Shermans and overmatching

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Wokelly
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Shermans and overmatching

Post by Wokelly » 22 Oct 2006 08:28

I've read through some of the threads on the shermans, and I noticed a few issues. First off people tend to point out the effective strenght of the sherman. While its hull was sloped, it suffered from armor quality issues and a distinct disadvantage of being prone to overmatching.

Throughout the war, sloped armor was used on an increasing number of tanks. However it seems overmatching was a phonemon not understood till after the war. I notice alot of people seem not to have heard of the effect. Basically overmatching was an event that occured if a shell hit sloped armor that was thinner then the diameter of the tank shell. This effect neglects the sloped armor advantage, causing the shell to simply smash though the armor, causing a much bigger hole and much more armor fragments to enter the tank compartment, killing and maiming. For example teh T-34, its effective armor should of also been around 100mm, but because the 75mm shell was so much thicker then the sloped 45mm hull, the shell would often just smash though armor instead of being deflected.

Now in regards to the sherman, it originally had 51mm of armor sloped at 31 degrees from the vertical. This in effect meant the hull was armound 100mm's of effective armor if hit, which proved adequit in the desert against Mark3s with the 50mm guns. Except for armor quailty issues, never fixed until the M4A3(75) and (76) versons in late 1943, they stood up well.

However, with the appearence of more and more long barreled Panzer 4s with Long 75mms, the shermans hull was all of a sudden being penetrated by these shells at ranges beyond where it should of statistically or technically should of stoped the round. Of coruse the culprit was a combination of overmatching and bad armor quality. The bad armor quailty reduced the strength of the armor, meaning in effect it was not as strong as its real 51mm, which meant the 75mm was more effective at overmatching the 51mm then it should of been.

However, by late 1943, the newer verson of the sherman was coming out (which ever versons had the 64mm at 43 degrees from the verticle hull armor, with 75 and 76mm guns). In order to get rid of the two near verticle spots where the commander and hull gunner sat, the hull slope was decreased, but more armor was added to compensate. This had the added benifit of actually preventing overmatching. As a result of finally ironing out the armor quailty issues, and 64mm against a 75mm shell, the hull finally became its effective 100mm or so of armor.

So basically, it wasn't until around late 1943 early 1944 the sherman finally was at its best probibly armor wise (except for the Jumbo). Except many thousands of shermans with the bad armor quailty and the easy to overmatch 51mm hull armor were in service with allied armies.

This probibly would of produced a mixed result in normandy, with some shermans easily being taken out by the german 75mms, and then the newer shermans which should technically be able to stop/deflect a the kwk 75/48 (probibly written wrong) from about 1000m if hit in the hull.

This basically means, the Sherman 76s and M4A3 (75)s would be a more survivable tank then the M4s, M4A1s, and early M4A3s. And for the brits, the Sherman Vs and IIs would be inferior to the Sherman III's (M4A2's ?).

Against its contemporary, the earlier shermans would be generally inferior to the Mark4s as a result of weak hull armor (see overmatching) and bad armor quaity. Probibly the Mark4s would have stronger hull armor and could be just a bit worse turret armor when factoring in armor quaily (as much as only 85% effective in early shermans - combat mission). On the other hand, the sherman 76s and M4A3 (75)'s would have superior armor, the hull armor being an effective 100mm and the turret being 89mm vs the mark 4s 80mm hull and 50mm turret.

So in effect, during WWII, some shermans were better then other shermans even if statisctially they should be on par. Now this is just me wondering out loud, but when discussions about the 3 Guard Divisions swaping for SHerman 76s, well this may of been a reason. The T-34 never ever had its hull armor increased, and thus throughout the war, it was extreamly vulnerable to hull shots, especially overmatching. I really dont know if they could have ever figured it out, but the Sherman 76 would of offered superior protection in the hull, though probibly the rounded turret on teh 34/85 was ballistically superior to the shermans turret. I cant help but wonder if maybe the Russian tankers noticed how rare it was for the sheramns hull armor to cave in like the T-34s would of after a hit by a 75mm shell. No evidence to back this up, just a guess.

Now, to be honest the sherman variants confuse the hell outta me, so basically when I refer to M4A3 (75)'s, I mean the 75mm armed shermans with the 64mm hull armor and 89mm turret armor. Sherman IIIs are the same deal, but some sites say they were M4A2s. Kinda confuses me a bit.

Anyways, this is my little addition, and sourcs are a pain to list. Really this is a mixture of info from Combat mission, WWIIOL motorpool discussions on overmatching, stuff Ive come by, and just putting the pieces together. Generally the stuff from the WWIIOL discussion forums is pretty accurate, cause people want a realilistic game, and thus soruces are used. If I still had access to the forums I would just be doing lots of coppying and pasting, but for now I have to go off memory.
Last edited by Wokelly on 22 Oct 2006 21:35, edited 1 time in total.

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Post by Carl Schwamberger » 22 Oct 2006 18:06

Nice to see someone who can actually cite a source & address the question a analytical way.

Can you discuss the "overmatching" phenomonon with other tanks. Last year I did a comparative study of the German & French tank armor of 1940. Nothing in depth, just a non scholarly brief. The inadaquacy of the German tank armor of 1939-1941 really jumps out.

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Post by Wokelly » 22 Oct 2006 21:11

Well overmatching seemed to be a bigger issue on allied tanks then Germans tanks. Now, if I read correctly from the WWIIOL forums, overmatching was not really understood till after the war.

NOTE - Armor is a very complicated thing to discuss. Armor types reacted to overmatching differently. For example, casting was not the best armor type, and I believe had issues with overmatching, though what issues I am unsure and dont want to speculate on. Also Face Hardened Armor (FHA), though generally strong, overmatched must worst the Rolled Homegenious Armor (RHA). On a side note to this note, I beleive I heard the army still has info on overmatching classified, possibly because many APCs had armor that could be overmatched by big caliber guns on modern tanks, and thus the information is still revelant, though armor now is so rediculously complicated to understand.

There is a really great guy on the WWIIOL forums who discussed this in detail and had sources, where I learne a bunch about it. Its actually quite difficult to explain.

Basically, if the shell has a greater thickness then hull armor ( a 75mm shell vs 45 mm of armor), there is a great possibility that the shell with overmatch. Basically what happens is the force of the shells impact is so overwhelming the armor doesnt have time to absorb the impact, and shatters.

I like to use the T34 for this. It had 45mm of armor sloped at 60 degrees from the verticle. This gave it 2.5x its effective strenght, being 45x2.5 = over 112.5mm of armor. Now, technically the German 75mm should of had some trouble against that, but it really did not end up being the case. Now of crouse, there is something abotu overmatching that I can not really explain. It has more to due then just with the size of the shell. For example, the low velocity 75mm on the early mark 4s were not particularily good T-34 killers, while the high velocity rounds were. So there is more to it then just the size of the shell. There must be another component in this as well.

Now I think what I read from Scotman, who works at a job where he has access to these kinds of documents and basically explained it to me, I think the shell must have the ability to penetrate the equivelent sloped armor at some range. Basically, the low velocity 75mm could not penetrate 112.5mm of armor (the effective armor of the T-34s 45mm at 60 degrees from the verticle) at any range while the high velocity 75mm could penetrate 112.5mm of armor at close ranges. I believe this difference meant that the high velocity could overmatch the armor while the loew velocity really did not, and thus the HV 75mm could destroy T-34s at long ranges while the LV could not even except possibly (cause I dont know) at close range.
Carl Schwamberger wrote:Nice to see someone who can actually cite a source & address the question a analytical way.

Can you discuss the "overmatching" phenomonon with other tanks. Last year I did a comparative study of the German & French tank armor of 1940. Nothing in depth, just a non scholarly brief. The inadaquacy of the German tank armor of 1939-1941 really jumps out.
Overmatching would probibly not have been a big thing back in the french campaign. When you look at it, the largest gun was a LV 75mm, and the common guns were in the range of 30-47mm against armor that was generally 30mm thick, and really sloped armor was not used greatly except maybe on the S-35 and StugB's. The only armored vehicles that probibly suffered from overmatching would of been armored cars like the 232's. German armor in france were simply to lighly armored to face the French and British ATGs of the time. Wasnt really an issue of overmatching as in tanks too lightly armored to withstand the allied ATGs.

The effect of overmatching started to frequently appear in 1941 in North Africa when allied tanks like the crusader 2 came up against 88s and the 75mms and sometimes 50mms. The Early stuarts were prone to this, and pictures were shown on the WWIIOL forums. Personally, I no longer have acess because my suscription is out, otherwise I would be doing lots of coppying and pasting. Basically, by sometime after WWII phonemon know as overmatching was finally dicovered, but too late for many of the dead allied crews sent out it tanks that had sloped armor supposedly able to take certain hits when in reality the armor just gave way to the shell. Really not a fault of the allied designers in this case.

Now the allies seemed to suffer from overmatching the most. From the earlier sherman models to their TDs, they all used sloped armor, but against the 75mm this slope was pretty much useless.

The germans on the other hand, probibly not because they understood the whole concept of overmatching, just happened to usually apply anywhere from 60mm to 80mm of sloped armor, basically making them immune to overmatching from the common allied 75mm or 76mm. However, due to the large caliber guns the soviets used, the germans would of experianced overmatching as well, especially against the 85mm, 100mm and 122mm.

Again, I want to stress I would not consider myself the best source on this, but alot of this I read from a guy who has access to these forums. Scotman is his game name, and he has helped the rats by getting them WWII testing documents, and is very reliable because he has access to these things, and tends to try to get this stuff declassified (17 pounder HE data was classified until just recently!).

Tanks that could be overmatched throughtout the war from the front: (no evidenc to back up, but generally had weak frontal armor that was over 10mm thinner then the equivent gun they would be facing)

-T-34s (all versons) vs 75mms and greater
-T-60's adn 70's vs 50mm and greater
-Many of the early SU Tank Destroyers with 45mm hulls vs 75mm and greater
-Early shermans vs 75mm and greater
-Late shermans vs 88mm and greater
-All US TDs vs 50mm and greater (all had hull armor of 38mm)

Just a small list, not complete, and some could quite well be incorrect. Generally the rule of thumb I'd place it at would be anything that was 20-30mm thinner then the shell that hit it would usually cause overmatching. Again, dont interpret this as the word of God, just my view and could quite possibly be incorrect. T-34s 45mm vs a 75mm means the shell is 30mm thicker, and Early Shermans 51mm vs 75mm means 24mm difference. On the other hand, Late shermans 64mm vs 75mm is 11mm difference.

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Post by Carl Schwamberger » 23 Oct 2006 04:00

I have to second your remark about how complex a subject this is. I've not done a indepth study, but have skimmed thru many documents on the subject of armor & AP ammunition. From that its clear very few folks remarking on tank armor have any idea of the real details. I'd not seen the term "overmatching" before, or seen it described as you do. Some of the articals on AP ammunition do refer to armor plate failing catastophicly from large caliber ammunition, but the details of how this happens are not addressed.

One thing I do see often described is the phenomenon of the AP projectile rotating its long axis as it strikes the armor face. This rotation continues until the axis is perpedicular to the armor face. At that point the projectile either penetrates the armor, or failing that continues to rotate & eventually seperates ricochting off.

Facinating stuff.

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Post by Wokelly » 23 Oct 2006 04:33

I'll see if I can get some of those pictures from the WWIIOL forums of overmatched tanks. I hope someone will copy and paste the article to me with all the pics.

I also want to say, my post is not 100% proven stuff. There could be errors, though I think most of what I have posted is true. Again, like everything posted on a forum, dont take it like its the word of God.

However, I have to agree on the fact little exists out there about overmatching. It would of been extreamly common I'd think in WWII for this to occur to allied vehicles. I mean the T-34s especially would have suffered throughout because of the sheer amount of 75mm guns in use by the Germans.

But it really does make sense. If overmatching didnt exist, allied tanks would of ended up being much better tanks in my opinion. I mean imagine the T-34-85 with that turret and a hull that was effecively 112.5mm thick. Thing would of been near as strong as a tiger, and there would of been thousands of them.

Again, in my opinion one of the reason allied tanks fared like they did vs german guns was because of the distinct lack of knowledge on overmatching during WWII. The allies really relied on the effective strength of sloping thin armor to protect their tanks. The US TDs and T-34s are prime examples of this.

The reason I bolded in my opinion is because I am getting the feeling not many people understand overmatching and are relying on my posts to inform them. I just dont want you to take my opion for fact, as they are simply my opions.

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Re: Shermans and overmatching

Post by cbo » 23 Oct 2006 11:23

Wokelly wrote:I've read through some of the threads on the shermans, and I noticed a few issues. First off people tend to point out the effective strenght of the sherman. While its hull was sloped, it suffered from armor quality issues and a distinct disadvantage of being prone to overmatching.

Throughout the war, sloped armor was used on an increasing number of tanks. However it seems overmatching was a phonemon not understood till after the war. I notice alot of people seem not to have heard of the effect. Basically overmatching was an event that occured if a shell hit sloped armor that was thinner then the diameter of the tank shell. This effect neglects the sloped armor advantage, causing the shell to simply smash though the armor, causing a much bigger hole and much more armor fragments to enter the tank compartment, killing and maiming.
"Overmatching" is what you get when the thickness of the armour (T) is less than the diameter of the AP round (D). If T/D is less than 1, then you have overmatch. But that does not mean that things suddenly changes dramatically.

You may see the AP round plugging a hole - i.e. knocking out a piece of armour about the size of the incomming round - instead of pushing aside the armour material, but that does not necessarily mean that you get a lot more damage, nor does it seem to happen unless T/D is considerably below 1. Catastrophic failure of the armour probably has more to do with the quality of the armour than any "overmatching".
Also, if T/D drops below 1, it tends to reduce the effects of sloping armour, but that happens primarily against high angles of slope.

Of course, T/D works both ways. If it raises above 1, then the effects of slope is increased, particularily against high angles. That is why the T-34s armour was quite effective against the most common threats it faced around the time it was designed and introduced to combat. Those threats were 20mm - 50mm guns, which the T-34s sloped 45mm hull armour dealt with quite well.

Still, "overmatching" - T/D ratio - is not that significant by itself, it is just one of the numerous factors that are at play when an AP round meets armour plate. Things like weight and velocity (i.e. energy) is rather more important.

In your example about the German low velocity (385 m/s) 75mm gun (7,5cm KwK 37 L/24) vs the high velocity (740 m/s) 75mm gun (7,5cm KwK 40 L/48) vs the 45mm/60 degree hull of the T-34, the T/D ratio is the same (45mm armour/75mm AP round = 0.6) and the two projectiles have the same weight. However, the low velocity projectile will be struggling to make an impact while the high velocity projectile will stand a fair chance of penetrating. The main difference is velocity, which means that the high velocity round has more than 3 times the kinetic energy compared with the low velocity round.

As you, I thought that T/D was not a well understood phenomenon prior to WWII, but someone here posted a pre-war penetration formula (Krupp?) which clearly included T/D, so the phenomenon was known.

Claus B

PS: Even though many people have issues with it, the booklet "Armour and Gunnery" by Lorrin Bird and Robert Livingston have some interesting discussions of the various phenomenon involved. It is mainly an attempt to get to grips with WWII armour penetration mechanics and beat it into formulas that can be (and are) used in wargames. It is a good starting point, in my view. They were selling it over the Internet, but I dont know if you can still get it?

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Post by Wokelly » 23 Oct 2006 21:18

thanks cbo, this is also a learning experiance for me, and I am hoping that this thread will eliminate any errors in my knowledge of overmatching.

I was under the assumption that overmatching was a much more common phonemon then you say it is. Hopefully more people can add and we can get a better understanding of this issue.

I also had read about the whole T/D forumula, but was unsure if it was correct, and thus did not post it. This should really replace the rule of thumb I used in my second reply.

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Post by Carl Schwamberger » 24 Oct 2006 02:59

cbo's post rang a bell. I have to reread it in detail, but I have here a artical from the Royal Field Artillery Journal that discusses the basics known in 1943 of Shot vs Armor. (Infact thats the name of the artical.) Although the term Overmatching is not used it seems from my quick glance that it is implied. There is also some discussion of armor hardness and how this all relates to design of the AP shot. It not a in depth essay, but a information piece for artillerymen busy at war.

From that artical and some other items I am getting the impression that the overmatching phenomenon is related somehow to the projectile rotation. That is the projectile has to transfer its energy to the armor for failure to occur & the necesary level off energery transfer does not occur if the projectiles axis of travel is to far from perpendicular to the face of the armor.

On this web page, in Section IV there is a brief overview of theory on shot v armor. Similar to the RJA artical. Perhaps I am reading too much into it, but the concept of overmatching seems to be approached in the illustrations and formulas. Clearer is the importance of shot rotation when in contact to the armor face.

http://www.battlefield.ru/index.php?opt ... &Itemid=64

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Post by cbo » 24 Oct 2006 11:40

Carl Schwamberger wrote:From that artical and some other items I am getting the impression that the overmatching phenomenon is related somehow to the projectile rotation. That is the projectile has to transfer its energy to the armor for failure to occur & the necesary level off energery transfer does not occur if the projectiles axis of travel is to far from perpendicular to the face of the armor.

On this web page, in Section IV there is a brief overview of theory on shot v armor. Similar to the RJA artical. Perhaps I am reading too much into it, but the concept of overmatching seems to be approached in the illustrations and formulas. Clearer is the importance of shot rotation when in contact to the armor face.

http://www.battlefield.ru/index.php?opt ... &Itemid=64
That website discusses how a blunt nosed projectile design can reduce some of the effects of sloping armour. First by preventing the projectile from bouncing away from the armour, secondly by turning the projectile towards the perpendicular, thus reducing the amount of armour material, the projectile has to go through. So I guess you can say that the blunt-nosed projectile "rotates" towards the armour, rather than away from it. The latter process is what the website calls "normalization".

"Overmatching" - i.e. T/D less than one - relates only to the relative size of the projectile vs the armour, not to projectile design or the phenomenon of "normalization" and I dont think "overmatching" is discussed on that site at all. It deals primarily with projectile types and how their design effects penetration.

But clearly, if you combine the factors discussed - overmatching (T/D < 1), lots of energy (weight and velocity) and a flat-nosed projectile, you will get relation between armour and AP projectile which can seriously reduce the effects of sloping armour. But they are separate phenomenon, each having their own effect.

Claus B

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Post by Carl Schwamberger » 24 Oct 2006 14:05

I can follow that. I was reading in the implications of undesireable rotation for projectiles with sharp points. Thats illustrated better in the JRA artical. Plus it has a few remarks about energy transfer, which would seem to relate to the sucess/failure of overmatched projectiles. Again I may be not understanding this, but energy distribution on impact seems to be the core of shot vs armor. Rotation, size/mass, shape, velocity all seem to tie together in energy tranfer to the armor.

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Post by Wokelly » 07 Nov 2006 03:15

From WWIIOL forums:

I hope when you say "cannot" you mean "currently is not" DOC because there is no theoretical reason why overmatch factors as an influence on the effectiveness of sloped armor cannot be quite easily included in a ballistics simulation.

Overmatch is calculated as the T/d ratio - namely, ratio of thickness of armor (a known quantity) to diameter of shell (again, a known quantity). As the T/d ratio drops the influence slope has as an armor multiplier (again a known quantity for each angle) declines.

To take a completely hypothetical example, say a 90mm shell hits 40mm armor sloped at 30 degrees. Ordinarily the slope multiplier would be about 1.25 for 30 degree slope, giving about 50mm effective resistance.

However, because you have a high degree of overmatch here (90mm v 40mm) your algorithm calculates that for a 40/90 overmatch ratio the overmatch factor is say 0.2 (ie an overmatch factor of 1.00 represents no overmatch, the lower the figure the higher the overmatch)

Then the effective armor of the target would be:

effective armor thickness*(1+( (slope multiplier -1)*overmatch factor))

so in the example above the calculation would be:

40mm * (1+((1.25-1)*0.2))
=
40mm * (1+(0.25*0.2))
=
40mm * (1+0.05)
=
40mm * 1.05 = 42mm effective

so the effect of the high degree of overmatch has been to reduce the slope multiplier from 1.25 to 1.05 and the effective armor is now 42mm instead of 50mm.

T/d ratio also has an influence on the penetration of unsloped armor, but that would be a lot more complicated to model. At least including overmatch factors in hits on sloped armor would eliminate some of the more obvious discrepancies between real life and ingame weapon performance.

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Post by cbo » 08 Nov 2006 01:13

Wokelly wrote:From WWIIOL forums:

I hope when you say "cannot" you mean "currently is not" DOC because there is no theoretical reason why overmatch factors as an influence on the effectiveness of sloped armor cannot be quite easily included in a ballistics simulation.
This is of course true and there are games out there which does use formulae which takes into account T/D. And there are, to my knowledge, many published penetration formulas which takes T/D into account. :)
To take a completely hypothetical example, say a 90mm shell hits 40mm armor sloped at 30 degrees. Ordinarily the slope multiplier would be about 1.25 for 30 degree slope, giving about 50mm effective resistance.
Just for the fun of it..... :)

Using this example (40mm armour, 90mm shell, T/D = 0.44) and applying Bird & Livingstons formulae, you'll get the following figures assuming APC projectile:

Impact angle - armour resists like:
10 degree - 40mm
20 degree - 43mm
30 degree - 48mm
40 degree - 56mm
50 degree - 71mm
60 degree - 101mm

If you assume 40mm armour vs 40mm shell, T/D = 1, you get:

10 degree - 41mm
20 degree - 44mm
30 degree - 51mm
40 degree - 63mm
50 degree - 84mm
60 degree - 123mm

The example given in your quote overestimates the effects of "overmatching" at 30 degrees for APC.

Assuming an uncapped AP projectile, for T/D 0.44 you get:

10 degree - 37mm
20 degree - 37mm
30 degree - 42mm
40 degree - 53mm
50 degree - 66mm
60 degree - 84mm

If you assume 40mm armour vs 40mm shell, T/D = 1, you get:

10 degree - 39mm
20 degree - 41mm
30 degree - 49mm
40 degree - 65mm
50 degree - 106mm
60 degree - 163mm

This is more like the example you quoted; APC is not as effected by T/D as AP is, probably because of the "normalization" effect discussed previously. On the other hand, "overmatching" AP wastes no energy to that process, reducing armour resistance considerably.

Claus B

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Post by Wokelly » 11 Nov 2006 06:57

So would this be how to calculate the T-34s hull armor effective strenght with overmatching?

Armor = 45mm
Shell = 75mm
Slope = 60 degrees
Slope multiplier = 2.5
T/D = 0.6

effective armor thickness*(1+( (slope multiplier -1)*overmatch factor))

45mm * (1+((2.5-1)*0.6))
=
45mm * (1+(1.5*0.6))
=
45mm * (1+0.9)
=
45mm * 1.9 = 85.5 mm effective

Is that correct? Or do the 1s become 2s in the formula?

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Post by Carl Schwamberger » 11 Nov 2006 15:23

I wonder how one adds in energy of the projectile to adjust the overmatch event? At some point there must a a threshold where the projectile energy wont be suffcient for overmatching to be relevant.

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Post by Wokelly » 11 Nov 2006 22:02

Wouldn't they just use penetration tables. Those take into account the loss of velocity. Just if the shell can penetrate x armor or not.

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