Effective Thickness vs. Line-of-Sight Thickness?

[serfCharlemagne]

I was recently on the Wikipedia page for the Sherman and noticed that in the Sherman Jumbo section, it mentions a much higher effective thickness than line-of-sight thickness. To quote, it says the Jumbo had "a total thickness of 101.6 mm (4.00 in), which resulted in a glacis of 148.97 mm (5.865 in) line-of-sight thickness, and over 180 mm (7.1 in) effective thickness."

Why is effective thickness so much higher than LOS thickness? Is it because the probability of deflecting the hit was very high for sloped armor during WWII? If so, would effective thickness be much higher than LOS for all sloped armor tanks? Or is Wikipedia just wrong on this one?

TIA

[Mobius]

There is a slope multiplier that is higher than LOS. It depends on the shell. Typically the Jumbo would face a German 75mm Pzgr. 39 and this adjusts to about 1.23 x the LOS thickness. Yes, the shell starts to ricochet.
Other shells differ somewhat. Also, it doesn't include shock effect nor armor quality.

[serfCharlemagne]

I see. Does it also depend on angle of the slope and thickness of the armor, or just on the shell? i.e., is it the same 1.23xLOS for regular Shermans, Jumbos, and T-34s?

Also, does the multiplier tend to grow or shrink with shell caliber/muzzle velocity? Are there charts or formulas out there for this?

[Mobius]

The 1.23 x is generally associated with 30°. The 0° penetration is 1.23 that of the 30°. I think there is another thread around here that has a table of the different multiples for other angles.
So the wiki figure probably would not go along with the 47° ratio. But, to find the calculated effective penetration to a higher degree you would have to plug the numbers into a formula that also includes shell cross-sectional density, KE and armor thickness to shell diameter ratio.
[Edit]I had to re-enter the numbers.
One formula has the 75mm 790m/s Pzgr. 39 penetrating at 1 m but not at 100 m.

[serfCharlemagne]

Gotcha. Just using the multiplier, though, that would give the Sherman (LOS thickness ~90mm) almost as much frontal armor as the Tiger I (1.23x90 = 110mm vs. 120mm for Tiger 1). And that's using the 30° multiplier when the actual slope was even greater. That can't be right, can it?

[Mobius]

At 47° it seems to be 1.489 x the 0° penetration. So the 64mm@ 47° is 95mm. Class "B" US Naval armor.

[serfCharlemagne]

So the multiplier is applied to the true thickness, NOT the line of sight thickness?

[Mobius]

So the multiplier is applied to the true thickness, NOT the line of sight thickness?

In my calculation it is. Like I indicated the table that was posted in a thread here applies it to the LOS thickness. It would work both ways. Just that the number applied would be different.

Check this thread out.
viewtopic.php?f=47&t=243793

[serfCharlemagne]

Got it, thank you for the help! It's greatly appreciated.

[serfCharlemagne]

Note for anyone who stumbles upon this thread later and is interested: I found this blog post that has a table of slope modifiers for effective armor thickness:
https://ruhrpottpatriot.tumblr.com/post ... what-do-we
It appears to quote post-war British tests labeled "WO 185/118, DDG/FV(D), Armour Plate Experiments." This War Thunder forum quotes the same table:
https://forum.warthunder.com/index.php? ... overmatch/

However, both those posts mention that "armor overmatch" also becomes an issue. Basically, once the diameter of the shell becomes greater than the actual thickness of the armor, the benefits of sloping start to be negated and the effective thickness goes down. For medium tanks like the Sherman and T-34, overmatch was always almost an issue later in the war since they were dealing mostly with 75mm guns and above (while their actual armor thickness was around 50mm for standard models)
See this post for more:
https://ruhrpottpatriot.tumblr.com/post ... d-why-some

[Peasant]

-- snip --

You can use this one to estimate the approximate effective thickness. Only for APC(BC) shells attacking RHA. Projectile is assumed to remain intact.
https://docs.google.com/spreadsheets/d/ ... sp=sharing

[serfCharlemagne]

Wow that's great, thank you! Very impressive calculator.

[critical mass]

I was recently on the Wikipedia page for the Sherman and noticed that in the Sherman Jumbo section, it mentions a much higher effective thickness than line-of-sight thickness. To quote, it says the Jumbo had "a total thickness of 101.6 mm (4.00 in), which resulted in a glacis of 148.97 mm (5.865 in) line-of-sight thickness, and over 180 mm (7.1 in) effective thickness."

Why is effective thickness so much higher than LOS thickness? Is it because the probability of deflecting the hit was very high for sloped armor during WWII? If so, would effective thickness be much higher than LOS for all sloped armor tanks? Or is Wikipedia just wrong on this one?

TIA

If I am not mistaken, the Jumbo glacis plate is a laminate of two armor plates, one 64mm plus a Second 38mm plate laminated tightly together by means of welding. The effective total thickness is slightly smaller than the nominal thickness due to the lamination representing a weakness. It’s noticeable, but not severe, approx. 94mm äquivalent of a single layer plate, presuming that the penetrator is whole and the failure is by ductile holing. This is slightly smaller than the nominal thickness of 102mm.

However, the plate is also tilted away from the vertical by 46deg, thus, representing a more difficult target to perforate. The cosine function alone increases the resultant resistance to 136mm äquivalent single layer RHAe vertical plate. Because this angle also causes many projectiles to loose energy by turning the axis of the projectile first away from the plate when it impacts, and then, in case it dug in deeply enough, into the plates back again, the resistance is even greater then the simple cosine if the plate fails in a ductile manner.

Against a low quality shell, which does also break up, and presuming that the plate fails only by ductile forms of holing (ductile plugging in this case) the resistance is equal to almost double that of a single layer RHAe vertical, which is considerably larger than the cosine. This equals 181mm RHAe. Because US APC do suffer break up if attacking cal/t plate at obliquities exceeding 25 def it is relevant in terms of rated resistance.

Against a more robust solid penetrator, such as a capped, highly resistent AP Shot, which does not break up, the resistance offered by this plate is in between both aforementioned extremes.

at even higher obliquities, projectile break up may improve chance of perforation, because break up inhibits ricochet. The effect is stronger, of course, for plates with low degrees of plasticity, which will cause more violent changes of axis of the projectiles (a more severe base slap) but which will also result in lower resistance to shear failures by brittle plugging.