Hood: Battlecruiser or Fast Battleship?

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Karl Heidenreich
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Hood: Battlecruiser or Fast Battleship?

Post by Karl Heidenreich »

I´ve been studying the comparison of the macro characteristics of the Capital Ships in service at 1941; in special the Hood because I wonder why she is classified as a Battlecruiser and not as a (Fast) Battleship.
In order I will resume my thoughts:

I. Main Armament.
Bismarck: 15"
Hood: 15"
KGV Class: 14"
Nelson Class: 16"
North Carolina Class: 16"
The Hood had an armament equal or superior than her contemporary Battleships.

II. Displacement and size.
Bismarck: 50,900 tons full load @ 251 meters lenght.
Hood: 49,136 tons full load @ 262.2 meters lenght.
KGV: 44,400 tons full load @ 227.1 meters lenght.
Nelson: 41,000 tons full load @ 216 meters lenght.
North Carolina: 37,178 tons full load @ 222 meters lenght.
The Hood, again, was heavier and longer than KGV, Nelson and North Carolina, all of them which were classified as BBs. Even she was longer than the Bismarck. No doubt she was cataloged as the world´s biggest warship at her time. Only the Yamatos and Iowas turned to be longer years later.

III. Armour and armour rate.
Bismarck: 17,540 tons @ PC/TC 17/22 r= 0.773
Hood: 13,650 tons @ PC/TC 12/19 r= 0.632
KGV: 12,612 tons @ PC/TC 9/17 r= 0.529
Nelson: 14,250 tons @ PC/TC 10/21 r= 0.476
North Carolina: 15,087 tons@ PC/TC 9/18 r= .5
As you can see, at least in numbers, the Hood had more armour than the KGV, a battleship, and was slightly behind the Rodney but her armour rate was, in fact, bigger than KGV, Nelson and North Carolina, only second to the Bismarck. The distribution of that armour, I agree, is another matter. Anyway, the issue here is that Hood was longer, heavier, better armoured and with armament that in much cases make her superior to other warships, BBs included.

Being this the case and with speed as a classification factor, then, the Hood must be reclassified as a Fast Battleship and not as a Battlecruiser. :!:
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Post by marcelo_malara »

The vertical armour of Hood was equal to other battleships. The horizontal armour was thinner.
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Post by Dave Saxton »

It has been put forward by many that Hood was the prototype fast battleship. Nonetheless, prototypes by diffinition, do not repesent the final or optimal arangement.

Hood, although called a battlecruiser, was in reality the best protected ship in the Royal Navy from the time of her completion until the Nelson joined the fleet. Hood had better deck protection than the Royal Sovereign class battleships or even the Queen Elizabeth class battleships.

The overall thickness of Hood's deck protection wasn't that bad; totalling more than the R's or the Queens. It was probably heavier than most forgien battleships of her time too. Of greater concern, were the materials used for the PDP, or protective deck plating used during this period of time. AFAIK, Hood's PDP was simply high tensile steel, not the armour grade homogenious materials, used by the capital ships of the later generation.
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Post by marcelo_malara »

Dave, you are right. I withdraw my previous opinion about the horizontal protection of Hood. Yesterday I compared Hood and Warspite in AOTS and they are very similar.
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Post by Karl Heidenreich »

There is another data that supports the idea of the reclassification of Hood as a Fast Battleship instead of Battlecruiser:

Armour comparison with other British Warships:

Hood
Main Belt: 305 mm
Upper Deck: 32 mm
Armoured Deck: 50 mm
Turrets: 130-380 mm
Protected lenght: 65%
Speed: 31 knots

KGV Class
Main Belt: 350-375 mm
Upper Deck: no!
Armoured Deck: 150 mm
Turrets: 150-324 mm
Protected lenght: 60%
Speed: 28 knots

Nelson Class
Main Belt: 355 mm
Upper Deck: no!
Armoured Deck: 160 mm
Turrets: 185-405 mm
Protected lenght: 50%
Speed: 23 knots

As as you can see, the weak point of the Hood, as we already know, is the Armoured Deck which is 3 times thinner than the KGV or Rodney. But all the other features are the same or of a bigger scale. Let´s take the protected lenght: it is a larger percentage than that of KGV (slightly) or the Nelson. The astounding characteristic is that KGV and Nelson class ships (both classified as Battleships) didn´t have armour at their upper decks, meanwhile Hood (a "Battlecruiser") did have armour on that deck.
The main belt armour was slightly thinner than that of the other two ships, but of the same scale. I do remember that Tirpitz main belt armour was very similar to this (But we must agree that the quality and mechanical properties of the Krupp steel were different from the British ones).
The main factor that make apart the BBs from the BCs is the relation "armour-speed" (the factor that created the BCs in first place) being the BC armour lighter to ensure higher speeds: "Outgun smaller vessels and outrun armoured ones".
Using, again, the speed as the element of classification we can see that Hood, in it´s overall characteristics was a Fast Battleship instead of a Battlecruiser.
Best regards
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Post by marcelo_malara »

Karl, you are making the same mistake as I did judging the horizontal protection based on the raw data found on books.
The forecastle deck on Hood is 32 mm near the fore turrets, 51 around funnels and 32 mm again behind.
The upper deck is 19 mm below the forecastle and and 51 and 26 behind it.
The main deck is more complex yet: 37 mm on boilers and machinery, 51 mm on the slopes, and 51/76 over magazines.
The lower deck (that exists only fore and aft of boilers and machinery) is 51 mm over magazines.
So for example the magazines had 177 mm over them (32+19+76+51), the boilers 108 (51+19+37) and the machinery 89 (32+19+37).
As Dave posted, that compared favourably with contemporary designs.

After the war the design philosophy changed and the horizontal armour was concentrated on a single deck at the lower end of the vertical belt (I don´t know if this is part of the all or nothing scheme). The idea was that to reach the armoured deck directly without passing thru the vertical belt, the shell had to come in a descending angle, fired a long distance away and so would strike with low velocity.
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Post by Karl Heidenreich »

Agreed. Two things:

1. Thanks for clearing that information :oops: , it´s obvious that the layers of armour one over the other bring more defense than the nominal. That´s the problem when you just see numbers by themselves.
Where did you get the info, anyway? I´ll like to have a look.

2. The real subject of this topic is reinforced by your afirmations because that´s proof about the Hood more compatible to the definition of a Fast Battleship than of a Battlecruiser, don´t you think? :lol:
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Post by marcelo_malara »

Yes Karl, I agree with you. It seems that we will have to reclassify Hood, at least for us.
For the protection scheme of Hood, I took the data from the volume of Conway´s Anatomy of the Ship, The Battlecruiser Hood. Wonderful volume, John Roberts did a magnificent work here, only surpased by his Battleship Dreadnought, of the same series. The level of detail of the drawings is really amazing, ranging from details of the construction like beams, plates, perspective drawings, to cutaways of condensers, distilling machinery and so on. Buy it, its worth the money.
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Post by Dave Saxton »

What is classified as a battle cruiser or a fast battleship is so ambiguous that it really doesn't matter that much. I will always call HMS Hood a battlecruiser, simply because that's what the Royal Navy called her at the time.
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Post by Dave Saxton »

marcelo_malara wrote:...........
After the war the design philosophy changed and the horizontal armour was concentrated on a single deck at the lower end of the vertical belt (I don´t know if this is part of the all or nothing scheme). The idea was that to reach the armoured deck directly without passing thru the vertical belt, the shell had to come in a descending angle, fired a long distance away and so would strike with low velocity.
Shortly before Hood's completion the British built a full scale mock up of a Hood section and fired at using a 15" APC from 25,000 yards. The shell defeated all the protected decks. This lead to the change in philosophy. The new British designs (that the Nelson was a scaled down version of) had an internal armoured raft. This internal raft resembled an inverted shoe box. The single main armoured deck rested on the upper edges of the belts that were inclined 15*. This was sometimes referred to as the "strict all or nothing system". The term "all or nothing" was first applied to USN battleships, beginning with the Nevada class. This didn't mean all deck armour consolidated into one single deck, as all AoN USN battleships always had two armoured decks. Originally, it meant mainly that no heavy armour was comitted to areas outside of the armoured citadal.

In some cases, but not all, several armoured plates don't have an effective thickness equal to their sum total thickness. There's two main theories why this may be so. One reason may be a loss of total shear strength. The shear strength of a steel can be quantified as roughly 60% of it's ultimate tensile strength, but is also has a function based on continious thickness. The other main theory is that shells passing through the first layer may have a change in trajectory, giving it a more favorable striking angle vs the next plate it encounters.

If one uses a design with more than one plate, care must be taken to not give the projectile more favorable striking angles.

If the first plate is of tough mechanical properties, and it's of sufficiant thickness, it will cause a penetrating projectile to yaw. Yawed projectiles require greater velocities to achieve penetration. This essentially increases the effective thickness of the second plate. This may have been the rational for the 1 1/2-inch weather deck armour on newer USN battleships. Shells striking at less favorable angles, yaw more than those that strike at right angles. Another factor effecting yaw is de-capping. Decapped projectiles are yawed more.

One problem with the older designs such as the Royal Sovereign class, was that each layer in sequance was not stout enough to de-cap, yaw, or breakup projectiles. For example the R's had an upper deck of only 25mm HTS, followed by the next deck of 30mm HTS, followed by 50mm HTS main deck. A better distribution would be all armour (and it should be homogenious armour, not just high tensile steel) in one deck of 105mm. If two decks are used, the upper deck should be just enough to induce yaw, (and hopefully de-capping) leaving the second plate, or the resitance plate, as thick as possible.

Against horizontal armour at very long range, the angle of fall gradually becomes more important than the velocity. As the striking angle becomes more favorable to the shell, the penetration potential of horizontal plates increases significantly, despite decreases in velocity. Past a battle range of about 25,000 meters, the velocity of almost all battleship shells will be less than 475 meters per second.
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Post by marcelo_malara »

Thank you for the new data Dave.
Just two questions. I didn´t ever see a section of Nelson´s hull, I don´t understand why was the protective deck sited over the belt:

-It can be struck directly quite easily by a shell coming at any descending angle, isn´t it dangerous?
-What is behind the belt? Machinery and boilers? Are they partly above the water line? Or is the belt totally underwater?
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Post by Dave Saxton »

marcelo_malara wrote:Thank you for the new data Dave.
Just two questions. I didn´t ever see a section of Nelson´s hull, I don´t understand why was the protective deck sited over the belt:

-It can be struck directly quite easily by a shell coming at any descending angle, isn´t it dangerous?
-What is behind the belt? Machinery and boilers? Are they partly above the water line? Or is the belt totally underwater?
I'm only passing on information discovered by my betters, from generations past.

The hull protection of the Nelson, and just about all battleships thereafter, indeeds resembles a bottomless rectangular box. The lid of the box is the main armoured deck, and the belts are the sides. The ends are enclosed by traverse armoured bulkheads; just aft of the after magzine/machinery space, and just forward of the forward most magazine.

In Nelson specificaly, the belts extend from one deck level above the water line to just below the water line. The Nelson belts are inclined 15* and are well inside the ship's outer shell plating.

There's very little in protective plating behind these primary armoured barriers on these type of designs, in keeping with the philosophy of putting most available armour tonnage into the primary plates, making them as thick as possible. Most such designs had thin anti -splinter plating of 1/2 -1 1/2 inches behind this. This is one of the trade offs of the philosophy.

The armoured deck can be hit directly by capped shells, passing through the upper two un-armoured decks (in the case of Nelson), or by passing over the main belts. Nonetheless, the striking angle would be so accute at the ranges the system is designed to protect the vital spaces, that roughly 5-inches is sufficiant.

In the case of the typical 15-16-inch shell; it's angle of fall is roughly 30-35* at the range of 30,000 meters, or 32,800 yards. At 30*of fall, and a velocity of around 460 meters per second, the penetration is about 120-130mm of the best quality deck armour. Many would be surprized that increasing the effective thickness to 150mm or 175mm, only buy's at most 1000-2000 meters in additional protected range. This is because the penetration of horizontal armour increases as a differential equation with increasing range. This fact sets the maximum practical protected range of 35-45000 ton battleships to about 30,000 meters-plus or minus a couple thousand meters.

A layered deck system, or a two deck system array, does have the added fail safe of prossibly destroying the plunging shell before penetration of the final primary armoured barrier. For example, Warspite hit the Italian Ceaser from 26,000 yards with a 15" APC. This shell was burst before reaching the main armoured deck, after being fuzed by the upper 40mm thick armoured deck. In any case the modernized Ceaser had sufficiant armour in three decks, to protect it to ~30,000 meters, vs 15-inch shells.


There's the philosophy of "stopping" shells, and there's also the philosophy of destroying shells, or both, that can be at work in a protection philosophy.
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Post by Dave Saxton »

marcelo_malara wrote:.... I don´t understand why was the protective deck sited over the belt:

-......?
This is one of the potential weakness's of the newer philosophy. By placing the deck above the belts, the deck can't provide an additional barrier against projectiles penetrating the belts, nor against plate debris. However, it protects the areas between the deck and the weather deck better, and doesn't require dividing the total deck armour thickness among two decks. The philosophy is of course contingent on the belts, not being penetrable during "normal" battle situations, and ranges.

By WWII the newer battleship guns were so powerful vs belts, that penetration was actually likely at normal ranges, however. For example, Bismarck's 15" gun could penetrate a 12" belt to a range of 31,000 meters. Of course, this would only likely happen with a broadsides on target angle. By offering a offset target angle; the target angle adds to the total striking angle, creating a less favorable striking angle. For a similar reason, belts are ofton inclined up to 19*. The angle of the belt slope, plus the target angle, plus the angle of fall, is the total striking angle. This could expand the inner protected zone of a 12" belt to about 20,000 meters, provided the quality of the armour was top drawer. Nonetheless, the belt protection vs heavy caliber shells, at likely WWII battle ranges, remains the AoN system's perhaps greastest short coming.

Even with a sloped belt giving the eqivilent of a 15-16" belt, the newer 15-inch guns could penetrate such a belt out to 25,000 meters. Almost all WWII gun battles between ships, were at ranges of less than 25,000 meters. The longest range hit ever, being less than 25,000 meters. The likelyhood of hits being scored beyond 25,000 meters are not great. Most of the few battles fought were actually less than 20,000 meters.
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Post by marcelo_malara »

Thanks for the explanation. Is there any book that contains all this?
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Post by Dave Saxton »

I don't know of a single work that condenses it all. Hmm.. perhaps something should be attempted? It's a such vast subject and we are always learning more and more though. I have learned a lot from the secondary writings (published and unpublished) of modern experts such as Bill Garzke, Bill Jurens, Raven and Roberts, and George Elders. Of course the real meat is in the primary documents left behind by the true experts that were there doing it during the time period. These documents, be they American, British, German, Japanese, Italian.... can be be like a revelation.

Oh, BTW I left a misleading statement in the above post, that I couldn't edit. A single deck above the belts doesn't protect the area between the deck and the weather deck better. It doesn't provide any protection for this area at all. It does expand the volume inside of the belts and deck though. This is particularly true of later designs like the KGV, that raised level of the main armoured deck one more deck level.
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