment for head shape could only be by altering constants, but since relative penetration is dependent on angle of incidence, it would be wrong to speak about constants at all.
I'd be interested to hear from anyone who could explain what assumptions are made in, say, the Krupp formula or the final American formula.
The official German periods of immunity were perhaps bureaucratic in nature (the machinery had various cut-off switches which were required by law), and it was sensible to keep the real strength of a ship secret. Perhaps this is also the reason for the scarcity of information on American testing before and during the War.
In practice, the short 16 in shells of the Rodney probably posed as much of a threat to the Bismarck at very short range (3000 yds) as any other, apart from the Yamato or perhaps the Vittorio Veneto. From the detailed accounts of survivors given in Brennecke there is no evidence that any shells did penetrate below the panzer deck. I do not know much about the various expeditions to the wreck, but I believe the last, most comprehensive, one found no evidence from those parts visible of penetrations of the torpedo bulkhead. And shells passing through both the belt and scarp would have needed to go through this to in order to reach the vitals.

paul.mercer
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Re: Plunging fire

Post by paul.mercer » Fri Mar 11, 2011 9:27 pm

Gentlemen,
Many thanks for your answers.
I believe that some of the expeditions to Bismarck did claim that Rodneys shells had penetrated through her side armour, (I supppose that a ton of shell moving at almost 3000 fps would go through most armour) if that is the case, would the 14" from KGv also have gone through if fired at the same close range - if so, would it not have been better to fire at close range rather than attempt plunging fire at longer range which would probably result in less hits? As I said in another post after reading the awful account of the final minutes in 'Killing the Bismarck' by Ian Ballantyne, it would appear that Rodney almost shot her to pieces.

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Re: Plunging fire

Post by Bgile » Fri Mar 11, 2011 10:10 pm

It is very difficult to hit Bismarck's citadel at 3 km. There is a very small space in which to hit the belt without the shell entering the water. If the shell enters the water at that range it will tend to re-emerge and hit Bismarck's superstructure. As Bismarck settles this becomes even more of a problem. Of course there is always the possibility of hitting the belt in a wave trough.

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Re: Plunging fire

Post by Thorsten Wahl » Wed Mar 16, 2011 8:06 pm

Flight time at 3 km is ~4 sec, I suspect its a direct shot, height of trajectory shouldnt exceed 15 m or so. danger space estimated ~360 m. It should be real hard to miss the target.

probability for a hit can be expcted ~90%
for a hit at main belt ~25%
upper belt ~25%
remainder superstructure

but general course of bismarck was north west zig-zagging
and Rodneys position therfore was -90 deg to +90 deg constantly changing from the bow of Bismarck

Due to target angle a lot of possible hits should be expected at the forward superstructure and the forward hull from my view
Meine Herren, es kann ein siebenjähriger, es kann ein dreißigjähriger Krieg werden – und wehe dem, der zuerst die Lunte in das Pulverfaß schleudert!

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Re: Plunging fire

Post by Bgile » Wed Mar 16, 2011 8:11 pm

Thorsten, when I say the "citadel" I mean the belt. Of course there will be lots of hits. The belt was almost submerged by the time Rodney was that close. She was firing "down 200" or so in an effort to get underwater hits. It was almost impossible because hits short would come back out of the water.

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Re: Plunging fire

Post by delcyros » Thu Mar 17, 2011 2:01 pm

How do You come to the conclusion that BISMARCK´s main belt was almost submerged? As far as I understand, it extended rather high above the designed waterline (even higher towards A- and D- turrets) and the flooding sustained more or less equalled the fuel and ammo weights spend before the final battle. Thus, it may be considered really difficult to see the main belt submerging unless more than 15,000ts water are inside the ship, equally distributed around the waterplan.

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Re: Plunging fire

Post by Bgile » Thu Mar 17, 2011 5:09 pm

From the Baron's account where he talks about water lapping over the deck around the after turrets at the end, and iirc Rodney's account where the ship seemed to be quite low in the water at the end of the engagement. It didn't list very much until the very end, and of course scuttling charges were set off.

Do you think this is somehow my idea? It's like you are riduculing the idea that 16" shells might reemerge from the water at low trajectories, withe the effect of a richochet. I thought this was common knowledge. We know from the wreck that there were few major caliber impacts on the belt. Why the argument? Are you saying that what happened was impossible?

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Re: Plunging fire

Post by dunmunro » Thu Mar 17, 2011 5:47 pm

By "belt" I presume you are both talking about the 320mm portion of Bismarck's main belt:
Image
The majority of the belt was located above the waterline (3.0/1.8 meters as designed, but 2.6/2.2 meters in practice), with the reasoning that shells are more likely to hit above than below the waterline.
Bismarck's draft increased by 1.3 meters at 53000 tonnes (extreme deep) from 45.5k tonnes (design). Given the underwater damage and counterflooding required to correct lists prior to the last battle, it seems reasonable that Bismarck was probably displacing somewhere between 52-55k tonnes at the start of the battle and somewhat more than this by the time Rodney closed the range. At 53k tonnes, only 1.7m of the main belt would be exposed above the waterline, and at 57k tonnes only about 1 metre of the main belt would be exposed, and this only represents a small fraction of average target height. At very close range, shells are unlikely to plunge when striking the water and given the gale force weather, shells with a trajectory that would carry them into the main belt are very likely to strike a wave prior to hitting the belt.

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Re: Plunging fire

Post by Byron Angel » Fri Mar 18, 2011 3:54 am

delcyros wrote:How do You come to the conclusion that BISMARCK´s main belt was almost submerged? As far as I understand, it extended rather high above the designed waterline (even higher towards A- and D- turrets) and the flooding sustained more or less equalled the fuel and ammo weights spend before the final battle. Thus, it may be considered really difficult to see the main belt submerging unless more than 15,000ts water are inside the ship, equally distributed around the waterplan.

..... Hmmm. Given that - ( a ) BISMARCK's main battery projectiles weighed IIRC about 1800 pounds; ( b ) the propellant charge for each could not have been more than about 600 lbs; ( c ) her main battery ammunition outfit was likely not more than 150 rounds per gun - we cannot be talking about more than perhaps 1,300 MT worth of ammunition overall having been expended for all calibers. As to fuel stores, BISMARCK's gross stowage was only about 8,300 MT according to Dulin & Garzke. And I do not believe that BISMARCK had by any means exhausted either her entire ammunition outfit or her fuel by the time of her sinking.

In any case, the real issue for BISMARCK IMO was not whether there was flooding below the protective deck; it is the degree of flooding that occurred above the protective deck and the dangerous instability produced by free surface effect. According to Nathan's penetration software, even the British pre-Cardonald 15-inch projectile would hole 13.5 inches of KC n/a at 10,000 yards and up to 40 degrees obliquity.

Strictly my opinion, of course

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Re: Plunging fire

Post by RobertsonN » Fri Mar 18, 2011 1:52 pm

During the First War, shells richocheting off the water surface were not uncommon, and this was at ranges of considerably more than 3000 yards. This is probably why fewer hits were scored on vessels' sides at and a little above the waterline than further up. Okun's analysis showed that shells could easily penetrate the main belt at this range but that the subsequent impact on the scarp would be so oblique that penetration was nearly impossible. The decapped shell would most likely richochet if it did not break up. On the other hand, everything above the panzer deck was wrecked and the Rodney did also attempt torpedoeing the Bismarck, for which to have any chance of success it needed to close to about 3000 yds. KGV had no torpedoes, which may be why it not close as much.

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