Iowa Class 1.5-inch STS decapping plate

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Javier L.
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Iowa Class 1.5-inch STS decapping plate

Post by Javier L. »

Is the 1.5-inch STS decapping outer plate in the Iowa Class battleships enough to decap a heavy shell before it hits the main belt?

What is exactly decapping a projectile?

Wouldn't have been better to add this 1.5-inch thickness to the main 12.1-inch belt and form a single thicker plate of 13.6 inches?

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Tiornu
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Re: Iowa Class 1.5-inch STS decapping plate

Post by Tiornu »

See the decapping articles at
http://www.navweaps.com/index_tech/index_tech.htm .
The outer hull plating was not meant as a decapper, to the best of my knowledge. (If it was meant as a decapper, it was a rather poor one.) Friedman traces its development to the need to prevent splinter damage.
You can't just add the outer plate thickness to the belt, because then you'd have no outer hull plating.
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Javier L.
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Post by Javier L. »

Thank you Tiornu,

I read Nathan Okun article here http://www.navweaps.com/index_tech/tech-045.htm and Tony DiGiulian says:

"On the South Dakota's, this shell plating is 1.25" thick (3.2 cm) and on the Iowa's it is 1.5" thick (3.81 cm). Using Nathan's formula above, the South Dakota's plating would be sufficient to decap any projectile up to 15.5" (39.4 cm) and the Iowa's plating would be sufficient to decap any projectile up to 18.6" (47.3 cm). This would imply that the Japanese Type 91 18.1" (46 cm) APC projectiles fired by the Yamato would be decapped by the Iowa's shell plating before they reached the main armor belt."

In the Bismarck article of Mr. Okun (http://www.combinedfleet.com/okun_biz.htm) this subject is addressed too:

"There was a 1.25" (31.8 mm) vertical STS outer hull (increased to 1.5" (38 mm) in the IOWA) and a 0.5" (12.7 mm) mild steel or HTS second bulkhead plate spaced a few feet in front of the main belt. This added outer plating would only reduce the 38 cm projectile's striking velocity on the main belt by roughly 10 feet/second (3 m/sec), but it was (barely) thick enough to decap the German 38 cm projectile (), resulting in shatter when the projectile hits the face-hardened Class 'A' belt. [It should be noted that the South Dakota's outboard plating would certainly de-cap any projectiles under 16" (40.6cm). De-capping 16" projectiles is possible but not certain, and no larger projectile would usually be de-capped. The Iowa class' slightly thicker outer plating, however, would de-cap all projectiles up to 18.6" (47.3cm) in diameter.]"

I just can't imagine how a thin 1.5-inch (38 mm) plate could decap an 18-inch shell from the Yamato!!
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Post by Tiornu »

There are more articles to read at NavWeaps.
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Post by Plamen74 »

Minimum plate which is nessesary to de-cap 38cm shell is 100mm. Look at the article of Nathan Okun Decaping revisited.

http://www.navweaps.com/index_tech/tech-085.htm
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Matthias
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Post by Matthias »

Old discussion but always actual.
I guess the decoupling power of a STS plate added before a main armour belt is due not only to the thickness of the armour but also to the quality of the shell itself.

For example, british AP shells at the Jutland were proved to get decapped or completely broken, anyway not exploding in the most cases, after have hit the armour plates of german battlecruisers, which we all know to be well protected and of though costitution.
But some of those shells were really big, 381, 356 mm and so on, so at least in theory they had enough mass and striking power to penetrate german armours and then explode into the main ship compartiments.But we all know that all the german battlecruisers apart Lutzow came back safe in Wilhelmshaven, although the many hits received (21 in Seydlitz case).

So, it was evident that english ammunition quality was not enough to operate properly against a good steel armour belt.
The problem is, Iowa and Yamato classes ships never received a shell hit in their whole opertaing lfe, so we will never be certain of the effectivity of their decapping plates and armours...
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Post by Dave Saxton »

The British improved the cap on their APC after Jutland.

Many assume that removing the cap only matters if face hardened armour is encountered. Although removing the cap increases shell breakup potential, in the case of impact against face hardened plates, it also reduces the shell's penetration potential vs homogeneous plates too.

Obliquity is a major factor. The greater the striking angle, the less the thickness required to de-cap. The more toward a right angle striking angle, the greater the thickness required. The Italians installed a 70mm decapping plate over their belts, but both the de-capping plate and the belt were sloped 15*. Obviously, both 70mm and 100mm are much greater than 38mm.

The 38mm as a de-capping plate was an idea that started in the 80's. It was assumed that 38mm vertical would de-cap large caliber projectiles, but further study showed this to not be so. It may de-capp at very large striking angles, but a shell striking at such total obiquity would not defeat the main belt anyway.

The 38mm was probably installed to provide some modest protection to the water plane against splinters, as well as increase structural stiffness.
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Post by Matthias »

Dave Saxton wrote:The British improved the cap on their APC after Jutland.
Indeed, I was speaking of 1916 situation infact.;)
"Wir kämpfen bis zur letzten Granate."

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