Some info. on radar, armament and wartime mods! 'Ya pays ya money and yer takes yer choice'! π
"Armament, electronics and protection
The main armament of the Illustrious class consisted of sixteen quick-firing (QF) 4.5-inch (110 mm) dual-purpose guns in eight twin-gun turrets, four in
sponsons
on each side of the hull. The roofs of the gun turrets protruded above the level of the flight deck to allow them to fire across the deck at high elevations.[11] The gun had a maximum range of 20,760 yards (18,980 m).[12] Her light anti-aircraft defences included six octuple mounts for QF 2-pounder ("pom-pom") anti-aircraft (AA) guns, two each fore and aft of the island and two in
sponsons
on the port side of the hull.[11] The 2-pounder gun had a maximum range of 6,800 yards (6,200 m).[13]
The completion of Illustrious was delayed two months to fit her with a Type 79Z early-warning radar; she was the first aircraft carrier in the world to be fitted with radar before completion.[11] This version of the radar had separate transmitting and receiving antennas which required a new mainmast to be added to the aft end of the island to mount the transmitter.[14]
The Illustrious-class ships had a flight deck protected by 3 inches (76 mm) of armour and the internal sides and ends of the hangars were 4.5 inches (114 mm) thick. The hangar deck itself was 2.5 inches (64 mm) thick and extended the full width of the ship to meet the top of the 4.5-inch waterline armour belt. The belt was closed by 2.5-inch transverse bulkheads fore and aft. The underwater defence system was a layered system of liquid- and air-filled compartments backed by a 1.5-inch (38 mm) splinter bulkhead.[15]
Wartime modifications
While under repair in 1941, Illustrious's rear "round-down" was flattened to increase the usable length of the flight deck to 670 feet (204.2 m).[16] This increased her aircraft complement to 41 aircraft by use of a permanent deck park.[17] Her light AA armament was also augmented by the addition of 10 Oerlikon 20 mm autocannon in single mounts with a maximum range of 4,800 yards (4,400 m).[18] in addition the two steel fire curtains in the hangar were replaced by asbestos ones.[19] After her return to the UK later that year, her Type 79Z radar was replaced by a Type 281 system and a Type 285 gunnery radar was mounted on one of the main fire-control directors.[16] The additional crewmen, maintenance personnel and facilities needed to support these aircraft, weapons and sensors increased her complement to 1,326.[7]
During her 1943 refits, the flight deck was modified to extend its usable length to 740 feet (225.6 m), and "outriggers" were probably added at this time. These were 'U'-shaped beams that extended from the side of the flight deck into which aircraft tailwheels were placed. The aircraft were pushed back until the main wheels were near the edge of the flight deck to allow more aircraft to be stored on the deck. Twin Oerlikon mounts replaced most of the single mounts. Other twin mounts were added so that by May she had a total of eighteen twin and two single mounts. The Type 281 radar was replaced by an upgraded Type 281M, and a single-antenna Type 79M was added. Type 282 gunnery radars were added for each of the "pom-pom" directors, and the rest of the main directors were fitted with Type 285 radars. A Type 272 target-indicator radar was mounted above her bridge.[16] These changes increased her aircraft capacity to 57[20] and caused her crew to grow to 1,831.[7]
A year later, in preparation for her service against the Japanese in the Pacific, one starboard octuple "pom-pom" mount, directly abaft the island, was replaced by two 40 mm Bofors AA guns;[21] which had a maximum range of 10,750 yards (9,830 m).[22] Two more twin Oerlikon mounts were added, and her boilers were retubed.[21] At this time her complement was 1,997 officers and enlisted men.[7] By 1945, accumulated wear-and-tear as well as undiagnosed shock damage to Illustrious's machinery caused severe vibrations in her centre propeller shaft at high speeds. in an effort to cure the problem, the propeller was removed, and the shaft was locked in place in February; these radical measures succeeded in reducing, but not eliminating, the vibrations and reduced the ship's speed to about 24 knots (44 km/h; 28 mph).["