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British vs. US Carrier Design in WWII


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#1 BuzzLightyear

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Posted 09 January 2005 - 02:15 AM

Found an interesting article on another board and thought it would make an interesting topic of discussion.

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Were Armored Flight Decks on British Carriers Worthwhile?
by Stuart Slade and Richard Worth
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Armor Protection on American and British Carriers
©2000 Stuart Slade

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This is a VERY complex design issue that defies easy answers. The question is not so much whether armor is useful (both US and British designs had very roughly comparable armor protection in terms of weight) but where does the designer put it.

US designers treated the entire flight deck, hangar deck and island assemblies as superstructure. The strength deck was the hangar deck and this is where they put the armor. The plusses of this configuration are that it carries the heavy weight of armor low, making stability problems less dreadful, permits a very light deck structure that's easy to repair and allows a long flight deck that makes operating aircraft easy. That light structure also initiates bombs, hopefully ensuring that damage is confined above the armor deck. The big negative is that it means the hangar deck is essentially unprotected.

British design practice with the Illustrious and Implacable classes was to armor the flight deck, making the flight deck the strength deck. The plusses here are that if the armor holds, bombs can be kept out of the ship completely. The negatives are that the size of lifts is restricted, stability problems are hellish and the airgroup capacity is comparatively small.

The US went the way it did because they had plenty of aircraft, used deck parks and envisaged launching mass strikes. They were able to base carrier defense on having fighters. The British were hobbled by the RAF that allocated few resources to the FAA, so the carriers had few and obsolete fighters. They had to build their carriers to take damage.

In fact, the British designs failed. Off Okinawa, the resistance of the British carriers seemed impressive but in reality the damage they took was severe. Having the hangar inside the hull girder made the hull structure weak and the ships were deformed by comparatively minor damage. Note how quickly nearly all the armored carriers were scrapped postwar - surveys showed they had irreparable hull damage. In contrast, the Essex's, which suffered much more severe damage, lasted for decades.

The severe damage suffered by the British armored carriers is documented by their post-war surveys. These surveys were carried out to determine the suitability of the ships for modernization.

Of the British armored carriers, Formidable and Illustrious were write-offs due to war damage. By the end of the war, Illustrious was in very poor condition; her centerline shaft was history due to structural deformation and her machinery was shot. Formidable had raped herself when a Firefly (sic) rolled off a lift and raked the hangar with 20 mm gunfire. This started a very bad fire which was contained within the hangar and acted like a furnace. The heat deformed the hull and that was it.

Indomitable was actually used in the post-war fleet and was modernized (lightly). In 1951 she had a gasoline explosion in her hangar deck. This was actually quite minor (an Essex would have shrugged it off) but the fact it was contained and was within the hull girder caused severe damage. She was patched with concrete for the Coronation Review, then scrapped.

Victorious was surveyed, found to be in reasonably good condition and rebuilt. The rebuild was fiendishly expensive, largely because the flight deck was the hangar deck and partly due to idiotically bad project planning.

The Victorious conversion was one of those tragedies that was almost comical. The original plans did not include re-engining the ship; this was a decision taken late in the rebuild process by which time most of the hull work (about 80 percent) had been completed. A machinery survey showed that the boilers had only about ten years of life left and it was decided they should be replaced. This meant that a lot of work had to be undone and then redone. The awful bit is that she was still within that ten year period when she was prematurely decommissioned. The Ship's Cover is pretty sulphurous in places. Another tragedy is that this monumental mess disillusioned the fleet with any sort of rebuild program (which had echoes in all sorts of places including the Type 15 program).

Another point which should be brought up is that the armored box hangar on the RN CV's was restricted to a height of 16 feet maximum and was as low as 14 feet in the upper hangars on the Indomitable, Implacable and Indefatigable. This restricted the use of the F4U Corsair fighter in the 14 feet hangars. This also hampered the usefulness of the British carriers postwar as aircraft grew in size. By contrast, the USN carriers had a hangar clear height of 20 feet in the Lexington class, 17 feet 3 inches in the Yorktown class and 17 feet 6 inches in the Essex class. This greater height allowed the Essex class to easily adapt to the much larger postwar jet aircraft.

The planned refits of the Implacable and Indefatigable would have seen the two hangars merged into one which would have made these ships much more capable. Sadly, the problems with the Victorious rebuild killed that plan off. In retrospect, they should have gone through the upgrade process first; as ships, they were much better than the first four armored carriers and were in good condition.

We also have to be very careful when looking at apparent ship histories in the 1945 - 1955 period. There is a lot of statistical deceit used here (Eric Grove in "Vanguard to Trident" makes an eye-opening read). Ships that were apparently in good condition and in service were actually laid up or otherwise non-operational. Illustrious is a good example. Her Ship's Cover is quite clear that she had never recovered from the damage she'd taken in WW2 and was limited to around 22 knots for all practical purposes. That's why she was used for experimental purposes - she wasn't much use for anything else. Indomitable is another example of statistical deceit. After her 1950 gasoline explosion (shortly after she finished her refit), she was completely useless and had to be towed to Spithead for the Coronation Review. As soon as that was over, she went to the breakers.

Two books, the Eric Grove "Vanguard to Trident" and Norman Friedman's "British Carrier Aviation" give a feel for this rather depressing period in history. Grove's book in particular is superb for providing a feel for the interplay between technology and politics that went on during this period. One interesting point that he brings out is that a great problem the RN had was in manning ships, even when money was available.

It is also not true that the Illustrious class carriers were worn out by hard war service. The last pair were only used for a couple of years and didn't work that hard. They certainly did not do the long deployments undertaken by the US carriers during the war. Indefatigable and Implacable were badly built (as were most British wartime ships - Admiralty records related to planned reconstructions quoted in Vanguard to Trident implicitly give war-built cruisers a life of only ten years). No criticism intended there - emphasis was on quantity rather than quality.

The Midway class is a much more complex design problem than just the adoption of an armored deck. In fact, the armored deck was not actually adopted - it grew out of other factors in the ship's design. Norman Friedman's "US Carriers - An Illustrated Design History" goes into this in detail, but, in summary, the Midway's were the first non-treaty restricted carrier designs in the US Fleet. British input to the design was actually very mixed - even after the Illustrious bombing (usually quoted as an example of the value of an armored deck), some British comment to the US Navy was very anti-deck armor. Originally, the Midway's were to have had a heavy (8 inch) deck gun battery. Eventually, this was discarded and the weight saved was used to provide two inches of flight deck armor. This was in addition to the 3.5 inches of hangar deck armor sported by the Essex's. The suggestion that they are a response to the UK armored carrier designs is largely a myth - the discussions that lead to the Midway's actually predate the Illustrious class.

Don't get me wrong; the strategic and operational logic that resulted in the Illustrious class was (for the Royal Navy) quite correct - the vulnerability of the ships to internal damage was unexpected and surprising. That vulnerability made their designs essentially failures since the sacrifices made to give them their heavy protection were not fully justified by their performance. That could not have been known pre-war, nor could the rapid escalation in weapons lethality that degraded the value of their deck armor.

The British dumped the armored deck for their last carrier designs and adopted a very Essex-like approach. It is a shame those ships didn't get built - they were really good-looking designs.

I think there is an important point here which should be included. We've been discussing the Essex/Illustrious classes in terms of armored flight deck versus armored hangar deck. In fact, this is not quite the key differential. The real point of difference is that the Essex class had an external hangar, that is, the hangar is located outside the ship's girder while the Illustrious class had an internal hangar; that is, the hangar is contained within the ship's girder.

An external hangar offers large side openings so that aircraft can be warmed up on the hangar deck, loading and unloading aircraft is made easier, underway replenishment becomes easier and safer and, most importantly, flight deck damage and hangar deck fires are outside the main hull and therefore of less structural consequence. Deck edge lifts are also very easy to install.

An internal hangar is contained within the ship's girder and is enveloped by the ship's hull. It is easier to protect, has better access to machinery shops and maintenance facilities and offers much better protection for the aircraft against bad weather. Deck edge lifts are difficult to install, of questionable value and have serious structural implications.

In structural terms, having an external hangar means that the upper strength deck is the hangar deck. This then means that the hull girder is shallower and thus more highly stressed. The best way to offset this is to thicken up the hangar deck so protection (armor) here grows naturally out of the design concept. If the flight deck is to be armored, that armor has to be in addition to the hangar deck protection. It is not often realized that Midway started life as a parallel design to Essex, intended to explore the effect of that extra protection on the Essex design. On 27,000 tons, it was found that deck protection had a disastrous effect on airgroup capacity (as few as 60 aircraft rated capacity at a time when Essex was rated at 110). This bought protection against 250 pound bombs. After 1940, the Midway design went its own way, becoming a quite different program to the Essex class. Note though, that the hangar deck remained the strength deck.

Starting with the Forrestal class, the size of the carriers meant that stress requirements forced the abandonment of the external hangar and hangar deck as strength deck concepts. A shallow hull of that size is a design impracticality. In the Forrestal and after, the flight deck is the strength deck, protection considerations had no influence whatsoever on the flight deck design. In fact, these carriers do not have armored flight decks. By the way, there is a construction trick that allows the Forrestal and later carriers to have their flight decks as strength decks and deck edge lifts without compromising hull strength. That trick is still highly classified.

The advantage of the internal hangar was that, by using the flight deck as strength deck, the British carriers had a much deeper hull girder, so the designers could use substantially lighter hull structural members, giving them a larger carrier for a given displacement. In fact, the Illustrious class are so well designed in weight economy terms that even today it is impossible to find areas to make additional savings. The big problem was that, since the flight deck was the strength deck, holes (lifts, etc.) had to be kept to a minimum, so the internal hangar concept immediately translated into fewer and smaller lifts - compromising the ability to launch and recover aircraft.

Unfortunately, there was a hidden problem that no one realized at that time. The hangar forms a large open void in the ship's hull girder. When faced with shock, this allows the girder to deform and, once deformed, the damage is irreversible. The gravity of the shock problem only became apparent with the magnetic mines used in 1939 and when the ships started taking near misses. In effect, the hulls became progressively twisted and rippled as damage mounted up. This killed Formidable and Illustrious (both ships were surveyed in 1947 to assess the expenditure required to repair them and it was found that both were beyond economical repair. In effect, they needed their hulls completely reconstructed and plans to rebuild them were abandoned). The gasoline explosion on Indomitable had the same effect; again hull damage was beyond economical repair.

Technically speaking, this is also a risk with the Forrestals and their descendants. Two things help out, though. One is the sheer size of the US carriers - they are much bigger in proportion to the void represented by their hangars. Another is the density of construction. The second thing is that US Carriers are built incredibly tough. Not only were they designed to take a terrible battering, but the latest ones were put together by the finest shipyard in the world. In contrast, the Illustrious class were built under treaty requirements and used great weight discipline throughout. This resulted in a lot of design compromises in power train, hull structure, etc., etc. Some of these were put right with the Implacable class.

Ark Royal and Eagle were the last gasp of the British pre-war carrier design. They were effectively enlarged Implacables. By the time their design was finished (1942), the British had realized that the sacrifices they were making for the heavy protection of an internal hangar could not be justified and they went to an external hangar much along the US lines. Okinawa proved this point; although often quoted as pointing to the value of an armored deck, careful analysis does not bear this out. The British carriers never came under the weight of attack that the US carriers suffered and never took the same density of hits. It is not immediately apparent, but most Kamikaze hits bounced off US carriers doing little or no damage - the ones that started the big fires were the exception. A critical factor seems to have been deck parks - if the stricken carrier had deck-parked aircraft, she was in trouble regardless of where her hangar was.

This debate between the virtues of internal and external hangars is history now; structural considerations mean that most modern carriers have to have internal hangars, regardless of their relative merits and limitations. Where and how carrier protection was designed grew out of that debate. What it does illustrate is one very important thing - carriers are unique in that their design is dictated by the aircraft they carry and how those aircraft are to be used. It is not correct to say that the USN was right and the RN was wrong or vice versa or that one design was better than another. The navies used their aircraft in very different ways and their carrier designs reflected that difference. When they started to use their aircraft in the same way, their carrier designs converged.



#2 BuzzLightyear

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Posted 09 January 2005 - 02:20 AM

Here's the link to the essay I posted above. There's more to it that you may be interested in.

http://www.navweaps....ch/tech-030.htm

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Here is another interesting link looking at the effect of Zamikaze strikes on different carrier designs:

http://www.navweaps....ch/tech-042.htm

#3 PMN1

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Posted 09 January 2005 - 03:00 AM

Ahh Tiornu's article.

The value of armoured box carriers of the RN vs the open unarmoured carrier designs of the US is always a good topic to strat an argument.

:)

#4 Tony Williams

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Posted 09 January 2005 - 03:34 PM

You have to bear in mind that the RN assumed they would have to operate within range of enemy land-based aircraft, and before the advent of radar there would not have been sufficient warning to get enough fighters airborne to fight off an enemy bombing attack. So their doctrine was to fight off air attacks with AA guns while keeping the planes safely in their armoured hangars. So they didn't place any great stress on procedures for fast aircraft launch and recovery, and onboard air groups were small.

In contrast, the Americans and Japanese, who envisaged open-ocean fighting, went in for much faster aircraft launch and recovery rates with big air groups to defend the fleet.

Of course, the development of radar clearly gave the advantage to the USN/IJN approach in almost all circumstances, and there is little doubt that the external hangar approach would have been better for the RN as it permitted larger-capacity hangars - but they would have had to change the handling procedures and get better aircraft to make that work. Needless to say, I have the RN going down that road in my alternative WW2 novel, 'the Foresight War' :D

Tony Williams: Military gun and ammunition website and Discussion forum

#5 Flo

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Posted 26 July 2011 - 05:18 PM

I read this post with a growing sense of bewilderment. Can anyone else see the basic flaw in the finely wrought arguments above?

A carrier without a flight deck or hanger is no longer a carrier. While it's marvelous that many Essex class carriers hit by Kamikazies were brought back into service, none did so during the war. By comparison, the armoured carriers, though junk by their next refit, were flying again within hours. Comments about how much greater the volume of attacks on USN carriers was are smoke and mirrors. A glancing hit on an armoured carrier would do no more damage than that on an unarmoured one. You could mention the heavier AAA armament of armoured carriers, or their smaller, harder to hit dimensions, but that too is beside the point. One solid hit on a wartime Essex immediately totalled the vessel with teriffic loss of life, the same hit prevented a days flying at worst, with only modest losses on an armoured ship.
The pathetic standard of British wartime build and the appaling mess and waste exposed in post war refit yards are no argument against the effectiveness of the Illustrious & Implacable classes. The very fact that Illustrious survived the war, in spite of multiple bomb and Kamikaze attacks makes the point more eloquently than I ever could.
Put into another form; if the British had built unarmoured carriers (improved Ark Royals, say) they would have been out of carriers half way through the war, without the industrial muscle to build more. Malta would almost certainly have fallen, North Africa shortly after. With access to the oil fields of the middle East granted to him Hitler could well have become unstopable.
"In fact, the British designs failed". :confused: More than any other class of ship on the RN list, the Illustrious class were war winners. It would have been nice, given funding, resources and some realistic aircraft choices, to build large, modern air groups for them to carry, but given the tools to hand they consistently produced amazing results.

"When they started to use their aircraft in the same way, their carrier designs converged." Alas, were it only true. British fixed wing carrier design ended in the 40s. Innovations like angled flight decks and improved catapults were compensating for the flaws in the only ships available at the end of empire...
...unless, of course, we actually put a Queen Elizabeth into service.

#6 PMN1

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Posted 27 July 2011 - 06:22 PM

The very fact that Illustrious survived the war, in spite of multiple bomb and Kamikaze attacks makes the point more eloquently than I ever could.
Put into another form; if the British had built unarmoured carriers (improved Ark Royals, say) they would have been out of carriers half way through the war, without the industrial muscle to build more. Malta would almost certainly have fallen, North Africa shortly after. With access to the oil fields of the middle East granted to him Hitler could well have become unstopable.


Enterprise suffered quite a few hits as well....a very good line i've seen that sums up the situation is ' despite the inferno in the hanger (caused by bombs coming down the lifts and so negating the armoured box) the armouring kept the bombs out of the machinery and magazines'.

Now that can be done with flight deck armour or hanger deck armour and flight deck armour as the article says is not going to stop a torpedo whereas more fighters might.

Its also important to recognise that the armour didn't cover the whole flight deck, just the areas above the hanger so the design should really be called armoured box carriers.

Another point is by the time the ships were entering service, bomb weights had gone up and were capable of penetrating the armouring scheme - all the bombs that hit Illustrious were 550kg and the scheme was designed to withstand 250kg.

Most of the hits, including during the much hyped attack on Illustrious, that the RN carriers took were outside the armouring scheme or inside it via the lifts and in the case of Formidable, spalling from the flight deck armour made one hell of a mess of the centre machinery spaces - Brown lists the hits in his book Nelson to Vanguard and in only one hit does he describe the armouring as invaluable, all the others were such that even a non armoured deck would have probably withstood the hit.

Why armouring was retained along with trying to stay in Treaty limits for Implacable and Indetafigable when radar was on the way and the Treaties were dead is beyond me.

Popular history has Malta being vital to interdicting Rommel's supplies whereas a closer look would show far more was held up in ships due to lack of capacity at ports and on the docks due to lack of transport in North Africa - all there was was a single road that was liable to flooding - a good read in Van Crefeld's 'Supplying War' and a good thread is here

http://forum.axishis...hp?f=56&t=99035


Given the resources Rommel had and could get hold of even if Barabarossa was cancelled and he had all the resources of Germany at his disposal, getting to the ME oil fields was even less likely than a successful Sea Lion, there is simply not the port capacity for unloading nor the infrastructure for carrying the supplies in North Africa

Keeping Malta supplied may have actually cost the British more than it cost the Axis.

Edited by PMN1, 27 July 2011 - 06:26 PM.


#7 GregP

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Posted 28 July 2011 - 12:22 AM

I agree with you, Flo. The carriers that survived to fight the war to a successful conclusion cannot be called failures. They may have been unfit for refit, but they DID succeed.

I'd call them effective in the intended role for the intended time of service, which was the war. After that, a ship survey might reveal a ship unfit for continued use, but it DID the job when it was needed.

#8 ickysdad

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Posted 29 July 2011 - 05:21 AM

Say what you want but the USN got it right with it's Essex's as compared to the RN's AFD CV's ,in fact the Yorktown's on a hull several thousand tons smaller carried a far larger airgroup. The carrier isn't just there carrying aircraft to protect itself ,it also protects the whole fleet and being able to carry more a/c even while giving up protection is very worthwhile. Now imagine a USN with Illustrious/Implacable types off of Okinawa you then have fewer fighters so more kamikazes get through plus you have fewer carrier aircraft that can conduct close air support for your amphib force. Also what about ordnance & avgas supplies??/ Range of RN versus USN CV's??? Could RN AFD CV's warm up their engines in the hangar??? Also just how much damage did it take to sink Yorktown,Hornet or Lexington?? Sorry but alot of Essex's returned after being hit by kamikazes.

Now it would be nice to have both Essex class capacity and an AFD/Box hangar but then your talking more like the Midway class.

here's a discussion a while back on warshps1 but it's 29 pages long.
http://warships1disc...com/topic/16179

Edited by ickysdad, 29 July 2011 - 05:31 AM.


#9 ickysdad

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Posted 29 July 2011 - 05:51 AM

Illustrious doesn't carry any heavier AA outfit then Yorktown when you consider the latter carried 2 MK 37 directors alongst with a heavier outfit of 40mm & 20mm.

#10 PMN1

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Posted 29 July 2011 - 05:21 PM

Also what about ordnance & avgas supplies??/ Range of RN versus USN CV's??? Could RN AFD CV's warm up their engines in the hangar??? [/url]


Well, the avgas supplies were less to do with the armouring, more to do with the way the RN stored the fuel...very successfully.

Also the warming up in the hanger was less to do with the armouring, more to do with the hanger being a closed design - the earlier Ark Royal with its closed hanger didn't have the armoured box and was just as incapable of warming its aircraft up in the hanger.

What would have been interesting is if the RN had abandoned the Treaty Limits for Implacable and Indefatigable and made them bigger - possibly Audacious size, would probably have commissioned later but they didn't contribute much anyway but the RN would have been better off post war.

Edited by PMN1, 29 July 2011 - 05:25 PM.





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