Rule of Thumb for Armour Penetration?

[Chad Harrison]

"It is a rule of thumb that armour is penetrable by rounds equal in diameter to its thickness" John Keegan, Author of "The Second World War".

Grogs, is this true? Because its not in CM.

Chad Cooper

[Kuma of Finland]

Originally posted by Chad Harrison:

"It is a rule of thumb that armour is penetrable by rounds equal in diameter to its thickness" John Keegan, Author of "The Second World War".

Grogs, is this true? Because its not in CM.

I don't consider myself grog, but give away my opinions anyway

I think this rule of thumb applies to "normal" rounds (HE on guns and full metal on small arms). Hollow charge, AP, tungsten etc. rounds are story of their own and they sure penetrate way more than their diameter.

Therefore, in theory, 8 mm steel plate should be enough protection from regular fire arms like 7,62 mm rifles. But as real life and CMBO have many times shown, this isn't completely true.

I guess the reason for this rule of thumb lies behind attacker's need to approximate the force needed to take out an armoured target, ie. some sort of "worst case" scenario.

Now everybody, correct me!

/kuma

[Andreas]

I think the reason this seems not to be in CM is that it is a rule of thumb. CM uses a very complex set of penetration equations, and variations in armour quality (fixed), weak points, etc.pp. All in a 3D environment, i.e. angle of incoming round to plate matters (which it does not under this rule of thumb).

As a rule of thumb, I am sure it works in CMBO.

75mm Sherman does not penetrate 82mm side armour of Tiger? Works.

88mm round penetrates 45mm T34 armour? Works.

[Chad Harrison]

I konw that CM algorithms are much more beyond just a mere rule of thumb. But this author, who ever he is, was too off with his historical and technical information. My original purpose was to see if anyone else has hear of this "Rule of Thumb" before; I hadn't. But to compare his "Rule of Thumb" to CM, makes one or the other *off*.

As I compared that theory to CM, it only works, roughly, for smaller calliber weapons. It comes within 20mm or so at 500m, with 0 degree armour slope. Compare for yourself:

37mm Greyhound: 56mm

57mm At Gun: 91mm

75mm Sherman: 89mm

Both pretty vanilla guns, and not too far off from the "Rule of Thumb". But the bigger brothers:

76mm Firefly: 157mm

88mm Tiger (Mark VI): 143mm

88mm JagdTiger: 205mm

To me, this rule of thumb seems to be bogus in the later years of the war due to higher velocity guns and other factors.

So has anyone else heard of this "Rule of Thumb" and is there any truth to it in Reality for more than a few lower calibre, early war guns? He referenced the rule in talking about the Battle of Falaise, which was late war where it seemed to be that this rule would not apply, atleast from what CM tells us.

Chad Harrison

[apex]

Using that rule of thumb, a 75mm/L24 and a 75mm/L70 would have the same penetration. So forget about it.

apex

[Michael Emrys]

The rule of thumb is fairly applicable to naval combat, which is the only place I have ever heard it mentioned myself. Frankly, if Keegan is trying to apply it to ground combat I am surprised as he usually shows far more acumen than that.

Michael

[Redwolf]

Well, if the reader is starting at zero knowledge, or at CNN-gained knowledge (which means less than zero , then this rule of thumb is certainly better than nothing. It also applies reasonably well for most guns for the first part of WW2.

Which Keegan book was than, again?

Anybody interested in this should get rexford's book.

[Shrike]

"It is a rule of thumb that armour is penetrable by rounds equal in diameter to its thickness" John Keegan, Author of "The Second World War".

You guys are reading this incorrectly. All he has said is that armor of X thickness will generally be penetrated by a round of X diameter. That means that 88mm of armor would generally be penetrated by an 88mm round. It doesn't mean that an 88mm shell's penetration is 88mm.

In the upper cases that were shown you have something like 150mm of penetration. Applying the rule of thumb all it says is that a 150mm round will generally penetrate 150mm of armor. It does NOT say that it REQUIRES a 150mm round to penetrate 150mm of armor.

Unless there was another intended meaning, this is all the quote literally says.

-Shrike

[Chad Harrison]

I went back to the book and took out the entire paragraph to put it more into context. Again the book is "The Second World War" by John Keegan, published by Viking 1989. The entire text of the paragraph follows:

"By 1944 each British and American infantry division had 60-100 anti-tank guns, as well as several hundred hand-held anti-tank missile projectors; the latter were weapons of last resort, but the former were genuine tank-destroyers. The enhanced effectiveness of the anti-tank gun derived not only from the growth in its distribution but also from the greatly increased calibre of those on issue to the infantry by mid-war - 57mm was standard, 75mm common and the heavier 80mm and 90mm available in specialist units. It is a rule of thumb that armour is penetrable by rounds equal in diameter to its thickness, and only the thickest tank armour exceeded 100mm. Therefore, as the German armoured divisions engaged in Operation Luttich found in their attack on the American 30th Division at Mortain, infantry could now hold their positions and inflict losses on the enemy under the weight o concentrated armoured attack."

I think that we could of been reading it wrong as Shrike pointed out, but still that rule of thumb has its flaws for higher velocity weapons. For a quick example, the 57mm American AT gun has about the same penetration as the US 75mm gun. Also, above all, just compare the short barrel early war German 75mm to the Panthers 75mm gun.

So as a "CNN-gained knowledge", it works. I was just really curious as to whether or not anyone else had ever heard of this "rule" before.

BTW, Michael, you sound familiar with Keegans works. Is he generally a good author?

Chad Harrison

[Michael Emrys]

Originally posted by Chad Harrison:

BTW, Michael, you sound familiar with Keegans works. Is he generally a good author?

Generally, yes. I am not as enthusiastic for him as some readers are, but then I haven't read all his books yet. He is definitely worth a look, but as in all other cases, I would be leery of expecting any single writer/historian to give the complete lowdown.

BTW, I think the reason the rule of thumb is more reliable for naval warfare is that the parameters effecting gun/armor performance are more apt to fall within a predictably narrow range. These include muzzle velocities and the ranges that fire is opened, just to name two. These are highly variable for tank vs. tank combat for any given caliber of weapon, but less so for naval guns.

Michael

[Chad Harrison]

Originally posted by Michael emrys:

</font><blockquote>quote:</font><hr />Originally posted by Chad Harrison:

BTW, Michael, you sound familiar with Keegans works. Is he generally a good author?

Generally, yes. I am not as enthusiastic for him as some readers are, but then I haven't read all his books yet. He is definitely worth a look, but as in all other cases, I would be leery of expecting any single writer/historian to give the complete lowdown.

BTW, I think the reason the rule of thumb is more reliable for naval warfare is that the parameters effecting gun/armor performance are more apt to fall within a predictably narrow range. These include muzzle velocities and the ranges that fire is opened, just to name two. These are highly variable for tank vs. tank combat for any given caliber of weapon, but less so for naval guns.

Michael</font>

[Tarquelne]

Regardless, thanks for the feedback on Keegan, I am looking at reading his "Six Armies in Normandy".

Chad[/QB]

Carefull about reading this forum, its full of spoilers.

[Firefly]

Originally posted by Tarqulene:

]Carefull about reading this forum, its full of spoilers.

So is CM:BO .

Keegan is a former lecturer in military history at Sandhurst and is currently considered by many to be the leading military historian in Britain. He exploded onto the scene with his excellent first book The Face of Battle, which is an excellent read, although it does not pertain directly to WW2 (it deals with the changing experience of the common soldier through history by comparing three battles: Agincourt, Waterloo and the Somme). Six Armies in Normandy is a very good look at the battle of Normandy and combines personal recollections by the participants, with a good overview of the operational aspects of the battle (D'Este's Decision in Normandy, IMO, is better for the strategic level).

I find him less convincing when he tries to tackle more sweeping areas as he did in his History of Warfare; he is a better at being a military historian than a general historian.

[ August 20, 2002, 08:07 PM: Message edited by: Firefly ]

[John D Salt]

Originally posted by Chad Harrison:

"It is a rule of thumb that armour is penetrable by rounds equal in diameter to its thickness" John Keegan, Author of "The Second World War".

[snips]

I'd be prepared to guess that John Keegan is mildly misremembering or misinterpreting a traditional rule of designatino which, I think, was more popular with the Royal Navy than with the army. This was that rounds capable of penetrating an armour thickness equal to or greater than their calibre would be designated "armour piercing", and those that could only penetrate less than their calibre are called "semi-armour-piercing". A lot of naval SAP was what would later, in land service, have been called APHE.

All the best,

John.

[Michael Emrys]

I'm not certain, but I believe that the US Navy didn't have an SAP category in its artillery, but did for its aerial bombs. They had AP and HC (or high capacity, a thin-walled shell carrying a greater amount of HE). Even the AP shells carried a modest amount of HE. Correct me if I am mistaken, but didn't the AP shells of the RN also have a blasting charge?

Michael

[ August 22, 2002, 04:16 PM: Message edited by: Michael emrys ]

[rexford]

"It is a rule of thumb that armour is penetrable by rounds equal in diameter to its thickness" John Keegan, Author of "The Second World War".

Rules of thumb are full of holes when it comes to armor penetration. Velocity is a big factor. Nose hardness is also big.

Russian 152mm will not penetrate 152mm of armor, velocity is too low.

In many cases, projectiles at 500m will penetrate a thickness greater than the ammo diameter.