Yaw deck?

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Billy
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Yaw deck?

Postby Billy » Mon May 01, 2017 11:11 pm

The top deck of American battleships was sometimes officially known as the yaw deck. There has been much debate over the function of yawing a heavy projectile. Some people saying of course it does others decrying the notion and refer to it as normalising, where the shell direction is slightly different but no actual yaw taking place.

Well one thing I think hasn't been considered yet is the gyroscopic effect. The Gyroscopic effect is basically if you have rapidly rotating object ( ie. a shell ) then if any deflecting is force is applied, like shell hitting deck armour and is delfected from its initial course ( commonly refered to as normalizing ) then a corresponding force will be applied to the object at 90° to the applied force - read yaw.

I believe that is why when an analysis of Tirpitz's armour scheme that the combined values of a spaced array is worth 5-6 inches deck armour. Or if I remember correctly the German value gave it a 30% advantage over single plates. I also fail to see any other effect giving this advantage of an intact projectile on armour. Although I do feel that the effects of decaping, shell damage and yaw are accumulative. Not to mention it exploding before it hits the second plate.

If German, British and American constuctors infer the positive effects of yaw in plate resistance and they seemed to be the people who studied it first hand and had the funding backing and experimental evidence at their disposal, to me its a given at least.

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Dave Saxton
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Re: Yaw deck?

Postby Dave Saxton » Tue May 02, 2017 2:55 am

There are two types of eccentric motion of a yawed shell. These are nutation and precession. Nutation is a rapid eccentric rotation (the rate of the shell's spin) around the longitudinal axis of the shell. Since it is around the longitudinal axis of the shell and it is eccentric, it prevents the shell from striking nose on in all cases. Precession is a slow eccentric rotation around an off-axis which passes through the shell's center of gravity. Since precession becomes manifest slowly, it requires some distance after yaw is induced before it becomes significant. This is why two plates in direct contact with each other provide less effective thickness than if the plates have space between them. If the distance between the plates is great enough (such as two deck levels rather than one or less) then the combined effects of nutation and precession become more fully manifest providing a greater effective thickness to the armour system.

The axis around which both nutation and precession rotates should not be confused with the path axis of the shell's trajectory. There is usually a trajectory change more toward the normal when a shell passes through heavy armour. In the case of battleship caliber shell passing through the Bismarck class' 50mm upper deck the amount of trajectory change toward the normal is not significant. If I recall correctly the deck would need to be around 7-inches thick for it to provide net significant trajectory change toward the normal. Nonetheless, studies indicate it is the reduction of kinetic energy that mainly induces yaw, rather than trajectory change.
Although I do feel that the effects of decaping, shell damage and yaw are accumulative.


De-capping provides its own reduction of the shell's penetration potential in several ways. Firstly, it reduces the mass of the shell by around 13%. This in turn reduces the kinetic energy of the shell in addition to the reduction already caused by the energy expended by penetrating the de-capping plate. The cap also helps the shell to penetrate the remaining armour so its loss reduces the penetrative capability of the shell beyond just the reduction of its mass and kinetic energy. German research revealed that if the primary armour had a tensile strength of 80kg/mm2 or higher, it required that a de-capped shell would require more velocity to obtain penetration than the same weight and caliber of shell that was capped. So we indeed see an accumulation of factors that each require more velocity of the shell to obtain penetration, at least off setting the expected reduction of effective thickness by using divided armour thickness-provided the system is designed correctly. The Germans calculated that their spaced array deck armour provided at least the sum thickness of the plates or a little more in terms of effective thickness.

American tests during WW2 of some spaced array systems indicated that yaw could provide as much as a + 30% enhancement of effective thickness in some cases.
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.

Thorsten Wahl
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Re: Yaw deck?

Postby Thorsten Wahl » Tue May 02, 2017 12:27 pm

American tests during WW2 of some spaced array systems indicated that yaw could provide as much as a + 30% enhancement of effective thickness in some cases.


during and after WW2 -30% enhancement -

complete description W. Hurlich; Spaced Armor 1950
http://oai.dtic.mil/oai/oai?verb=getRec ... =ADA954865
-------------------------------------------------------------------------------------------------------------------
wrongly implement "Spaced Arrays" may have reduced protection compared to the total thicknesses of the plates in series, as a so called "optimal yaw" may increase considerably the penetrative capability of certain projectile types against the main armor plate compared to unyawed Impact, especially if distance between fore and main plates was too low.
british ADM Reports Series HIGH OBLIQUITY ATTACK of DECK TARGETS I - III
Report I (from 1945) couldnt find at the National Archives London
Summary Report SUPP 6/910 "the pentration of armour plate" section complex Targets

the typical projectile failure type against a german deck type armour arrangement has been described as "projectile topple", wich can be described as a somersault of the projectile after the impact becaus of to much yaw.
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Paul L
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Re: Yaw deck?

Postby Paul L » Fri May 12, 2017 3:04 am

The so called deflecting shells have little effect on follow on penetration , since the t/d is too low. I recall the path of American shell through French battleship deck it was only a couple of degrees at most, well within measurement error.

All projectiles that pass through spaced plate have yaw which also appears to weaken the penetrator. It wasn't until the 1960s/70s that this phenomena could be explored scientifically.

My estimations put the accumulative effects at 15%. With the KM decks they have both vertical and horizontal plates that can seriously f%@k up slanted penetration, although the accumulated effects are small changing 5.1" armor to 5.9" armor.
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Bill Jurens
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Re: Yaw deck?

Postby Bill Jurens » Sat May 13, 2017 11:35 pm

Billy wrote: "The top deck of American battleships was sometimes officially known as the yaw deck."

I find this an interesting statement. Can you provide a primary source reference for this terminology?

Thanks.

Bill Jurens

Thorsten Wahl
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Re: Yaw deck?

Postby Thorsten Wahl » Mon May 15, 2017 7:52 am

The statement w/ yawdeck was placed by Dave Saxton some years ago on this board. He refers to correspondence between Bu.ord and someone else, If I remember correctly.
Meine Herren, es kann ein siebenjähriger, es kann ein dreißigjähriger Krieg werden – und wehe dem, der zuerst die Lunte in das Pulverfaß schleudert!

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Dave Saxton
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Re: Yaw deck?

Postby Dave Saxton » Mon May 15, 2017 1:35 pm

I can't speak for Billy, but I do have a copy of a letter from the Bureau of Ordinance to Carnegie Steel that describes the upper deck of the new fast battleships as a "Yaw deck."
Entering a night sea battle is an awesome business.The enveloping darkness, hiding the enemy's.. seems a living thing, malignant and oppressive.Swishing water at the bow and stern mark an inexorable advance toward an unknown destiny.


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