Graf Zeppelin-class aircraft carrier

Graf Zeppelin at Kiel, June 1940, displaying her newly rebuilt bow. Also visible are her 15 cm casemate guns, before their removal to defend occupied Norway. The photo is marked Geheim ("secret").

The Graf Zeppelin class's hull was divided into 19 watertight compartments, the standard division for all capital ships in the Kriegsmarine.[11] Their belt armor was to vary from 100 mm (3.9 in) over the machinery spaces and aft magazines, to 60 mm (2.4 in) over the forward magazines and tapered down to 30 mm (1.2 in) at the bows. Stern armor was kept at 80 mm (3.1 in) to protect the steering gear. Inboard of the main armor belt was a 20 mm (0.79 in) anti-torpedo bulkhead.[12]

Horizontal armor protection against aerial bombs and plunging shellfire started with the flight deck, which acted as the main strength deck. The armor was generally 20 mm (0.79 in) thick except for those areas around the elevator shafts and funnel uptakes where thickness increased to 40 mm (1.6 in) in order to give the elevators necessary structural strength and the critical uptakes greater splinter protection.[12] Beneath the lower hangar was the main armored deck (or tween deck) where armor thickness varied from 60 mm (2.4 in) over the magazines to 40 mm (1.6 in) over the machinery spaces. Along the peripheries, it formed a 45 degree slope where it joined the lower portion of the waterline belt armor.[12]

The Graf Zeppelins' original length-to-beam ratio was 9.26:1, resulting in a slender silhouette. However, in May 1942, the accumulating top-weight of recent design changes required the addition of deep bulges to either side of Graf Zeppelin's hull, decreasing that ratio to 8.33:1 and giving her the widest beam of any carrier designed prior to 1942.[13] The bulges served mainly to improve Graf Zeppelin's stability but they also gave her an added degree of anti-torpedo protection and increased her operating range because selected compartments were designed to store approximately 1500 additional tons of fuel oil.[14]

Graf Zeppelin's straight-stemmed prow was rebuilt in early 1940 with the addition of a more sharply angled "Atlantic prow", intended to improve overall seakeeping. This added 5.2 m (17 ft) to her overall length.[11]

The Graf Zeppelin class's power plant was to consist of 16 La Mont high-pressure boilers, similar to those used in the Admiral Hipper-class heavy cruisers. Their four sets of geared turbines, connected to four shafts, were expected to produce 200,000 shp (150,000 kW) and propel the carrier at a top speed of 35 knots (40 mph; 65 km/h). With a maximum bunkerage capacity of 5000 tons of fuel oil (prior to the addition of bulges in 1942), the Graf Zeppelins' calculated radius of action was 9,600 miles (15,400 km) at 19 knots (35 km/h; 22 mph). However, wartime experience on ships with similar power plants showed such estimates were highly inaccurate, and actual operational ranges tended to be much lower.[15]

Two Voith-Schneider cycloidal propeller-rudders were to be installed in the forward bow of the ship along the center-line. These were intended to assist in berthing the ship in harbor and also in negotiating narrow waterways such as the Kiel Canal where, due to the carrier's high freeboard and difficulty in maneuvering at speeds below 8 knots (15 km/h; 9.2 mph), gusting winds might push the ship into the canal sides. In an emergency, the units could have been used to steer the ships at speeds under 12 knots (22 km/h; 14 mph) and, if the ships' main engines were rendered inoperable, could propel the vessel at a speed of 4 knots (7.4 km/h; 4.6 mph) in calm seas. When not in use, they were to be retracted into their vertical shafts and protected by water-tight covers.[15]

The Graf Zeppelins' steel flight deck, overlaid with wooden planking, was 242 m (794 ft) long by 30 m (98 ft) wide at its maximum. It had a slight round down right aft and overhung the main superstructure but not the stern; being supported by steel girders. At the bow, the carriers were to have an open forecastle and the leading edge of her flight deck was uneven (mainly due to the blunt ends of her catapult tracks), but it did not appear likely that would have caused any undue air turbulence. Careful wind-tunnel studies using models confirmed this, but they also revealed that their long low island structure would generate a vortex over the flight deck in these tests when the ship yawed to port. This was considered to be an acceptable hazard when conducting air operations.[16]

The Graf Zeppelin class's upper and lower hangars were long and narrow with unarmored sides and ends. Workshops, stores and crew quarters were located outboard of the hangars, a design feature similar to that of British carriers.[16] The upper hangar measured 185 m (607 ft) x 16 m (52 ft); the lower hangar 172 m (564 ft) x 16 m (52 ft). The upper hangar had 6 m (20 ft) vertical clearance while the lower hangar had 0.3 m (1 ft 0 in) less headroom due to the ceiling braces. Total usable hangar space was 5,450 m² (58,700 sq ft) with stowage for 43 aircraft: 20 Fieseler Fi 167 torpedo bombers, 18 in the lower hangar, two in the upper hangar; 13 Junkers Ju 87C dive bombers in the upper hangar and 10 Messerschmitt Bf 109T fighters in the upper hangar.[17]

The Graf Zeppelin class had three electrically operated elevators positioned along the flight-deck's center-line: one near the bow, abreast the forward end of the island; one amidships; and one aft. They were octagonal in shape, measuring 13 m (43 ft) x 14 m (46 ft), and were designed to transfer aircraft weighing up to 5.5 tons between decks.[18][19]

Two Deutsche Werke compressed air-driven telescoping catapults were installed at the forward end of the flight deck for power-assisted launches. They were 23 m (75 ft) long and designed to accelerate a 2,500 kg (5,500 lb) fighter to a speed of approximately 140 km/h (87 mph) and a 5,000 kg (11,000 lb) bomber to 130 km/h (81 mph).[19]

A dual set of rails led back from the catapults to the forward and midship elevators. In the hangars, aircraft were to be hoisted by crane - a method also proposed for the Essex-class carriers of the United States Navy, but rejected as too time-consuming - onto collapsible launch trolleys. The aircraft/trolley combination would then be lifted to flight deck level on the elevator and trundled along the rails to the catapult start points. When the catapults were triggered, a burst of compressed air would propel moveable slideways within the catapult track wells forward.[20]

As each plane lifted off, its launch trolley would reach the end of the slideway but remain locked in place until the tow attachment cables were released. Once the slideways were retracted back into the catapult track wells and the tow cables unhooked, the launch trollies would be manually pushed forward onto recovery platforms, lowered to the forecastle on "B" deck, then rolled back into the upper hangar for re-use via a secondary set of rails.[20] When not in use, the catapult tracks were to be covered with sheet metal fairings to protect them from harsh weather.[19]

Eighteen aircraft could have theoretically been launched at a rate of one every 30 seconds before exhausting the catapult air reservoirs. It would then have taken 50 minutes to recharge the reservoirs. The two large cylinders holding the compressed air were housed in insulated compartments located between the two catapult tracks, below flight deck level but above the main armored deck. This positioning afforded them only light protection from potential battle damage. The insulated compartments were to be electrically heated to a temperature of 20 °C (68 °F) in order to prevent ice from forming on the cylinder piping and control equipment as the compressed air was vented during launches.[21]

It was intended from the outset that all of the Graf Zeppelins' aircraft would normally launch via catapult. Rolling take-offs would be performed only in an emergency or if the catapults were inoperable due to battle damage or mechanical failure. Whether this practice would have been strictly adhered to or later modified, based on actual air trials and combat experience is open to question, especially given the limited capacity of the air reservoirs and the long recharging times necessary between launches.[19] One advantage of such a system, however, was that the Graf Zeppelins could have launched their aircraft without need for turning the ship into the wind or under conditions where the prevailing winds were too light to provide enough lift for her heavier aircraft. They could also have launched and landed aircraft simultaneously.[22]

To facilitate rapid catapult launches and eliminate the necessity of time-consuming engine warm-ups,[Note 1] up to eight aircraft were to be kept in readiness aboard the German carriers on their hangar decks by the use of steam pre-heaters. These would keep the aircraft engines at an operational temperature of 70 °C (158 °F). In addition, engine oil was to be kept warmed in separate holding tanks, then added via hand-pumps to the aircraft engines shortly before launch. Once the aircraft were raised to flight deck level via the elevators, aircraft oil temperature could be maintained, if need be, through the use of electric pre-heaters plugged into power points on the flight deck. Otherwise, the aircraft could have been immediately catapult-launched as their engines would already have been at or near normal operating temperature.[23]

Four arrester wires were positioned at the after end of the flight deck with two more emergency wires located afore and abaft of the amidships elevator. Original drawings show four additional wires fore and aft of the forward lift, possibly intended to allow recovery of aircraft over the bows, but these may have been deleted from the ship's final configuration.[18] To assist with night landings, the arrester wires were to be illuminated with neon lights.[22]

Two 4 m (13 ft) high, slitted steel wind barriers were installed afore the midships and forward elevators. These were designed to reduce wind velocity over the flight deck to a distance of approximately 40 m (130 ft) behind them. When not in use they could be lowered flush with the deck to allow aircraft to pass over them.[18]

The position of Graf Zeppelin's superstructure in relation to the flight deck.

The Graf Zeppelins' starboard-side island housed the command and navigating bridges and charthouse. It also served as a platform for three searchlights, four domed stabilized fire-control directors and a large vertical funnel. To compensate for the weight of the island, the carrier's flight deck and hangars were offset 0.5 m (1 ft 8 in) to port from her longitudinal axis.[11] Design additions proposed in 1942 included a tall fighter-director tower, air search radar antennas and a curved cap for her funnel, the latter intended to keep smoke and exhaust gases away from the armored fighter-director cabin.[24]

The Graf Zeppelins were to be armed with separate high and low angle guns for AA and anti-ship defense at a time when most other major navies were switching to dual-purpose AA weapons and relying on escort ships to protect their carriers from surface threats.[15] Her primary anti-shipping armament consisted of sixteen 15 cm (5.9 in) SK C/28 guns paired in eight armored casemates. These were mounted, two each, at the four corners of the carriers' upper hangar deck, positions that raised the possibility the guns would be washed out in heavy seas, especially those in the forward casemates.[15]

Chief Engineer Hadeler had originally planned for only eight such weapons on the carriers, four on each side in single mountings. However, the Naval Armaments Office misinterpreted his proposal to save space by pairing them and instead doubled the number of guns to 16, resulting in a need for increased ammunition stowage and more electrically operated hoists to service them.[25] Later in Graf Zeppelin's construction, some consideration was given to deleting these guns and replacing them with 10.5 cm (4.1 in) SK C/33 guns mounted on sponsons just below flight deck level. But the structural modifications needed to accommodate such a change were judged too difficult and time-consuming, requiring major changes to the ship's design, and the matter was shelved.[26]

Primary AA protection came from 12 10.5 cm (4.1 in) guns, paired in six turrets positioned three afore and three aft of the carrier's island. Potential blast damage to planes sited on the flight deck when these guns fired to port was an unavoidable risk and would have limited any flight activity during an engagement.[16]

The Graf Zeppelin class's secondary AA defenses consisted of 11 twin 37 mm (1.5 in) SK C/30 guns mounted on sponsons located along the flight deck edges: four on the starboard side, six to port and one mounted on the ship's forecastle. In addition, seven 20 mm (0.79 in) MG C/30 guns were installed on single-mount platforms on either side of the carrier: four to port and three to starboard. These guns were later changed to Flakvierling mountings.[27]

Work started on Graf Zeppelin, ordered as Flugzeugträger A, in 1936. She was laid down on 28 December that year, and launched on 8 December 1938. She was incomplete by April 1940, when a changed strategic situation led to work on her being suspended.[41] By early 1942 the usefulness of aircraft carriers in modern naval warfare had been amply demonstrated, and on 13 May 1942, with Hitler's authorization, the German Naval Supreme Command ordered work resumed on the carrier.[42]

With technical problems, such as the demand for newer planes specifically designed for carrier use, and the need for modernization, progress was delayed. The German naval staff hoped all these changes could be accomplished by April 1943, with the carrier's first sea trials taking place that August. By late January 1943 Hitler had become so disenchanted with the Kriegsmarine, especially with what he perceived as the poor performance of its surface fleet, that he ordered all of its larger ships taken out of service and scrapped. As of 2 February 1943, construction on the carrier ended for good.

Graf Zeppelin languished for the next two years in various Baltic ports. On 25 April 1945, she was scuttled at Stettin (now Szczecin, Poland), ahead of the advancing Red Army.[43] The ship was subsequently raised by the Soviets[44] and was used for target practice and sunk in 1947. Her wreck was discovered in 2006 by Polish researchers in the Baltic off Władysławowo, at the head of the Hel Peninsula.[45]

The contract to build the ship was awarded to the Friedrich Krupp Germaniawerft in Kiel in 1938, with a planned launch date on 1 July 1940. Work on Flugzeugträger B began in 1938 but was halted on 19 September 1939 because, now that Germany was at war with Great Britain and France, priority had shifted to U-boat construction. The hull, completed only up to the armored deck, sat rusting on its slipway until 28 February 1940, when Admiral Raeder ordered her broken up and scrapped.[46] Scrapping was completed four months later.

The Kriegsmarine never named a vessel before it was launched, so it was only given the designation "B" ("A" was Graf Zeppelin's designation before launch). Had it been completed, the aircraft carrier could have been named Peter Strasser in honor of the World War I leader of the naval airships Peter Strasser.[47]

In 1937, the Kriegsmarine planned two additional aircraft carriers of the Graf Zeppelin class with the official designations C and D. Both these carriers were planned to be operational by 1943. However, by the end of 1938, this plan was changed to only build these two carriers plus any further units as smaller carriers.[48]