Iowa Class real performance

[Tiornu]

Is that the effective limit, naval limit...?
I believe the angle of the faceplate is closer to 35deg, if that matters.

[José M. Rico]

According to 100/40 g.Kdos, at 24,000 meters (26,246 yards) a 38 cm Psgr. L/4,4 Armour Piercing shell falls at 22º from the horizontal with a striking velocity of 479 meters/second (1,571 feet/second) and can penetrate a 350 mm face hardened vertical plate with a striking angle of 68º. Against a back sloped 30º plate the striking angle would be 82º.

Those of you who have my Bismarck e-book check page 43 to see the complete Range and Penetration Table for the 38 cm SK C/34.

[Tiornu]

The Americans did not use face-hardened armor on their turret faces, which is why you have to use M79APCLC instead of FACEHARD. I believe they planned to use face-hardened armor but had trouble with manufacturing it--yet I don't know why that would be the case, as the old battleships had 18in turret faces.
On the other side of the spectrum, the French and Japanese used face-hardened armor for their turret roofs.

[Nlneff]

Is that the effective limit, naval limit...?
I believe the angle of the faceplate is closer to 35deg, if that matters.

Thats the Navy Ballistic Limit, according to M79APCLC.

Changing the faceplate angle to 35 degrees would improve things, but not enough to note with the 2000 yard differences given in NAVWEAPS (If I have time later, I could try averaging out the numbers to get 1000 yard increments, but its a chore). At 16000 yards, the Navy Ballistic limit is 1881 fps, the 38cm shell has 1889 fps, so it just penetrates. Again, a used liner would drop the velocity below this.

[Dave Saxton]

The curves I'm using are for homogenous armour similar to the Krupp curves but supposedly corrected to Class B. It's not much different from the original Krupp curves for homogenous armour though. This was supplied to me on a proprietory basis. I have more faith on the old school Krupp type curves than in modern calculators.

I don't for one minute believe that these plates had 100% effective thickness, for known metalurgical reasons, but I'm not factoring that in. At least not knownly. Is this factored into the curves already?

I don't believe that using homogenous armour in such thick plates was a good thing, contrary to the conventional wisdom of our time. Nonetheless, the use of Class B on the new construction was forced upon the builders by the continued rejection, for quality reasons, of the original planned Class A plates by BofO. Face hardened armour would perform better at damaging or breaking up attacking projectiles.

The problem of the enormous penetrative power of modern guns and projectiles was a difficult one. This problem was great at medium to medium long ranges, or the more practical battle ranges. At shorter ranges the striking velocity may exceed the critical velocities, and the necessary velocity may exceed the critical velocity. Hence a shell with a low critical velocity may perform poorly against a barbet, conning tower..ect... Angling a face plate or a corner plate lowers the critical velocity at short range, but there's a trade off at longer ranges.

[José M. Rico]

According to 100/40 g.Kdos, at 24,000 meters (26,246 yards) a 38 cm Psgr. L/4,4 Armour Piercing shell falls at 22º from the horizontal with a striking velocity of 479 meters/second (1,571 feet/second) and can penetrate a 350 mm face hardened vertical plate with a striking angle of 68º. Against a back sloped 30º plate the striking angle would be 82º.

The Americans did not use face-hardened armor on their turret faces

Wouldn't a 38 cm Psgr. L/4,4 AP shell then perform even better against homogeneous armour?

[Bgile]

According to 100/40 g.Kdos, at 24,000 meters (26,246 yards) a 38 cm Psgr. L/4,4 Armour Piercing shell falls at 22º from the horizontal with a striking velocity of 479 meters/second (1,571 feet/second) and can penetrate a 350 mm face hardened vertical plate with a striking angle of 68º. Against a back sloped 30º plate the striking angle would be 82º.

Those of you who have my Bismarck e-book check page 43 to see the complete Range and Penetration Table for the 38 cm SK C/34.

Ok but here we are talking about 457mm of armor.

[Tiornu]

Wouldn't a 38 cm Psgr. L/4,4 AP shell then perform even better against homogeneous armour?

Yes, definitely, unless there's something wrong with the face-hardened armor. The Americans most definitely wanted to use face-hardened armor and planned to do so, but were forced to make the switch.

[Tiornu]

"The curves I'm using are for homogenous armour similar to the Krupp curves but supposedly corrected to Class B."

Supposed by whom?

[Bill Jurens]

The hit on South Dakota is unlikely to have been from an armor-piercing round. The steel doesn't lie; an impact from an a.p. round would have caused much more damage at the impact point, and the amount of damaged and removed decking is much too large to have been removed by the relatively small charge in an a.p. round. The secondary damage lower on the barbette is likely to have been due to the impact of the base plug or some large piece of dislodged structure.

The possible penetration of an Iowa face plate by a Bismarck bullet represents an interesting topic, but one which I cannot devote very much time to at present. It's worth noting, however, that heavy homogeneous plates were rarely tested at near to normal impacts, so the actual data base is pretty rare. In that regard, I would view other than official (and reproduceable) data with considerable skepticism. GKdos 100 does not cover this type of impact at all, but with considerable extrapolation of lines which are already there, it would appear that the predicted penetration for a 450 m/s impact at 90 degrees would be about 13.5". But this is, admittedly, highly tentative.

Bill Jurens

[José M. Rico]

According to 100/40 g.Kdos, at 24,000 meters (26,246 yards) a 38 cm Psgr. L/4,4 Armour Piercing shell falls at 22º from the horizontal with a striking velocity of 479 meters/second (1,571 feet/second) and can penetrate a 350 mm face hardened vertical plate with a striking angle of 68º. Against a back sloped 30º plate the striking angle would be 82º.

Ok but here we are talking about 457mm of armor.

Yes I know, but you also have take into consideration the new striking angle which is closer to 90º, and the fact that the American turret faces were made of homogeneous armor, thus improving the penetration capabilities of the attacking shell.

I'm not saying that a 38 cm Psgr. L/4,4 AP shell could penetrate 457mm of homogeneus armour at 82º from 26,000 yards, but I'm afraid that without a real test there is not way to know that for sure.

[Karl Heidenreich]

Bill Jurens:

The possible penetration of an Iowa face plate by a Bismarck bullet represents an interesting topic, but one which I cannot devote very much time to at present.

A shame because Mr. Jurens reputation is among the great experts.

Best regards.

[Bill Jurens]

Well, you sure know how to flatter a guy...

I am working on a number of other things now, but your kind comment enticed me to do a quick 'back of the envelope' calculation using the USN's official ORD SK78841 as a guide. This suggests that penetration of an Iowa faceplate by a 15" bullet was improbable.

Anyhow, according to the Iowa armor plan, the faceplates were 17" thick with 2.5" backing plates making the total thickness about 19.5". (I KNOW that it's effectively slightly less than that, but let's not quibble...)

Bismarck would have achieved an obliquity of zero degrees against an Iowa class Class "B" faceplate at a range of about 32000 meters. At that range, the striking velocity is about 460 m/s, i.e. about 1509 f/s. The Navy ballistic limit for the plate for zero degree obliquity assuming a fairly good USN projectile would have been about 579 m/s, i.e. about 1.2 times the striking velocity of the bullet. So complete effective penetration is improbable.

If 579 m/s is required to penetrate at zero degrees, what happens if Bismarck's projectile hits at that velocity? A 579 m/s striking velocity corresponds to a range of about 12500 meters and an angle of fall of 8 degrees. Assuming a 35 degree slope on the plate, this leave a residual obliquity of 27 degrees, and ORD SK 78841 gives a required velocity of 693 m/s to penetrate effectively, again about 1.2 times the actual striking velocity that is experienced.

This is roughly in accordance with my interpretation of the German graphs of homogeneous armor penetration in GKDos 100, which suggested that 13.5" of penetration, or perhaps a bit more, was in the ballpark of what might be expected.

There have been comments made that the thicker versions of the US Class "B" were in some way inferior to equivalent thicknesses of Class "A" armor, but I think you will find that US experience was different, and that Class 'B" armor was actually better at low-obliquity, i.e. near-normal, impacts in these very thick guages.

I must emphasise that this represents only a quick calculation, and that ORD SK 78841 is not necessarily the best tool for the job, but it does serve to indicate that Iowa's faceplates were probably at least marginally able to keep out a Bismarck projectile at any reasonable range, and perhaps all ranges. More refined calculations might modify the exact penetration values slightly, but I doubt if the differences would really be significant.

Comments (and new calculations, if anyone would like to do them) welcome...

Bill Jurens

[Nlneff]

Well, tweaking my M79APCLC calulations, I had thought the faceplate was 17in on 2in, equivilant to about 18.4-18.5, revising things to 18.75 for a 2.5in backing plate, doesnt change much. At 16000 yards (not meters) with a new liner the 38cm just fails to penetrate, with the limit velocity being 1897 fps, and the shell hitting at 1889 fps. Assuming a used liner probably moves the range in a little. I would consider Bill Jurens calculations more definitive then mine, but both indicate penetration much beyond these ranges is unlikely.

[Bill Jurens]

Your results match my calcs almost exactly. A range around 14600 meters would seem to be near the optimum situation. ORD SK 78841 predicts a limit velocity of 1942 f/s, i.e. c. 591 m/s, and the impact velocity is at that range, picked quickly off a graph -- is about 575 or 580. About 98% as you noted.

It's a marginal situation to be sure; even if the projectile did not make it all the way through in an intact condition, the resulting damage to the face plate would almost certainly disable one or two guns due to distortion and jamming of inboard structures.

Bill Jurens