Why was it called "medium" tank

Usually when I make a comment like that our resident experts jump on it either pointing out how I was both right and wrong or just the latter. I'm sure that I've read that Jumbo's or at least Jumbo like mods were done in the field, or perhaps more correctly at the field service depots in Europe. Of course just because I've read it somewhere doesn't mean that it is right.

 

It might also reflect the urgency of being seen to do something the survivability of the M4. The British OR teams came to the conclusion that survivability would be better enhanced by putting the extra weight into improving the gun instead.

 

Jumbo wasn't really about the impetus to improve on M4 though. Another sport, with a very specific task in mind, though I suspect the reassurance factor may have played a part.
If they ever issued them in wider numbers than the relative handful fielded we'd be talking about the taking of Caen as a blitzkrieg by comparison to the lumbering bogged-down broken-tracked 'Jumbo age'.

I suppose the addition of extra armour was always fairly straightforward, even if it involved a lot of design and production effort for stuff like Jumbo, with known prices paid.
The addition of bigger guns, however; just a tiny smidge more complex.

First... find & supply your gun...

 

Again the British thought that in the 17 pdr they had a gun more than capable of knocking out any German tank, making a Firefly as good they might need.

British OR analysts did not find any statistical evidence that tanks in Normandy were more likely to be hit on the front than the side. They concluded there was no point in trying to add enough armour that might keep a 75mm or 88mm shot.

I am not sure what extra protection the Jumbo armour actually provided? Was it enough to deflect 75mm shot form a pak 40?.

 

The 17-pounder is all well and good...against tanks. Except the Sherman "Jumbo" was intended for the assault role, as in supporting an infantry assault, and not going toe-to-toe with a Panther or Tiger.

IIRC, the early 17-pounder HE shell was not all that good, and required redesign to make it effective. Even then, the 75mm still had a larger bursting charge than both 17-pounder HE shells.

 

17pdr was 'gun found', for absolute certain. 'Gun supply' though, a bugger, or I feel there might have been a lot more Fireflies out and about.
Mind you, if you've ever looked inside a Firefly turret it's a mess. Sideways gun and not much space to move. Reeks of a certain amount of imperfection.
Ammo supply was a double-bugger on 17pdr too, if I recall.

Hunnicutt's figures for Jumbo armour vs. standard mid-production M4a3:

M4a3/M4a3e2

Hull:
Front upper: 2"/4"
Front lower: 2"/5.5-4.5"
Side Upper: 1.5"/3"
Side Lower: 1.5"/1.5"
Rear: 1.5"/1.5"
Top: 0.75"/1"
Floor: 0.5-1"/0.5-1"

Turret:
Gun Shield: 5"/7"
Rotor shield:2"/?
Front: 3"/6"
Sides: 2"/6"
Rear: 2"/6"
Top: 1"/1"

As to deflection vs. standard German AT guns... dunno really. Probably something others can comment on more solidly as I get a bit... bored... with straight statistical penetration stuff.
Better than average I suppose, but that might not help much when the over-strained track has broken or you've sunk into a bit of Marsh and several enemy guns are on you...

 

On the plus side for the Sherman Jumbo...It use the a modified version of the 76mm gun mount for the Sherman. So, it was very easy to change out the 75mm for the better(AT-wise) 76mm. This became a fairly common field modification.

 

Those look comparable with the armour on the Churchill. So the 75,mm armed Jumbo M4 is a USA Infantry Tank.

The Churchill was far from invulnerable. . 4" looks thick enough to stop a pak 40 Pgr 39 round, but not one form a 7.5 cm KwK 42 mounted by the Panther or from an 88mm pak 43


Re 17 Pdr. There were [production problems. Industrial relations problems in 1944 prevented more than a handful of M10s being up-gunned to carry the 17 Pdr before D Day.. The 77mm mounted in the Comet was the 17 pdr redesigned to be used as a tank gun. .

 

I don't think people credit Churchill's front armour enough. 152 or 155mm, I forget, but the thickest standard front plate of the war.
Still indeed cut through like butter by HV guns at standard European engagement ranges, to the extent that anyone advocating heavier armour in that period really was rather missing the point.

(I know, I know, I've banged on about this HV/range thing so many times, but it's worth repeating until the lazier critiques of allied armour start to raise more eyebrows than they currently do.)

 

The thing they found out about armor was that sloping the plate was a lot more effective than just adding slabs of armor. This was especially important in '44 when AT projectiles were much more sophisticated then they were three years earlier.

 

Sloped, thick, thin, composite, rolled, homogenised, riveted or slab, they quickly discovered that at Normandy ranges and angles of attack, armour wasn't making a great deal of difference against modern guns.

 

I've got at least 1 interview that describes the effect of an "88" AP shell against the front of a Jumbo. He described it as if someone had taken an ice cream scoop to the front of the tank and scooped out about 4 inches. Then it bounced back out.

This was during the combat near the end of the Bulge, so I don't know if this was a confirmed 88 or just the everything was an 88 that was prevalent during the campaign.

 

I support the overall conclusion that there was no American tanks heavy enough to be classified a "Heavy".

As for this talk about the deflecting shells: I also agree that it didn't matter after the shells themselves became too powerful to truly defend against. A straight shot usually went through unless the velocity wasn't strong enough to push it. This is why inverted domes were found on many tanks that were not "Knocked Out". Their armor held or their enemy miscalculated and/or was too far to penetrate.

Unless you're talking about the unlucky shots that resulted in scenes like from th movie "Fury" where they encountered the German stationary guns and emplacements. One of the shots came so close that it bounced off the hull of an M4. Point being that it was rare for deflects and rounds that didn't fully hit.

Good long post.