Overmatching, why some tanks sloped armor just didnt work

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 not a lot of people seem not to have heard of the effect. Basically overmatching was an event that occurred if a shell hit sloped armor that was thinner then the diameter of the tank shell.

Basically overmatching neglected the effective slope of the armor, making the armor not as strong if hit by a drastically thicker shell. Overmatching can be determined by a simple formula T/D (Thickness of armor / diameter of shell). Overmatching will not occur if T/D = 1.00 or greater, but will occur if T/D is less then 1.00.

This can be factored into the formula:

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

To take a completely hypothetical example, say a 75mm shell hits 45mm armor sloped at 60 degrees, or basically a 75mm hitting the T-34s hull. Ordinarily the slope multiplier would be about 2.5 for 60 degree slope, giving about 112mm effective resistance.

This chart can be used to calculate the effective armor sloped armor.


However, because you have a high degree of overmatch here (75mm v 45mm) your algorithm calculates that for a 45/75 overmatch ratio the overmatch factor is say 0.6 (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:

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

So in effect, the armor loses 26.5 mm of effective strength by being hit by a thicker shell. Thats a massive difference when you think about it. Hell, the common German 75 would not be able to penetrate 112mm of armor at ranges of 800m, but with overmatching, they were easily able to pierce 85mm of armor at 2000m.

Now many tanks in WWII had sloped armor, especially American and Russian tanks. However, this event called overmatching made it so many of the tanks with sloped armor. The US TDs, and the sherman versons before the M4A2 and M4A3W series were all extreamly prone to this event. Also the T-34 always had this issue with its hull armor.

Now there was an EXTREAM effect of overmatching literally causes an entire section of armor to catastrophically fail. The effect looks like someone took a massive sledgehammer to that section of armor. Literally, a 88mm shell will create a hole 3 feet in diameter to appear, and an massive amount of armor to shatter and fly into the tank. Needless to say the crew is slaughtered and ripped appart.



This is all extreamly important to consider when talking about many tanks in WWII. For example the early shermans, as a result of poor armor quality (later fixed by 1943), and thin hull armor, was never as strong as it looked on paper with its effective armor. The shermans hull was suppost to be about 110mm of effective armor, but being 51mm, it suffered from overmatching. Its effective armor would likely be in the same range as the T-34 hulls armor, about 80-90mm, but adding in faulty armor quailty (85-90% according to some sources including Combat Mission), it could be as low as 70-80mm.

This problem was not fixed until the Sherman (75)W and (76)W series rolled off, and also the British Sherman III (M4A2?) series. To eliminate 2 weak spots on the hull, the frontal armor was increased to 64mm, with a lesser slope, in effect, the armor because much more stronger, and much more effective vs 75mm shells. Tests from Combat mission shows the common 75 is unable to pierce the hull armor of these shermans at ranges of 800-1000m, though while its a simulator, its also a game.

 

There is one thing that you do not account for velocity, evan if what you are saying is true the velocity is a factor. For example someone can just run with a 88mm AP round into a tank but that does not mean it will penetrate nor go off.

 

All penetration tables of course take velocity into account by measuring penetration across various distances.

Wo.Kelly, first of all, welcome to the forum.
I think there's a few things that can be said about your post. Firstly, the T-34 still derived a pretty massive enhancement of protection (a factor of 1.9 as you point out) from the slope of its armour. Overmatching or not, this is quite a phenomenal use of 45mm-thick plating. Secondly, it is not too surprising to find that larger, heavier shells from more powerful guns find it easier to penetrate thinner sheets of metal; if you study penetration tables you'll find that few guns ever fail to penetrate armour as thick as their calibre if said guns were made for AT purposes. I wonder how much of this is due to overmatching, rather than simple penetrative power. Thirdly, didn't overmatching also occur against unsloped plates?

 

Im not sure what you are trying to say here, but yes velocity is taken into account. As stated by roel (thanks for the welcome by the way), penetration tables take into account velocity. This is why you see the 75mm L/43s and L/48s penetrating the hull armor of T-34s and Sherman's while the 75mm L/24 not being able to penetrate the armor. While overmatching may of made the hull armor weaker, the shell from the short barrel 75mm didn't have the velocity to penetrate the armor.

Roel:

First off, while 1.9x is pretty impressive, you have to remember that 85mm compared to 112mm is a pretty substantial loss of protection. Plus the common German 75mm would be unable to pierce 112mm of armor at ranges of 800m or greater, which is common engagement ranges for a lot of tank battles, however it is able to pierce 85mm of armor at ranges of 2000m easily. Thats a pretty big difference right there.

Overmatching didn't occur on flat armor. However, I think you might be referring to the catastrophic effect of overmatching I mentioned. That event where the armor shatters is more due to poor armor quality then overmatching. Armor can fail like that on flat armored tanks as a result of simple bad armor quality.

 

4 comments and two very interesting posts ! I like your style...

On the subject of armor quality, I believe Russian and German armor was extremely brittle in comparison to that of Allied nations... German armor tended to be ~375 BHN, American armor only ~250 BHN, whereas it was not uncommon to find in excess of ~450 BHN on Russian tanks... The advantage of having harder armor was that, if it was thicker than the shell diameter, the shell would merely shatter harmlessly on impact, the disadvantage being that if the shell diameter was thicker than the armor the armor would splinter, likely cracking the entire tank chassis... Softer armor used by the Americans (and the Germans late-war) was designed to absorb the impact all throughout the tank's chassis, so I imagine that overmatching would have less devastating an effect on thinner low BHN armor than thinner high BHN armor. Of course not many tanks in WWII had armor sufficiently thicker than ~76mm or ~88mm anyway to be impervious, so softer armor was generally a better choice... Softer armor also tended to be less succeptible to AP rounds, by the end of WWII all nations had switched to production of lower BHN armor

 

You are correct. The Germans for many of their tanks used what is referred to as Face Hardened Armor (FHA). During North Africa especially, the British AP round would usually shatter at ranges of 800m (dont quote me on that though) or so against German armor. The original Grant tanks also had the same issue with their 37mm and 75mm shells. Hence, the adoptance of APCBC shells.

The western allies stuck with Rolled Homogenious Armor (RHA). The British were thinking of using FHA as well on their tanks, but realized the German APCBC round would actually do better against FHA then RHA. (Dont cite me on that though).

Regardless the Germans used FHA on a lot of tanks, and like anything you make extreamly hard, it shatters pretty easily. There was a nice pic on the WWIIOL forums of a panthers hull that had been shattered by a rocket typhoon AP rocket, just to show the brittleness of the hull on late war panthers, combined with their FHA hull.

Americas piss poor armor quality was a stupid decision by the US to entrust armor casting to Car companies. Instead of using the rail way companies in the US that had lots of experience with castings. It wasnt until late 1943 I believe the US finally got the armor quality issue under control.

 

Actually that explains a lot to me since I was wondering why the Canadians had their tanks produced in railway yards.

 

Yes, and a difference no source I can find is willing to confirm. Penetration of the 75mm L/48 at 2000 meters is given as 64mm. To penetrate 85mm the range would have to be 1000 meters or less, and even then it would be a doubtful case with APCBC (standard) ammunition if not with APCR. Secure penetration of a plate this thick would still be possible only at around 800 meter range.

Where did you get that penetration statistic?

That's not what I'm talking about. I'm talking about rounds failing to penetrate because the plate they hit is thicker than the round diameter, regardless of the angle of that plate. This is said to be one of the main advantages of thick straight armour slabs as seen on the Churchill and Tiger.

 

I would be very interested to see Tony William's views & comments in this thread.

 

As a highly exceptional event, I am allowing a banned member to post through me here, since he is very knowledgeable on this particular subject and provides the arguments that I do not have so readily available. Please note that all comments regarding this post will have to be directed at me, and that I will not allow the contribution of this banned member to go outside of this topic. His every word will also be subject to my personal judgment before I post his replies. I hope it is clear to everyone that this is not the same as allowing said member to rejoin the forums - it is all under my direct and absolute control.

 

Yes, I know about shatter gap. Instead of the shell being able to break though the armor, when faced with thicker T/D is greater then one the shell had to tunnel its way though the armor instead. Yes, that is why the tiger was such a hard tank to penetrate.

Yes, flat armor in a way can be overmatched, but not in the same way as sloped armor. Flat armor will not lose effective strength because it is flat armor and receives no effective strength as a result. Yes you are right the shell will not likely shatter if T/D is less then one, and I guess you could slide that in with overmatching in a way, but I am referring to the loss of the effective strength sloped armor is to provide as a result of being hit by a thicker shell.

I didn't necessarily mean this forum, but really from all the forums I have come across, its not a widely understood issue.

Well I was unaware the Germans used different armor types between production models. To my knowledge, and maybe I am guilty of generalizing, the Panzer 3 series and its derivatives (ie Stug series) and the panthers hull used FHA, of at least parts of the tank. Since those series made up such a large part of the German armored forces in the whole. However it appears I am wrong, so thank you for correcting me. The reason I post this information on forums instead of websites is because people can point out where I am wrong. Also, I have no idea how to create web sites....

As for the Russian armor being brittle, I can only guess it was how their armor was made. I cant say I am knowledgeable with how the Russians made their armor, as it is any writing on armor quality and overmatching isn't the most easiest thing to find. Its not like many other aspects of WWII were you can just Google it and get plenty of results.

However the western allied RHA to my knowledge was not as brittle as FHA armor was. However, thank you for the correction. I did not know that Russian RHA was brittle, or at least that brittle. I had heard things about bad Russian armor quality, but I did not understand in what way their armor was flawed.

Okay, well the picture isnt working for me, but it might for you guys. Anyways the panthers hull armor was shattered by a AP shot from a british Rocket. The guy who posted this is a great source for information, and he goes by the name Scotsman. His job allows him access to many government papers on things like overmatching, and he apperently has also conducted real life tests on overmatching.

To be honest, I dont really know if this contributes to the discussion at all, but i think it is interesting enough to include.

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Well you are correct in that saying piss poor is an overstatement, but early American armor was flawed in many respects. To be honest it bugs me that the armor on the shermans were flawed, besides being quite thin in a number of respects. Personally, I am not an expert on armor quality, but the US did something so when their M4A3(75)W series and the Sherman (76)W series rolled off the lines they had managed to correct the armor quality issues. However that means that throughout the war men were fighting in Shermans of varying quality.

Though:

The Sherman turrets were all cast shapes, and the M4A1 was a completely casted hull. So as you can see, there are some real issues with early war American Armor Quality.

http://www.freeweb.hu/gva/weapons/usa.html#Cast_Armour

http://www.freeweb.hu/gva/weapons/intro ... ous_Armour
Its a great web site, the bibliography is extensive and full of credible books.

Most of my sources come from others who have posted about this. As mentioned above, Scotsman, who has access to many of these documents, is a superb source. I may hjust end up creating an entire post with all his WWIIOL posts on the PS forums, otherwise I will try to track down all the information that I have posted. Unfortunately there is not a whole lot written about this, and a lot of these things extracted are not in one nice little compact book. I will do my best though.

 

Okay in my view I dont consider that overmatching. As the banned forum member said, its called shatter gap. You can use the T/D forumla to determine whether a shell has a chance of shattering, but really I dont slide them into the same category.

 

Incedentally (I'm taking a huge step back here), wasn't the 88 pretty much the only gun that could take out the T-34?

 

Originally until the long barrel 75s came out. The Germans first encountered the T-34 within the first week of barbarossa, and all they could really do was fling shells at it with little or no effect.

Antony Beevor's book "Stalingrad" makes reference to a German tankers letter home talking about how they managed to disable a T-34, and drove in point blank and started firing at the T-34. Eventually the crew came out, deafened by the deflections, but no a single penetration occurred.

Now originally it was required the 88, a stuka, or within a certain distance the Pak38 (50mm L/60). Most German tanks in Russia for all of 1941 had the short barrel 50mm (50mm L/42), and eventually the long barrel 50mm was added, this being pretty much the pak38 mounted on a tank. The long Barrel 50mm gave the Panzer 3s a chance vs the T-34s, you gotta remember the whole overmatching thing.

50mm gun vs 45mm of armor. In effect, the T-34s hull was about 110mm of effective armor against 50mm guns, and I believe the 50mm L/60 could only manage 100mm at best. In reality I don't know how well the 50mm worked against the T-34, but they obviously reversed the roll of the panzer 3 and panzer 4 for a reason, the panzer 3 becoming the MBT and the panzer 3 series getting the short barrel 75mm for infantry support.

But yes, in general they had to rely on the 88s, pak38s (not the best gun still), and stukas to kill off the T-34s, but only 1300 T-34s had been made by December 1941, many having already been destroyed. THere is one story of a russian tank commander driving his entire platoon of 4 or 5 T-34s into a swamp and losing them all. Bad training and doctrine allowed the Germans to destroy many T-34s much more easily then would have happened if they had been used properly.

 

on the subject of shell , the russian chose the 122 mm field gun for their
heavy tanks , it had an incredible punch against panthers on the testing
range at 1500m , reportedly penetrating the glacis plate , the engine and
pierced the rear plate ! ( s. zagola ,osprey )
There didn't seems to be problem with the russians tanks armor ,the welds
however were notoriously crappy .
The incident on a russian tank being hit for ever and the crew walking out ,
dazzled but alive is usuall given as a KV , a very tough oponent if its gear
box didn't break as it usually did .
Armor commanders doing stupid things are not an exclusive of the
russians
though they did set standards of sorts , :roll:

the record is the dumkopf who drove five brand new and secret tigers on a
local attack on the leningrad front in january 43 .
Using a road surrouded by marshes , the last tank got hit by a russian
ambush and the crews had to run for it leaving four brand new tigers and
one slightly damaged one to the grateful hand of the red army intelligence
mr Hitler was rightly furious .

 

Well it could have been a KV. I dont quite remember the guy in the letter specifying. Regardless, it proves the point that for awhile the best weapon against T-34s and KVs were the 88.

Okay, thanks for the correction then.

 

Phelps's point was, though, that rockets are not comparable to AT shells and should not be used to prove a statement about them.

The source you use (guns vs armour) is very good, in fact I got the statistics with which I disproved your point from that very site...

 

Which point? You mean the distance a 75mm could take out a T-34?

 

What an interesting post ! I’m no expert but I remember reading another post in a Armour forum about velocity and trajectory effect against sloped armour. It said something about lower velocity rounds had a higher trajectory to target, in other words its arc was higher therefore the round would connect at a steeper angle. This would in some cases negate the slope of the armour. Also the trajectory impact angle would increase the further the target was away.

 

We talked a lot about this on the WWIIOL forums. Its actually not true. I'll try to dig up the calculations someone did, but in reality low velocity shells drop a few degrees at most. Even the Sherman "low" velocity shell was traveling at 600 meters a second, which is by no means slow. At most, the lower velocity shells would drop of 5 degrees, but most likely only 2 or 3 degrees at 1000m or so.

The only real way to neglect the slope of an enemy tank is to get on high ground and fire down on them.