Of tanks and decapping plates

[SpuriusMaximus]

Those of you who are into warships too, will probably be familiar with the concept of a decapping plate. The short and skinny is that if you have a plate of homogenous steel or even iron that is 0.0805 times or higher of the incoming calibre, it will remove any balistic cap on the round.

Now the concept was obviously well known to all the major nations involved in WW2, seein' as they also had a navy. However it doesn't seem to be used much on tanks.

Mind you, there are a few examples, such the side skirts on the later Panthers, which at 4mm thickness would reliably decap a Soviet 45mm round. (So yeah, they weren't just against AT rifles.) But most tanks only get into spaced armour at all by the time the PIAT and Panzerfaust and such get used, and even then it's sometimes just mesh screens against those and with no real decapping ability against AT rounds in use at the time.

So that would be my question for the more learned people here: why wasn't it used more? Were ballistic caps on AT rounds uncommon in WW2, or what is the reason?

 

[abaddon]

Those of you who are into warships too, will probably be familiar with the concept of a decapping plate. The short and skinny is that if you have a plate of homogenous steel or even iron that is 0.0805 times or higher of the incoming calibre, it will remove any balistic cap on the round.

Now the concept was obviously well known to all the major nations involved in WW2, seein' as they also had a navy. However it doesn't seem to be used much on tanks.

Mind you, there are a few examples, such the side skirts on the later Panthers, which at 4mm thickness would reliably decap a Soviet 45mm round. (So yeah, they weren't just against AT rifles.) But most tanks only get into spaced armour at all by the time the PIAT and Panzerfaust and such get used, and even then it's sometimes just mesh screens against those and with no real decapping ability against AT rounds in use at the time.

So that would be my question for the more learned people here: why wasn't it used more? Were ballistic caps on AT rounds uncommon in WW2, or what is the reason?

I suppose it's like the water blivets in the Sherman, a battlefield mod that took time to work it's way through formal channels.

 

[jimbob]

I suppose it's like the water blivets in the Sherman, a battlefield mod that took time to work it's way through formal channels.

Do you have any more details on these? Google isn't helping, and it sounds interesting.

 

[abaddon]

Do you have any more details on these? Google isn't helping, and it sounds interesting.

The Sherman got the nick "tommy-cooker" from the inclination to burn on being hit. Even a non-penetrating hit caused internal spalling which in turn ignited fuel and ammo. Hanging filled blivets reduced penetrator/spalling velocity and acted as an effective fire suppressant. The ad-hoc wet stowage of ammo was adopted into the sherman A3 onward and retroed into the A1/2 variants where practical.

As for a reference, damned if I can remember which history of the Sherman I read but there are plenty of them out there. I could go rummage if you want.

 

[jimbob]

Cheers - I'd heard of the nickname.

 

[Andy_Ross]

Sherman didn't burn any more frequently than any other tank.

 

[SpuriusMaximus]

Well, by the end of the war, most German tanks didn't have fuel, so I bet they didn't burn as often as the Shermans

 

[MRC_Hans]

Those of you who are into warships too, will probably be familiar with the concept of a decapping plate. The short and skinny is that if you have a plate of homogenous steel or even iron that is 0.0805 times or higher of the incoming calibre, it will remove any balistic cap on the round.

Now the concept was obviously well known to all the major nations involved in WW2, seein' as they also had a navy. However it doesn't seem to be used much on tanks.

Mind you, there are a few examples, such the side skirts on the later Panthers, which at 4mm thickness would reliably decap a Soviet 45mm round. (So yeah, they weren't just against AT rifles.) But most tanks only get into spaced armour at all by the time the PIAT and Panzerfaust and such get used, and even then it's sometimes just mesh screens against those and with no real decapping ability against AT rounds in use at the time.

So that would be my question for the more learned people here: why wasn't it used more? Were ballistic caps on AT rounds uncommon in WW2, or what is the reason?

As far as I can find out (had to read up on the subject) Such rounds were not uncommon, at least not in the later part of the war.

So, if I should venture a guess, especially in light of the discussion on Shermans, later in this thread, it might not have been deemed efficient enough, compared to the extra weight and complexity. Possibly, the hit of a cannon shell, even if it did not penetrate, created such mayhem inside a tank that it would mostly be disabled anyway. This would explain why extra measures were only employed to counter hand-held devices which do not posses any impact energy to speak of.

Hans

 

[8enotto]

The German tanks most widely used in all battles were the 3 and 4 versions. Equally prone to lighting up on a hit.

It was the allied nations making the new sabot and super hard projectiles that sped up axis retrofits and as they were closer to home it came faster
The allied tanks had to field work any damn thing they could from logs to extra track links onto the hull to try to resist the German 88 and later 75 mm weapons.

Some Sherman had skirts in the drive across Europe. I never was able to know if it was field modifications or new replacement units from the states.

Russian armour up to the battle of Kursk were all open wheeled but T34 units in Berlin show use of extra add on armour plate. Probably due to so many panzerfaust weapons loose in the streets.

 

[SpuriusMaximus]

As far as I can find out (had to read up on the subject) Such rounds were not uncommon, at least not in the later part of the war.

So, if I should venture a guess, especially in light of the discussion on Shermans, later in this thread, it might not have been deemed efficient enough, compared to the extra weight and complexity. Possibly, the hit of a cannon shell, even if it did not penetrate, created such mayhem inside a tank that it would mostly be disabled anyway. This would explain why extra measures were only employed to counter hand-held devices which do not posses any impact energy to speak of.

Hans

Well, I suppose in the case of US tanks, decapping an 88mm would need a 7mm plate anyway, which would make the Sherman go well past the hard limit they had put on its weight. I'm more intrigued by the fact that the Germans and Russians didn't use it more, though. Neither had a problem with making their tanks overweight, or in the case of Germans downright obese, as far as I can tell.

I'd add the Italians too, somewhat, in that they certainly made decapping pretty much their naval design focus early, so you'd expect the notion to be the most obvious to them. But then again, it WAS Italy, so asking them to build good equipment is a bit unfair

 

[SpuriusMaximus]

@8enotto
Well, I wouldn't really include sabot rounds into why decapping plates were so rare in WW2. It is almost certainly the reason post-war, but back in WW2 IIRC only the Brits used sabot rounds, and they were rare, and... well, to quote Obi Wan, "only imperial stormtroopers are so precise." Really, to get hit by a sabot round anywhere past point blank, it must have been shot at someone else.

Not only were the first sabot rounds prone to destabilizing in flight anyway, but it turns out that rifled barrels are awful for shooting APFSDS. There's a reason modern tank guns are all smoothbore.

 

[Andy_Ross]

German Tanks didn't have 'decapping plates'

Their spaced armour and skirts were entirely concerned with disrupting shaped charges.

Same with the logs and old track plates etc hung on the turrets and hulls of Shermans.

Shermans didn't have 'skirts' They all came fitted with mud and dust shield. These were usually removed by the crews.

 

[SpuriusMaximus]

German Tanks didn't have 'decapping plates'

Hence my question why not.

Their spaced armour and skirts were entirely concerned with disrupting shaped charges.

AFAIK, that's a myth. When they were considering making the Panther II and just put the skirts on the existing one instead, shaped charges were not yet a problem.

 

[Andy_Ross]

Testing of de-capping plates showed that it takes around 20% of the shell diameter in thickness of armour grade material. On Battleships that meant a 16" shell would need a thickness of about 80mm of armour.
The theory is that a de-capped shell will shatter when it strikes the face hardened armour belt if it has enough striking speed. You need a main belt thick enough to stop penetration of shells striking at speeds below the this velocity. Shells without caps that hit with enough energy to penetrate will be shattered, and those that are going slow enough not to shatter can not penetrate.

Battleships are big enough to build these characteristics in to the hull. With tanks you have far less weight, area and hull volume to play with.

 

[MRC_Hans]

Battleships are big enough to build these characteristics in to the hull. With tanks you have far less weight, area and hull volume to play with.

Plus, if the shock-wave from the scattering shell propagates into the compartment behind and does even severe damage, the ship as such will not suffer seriously. But in a tank, there is only that compartment.

Hans

 

[SpuriusMaximus]

20% of shell diameter seems to be a bit off, considering that the decapping plate on an Iowa was only 1.5" thick. If 20% were the number, then an Iowa can't even decap an 8" shell from a cruiser, which would just beg the question: why bother, then?

In fact, the thickest decapping plate I've ever heard of was the 70mm on Italian battleships. I don't think anyone ever made an 80mm one.

 

[Andy_Ross]

Plus, if the shock-wave from the scattering shell propagates into the compartment behind and does even severe damage, the ship as such will not suffer seriously. But in a tank, there is only that compartment.

Hans

Well, the space between the outer armour and the main belt can flood and compromise the stability of the ship. Flooding 'in the wings' is always a problem.

 

[Andy_Ross]

20% of shell diameter seems to be a bit off, considering that the decapping plate on an Iowa was only 1.5" thick. If 20% were the number, then an Iowa can't even decap an 8" shell from a cruiser, which would just beg the question: why bother, then?

In fact, the thickest decapping plate I've ever heard of was the 70mm on Italian battleships. I don't think anyone ever made an 80mm one.

The Italian Vittorio Veneto class ships had their armour belts sloped at 15 degrees so they could get away with thinner plates. Plus you have to ask what size shell were they protecting against?

The outer hull plating of Iowa was not meant as a decapper, to the best of my knowledge, it was 'splinter protection' for the space outboard of the armour which was inboard of the ships hull plates. (If it was meant as a decapper, it was a rather poor one.)


Good piece here
http://www.navweaps.com/index_tech/tech-045.php

and a follow up here
http://www.navweaps.com/index_tech/tech-085.php

 

[SpuriusMaximus]

Well, even going with 20%, to decap a 76mm (which made up most of the tank guns the Americans, Brits and Soviets were using), you need about 15mm worth of plate even at perpendicular impact. A Panther had a maximum of 100mm armour on the front and sloped at that. So I have to wonder why not make it something like 85mm, space, the other 15mm.

That's basically my question. Was there a good reason why that wouldn't work?

(Also, yeah, I know, the Panther wasn't great anyway. I know.)

 

[Andy_Ross]

complexity of construction and marginal gains?


Standard British AT rounds were armour-piercing capped ballistic cap (APCBC)

That is the cap was just for aerodynamic purposes. 'decapping' it would have made no difference to it's penetration of armour.

AT rounds were solid shot. Not explosive filled shells with a hardened cap