Armor Protection on American and British Carriers
©2000 Stuart Slade
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 – aircraft that caused the damage was actually a Corsair) 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.
The Armored Box: The War's Verdict
©2002 Richard Worth
In Nelson to Vanguard, D.K. Brown critiques the choices made by designers of aircraft carriers. “I would suggest that both the RN and USN were right for the wars they planned to fight, the RN in narrow seas, facing shore-based aircraft while the USN expected to engage the Japanese fleet in the open Pacific.”1 Defenders of the armor-box carriers inevitably cite these differing scenarios as justification for the Illustrious design, yet they do so without Brown’s support - his comment was directed toward the question of open hangars versus enclosed hangars, and he is no defender of the armored box. He states outright that his choice for a British carrier design in that period would be an improved Ark Royal.2 The armor-box hangar, for all its legendary virtue, never justified the legend.
The box armor requirement dragged a crowd of design burdens on its coattails. Stuart has addressed the unforeseen structural issues. Lift configuration, freeboard, habitability, ship’s speed - the box restricted them all. But the salient fact, overshadowing all others, was the limit it imposed on air complement. Here, however, a fundamental misconception has clouded the armor debate; the leadership’s decision for smaller air groups preceded the flight deck armor, a feature subsequently superimposed on the preliminary design work.3 These two steps, though distinct, became inseparably meshed in the design’s wartime shortcomings and thus must be considered together. The small-group specification put the ships at an initial disadvantage, and the armor then canceled any hope for a remedy, cramping the hangars and reducing the space available for deck parks.
Wartime exploits in the Mediterranean have given a false impression that the armored carriers were intended specifically for that narrow-seas setting, but Britain had worldwide commitments. The Admiralty’s eyes, roving over the vastness of the Indian Ocean and South China Sea, never myopically fixed on a European land-based air threat. Difficult negotiations with Japan raised the immediate prospect of a Far East war, and Greene and Massignani specifically link this threat to the Admiralty’s prevailing battle doctrine.4 Fleet projections in the mid-1930’s revealed the true priorities, calling for the deployment of eight fleet carriers - five of them to the East.5 The narrow European seas, often touted as the reason for the armored box, exerted minimal influence. The Mediterranean hadn’t suddenly shrunk after Ark Royal was designed.
But something had indeed changed - the treaty situation - and this, rather than geography, gave birth to the armor-box carrier. As the treaty expiration date drew near, the British foresaw their freedom to build multiple carriers, which led them to think they need not crowd each individual ship with a powerful air complement6 - the dubious decision that made the armored carrier possible. It paralleled the navy’s willingness to build small but numerous cruisers, though along a different line of thought - design studies revealed that small carriers would prove cost-ineffective and operationally inferior; yet small air groups had an allure in placing few FAA eggs in any one basket, however large the basket might be.7 While no one dared apply such logic to battleships—arguing for only two or three guns per ship - it had some validity as applied to carriers in view of the limited FAA resources at the time. However, it forfeited the potential impetus for increased procurement inherent in a fleet of half-empty carriers,8 and it neglected any consideration of wartime mobilization.
The Illustrious project gestated amid unprecedented haste and informality in the British design bureau,9 permanently masking much of the designers’ rationale, yet hints have survived. The RAF mantra that “the bomber will always get through” certainly played a role.10 Given the contrasting “needs” for a small air group aboard a large carrier, the option to devote a hefty tonnage to protecting the air group seems an obvious one; and to armor the flight deck against the sort of weapon carried by the newest FAA bomber (the Skua’s 500-lb SAP bomb) indicates balanced thinking, if not foresight. Clues like these provide only a partial picture of the design process, but World War II would precisely gauge the design results.
As things turned out, the Mediterranean Campaign failed to fulfill its billing as the quintessential narrow-sea setting complete with a high incidence of bomb hits. During the entire war, only fifteen bombs scored hits on Allied carriers in the Mediterranean, a number surpassed in the first year of the fight against Japan. Of the fifteen hits, the Illustrious class flight deck armor defeated only one - Victorious shrugged off an anti-personnel bomb dropped at low altitude by an Re.2001 fighter.11 Indomitable took two 500-kg hits, but both of them avoided her armor which thus did nothing to preserve her flight deck; the ship was non-operational for the remainder of the action. Of the two 500-kg bombs dropped on Formidable, one struck her deck armor and sent pieces of it shooting all the way down into the ship’s machinery spaces. In the most famous Mediterranean incident, Illustrious survived numerous hits, but only one 500-kg bomb found her deck armor.12
So the armored box’s primary achievement in this narrow-sea setting was to detonate the two Formidable and Illustrious bombs high in the hull, which certainly enhanced survivability, though not in the way the designers intended. The hangars and their planes suffered increased damage, but crippling damage to the vitals became less likely. There’s no debating the advantage of this; yet debate continues, and properly so, because of the extra ounce of prevention the ships could have enjoyed with a larger CAP. Accepting a small fighter group meant accepting a greater probability of bomb hits, with the hopes of minimizing the damage those bombs caused - a strange set of priorities. Of course, the carrier’s escorts might dispute the entire notion of minimizing the damage - the armored box did them no good, in contrast to the universal blessing of a hefty CAP. And hangar armor, unlike fighters, could never counter a flight of torpedo planes. However, British planners had not foreseen that fighter interception would become an effective defense against fast, modern aircraft.
Apart from this self-defense issue, a Yorktown-sized air group would have greatly increased the ships’ offensive capability. With a larger airgroup, how much more could the British have accomplished at Taranto? Would Vittorio Veneto have survived Matapan if attacked by twice as many Albacores? Throughout the campaign, British carriers suffered from limited offensive muscle, which in turn allowed the enemy to retain a greater ability to strike back. The armored carrier design seemingly argued that the best defense was a weak offense.
It should be noted that the armored box was intended to defeat not only bombs but cruiser shells as well.13 Yet the “narrow sea” never proved so narrow as to force an armored carrier into the line of 6-inch gunfire. Naturally, the designers can’t be blamed for lacking postwar hindsight at a time when some carriers still sported cruiser-caliber guns, but war experience revealed that the compromises they accepted were unnecessary to the ships’ eventual mission.
It was in the open waters of the Pacific, late in the war, that the armored flight decks encountered a threat they could defeat - the kamikaze.14 The ensuing “sweepers, man your brooms” publicity properly underscored the potential benefits of flight deck armor, but also obscured the actual record; the Royal Navy’s own survey cited the flight deck armor as instrumental in defeating only one kamikaze. Even so, popular acclaim singled out the armor factor when the full story was much more revealing.
The British received relatively tame treatment from the kamikazes, as noted in David Hamer’s overview of the Okinawa campaign: “The Americans were operating four times as many fast carriers as the British, and the weight of Kamikaze attacks against them was many times greater again: ten Kikusui (massed suicide attacks) being flung against them whereas there were no such attacks on the British carriers.”15 A tally of Japanese aircraft lost during this time illustrates the disproportionate burden; the American TF 58 (including fifteen fast carriers) destroyed 1,908 Japanese planes, while the British TF 57 with its four fast carriers managed only 75 kills.16 Despite this glaring disparity, kamikazes damaged four carriers in each task force - every British carrier suffered at least one hit. The only armored carrier to reach war’s end without kamikaze damage was Implacable, which arrived on station at the end of the Okinawa campaign. What would have become of the British carrier fleet if it had faced the same intensity of attack as the Americans? The prospects are sobering.
The frequency of hits on British carriers did not equate to extensive casualties, and the hangar armor certainly saved lives once a plane actually struck, at least in one instance.17 But again, armor was not a solitary factor in limiting the casualties. The British never faced the prospect of a kamikaze hit amid an American-sized crowd of armed and fueled aircraft.18 The restricted air group had this ironic side-benefit; it provided less kindling in case of fire. Avgas storage, a proven killer of carriers, was severely limited19 as was aircraft weaponry.20 In this case, the fact that Illustrious presented a lesser threat to the enemy also made her a tougher target - analogous perhaps to sending a battleship into action with only a few rounds of ammunition in hopes of preventing a magazine explosion. Unlike a battleship, though, a fully stocked aircraft carrier can shoot down the enemy “shells” before they reach striking range.
So the Mediterranean experience recurred in the Pacific, with the ships showing occasionally increased resistance to hits that an enlarged fighter group might have prevented altogether. By this time, however, no one doubted the value of fighter interception. Brown puts it in simple terms: “More fighters would have been better protection than armour.”21 Without an advantage in defense, the armor-box layout could not justify its weaker offense. British planners, seeing the correlation of America’s open hangars and offensive muscle, turned about-face in their final fleet carrier design of the war, the Malta project. Initially featuring the hangar armor of its predecessors, Malta eventually abandoned not only the armored box, but all flight deck armor and even the enclosed hangar itself.22
Debates over the armor-box carriers can take many forms, focusing on the flight deck in particular, the armored box in general, the very concept of an enclosed hangar, or the entirety of the ship design. Discussions can account for external factors such as FAA mismanagement and radar advances, while noting the ships’ commendable war record and their inglorious postwar lingering. While the Illustrious design within its 1930’s context remains subject to varying critiques, it’s clear that in wartime the armored box limited the ships both offensively and defensively - and found no vindication during World War II.
1) The armor protection for the Illustrious class was as follows:
The Flight Deck over the hangar consisted of structurally worked 120 lbs. NC armor (3 inches), which was considered proof against 6 inch (100 lbs. projectiles) plunging fire below 23,000 yards and 500 lbs. SAP bombs dropped from 7,000 feet. The hangar sides were 180 lbs. C armor (4.5 inches) and the hangar bulkheads were of 80 lbs. NC plating. This was considered adequate to protect against 6 inch gun fire at ranges over 7,000 yards.
The armor protection for the magazines was 180 lbs. C armor on the ship's side and 120 and 100 lbs. NC armor on the hangar deck.
The side protection system was design to withstand a 750 lbs. TNT contact charge and consisted of an air-liquid-air "sandwich" system with a depth of 14 feet and ran between stations 61 and 121, 240 feet in all, and continuous from outer bottom to 9 inches above the main deck.
The Indomitable class had similar protection except that the hangar side protection was reduced from 180 lbs. to 60 lbs. C armor (1.5 inches).
The protection in Implacable and Indefatigable was also similar to the Illustrious class except that the hangar side and end protection was reduced to 60 lbs. NC while the lower hangar deck (top of citadel) was 60 to 100 lbs. NC armor. Protection to the steering gear was 120 lbs. NC. As a result of trials on Job 74 a protective bulkhead of 55 lbs. was fitted in lieu of the 60 lbs. bulkhead on previous ships.
2) It should be understood that of the eight hits suffered by HMS Illustrious off Malta and the further two hits inflicted while at Malta, only one struck the armored box and that one penetrated the 3 inch deck armor and exploded in the hangar, causing serious damage to the forward lift and a bad fire which destroyed several aircraft. According to D.K. Brown RCNC, writing about the damage in his Warship Issue No. 28 "Attack and Defence" article:
"It is interesting to note that the armoured hangar, provided at such great expense, proved of no value . . . The ship was also lucky in that several hits were close together so that the later ones added little to the earlier damage. On the other hand, to withstand 8 hits and 2 near misses from 500 kg bombs was a great achievement. Her designer, W.A.D. Forbes, Captain Boyd and the crew all had reason for pride in their work."The armored decks of these carriers were intended to resist 500 lbs. bombs, not the 500 kg (1,100 lbs.) bombs that the Germans used during these attacks. The failure of the armored deck on HMS Illustrious to resist the much larger bomb is thus not so surprising.