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20mm Oerlikon

Size	Image Description	Contributed By And/Or Copyright
90k	Three types of armament are visible - 5in/25cal guns, 40mm Bofors, and 20mm Oerlikons. Note the anti-skid patterns on the deck under the 20mm mounts and the floater net mounted on the 20mm gun tub, January 1945. The 20mm Oerlikon was a free swinging weapon, operated by maximum gas pressure, about 23 tons per square inch and by springs which were correspondingly powerful. Factors such as friction, the effects of different elevations, cold weather, rain were too small in proportion to the operating forces to cause stoppage. Therefore, it offered great reliability because of its reserve of power compared with the Hispano-Suiza gun used before its introduction to the Fleet. It had a high rate of fire and could come into action very quickly. One of the greatest advantages of the Oerlikon was that it had a barrel which could be replaced in about 30 seconds after prolonged fire at sea against multi-plane attacks. It could be bolted down almost anywhere. A talker stood behind the gunner with a loader to his right carrying a spare magazine. The magazine employed with the Oerlikon was superior and could be kept fully loaded without any tension on its springs and feeding the 490RPM was an easy task compared to the Hispano-Suiza magazine which held 690RPM. A canvas bag below the gun took expended cartridge cases. The MK.4 weighed about 1695lb and incorporated a feature that permitted the operator to raise or lower the trunnions which often corroded at sea, to accomodate his height. Before the appearance of the Bofors in numbers in 1943, it enjoyed a high percentage of aircraft kills in the Pacific Fleet, 32 percent for the period between Pearl Harbor and September 1944. However, as ranges increased and as the Japanese turned to night attacks, the Bofors and the VT-fuzed 5in guns became more and more significant. Thus, the Oerlikon's share in 1944 amounted to only 25 percent, compared to 48.3 for the second half of 1942. The Fleet demanded more Bofors as it could not defeat an attacker before he released a weapon. It had become a "revenge" gun, only able to prevent him coming home. One Captain reported that the 20mm had a negative psychological effect; "the saying among the crew being that when the 20mm Oerlikons opens fire, it's time to hit the deck". The Oerlikon was retained into the 1950's.	U.S. Navy Photograph submitted by Pieter Bakels. Photo added 08/10/08.
302k	20mm crews stand by at their stations on February 1945.	U.S. Navy Photograph submitted by Pieter Bakels.
188k	20mm firing. Note the gun shield does not yet have the cutout for gun sight Mk.14.Mod.6.	U.S. Navy Photograph submitted by Pieter Bakels.
662k	The 20mm AA gun & crew, from "The Blue Jackets's Manual", 1944. Shown here is a PDF for operating instructions for the 20mm Gunsight.	U.S. Navy Photograph submitted by Pieter Bakels. Photo added 08/10/08.
662k	Sailors man 20mm guns during an operation. Note the Gunsight Mk.14 Mod.6 atop gun mount.	U.S. Navy Photograph submitted by Pieter Bakels.
45k	The version on the above image,(Prototype Elco Thunderbolt with two 0.50-caliber machine guns and four 20mm in late 1942) the ELCO quad, a quadruple 20mm (Oerlikon), power-driven mount originally intended for PT boats (Mark 15) did not have the later Mark 14 lead-computing sight (see G.A. drawings given) that enhanced its A.A. capability but had a simple tube sight above the operator's seat. The "Thunderbolt" was an attempt to increase 20mm hitting power. At night, the continuous stream of tracers from the "Thunderbolt" gun, like the beam of a searchlight, hurled destructive 20mm projectiles at 1800 rounds per minute at an enemy. Designed as a dual-purpose weapon, it had an elevation/depression range of +85/-15 degrees and full traverse. Two units were mounted on the Massachusetts (BB-59) and Maryland (BB-46) to augment their batteries and retained the Mark 14 gun sight. By June 1944 it had been authorized also for Arkansas (BB-33), Colorado (BB-45), West Virginia (BB-48) and Washington (BB-56) and the training ship Wyoming (AG-17). The Pacific Fleet recommended further installations in view of the success of the first two but nothing came of the project, partly because BuOrd considered the 20mm most valuable as a free-swinging weapon which could still be used after all shipboard power had been lost. The mount consisted of a quadrant-shaped armoured gondola with four guns. The gondala sides and back were constructed of 1/4in plate with a 1/2in front plate. A tubular handle was attached to the rear of the gondola which permitted manual elevation and traverse of the mount in the event of an electric or hydraulic failure. It had a gunner's seat with joystick and firing treadle fitted to port. An oil filter and recuperator were fitted to starboard. There were slots for shell ejection and manual cocking of the 20mm cut in the rear. The gondola pivoted on two vertical yokes wit elevation and depression effected by an hydraulic ram on each yoke. The yokes were attached to a conical base which contained an electrically-driven hydraulic traversing motor. Installation of the mount required the addition of a 24-volt DC generator. The "Thunderbolt" weighed about the same (560lb) as the more powerful 40mm (single) mount.	U.S. Navy Photograph submitted by Pieter Bakels. Photo added 08/10/08.

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