06 July 2015

The Origins and Development of the American Escort Carrier

The USS Long Island as built, the first escort carrier
At the start of the Second World War with the German invasion of Poland in September 1939, the Kriegsmarine had fifty-seven U-boats in its fleet, but of those fifty-seven submarines, only twenty-two of them were suitable enough for carrying out combat patrols in the Atlantic and that small group consisted of the 626-ton Type VII boat and the larger 1,032 ton Type IX boat.  The Type VII was the most common U-boat used in the Battle of the Atlantic and with 703 hulls constructed, was the most widely built submarine class in naval history. The larger Type IX submarine was designed for extended long range patrols but lacked the maneuverability of the Type VII. Approximately 283 hulls of this larger class were built during the Second World War. Prior to the onset of the war, the head of the U-boat force, Rear Admiral Karl Doenitz, had stated that he would need at last ninety operational boats for the Atlantic at the start of hostilities and that eventually 300 would be needed to guarantee that he could choke off Great Britain's Atlantic supply routes. Fortunately for the Allies, Hitler didn't believe that the war would take long and dragged his feet on authorizing rapid expansion of the U-boat force for some time. The Atlantic U-boat menace is the context to understand the development of the escort carrier- in fact, as it would turn out, the escort carrier was one of the keys to victory in the Battle of the Atlantic. 

Led by the British construction of the HMS Argus which was commissioned in 1918 as the world's first aircraft carrier with a full-length deck (thereby setting the pattern of the aircraft carrier configuration) after conversion from a partially completed ocean liner, the United States Navy had the collier USS Jupiter converted to the first American aircraft carrier, the USS Langley, which was commissioned in 1922. Compared to the large fleet carriers that would become famous in the Second World War, both the HMS Argus and the USS Langley were much smaller vessels with the Argus coming in at 15,775 tons and the Langley at 11,050 tons. But both navies gained valuable experience in operating the ships, the US Navy in particular using the Langley to fine tune procedures for high tempo flight operations. The interwar period was a time of transition for both the US Navy and the Royal Navy as they transitioned from big gun warships to aircraft carriers- during the First World War, the first aircraft carrier in naval history, the HMS Furious, had started out as a cruiser converted with a partial flight deck before getting upgraded to a the pattern set by the HMS Argus with a full length deck. During it's time with a partial flight deck, though, the Furious did retain its aft gun turret, leading to some discussions in the Royal Navy about hybrid cruisers- vessels that had partial flight decks with cruiser guns. The ideas persisted into the 1920s and with the limits on battleship and aircraft carrier tonnage by the 1922 Washington Naval Treaty, the ideas of small aircraft carriers arose as a way of making better use of the allowed tonnage. A loophole in the treaty allowed the United States a chance to use up to 25% of its allotted cruiser tonnage for conversion to small aircraft carriers, but this was an idea that many "Big Gun" admirals were reticent to pursue. 

Bruce G. Leighton, early escort carrier advocate
In 1927, USN Lieutenant Commander Bruce G. Leighton wrote an influential paper on light carriers- he had presciently described roles for such carriers to include antisubmarine warfare, supporting larger fleet carriers, scouting and reconnaissance and support of amphibious landings while larger carriers could be freed up to hunt down the enemy's capital ships. He was also motivated by a concern that the loss of a large fleet carrier would be a bigger blow to the Navy's strength than the loss of a smaller carrier based on a cruiser hull. At the time of Leighton's paper, the USS Lexington and the USS Saratoga had just become operational and even though both vessels were much larger and more useful operationally than the USS Langley, they were still just the second and third aircraft carriers in the US fleet. 

With the clouds of war looming on the horizon and Japan pursuing an aggressive expansionist foreign policy in the Western Pacific, in 1935 the US Navy's Bureau of Construction and Repair had been considering conversion of ten fast passenger liners into light carriers. Conversion of an existing hull was believed to be the most expeditious way to get hulls into the fleet. The plans for light carriers might have died off had it not been for men like John S. McCain, the captain of the fourth US aircraft carrier built, the USS Ranger (the Ranger was first US carrier built from the keel up as an aircraft carrier; McCain was the grandfather of Arizona Senator John McCain). He and others in the fleet had wanted up until 1939 approximately eight "pocket-sized" aircraft carriers based on cruiser hulls that could act as outer defensive screens for the larger fleet carriers. The outbreak of the war in Europe furthered ideas of small aircraft carriers in the US Navy as observers watched the British battle the U-boats that were menacing their trans-Atlantic supply lines. Originally the USN thought that small aircraft carriers might have a dual role in providing air cover to the convoys in the Atlantic as well as delivering much-needed Lend-Lease aircraft to the Royal Air Force. As it would turn out, the cause of what would become the escort carrier got its biggest boost in October 1939 from President Franklin D. Roosevelt. He was a lifelong enthusiast and student of naval history and served as Assistant Secretary of the Navy from 1913-1918 under President Woodrow Wilson. Well-versed in naval affairs, Roosevelt instructed the US Navy to procure a merchant ship for conversion to an aircraft carrier. The ship was to displace 6,000 to 8,000 tons and have a speed of 15 knots and operate a unique air wing made up of either 12 helicopters or autogyros. The vessel would act as a convoy escort and perform antisubmarine warfare missions- the helicopters or autogyros wouldn't carry weapons, but use smoke bombs to mark U-boat locations for attack by destroyers.

Discussions with the Chief of Naval Operations, Admiral Harold Stark, in the weeks following Pearl Harbor led to the decision to obtain two diesel-powered C3-class merchant vessels from the Maritime Commission. The C3 class was designed by the US Maritime Commission as a 492-foot (150m) general purpose vessel that could be easily modified to a variety of roles. Helicopters and autogyros were ruled out as it was felt fixed-wing aircraft were more flexible and capable compared to the rotary winged aircraft of the day. This dictated the need for a full length flight deck. The Navy told the President that conversion to an escort carrier would take 18 months, but to Roosevelt, this was unacceptable. The speed of conversion was essential. He told Admiral Stark that the conversion should take no more than three months. At end of January 1940 two vessels were secured from the Moore-McCormick Line, the Mormacmail and the Mormacland. One ship after conversion would go to the US Navy and the other ship after conversion would go to the Royal Navy. The only organic air cover the British had at the time for the Atlantic convoys were Hawker Sea Hurricanes installed on CAM ships- catapult aircraft merchant ships. It was a one-way trip for the pilot who had no means of recovery other than ditching alongside a ship. Wholly unsuitable for antisubmarine warfare, the CAM ships were really geared towards going after the long-range Focke Wulf Fw 200 Condor maritime patrol aircraft that scouted the Atlantic for the U-boat fleet.

On 2 June 1940 just a few days before Roosevelt's three month deadline, the Mormacmail's conversion to an escort carrier was completed and she was commissioned as the USS Long Island at Newport News, Virginia and designated APV-1 but was soon changed to AVG-1 (Aircraft Escort Vessel). The designations changed two more times during the war as the value of the escort carrier was proven, to ACV (Auxiliary Aircraft Carrier) in August 1942 to CVE (Escort Carrier) in July 1943. As built, the USS Long Island was 492 feet (150m) and displaced 13,500 tons and could make 16.5 knots. The original flight deck was only 362 feet with the bridge below the forward edge of the flight deck. Testing showed the need for a longer flight deck, so in the summer after her commissioning the carrier went back into the yards to have a proper full length flight deck. There was one elevator aft and a single forward catapult on the port side. The opportunity was taken during the extension of the flight deck to make other improvements and when she came out of her refit, the ship now displaced 14,953 tons and was faster with a speed of just over 17 knots.

HMS Archer- note the small island not present on the Long Island
The Mormacland was converted a similar configuration but had a small island on the starboard side. She was transferred to the Royal Navy in November 1941 to become the HMS Archer. Despite the getting the second escort carrier built in the United States, the Royal Navy did set out to build an escort carrier of their own with the conversion of the German merchant vessel Hannover to become the HMS Audacity, commissioned in June 1941 to become the first British escort carrier with the HMS Archer as the second such ship. Compared to the USS Long Island and the HMS Archer, the HMS Audacity was very small escort carrier with a displacement of only 6,000 tons with an air wing of just six aircraft. The two escort carriers were a boon to British convoy operations and the Royal Navy ordered five more escort carriers from the United States based on the C3 class merchant vessel hull and based on the USS Long Island with further improvements. These four ships had a 440-foot flight deck and a small island on the starboard side (which the USS Long Island lacked)- they became the HMS Avenger, HMS Biter, HMS Dasher, and the HMS Charger. The last ship of the group was returned to the US Navy in 1942 and became the USS Charger which remained on the US East Coast as a training carrier.

USS Bogue
Much like how the USS Langley was used primarily for testing and experimentation, so too was the USS Long Island in its early career as the first American escort carrier. With the entry of the United States into the war in December 1941, Admiral Doenitz's U-boat fleet began to prosecute its attacks on the US East Coast with 400 ships sunk just in the first six months following Pearl Harbor. With the utmost urgency, a new class of escort carrier was ordered into production based on the experience with the USS Long Island. These vessels were converted from twenty four C3-class hulls as the Bogue-class. Because the Long Island and the first British escort carriers were diesel powered, they were thought to be too slow based on operational experience and the Bogue-class carriers would have steam turbines to drive them to 18 knots. The new escort carriers had a flight deck 442 feet long by 80 feet wide, bigger than the Long Island's flight deck. A single catapult was fitted on the forward port side but the ships had a larger hangar deck and two elevators instead of the Long Island's single elevator. A small island was located on the starboard side as well.

USS Sangamon
Of the twenty carriers based on C3-class hulls that became to Bogue-class, ten of them went to the Royal Navy. The new ships could carry twice the fuel of the Long Island/Archer-class ships and had an improved compartment layout in the hull. As the ships were four feet lower than the Long Island/Archer, they also had better seakeeping qualities. Since the 1942 escort carrier program called for twenty-four ships and only twenty C3-class hulls were available, four Cimarron-class fleet oilers were converted to escort carriers to become the Sangamon class. As these ships were based on larger hulls, they were longer at 553 feet with 23,250 tons displacement and could carry two squadrons of aircraft. Launched originally with just a single port side catapult, the four ships of the Sangamon class got a second catapult on the starboard side in 1944. Being former fleet oilers, the Sangamon class carried a significant amount of fuel which could also be used to top off the escorting destroyers in the task force. They were more stable at sea than the Bogue-class and could make 19 knots. The four ships of this class were the USS Sangamon (CVE-26), USS Suwanee (CVE-27), USS Chenango (CVE-28) and USS Santee (CVE-29). The Navy wanted more Sangamon-class carriers, but fleet oilers were also in demand for underway replenishment of the carrier battle groups in the Pacific.

Aware of the shortage of hulls for conversion to escort carriers, American shipbuilding magnate Henry J. Kaiser had proposed to President Roosevelt in 1942 the construction of an escort carrier design capable of 20 knots in quantity production from his shipyards which were turning out Liberty cargo ships at an impressive rate. Using prefabricated sections and mass production techniques used on the Liberty ships, the Casablanca-class escort carriers would become the most produced aircraft carrier class in naval history with fifty carriers built between July 1943 to the last Casablanca-class built in July 1944, an astounding achievement for the American wartime shipbuilding industry. The ships were built in Vancouver, Washington, and then delivered to Astoria, Oregon for final fitting-out before commissioning.

Schematic of the USS Casablanca
The Casablanca-class (the lead ship, USS Casablanca, was designated CVE-55) were 498 feet long with a 477-foot long by 80-feet wide flight deck that had two elevators and a single port side catapult. The engines were spaced apart to prevent a hit from taking out the engine room and the hangar deck was quite wide for an escort carrier. They were also the first all-welded carriers which made them lighter at 10,900 tons displacement but this gave the Casablanca-class superb maneuverability. Despite the quantity production and design features, the Casablancas weren't as good as the Sangamon-class carriers and the Navy insisted on something better if not as good as the Sangamon class. This resulted in the ultimate and final escort carrier class of the war, the Commencement Bay class.

USS Commencement Bay, the ultimate escort carrier
Based on all the operational experience of escort carriers so far, the Commencement Bay class ships were based on an improved Sangamon-class layout but longer with a length of 577 feet and a displacement of 24,900 tons. The flight deck was much stronger to operated heavier aircraft and the two elevators also operated faster to speed flight operations. Two forward catapults were standard with a bigger island as well. The ships could make 20 knots and had the heaviest defensive anti-aircraft armament of any escort carrier of the war. The lead ship, the USS Commencement Bay (CVE-105) was commissioned on 27 November 1944. Thirty-three ships were ordered but only nineteen were completed by war's end. Two of the completed carriers went straight into mothballs after completion. While the Commencement Bay-class carriers didn't see as much action as their predecessors, several of the ships did go on in the postwar period to become the first helicopter assault carriers and served until the arrival of the Iwo Jima-class LPH carriers in the 1960s.

Source: Hunter-Killer: U.S. Escort Carriers in the Battle of the Atlantic by William T. Y'Blood. Bluejacket Books/Naval Institute Press, 1983, pp 11-26. Photos: US Navy, National Archives, Wikipedia.

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