Comparison of armoured to unarmoured flight deck designs
Encyclopedia
An armoured flight deck is an aircraft carrier
flight deck
that incorporates substantial armour in its design.
Comparison is often made between some of designs of the Royal Navy
(RN) and the United States Navy
(USN). The two navies followed differing philosophies in the use of armour on carrier flight decks starting with the design of the RN's Illustrious
class and ending with the design of the Midway
class, when the USN also adopted armoured flight decks. The two classes most easily compared are the RN's Illustrious
class and Implacable
class and their nearest USN contemporaries, the Yorktown
and Essex
classes. The Illustrious class followed the Yorktown but preceded the Essex, while the Implacable class design predated the Essex but these ships were completed after the lead ships of the Essex class. The development of armoured flight deck carriers proceeded during World War Two and before the end of World War Two, both the USN, with USS Midway
, and the Imperial Japanese Navy
(IJN), with Taiho
and Shinano
, would also commission armoured flight deck carriers, while all USN fleet aircraft carriers built since 1945 feature armoured flight decks. The remainder of the IJN carrier force during World War II had unarmoured flight decks just like the Yorktown and Essex classes of the USN.
The carriers that were built with armoured decks fall into two distinct types - those with armour at the flight deck level protecting the hangar and those that only had armour between the hangar deck and the lower levels of the ship. The different thickness of armour, and how they were distributed, are described in the table below.
s of main deck armour that extended from the base of the hangar side-wall to the top of the main side belt. The latter protected the machinery, magazines and aircraft fuel and weaponry stores. The RN's closed and armoured hangars were capable of being environmentally sealed for protection against chemical weapon attack. The armoured design meant that it would have to be attacked with Armour Piercing (AP) bombs, which have much less blast effect than higher-capacity General Purpose (GP) bombs carrying about twice the explosive amount. GP bombs also caused severe hull damage if they exploded in the water close to the hull; AP bombs, much less so. The USN open hangar design allowed large numbers of aircraft to be warmed up while inside, theoretically reducing the time required to range and launch a strike, but stowage of fuelled and armed aircraft in an unarmoured hangar was extremely dangerous:
During the war, the British fitted immersion heaters to the oil tanks of its aircraft so minimal warm-up was required when they reach the flight deck.
US and earlier Japanese carriers had their armour placed at the hangar deck, essentially treating the hangar spaces and flight deck as superstructure - making these areas very vulnerable to the blast from GP bombs and other explosions, which in turn caused massive casualties in comparison to RN designs. A bomb that struck the flight deck would likely penetrate and explode in the hangar deck, but the armour there could still protect the ship's vitals — including the engine spaces and fuel storage. The flight deck could also possibly fuze light bombs prematurely, which would reduce the chance of them going through the hangar deck. Such a design allowed for larger, open-sided hangar bays (improving ventilation but making the ship very vulnerable to chemical weapon attack) and the installation of deck-edge elevators. USN carriers with hangar deck armour only, usually had wooden decking over thin mild steel flight decks which were easy to repair. The USN moved the structural strength deck to the flight deck, starting with the Forrestal class which had "...an enclosed..." hanger.
Aviation fuel delivery and stowage systems were extremely vulnerable. The Royal Navy stowed aviation fuel in cylindrical tanks, that in turn were surrounded by seawater. RN aviation fuel lines were purged with carbon dioxide
when not in use. The USN used a similar system, which was further improved after the two navies began exchanging information in 1940. Pre-war USN and IJN carrier designs used a fuel stowage system which was not as secure as that used by the RN, but allowed for much greater stowage capacity. Several USN and IJN carriers were lost due to aviation gas fume explosions.
and Mediterranean Sea
, under the umbrella of land-based enemy air forces. By contrast, both the USN and the Royal Navy expected to be operating within the vast expanses of the Pacific Ocean
but the USN did not have to worry about operating in the Mediterranean. The differences in construction were determined by doctrine that was largely driven by the different approaches to the same tactical problem: How to destroy the enemy's aircraft carriers while surviving the inevitable counter strike. Prior to WWII the RN and USN both recognized that the dive bomber could disable the flight decks of enemy aircraft carriers:
The RN was thus faced with designing a carrier that would be survivable under the conditions to be expected in the Atlantic, Mediterranean, and Pacific Oceans, and before the development of effective naval Radar; these conflicting demands resulted in the development of aircraft carriers whose decks were armoured against 500 lb Armour Piercing bombs and 1000 lb General Purpose bombs. The RN considered that an unarmoured carrier would be unlikely to be able fly off more than one deck load of strike aircraft prior to being attacked so the armoured flight deck carriers accepted a reduction in hangar capacity to the equivalent to one deck load of aircraft. USN, IJN some RN Fleet carriers such as Ark Royal had sufficient aircraft capacity to allow for two ranges, each equal to a full deck load of strike aircraft. The RN and IJN limited their aircraft carrier's aircraft capacity to the capacity of their hangars, and struck down all aircraft between operations. The USN, typically, used a permanent deck park to augment the capacity of their aircraft carrier's hangars. The use of a permanent deck park appeared to give USN carriers a much larger aircraft capacity than contemporary RN armoured flight deck carriers. The flight deck armour also reduced the length of the flight deck, reducing the maximum aircraft capacity of the armoured flight deck carrier, however the largest part of the disparity between RN and USN carriers in aircraft capacity was due to the use of a permanent deck park on USN carriers.
The Royal Navy also had the disadvantage that they entered into WWII with the Royal Navy being pitted against large, land based, air-forces whose aircraft also had superior performance to all existing naval aircraft, while the RAF's increased demand for high performance land based aircraft, after the Fall of France, actually retarded the production and development of Fleet Air Arm
aircraft. On the other hand, the RN rapidly introduced new technologies, such as radar which enhanced the defensive capability of aircraft carriers. The RN thus had to develop new operational doctrines during the war. The USN, in contrast, was able to benefit from technology transfers from the UK and the wartime experiences of the RN, which was freely shared with the USN, prior to its entry into the war, allowing it to anticipate the changes needed to prepare its carriers for the coming conflict with Japan. The USN designed the armoured flight decks of the Midway class carriers based upon an analysis of the effectiveness of RN armoured flight decks. The IJN also benefited from being able to observe the effectiveness of RN aircraft carriers in action, while both the USN and IJN were able to introduce new aircraft types, prior to their entry into WWII.
to which they complied, the Indomitable and Implacable-type carriers had to accept a reduction in hangar heights (to keep the metacentric height
within acceptable limits) and size, and as a result, had some restriction on aircraft types supplied via Lend-Lease
. IJN carriers typically had 16 feet (4.9 m) high hangars, including Taiho and Shinano. The USN Lexington class
had 20 ft (6.1 m) hangar heights while the Yorktown, Wasp, Essex, and Midway classes had 17 inch hangar heights.
attacks that actually struck their carriers, but the American carriers primarily relied on fighters to prevent the carriers from being hit in the first place. RN carriers such as Ark Royal or Illustrious had far heavier AA outfits than their USN counterparts, up to the introduction of the USN Essex class carriers. Ark Royal , in 1940, carried 16 x 4.5-inch guns, 32 x 40mm "Pom-pom"
and 32 x 0.5 inch
against 8 x 5", 16 x 28mm and 24 x .5" for Enterprise, in 1940. "In wartime, however, the US Navy found the armoured carriers fascinating. After having examined HMS Formidable in 1940, the US naval attaché commented that, were he crossing the Pacific, he would prefer her to a Yorktown, the closest US equivalent, on the basis that she might carry fewer aircraft, but she would be much more likely to get there". Late in the war when the USN operated many carriers together and had improved radar, their fighter and AA defence was reasonably effective, yet both conventional and kamikaze attacks were still able to penetrate USN defences. Bunker Hill and Franklin nearly succumbed in 1945. The larger air groups (80–110 planes, vs. 52–81 for late war British ships
) allowed for a more effective combat air patrol
(CAP) without reducing strike capability, improving the protection of the whole battle group and lessening the workload of the carrier escorts. Carrier fighters were able to shoot down far more kamikaze aircraft than any amount of deck armour would have protected against showing the value of absolute numbers, but in the early war period IJN aircraft had little difficulty in penetrating USN CAPs; near the end of the war, veteran American fighter pilots in superior F6F Hellcat
and F4U Corsair
fighters were able to defeat the young, inexperienced and ill-trained kamikaze pilots with ease and run up huge kill scores but attackers were still able to get through. (In addition to larger aircraft complements, the US Navy had larger fleets and more resources, so they could establish destroyer pickets and develop dedicated AAW ships such as the Atlanta-class
antiaircraft cruisers which would have also drawn attention away from the carriers.) On the surface, the record seems balanced.
British naval historian D.K. Brown put the practical difference between American and British design philosophies in no uncertain terms: "More fighters
would have been better protection than armour
." but that British designs were good for the circumstances in which they were meant to be used. Yet, even the Ark Royal
, Britain's newest carrier prior to WW2, never operated close to her theoretical aircraft capacity. Prior to the development of effective Radar and high speed monoplane fighters, a successful fighter defence was extremely unlikely for any navy thus calling into doubt D.K. Brown's conclusions. The benefits of flight deck armour were intended to counter these issues. Fewer aircraft meant a lower priority to attack than the more heavily-armed American carriers and the RN's operational doctrine dictated smaller airgroups, and the armoured hangar carriers had smaller Avgas and ammunition supplies to match. However, RN carriers carried far more aircraft later in the war, making use of the deck park when they adopted USN style operational doctrine. The 2nd generation RN armoured carriers, Indomitable and the Implacable class which had an additional half length lower hangar, were considerably less outmatched by their USN counterparts in the numbers of aircraft operated. The RN operating in harsher weather protected their aircraft from the elements and did not use a permanent deck park in the earlier part of the war. US carriers and their fighters shot down more than 1,900 suicide aircraft during Operation Kikusui (the last and largest Kamikaze attack in the Okinawa campaign
), versus a mere 75 for the British, yet both forces suffered the same number of serious hits (four), on their carriers. However the kamikazes made 173 strikes against other USN targets and the 4 USN carriers suffered a massive death toll, in contrast to the relatively light casualties on the RN carriers.
The kamikaze threat overall was serious, but allied defences neutralized it, and many kamikaze strikes missed the deck armour entirely, or bounced off the decks of both British or American carriers. In some cases, kamikazes either struck glancing blows that did only superficial damage that was fixed within minutes or hours, or missed entirely, due to the poor training and poorer flight experience of their pilots. The majority of kamikazes that did inflict harm caused no more damage than they would have against smaller ships. After a successful kamikaze hit, the British were able to clear the flight deck and resume flight operations in just hours, while their American counterparts often could do the same, but not always; in some cases repairs took a few days or even months. The U.S.N liaison officer on HMS Indefatigable
commented: "When a kamikaze hits a U.S. carrier it means 6 months of repair at Pearl [Harbor
]. When a kamikaze hits a Limey carrier it’s just a case of "Sweepers, man your brooms."”
American carriers of the Essex class suffered very high casualties from serious kamikaze hits, though they did survive. USS Franklin
was struck by two 250kg(550lbs) bombs, one semi-armour piercing (SAP) and one general purpose (GP), both of which penetrated into her hanger and set off ammunition, killing 724 personnel. USS Bunker Hill
was severely damaged by pair of kamikaze hits which killed 346 men. Each of these USN carriers suffered more casualties than all the British RN armoured carriers combined, illustrating the life saving features of RN carrier design. Illustrious, which had the highest toll, suffered 126 fatal casualties and 84 wounded when hit by six 1100 lb bombs on January 10, 1941. The USN studied the superior defensive qualities of Royal Navy armoured carriers and this analysis is partly revealed in the damage report following the attack on Franklin
on March 13, 1945:
These comments regarding the effectiveness of the armoured flight deck design are taken from the USS Franklin's damage report where Franklin was severely damaged and 1/3 of the crew killed or wounded by bombs which could not have penetrated the armoured flight deck of a Royal Navy aircraft carrier. This excerpt is an eloquent testimony as to why the USN decided to adopt armoured flight decks on all aircraft carriers starting with the Midway.
The only Allied carrier lost to a deck hit was the American Independence-class
light carrier, USS Princeton
(CVL-23). Indeed, many light and escort carriers were unarmoured, with no protection on the hangar or flight deck, and thus they fared poorly against deck hits.
(R67) was an excellent example of this; while she weathered a severe kamikaze hit in 1945 which cratered her deck armour, the hit caused severe internal structural damage and permanently warped the hull (damage worsened in a postwar aircraft-handling accident wherein a Vought Corsair
rolled off a lift and raked the hangar deck with 20mm cannon fire, causing a severe fire); but plans to rebuild her as per Victorious were abandoned due to budget cuts, not structural damage, and she lingered in reserve until 1956 before being towed off to the breakers. The Royal Navy planned to rebuild most of the armoured carriers in the early post war period:
Illustrious suffered a similar battering, especially off of Malta in 1941 when hit by German dive bombers and late in the the war was limited to 22 knots (41 km/h) because her centreline shaft was disabled due to accumulated wartime damage; she spent five years as a training and trials carrier (1948–53) and was disposed of in 1954. Indomitable was completely refit to like-new condition, only to suffer a severe gasoline explosion onboard, which caused "considerable structural and electrical damage to the ship". Indomitable was refitted between 1948 and 1950 and served as flagship of the Home Fleet then served a tour of duty in the Mediterranean, where she was damaged by the petrol explosion. She was partially repaired before proceeding under her own power to Queen Elizabeth II
's 1953 Coronation Review, before being placed in reserve in 1954. Indomitable was scrapped in 1956. The explosion which occurred on Indomitables hangar deck, while severe, would also have caused severe casualties and extensive damage to an Essex-class carrier, several of which returned to service after hangar explosions, primarily due to the USN's considerable financial and material resources. The postwar Royal Navy could only afford to rebuild Victorious and had to abandon plans to rebuild four other armoured carriers due to cost, and to provide crews to man the postwar built carriers, such as Ark Royal
, due to reductions in manpower.
Another factor is the advantage in resources that the US Navy had over the Royal Navy. The numerous and capacious American yards on the East and West Coasts allowed the US Navy to build and repair carriers at a more leisurely pace and higher quality individually, while producing ships collectively at a furious rate. The British with their strained facilities were forced to rush repairs (indeed the overloaded British shipyards had forced some vessels to be sent to the US for repairs) and some ships such as Illustrious, were forced into service even though not fully repaired. The RN was in a state of continual contraction after WWII, and simply did not have the resources or inclination to repair ships that it could no longer man.
design, the strength deck remained on the hangar level. Midway had originally been planned to have a very heavy gun armament (8 in weapons). The removal of these weapons freed up enough tonnage to add 3 inches (76 mm) of armour at the flight deck level. While this made a great deal of sense from an air group perspective, the Midway ships sat very low in the water for carriers (due to their much greater displacement), certainly much lower than the smaller Essex-class carriers, and had a great deal of difficulty operating in heavy seas. Flight deck armoured ships almost universally (except for the Midway class as completed) possessed a hurricane bow, where the bows were sealed up to the flight deck; wartime experience demonstrated that ships with the hurricane bow configuration (also including the American Lexington class) shipped less water than ships with an open bow. Late-life refits to Midway to bulge her hull and improve freeboard instead gave her a dangerously sharp roll, and made flight operations difficult even in moderate seas. This was therefore not repeated on Coral Sea (Roosevelt had been decommissioned years earlier). After the war, most of the Essex class ships were modified with a hurricane bow and in the case of Oriskany the wooden flight deck surface was replaced with aluminium for improved resistance against the blast of jet engines, making them appear to have armoured flight decks, but in fact their armour remained at hangar level.
The supercarriers of the postwar era, starting with the Forrestal class
— nearly 200 feet (61 m) longer and 40 feet (12.2 m) wider in the beam than their World War II counterparts — would eventually be forced to move the strength deck up to the flight deck level as a result of their great size; a shallow hull of those dimensions became too impractical to continue. The issue of protection had no influence on the change; the Forrestal class had an armoured flight deck of at least 1.5" thickness. Some of the follow on classes to the Forrestals also had armoured flight decks although deck armour is of little to no use against modern anti-ship missiles, it may help limit the damage from flight deck explosions. The experience of World War Two caused the USN to change their design policy in favour of armoured flight decks:
Aircraft carrier
An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...
flight deck
Flight deck
The flight deck of an aircraft carrier is the surface from which its aircraft take off and land, essentially a miniature airfield at sea. On smaller naval ships which do not have aviation as a primary mission, the landing area for helicopters and other VTOL aircraft is also referred to as the...
that incorporates substantial armour in its design.
Comparison is often made between some of designs of the Royal Navy
Royal Navy
The Royal Navy is the naval warfare service branch of the British Armed Forces. Founded in the 16th century, it is the oldest service branch and is known as the Senior Service...
(RN) and the United States Navy
United States Navy
The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...
(USN). The two navies followed differing philosophies in the use of armour on carrier flight decks starting with the design of the RN's Illustrious
Illustrious class aircraft carrier
The Illustrious class was a class of aircraft carrier of the Royal Navy that were some of the most important British warships in World War II...
class and ending with the design of the Midway
Midway class aircraft carrier
The Midway class aircraft carrier was one of the longest lived carrier designs in history. First commissioned in late 1945, the lead ship of the class, was not decommissioned until 1992, shortly after service in Operation Desert Storm in 1991.-History:...
class, when the USN also adopted armoured flight decks. The two classes most easily compared are the RN's Illustrious
Illustrious class aircraft carrier
The Illustrious class was a class of aircraft carrier of the Royal Navy that were some of the most important British warships in World War II...
class and Implacable
Implacable class aircraft carrier
The Implacable class was a class of aircraft carrier employed by the Royal Navy in the second half of Second World War. The class design was derived from the .- Design :...
class and their nearest USN contemporaries, the Yorktown
Yorktown class aircraft carrier
The Yorktown class was a class of three aircraft carriers built by the U.S. and completed shortly before World War II. They bore the brunt of early action in that war, and the sole survivor of the class was to become the most decorated ship in the history of the U.S...
and Essex
Essex class aircraft carrier
The Essex class was a class of aircraft carriers of the United States Navy, which constituted the 20th century's most numerous class of capital ships with 24 vessels built in both "short-hull" and "long-hull" versions. Thirty-two were originally ordered; however as World War II wound down, six were...
classes. The Illustrious class followed the Yorktown but preceded the Essex, while the Implacable class design predated the Essex but these ships were completed after the lead ships of the Essex class. The development of armoured flight deck carriers proceeded during World War Two and before the end of World War Two, both the USN, with USS Midway
USS Midway (CV-41)
USS Midway was an aircraft carrier of the United States Navy, the lead ship of her class, and the first to be commissioned after the end of World War II...
, and the Imperial Japanese Navy
Imperial Japanese Navy
The Imperial Japanese Navy was the navy of the Empire of Japan from 1869 until 1947, when it was dissolved following Japan's constitutional renunciation of the use of force as a means of settling international disputes...
(IJN), with Taiho
Japanese aircraft carrier Taiho
was an aircraft carrier of the Imperial Japanese Navy during World War II. With a heavily armored hull and flight deck , she represented a major departure in Japanese carrier design and was expected to not only survive multiple bomb, torpedo or shell hits but also continue fighting effectively...
and Shinano
Japanese aircraft carrier Shinano
named after the ancient Shinano Province, was an aircraft carrier of the Imperial Japanese Navy during World War II. Initially laid down as the third of the battleships, Shinano′s partially complete hull was converted to an aircraft carrier in 1942, midway through construction. Over the next two...
, would also commission armoured flight deck carriers, while all USN fleet aircraft carriers built since 1945 feature armoured flight decks. The remainder of the IJN carrier force during World War II had unarmoured flight decks just like the Yorktown and Essex classes of the USN.
Design
In choosing the best design for their carriers, the British had to consider the advantages and disadvantages of hangar design. There was a choice between open or closed hangar and the position of the armour. The placing of the strongest deck affected the strength of the hull. The further apart the deck and the keel, the stronger the design. If the flight deck was placed above the main deck then it had to be built to allow for movement with expansion sections. A closed hangar design was the strongest structurally and made for a lighter hull and the RN carried this concept one step further and designed the armoured flight deck to also act as the strength deck without any underlying plating, thus achieving an armoured flight deck on the lowest possible displacement.The carriers that were built with armoured decks fall into two distinct types - those with armour at the flight deck level protecting the hangar and those that only had armour between the hangar deck and the lower levels of the ship. The different thickness of armour, and how they were distributed, are described in the table below.
| Class (standard displacement) | Flight deck | Main deck | Side belt |
|---|---|---|---|
 Yorktown classYorktown class aircraft carrier The Yorktown class was a class of three aircraft carriers built by the U.S. and completed shortly before World War II. They bore the brunt of early action in that war, and the sole survivor of the class was to become the most decorated ship in the history of the U.S... (19,875 tons) |
n/a | 1.5 in (38 mm) | 2.5 – 4 in (64 – 101 mm) |
| Ark Royal class HMS Ark Royal (91) HMS Ark Royal was an aircraft carrier of the Royal Navy that served during the Second World War.Designed in 1934 to fit the restrictions of the Washington Naval Treaty, Ark Royal was built by Cammell Laird and Company, Ltd. at Birkenhead, England, and completed in November 1938. Her design... (22,000 tons) |
.75 in (19 mm) | 3.5 in (89 mm) | 4.5 in (114 mm) |
| Illustrious class Illustrious class aircraft carrier The Illustrious class was a class of aircraft carrier of the Royal Navy that were some of the most important British warships in World War II... (23,000 tons) |
3 in (76 mm) | 2 in (51 mm) | 4.5 in (114 mm) |
 Shōkaku classJapanese aircraft carrier Shokaku Shōkaku was an aircraft carrier of the Imperial Japanese Navy, the lead ship of her class. Along with her sister ship , she took part in several key naval battles during the Pacific War, including the attack on Pearl Harbor, the Battle of the Coral Sea and the Battle of the Santa Cruz Islands... (25,600 tons) |
n/a | 3.9 in over machinery (99 mm) | 1.8 in (46 mm) |
| Essex class Essex class aircraft carrier The Essex class was a class of aircraft carriers of the United States Navy, which constituted the 20th century's most numerous class of capital ships with 24 vessels built in both "short-hull" and "long-hull" versions. Thirty-two were originally ordered; however as World War II wound down, six were... (27,200 tons) |
n/a | 2.5 in (64 mm) | 4.75 in (121 mm) |
| Taiho Japanese aircraft carrier Taiho was an aircraft carrier of the Imperial Japanese Navy during World War II. With a heavily armored hull and flight deck , she represented a major departure in Japanese carrier design and was expected to not only survive multiple bomb, torpedo or shell hits but also continue fighting effectively... (29,770 tons) |
3.1 in (80 mm) | 1.3 in (32 mm) | 2.2 in abreast machinery(60 mm) |
| Implacable class Implacable class aircraft carrier The Implacable class was a class of aircraft carrier employed by the Royal Navy in the second half of Second World War. The class design was derived from the .- Design :... (23,500 tons) |
3 in (76 mm) | 2.5 in (64 mm) | 4.5 in (114 mm) |
| Shinano Japanese aircraft carrier Shinano named after the ancient Shinano Province, was an aircraft carrier of the Imperial Japanese Navy during World War II. Initially laid down as the third of the battleships, Shinano′s partially complete hull was converted to an aircraft carrier in 1942, midway through construction. Over the next two... (62,000 tons) |
3.1 in (80 mm) | 3.9 in (99 mm) | 5 in (127 mm) |
| Midway Midway class aircraft carrier The Midway class aircraft carrier was one of the longest lived carrier designs in history. First commissioned in late 1945, the lead ship of the class, was not decommissioned until 1992, shortly after service in Operation Desert Storm in 1991.-History:... (45,000 tons) |
3.5 in (89 mm) | 2 in (51 mm) | 7.9 in (200 mm) |
Theory
Armour at the flight deck level would protect the hangar deck and the aircraft stored there from most bombs. The armour of the Illustrious class was intended to protect against 1,000 pound bombs. In the Illustrious class, the armoured flight deck extended for about two-thirds of the length of the ship, bounded by the two aircraft lifts (which were without the armour). The deck was closed by 4.5 inches (114.3 mm) armoured sides and bulkheads, forming an armoured box. The bulkheads had sliding armoured portals to allow access between the hangar and the aircraft lift. There were 3 inches (76.2 mm) lateral strakeStrake
A strake is part of the shell of the hull of a boat or ship which, in conjunction with the other strakes, keeps the sea out and the vessel afloat...
s of main deck armour that extended from the base of the hangar side-wall to the top of the main side belt. The latter protected the machinery, magazines and aircraft fuel and weaponry stores. The RN's closed and armoured hangars were capable of being environmentally sealed for protection against chemical weapon attack. The armoured design meant that it would have to be attacked with Armour Piercing (AP) bombs, which have much less blast effect than higher-capacity General Purpose (GP) bombs carrying about twice the explosive amount. GP bombs also caused severe hull damage if they exploded in the water close to the hull; AP bombs, much less so. The USN open hangar design allowed large numbers of aircraft to be warmed up while inside, theoretically reducing the time required to range and launch a strike, but stowage of fuelled and armed aircraft in an unarmoured hangar was extremely dangerous:
During the war, the British fitted immersion heaters to the oil tanks of its aircraft so minimal warm-up was required when they reach the flight deck.
US and earlier Japanese carriers had their armour placed at the hangar deck, essentially treating the hangar spaces and flight deck as superstructure - making these areas very vulnerable to the blast from GP bombs and other explosions, which in turn caused massive casualties in comparison to RN designs. A bomb that struck the flight deck would likely penetrate and explode in the hangar deck, but the armour there could still protect the ship's vitals — including the engine spaces and fuel storage. The flight deck could also possibly fuze light bombs prematurely, which would reduce the chance of them going through the hangar deck. Such a design allowed for larger, open-sided hangar bays (improving ventilation but making the ship very vulnerable to chemical weapon attack) and the installation of deck-edge elevators. USN carriers with hangar deck armour only, usually had wooden decking over thin mild steel flight decks which were easy to repair. The USN moved the structural strength deck to the flight deck, starting with the Forrestal class which had "...an enclosed..." hanger.
Aviation fuel delivery and stowage systems were extremely vulnerable. The Royal Navy stowed aviation fuel in cylindrical tanks, that in turn were surrounded by seawater. RN aviation fuel lines were purged with carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
when not in use. The USN used a similar system, which was further improved after the two navies began exchanging information in 1940. Pre-war USN and IJN carrier designs used a fuel stowage system which was not as secure as that used by the RN, but allowed for much greater stowage capacity. Several USN and IJN carriers were lost due to aviation gas fume explosions.
Doctrine and design
The Royal Navy had to be ready to fight a war in the confines of the North SeaNorth Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...
and Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...
, under the umbrella of land-based enemy air forces. By contrast, both the USN and the Royal Navy expected to be operating within the vast expanses of the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
but the USN did not have to worry about operating in the Mediterranean. The differences in construction were determined by doctrine that was largely driven by the different approaches to the same tactical problem: How to destroy the enemy's aircraft carriers while surviving the inevitable counter strike. Prior to WWII the RN and USN both recognized that the dive bomber could disable the flight decks of enemy aircraft carriers:
The RN was thus faced with designing a carrier that would be survivable under the conditions to be expected in the Atlantic, Mediterranean, and Pacific Oceans, and before the development of effective naval Radar; these conflicting demands resulted in the development of aircraft carriers whose decks were armoured against 500 lb Armour Piercing bombs and 1000 lb General Purpose bombs. The RN considered that an unarmoured carrier would be unlikely to be able fly off more than one deck load of strike aircraft prior to being attacked so the armoured flight deck carriers accepted a reduction in hangar capacity to the equivalent to one deck load of aircraft. USN, IJN some RN Fleet carriers such as Ark Royal had sufficient aircraft capacity to allow for two ranges, each equal to a full deck load of strike aircraft. The RN and IJN limited their aircraft carrier's aircraft capacity to the capacity of their hangars, and struck down all aircraft between operations. The USN, typically, used a permanent deck park to augment the capacity of their aircraft carrier's hangars. The use of a permanent deck park appeared to give USN carriers a much larger aircraft capacity than contemporary RN armoured flight deck carriers. The flight deck armour also reduced the length of the flight deck, reducing the maximum aircraft capacity of the armoured flight deck carrier, however the largest part of the disparity between RN and USN carriers in aircraft capacity was due to the use of a permanent deck park on USN carriers.
The Royal Navy also had the disadvantage that they entered into WWII with the Royal Navy being pitted against large, land based, air-forces whose aircraft also had superior performance to all existing naval aircraft, while the RAF's increased demand for high performance land based aircraft, after the Fall of France, actually retarded the production and development of Fleet Air Arm
Fleet Air Arm
The Fleet Air Arm is the branch of the British Royal Navy responsible for the operation of naval aircraft. The Fleet Air Arm currently operates the AgustaWestland Merlin, Westland Sea King and Westland Lynx helicopters...
aircraft. On the other hand, the RN rapidly introduced new technologies, such as radar which enhanced the defensive capability of aircraft carriers. The RN thus had to develop new operational doctrines during the war. The USN, in contrast, was able to benefit from technology transfers from the UK and the wartime experiences of the RN, which was freely shared with the USN, prior to its entry into the war, allowing it to anticipate the changes needed to prepare its carriers for the coming conflict with Japan. The USN designed the armoured flight decks of the Midway class carriers based upon an analysis of the effectiveness of RN armoured flight decks. The IJN also benefited from being able to observe the effectiveness of RN aircraft carriers in action, while both the USN and IJN were able to introduce new aircraft types, prior to their entry into WWII.
Aircraft restrictions
All RN fleet carriers had 16 ft (4.9 m) hangar heights, except the two Implacable class ships, which had 14 ft (4.3 m) heights, and Indomitable which had a 16 ft (4.9 m) lower hangar and a 14 ft (4.3 m) upper hangar. The Illustrious class had a single 16 ft (4.9 m) high hangar that was 468 ft (142.6 m) long. Within the confines of ship design, and the Second London Naval TreatySecond London Naval Treaty
The Second London Naval Disarmament Conference opened in London, the United Kingdom, on 9 December 1935. It resulted in the Second London Naval Treaty which was signed on 25 March 1936.- Description :...
to which they complied, the Indomitable and Implacable-type carriers had to accept a reduction in hangar heights (to keep the metacentric height
Metacentric height
The metacentric height is a measurement of the static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre . A larger metacentric height implies greater stability against overturning...
within acceptable limits) and size, and as a result, had some restriction on aircraft types supplied via Lend-Lease
Lend-Lease
Lend-Lease was the program under which the United States of America supplied the United Kingdom, the Soviet Union, China, Free France, and other Allied nations with materiel between 1941 and 1945. It was signed into law on March 11, 1941, a year and a half after the outbreak of war in Europe in...
. IJN carriers typically had 16 feet (4.9 m) high hangars, including Taiho and Shinano. The USN Lexington class
Lexington class aircraft carrier
The Lexington class aircraft carriers were the first operational aircraft carriers in the United States Navy. There were two ships in the class: and ....
had 20 ft (6.1 m) hangar heights while the Yorktown, Wasp, Essex, and Midway classes had 17 inch hangar heights.
Defences
The British approach of armoured flight decks was an effective form of passive defence from bombs and kamikazeKamikaze
The were suicide attacks by military aviators from the Empire of Japan against Allied naval vessels in the closing stages of the Pacific campaign of World War II, designed to destroy as many warships as possible....
attacks that actually struck their carriers, but the American carriers primarily relied on fighters to prevent the carriers from being hit in the first place. RN carriers such as Ark Royal or Illustrious had far heavier AA outfits than their USN counterparts, up to the introduction of the USN Essex class carriers. Ark Royal , in 1940, carried 16 x 4.5-inch guns, 32 x 40mm "Pom-pom"
QF 2 pounder naval gun
The 2-pounder gun, officially designated the QF 2-pounder and universally known as the pom-pom, was a 1.575 inch British autocannon, used famously as an anti-aircraft gun by the Royal Navy. The name came from the sound that the original models make when firing...
and 32 x 0.5 inch
Vickers machine gun
Not to be confused with the Vickers light machine gunThe Vickers machine gun or Vickers gun is a name primarily used to refer to the water-cooled .303 inch machine gun produced by Vickers Limited, originally for the British Army...
against 8 x 5", 16 x 28mm and 24 x .5" for Enterprise, in 1940. "In wartime, however, the US Navy found the armoured carriers fascinating. After having examined HMS Formidable in 1940, the US naval attaché commented that, were he crossing the Pacific, he would prefer her to a Yorktown, the closest US equivalent, on the basis that she might carry fewer aircraft, but she would be much more likely to get there". Late in the war when the USN operated many carriers together and had improved radar, their fighter and AA defence was reasonably effective, yet both conventional and kamikaze attacks were still able to penetrate USN defences. Bunker Hill and Franklin nearly succumbed in 1945. The larger air groups (80–110 planes, vs. 52–81 for late war British ships
Implacable class aircraft carrier
The Implacable class was a class of aircraft carrier employed by the Royal Navy in the second half of Second World War. The class design was derived from the .- Design :...
) allowed for a more effective combat air patrol
Combat air patrol
Combat air patrol is a type of flying mission for fighter aircraft.A combat air patrol is an aircraft patrol provided over an objective area, over the force protected, over the critical area of a combat zone, or over an air defense area, for the purpose of intercepting and destroying hostile...
(CAP) without reducing strike capability, improving the protection of the whole battle group and lessening the workload of the carrier escorts. Carrier fighters were able to shoot down far more kamikaze aircraft than any amount of deck armour would have protected against showing the value of absolute numbers, but in the early war period IJN aircraft had little difficulty in penetrating USN CAPs; near the end of the war, veteran American fighter pilots in superior F6F Hellcat
F6F Hellcat
The Grumman F6F Hellcat was a carrier-based fighter aircraft developed to replace the earlier F4F Wildcat in United States Navy service. Although the F6F resembled the Wildcat, it was a completely new design powered by a 2,000 hp Pratt & Whitney R-2800. Some tagged it as the "Wildcat's big...
and F4U Corsair
F4U Corsair
The Vought F4U Corsair was a carrier-capable fighter aircraft that saw service primarily in World War II and the Korean War. Demand for the aircraft soon overwhelmed Vought's manufacturing capability, resulting in production by Goodyear and Brewster: Goodyear-built Corsairs were designated FG and...
fighters were able to defeat the young, inexperienced and ill-trained kamikaze pilots with ease and run up huge kill scores but attackers were still able to get through. (In addition to larger aircraft complements, the US Navy had larger fleets and more resources, so they could establish destroyer pickets and develop dedicated AAW ships such as the Atlanta-class
Atlanta class cruiser
The Atlanta-class cruisers were United States Navy light cruisers originally designed as fast scout cruisers or flotilla leaders, but later proved to be effective anti-aircraft cruisers during World War II. They were also known as the Atlanta-Oakland class. The lead ship Atlanta was sunk in action...
antiaircraft cruisers which would have also drawn attention away from the carriers.) On the surface, the record seems balanced.
British naval historian D.K. Brown put the practical difference between American and British design philosophies in no uncertain terms: "More fighters
Fighter aircraft
A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...
would have been better protection than armour
Armour
Armour or armor is protective covering used to prevent damage from being inflicted to an object, individual or a vehicle through use of direct contact weapons or projectiles, usually during combat, or from damage caused by a potentially dangerous environment or action...
." but that British designs were good for the circumstances in which they were meant to be used. Yet, even the Ark Royal
HMS Ark Royal (91)
HMS Ark Royal was an aircraft carrier of the Royal Navy that served during the Second World War.Designed in 1934 to fit the restrictions of the Washington Naval Treaty, Ark Royal was built by Cammell Laird and Company, Ltd. at Birkenhead, England, and completed in November 1938. Her design...
, Britain's newest carrier prior to WW2, never operated close to her theoretical aircraft capacity. Prior to the development of effective Radar and high speed monoplane fighters, a successful fighter defence was extremely unlikely for any navy thus calling into doubt D.K. Brown's conclusions. The benefits of flight deck armour were intended to counter these issues. Fewer aircraft meant a lower priority to attack than the more heavily-armed American carriers and the RN's operational doctrine dictated smaller airgroups, and the armoured hangar carriers had smaller Avgas and ammunition supplies to match. However, RN carriers carried far more aircraft later in the war, making use of the deck park when they adopted USN style operational doctrine. The 2nd generation RN armoured carriers, Indomitable and the Implacable class which had an additional half length lower hangar, were considerably less outmatched by their USN counterparts in the numbers of aircraft operated. The RN operating in harsher weather protected their aircraft from the elements and did not use a permanent deck park in the earlier part of the war. US carriers and their fighters shot down more than 1,900 suicide aircraft during Operation Kikusui (the last and largest Kamikaze attack in the Okinawa campaign
Battle of Okinawa
The Battle of Okinawa, codenamed Operation Iceberg, was fought on the Ryukyu Islands of Okinawa and was the largest amphibious assault in the Pacific War of World War II. The 82-day-long battle lasted from early April until mid-June 1945...
), versus a mere 75 for the British, yet both forces suffered the same number of serious hits (four), on their carriers. However the kamikazes made 173 strikes against other USN targets and the 4 USN carriers suffered a massive death toll, in contrast to the relatively light casualties on the RN carriers.
HMS Indefatigable (R10)
HMS Indefatigable was an Implacable-class aircraft carrier of the British Royal Navy. Indefatigable was present at the formal surrender of the Japanese on 2 September in Tokyo Bay. She later helped to repatriate Allied POWs held in Japan and was used as a spotting ship for later US nuclear tests in...
commented: "When a kamikaze hits a U.S. carrier it means 6 months of repair at Pearl [Harbor
Pearl Harbor
Pearl Harbor, known to Hawaiians as Puuloa, is a lagoon harbor on the island of Oahu, Hawaii, west of Honolulu. Much of the harbor and surrounding lands is a United States Navy deep-water naval base. It is also the headquarters of the U.S. Pacific Fleet...
]. When a kamikaze hits a Limey carrier it’s just a case of "Sweepers, man your brooms."”
American carriers of the Essex class suffered very high casualties from serious kamikaze hits, though they did survive. USS Franklin
USS Franklin (CV-13)
The USS Franklin , nicknamed "Big Ben," was one of 24 s built during World War II for the United States Navy, and the fifth US Navy ship to bear the name. Commissioned in January 1944, she served in several campaigns in the Pacific Theater of Operations, earning four battle stars...
was struck by two 250kg(550lbs) bombs, one semi-armour piercing (SAP) and one general purpose (GP), both of which penetrated into her hanger and set off ammunition, killing 724 personnel. USS Bunker Hill
USS Bunker Hill (CV-17)
USS Bunker Hill was one of 24 s built during World War II for the United States Navy. The ship, the second US Navy ship to bear the name, was named for the Battle of Bunker Hill. Bunker Hill was commissioned in May 1943, and served in several campaigns in the Pacific Theater of Operations, earning...
was severely damaged by pair of kamikaze hits which killed 346 men. Each of these USN carriers suffered more casualties than all the British RN armoured carriers combined, illustrating the life saving features of RN carrier design. Illustrious, which had the highest toll, suffered 126 fatal casualties and 84 wounded when hit by six 1100 lb bombs on January 10, 1941. The USN studied the superior defensive qualities of Royal Navy armoured carriers and this analysis is partly revealed in the damage report following the attack on Franklin
USS Franklin (CV-13)
The USS Franklin , nicknamed "Big Ben," was one of 24 s built during World War II for the United States Navy, and the fifth US Navy ship to bear the name. Commissioned in January 1944, she served in several campaigns in the Pacific Theater of Operations, earning four battle stars...
on March 13, 1945:
These comments regarding the effectiveness of the armoured flight deck design are taken from the USS Franklin's damage report where Franklin was severely damaged and 1/3 of the crew killed or wounded by bombs which could not have penetrated the armoured flight deck of a Royal Navy aircraft carrier. This excerpt is an eloquent testimony as to why the USN decided to adopt armoured flight decks on all aircraft carriers starting with the Midway.
The only Allied carrier lost to a deck hit was the American Independence-class
Independence class aircraft carrier
The Independence class aircraft carriers were a class of light carriers built for the United States Navy that served during World War II.This class were a result of President Franklin D. Roosevelt's interest in Navy shipbuilding plans...
light carrier, USS Princeton
USS Princeton (CVL-23)
The fourth USS Princeton was a United States Navy lost at the battle of Leyte Gulf in 1944.-Construction and deployment:The ship was laid down as the Tallahassee by the New York Shipbuilding Corporation, Camden, New Jersey, 2 June 1941...
(CVL-23). Indeed, many light and escort carriers were unarmoured, with no protection on the hangar or flight deck, and thus they fared poorly against deck hits.
Post war analysis
What was not discovered until late in the war was that the kamikaze impacts proved to have a long term effect on the structural integrity of British carriers and their post war life was shortened, as the RN lacked the financial resources to repair the damage, in contrast to the vast resources of the USN which was even able to rebuild carriers such as Franklin that had been completely gutted and the crew decimated by IJN attacks. HMS FormidableHMS Formidable (R67)
HMS Formidable was an Illustrious class aircraft carrier of the Royal Navy during World War II. She was constructed by Harland & Wolff, Belfast, and launched on 17 August 1939...
(R67) was an excellent example of this; while she weathered a severe kamikaze hit in 1945 which cratered her deck armour, the hit caused severe internal structural damage and permanently warped the hull (damage worsened in a postwar aircraft-handling accident wherein a Vought Corsair
Vought Corsair
Vought Corsair is the name of several former aircraft of the US Navy*Vought O2U Corsair, a biplane scout and observation aircraft.*Vought F4U Corsair, a single-engined, single-seat shipborne fighter*LTV A-7 Corsair II, a single-seat light jet attack aircraft...
rolled off a lift and raked the hangar deck with 20mm cannon fire, causing a severe fire); but plans to rebuild her as per Victorious were abandoned due to budget cuts, not structural damage, and she lingered in reserve until 1956 before being towed off to the breakers. The Royal Navy planned to rebuild most of the armoured carriers in the early post war period:
Illustrious suffered a similar battering, especially off of Malta in 1941 when hit by German dive bombers and late in the the war was limited to 22 knots (41 km/h) because her centreline shaft was disabled due to accumulated wartime damage; she spent five years as a training and trials carrier (1948–53) and was disposed of in 1954. Indomitable was completely refit to like-new condition, only to suffer a severe gasoline explosion onboard, which caused "considerable structural and electrical damage to the ship". Indomitable was refitted between 1948 and 1950 and served as flagship of the Home Fleet then served a tour of duty in the Mediterranean, where she was damaged by the petrol explosion. She was partially repaired before proceeding under her own power to Queen Elizabeth II
Elizabeth II of the United Kingdom
Elizabeth II is the constitutional monarch of 16 sovereign states known as the Commonwealth realms: the United Kingdom, Canada, Australia, New Zealand, Jamaica, Barbados, the Bahamas, Grenada, Papua New Guinea, the Solomon Islands, Tuvalu, Saint Lucia, Saint Vincent and the Grenadines, Belize,...
's 1953 Coronation Review, before being placed in reserve in 1954. Indomitable was scrapped in 1956. The explosion which occurred on Indomitables hangar deck, while severe, would also have caused severe casualties and extensive damage to an Essex-class carrier, several of which returned to service after hangar explosions, primarily due to the USN's considerable financial and material resources. The postwar Royal Navy could only afford to rebuild Victorious and had to abandon plans to rebuild four other armoured carriers due to cost, and to provide crews to man the postwar built carriers, such as Ark Royal
HMS Ark Royal (R09)
HMS Ark Royal was an Audacious-class aircraft carrier of the Royal Navy and, when she was decommissioned in 1978, was the Royal Navy's last remaining conventional catapult and arrested-landing aircraft carrier...
, due to reductions in manpower.
Another factor is the advantage in resources that the US Navy had over the Royal Navy. The numerous and capacious American yards on the East and West Coasts allowed the US Navy to build and repair carriers at a more leisurely pace and higher quality individually, while producing ships collectively at a furious rate. The British with their strained facilities were forced to rush repairs (indeed the overloaded British shipyards had forced some vessels to be sent to the US for repairs) and some ships such as Illustrious, were forced into service even though not fully repaired. The RN was in a state of continual contraction after WWII, and simply did not have the resources or inclination to repair ships that it could no longer man.
Midway and Forrestal classes
While flight deck level armour was eventually adopted by the Americans for the MidwayMidway class aircraft carrier
The Midway class aircraft carrier was one of the longest lived carrier designs in history. First commissioned in late 1945, the lead ship of the class, was not decommissioned until 1992, shortly after service in Operation Desert Storm in 1991.-History:...
design, the strength deck remained on the hangar level. Midway had originally been planned to have a very heavy gun armament (8 in weapons). The removal of these weapons freed up enough tonnage to add 3 inches (76 mm) of armour at the flight deck level. While this made a great deal of sense from an air group perspective, the Midway ships sat very low in the water for carriers (due to their much greater displacement), certainly much lower than the smaller Essex-class carriers, and had a great deal of difficulty operating in heavy seas. Flight deck armoured ships almost universally (except for the Midway class as completed) possessed a hurricane bow, where the bows were sealed up to the flight deck; wartime experience demonstrated that ships with the hurricane bow configuration (also including the American Lexington class) shipped less water than ships with an open bow. Late-life refits to Midway to bulge her hull and improve freeboard instead gave her a dangerously sharp roll, and made flight operations difficult even in moderate seas. This was therefore not repeated on Coral Sea (Roosevelt had been decommissioned years earlier). After the war, most of the Essex class ships were modified with a hurricane bow and in the case of Oriskany the wooden flight deck surface was replaced with aluminium for improved resistance against the blast of jet engines, making them appear to have armoured flight decks, but in fact their armour remained at hangar level.
The supercarriers of the postwar era, starting with the Forrestal class
Forrestal class aircraft carrier
The Forrestal-class aircraft carriers were a four-ship class designed and built for the United States Navy in the 1950s. It was the first class of so-called supercarriers, combining high tonnage, deck-edge elevators and an angled deck...
— nearly 200 feet (61 m) longer and 40 feet (12.2 m) wider in the beam than their World War II counterparts — would eventually be forced to move the strength deck up to the flight deck level as a result of their great size; a shallow hull of those dimensions became too impractical to continue. The issue of protection had no influence on the change; the Forrestal class had an armoured flight deck of at least 1.5" thickness. Some of the follow on classes to the Forrestals also had armoured flight decks although deck armour is of little to no use against modern anti-ship missiles, it may help limit the damage from flight deck explosions. The experience of World War Two caused the USN to change their design policy in favour of armoured flight decks: