The Swiss-designed Oerlikon 20mm gun was one of the most widely used anti-aircraft cannons during World War II. Like the Bofors 40mm L/60 gun, the Oerlikon was used by navies on both sides of the war such as the United States, Britain, Japan, Germany, and Italy. In the postwar era, the Republic of Singapore Navy (RSN) fitted the Oerlikon on board several ships such as the Independence-class patrol craft, Swift-class coastal patrol craft, the diving support ship RSS Jupiter (M102), and the Navy’s first ship, RSS Panglima. The Oerlikon was sometimes referred to as the Y Gun, as it was often placed at the aft, or rear, of the RSN’s ships.
The open-breech air-cooled Oerlikon 20mm was based on the Becker Type M2 20mm cannon, a German autocannon from World War I. An early Oerlikon model was made in the early 1930s, but the British Admiralty and United States Navy (USN) rejected the design due to its low muzzle velocity and poor rate of fire. Coupled with other financial difficulties, this almost caused the Swiss manufacturer to go bankrupt. However, the Imperial Japanese Navy saved the company by purchasing the gun’s license rights, and these funds allowed Oerlikon to make further enhancements to the 20mm cannon. The company improved the gun’s muzzle velocity and rate of fire and, importantly, made it easy to maintain and operate. The Oerlikon 20mm then went on to become one of the most prolific anti-aircraft weapons of World War II. Ironically, these guns on board the USN’s vessels were responsible for downing 32 per cent of Imperial Japanese aircraft from 1941 to 1944.
When the Japanese first began their kamikaze attacks on 25 October 1944 during the Battle of Leyte Gulf, the Allied forces quickly realised that the rapid approach speeds of kamikaze aircraft outmatched the Oerlikon’s firepower. The cannons were thus quickly replaced by the Bofors 40mm, with its longer range, increased volume, and higher accuracy.