The Boeing Vertol CH-46 Sea Knight is an American medium-lift tandem-rotor transport helicopter powered by twin turboshaft engines. It was designed by Vertol and manufactured by Boeing Vertol following Vertol's acquisition by Boeing. Development of the Sea Knight, which was originally designated by the firm as the Vertol Model 107, commenced during 1956. It was envisioned as a successor to the first generation of rotorcraft, such as the H-21 "Flying Banana", that had been powered by piston engines; in its place, the V-107 made use of the emergent turboshaft engine. On 22 April 1958, the V-107 prototype performed its maiden flight. During June 1958, the US Army awarded a contract for the construction of ten production-standard aircraft, designated as the YHC-1A, based on the V-107; this initial order was later cut down to three YHC-1As. During 1961, the US Marine Corps (USMC), which had been studying its requirements for a medium-lift, twin-turbine cargo/troop assault helicopter, selected Boeing Vertol's Model 107M as the basis from which to manufacture a suitable rotorcraft to meet their needs. Known colloquially as the "Phrog" and formally as the "Sea Knight", it was operated across all US Marine Corps' operational environments between its introduction during the Vietnam War and its frontline retirement during 2014. The Sea Knight was operated by the USMC to provide all-weather, day-or-night assault transport of combat troops, supplies and equipment until it was replaced by the MV-22 Osprey during the 2010s. The USMC also used the helicopter for combat support, search and rescue (SAR), casualty evacuation and Tactical Recovery of Aircraft and Personnel (TRAP). The Sea Knight also functioned as the US Navy's standard medium-lift utility helicopter prior to the type being phased out of service in favor of the MH-60S Knighthawk during the early 2000s. Several overseas operators acquired the rotorcraft as well. Canada operated the Sea Knight, designated as CH-113; the type was used predominantly in the SAR role until 2004. Other export customers for the type included Japan, Sweden, and Saudi Arabia. The commercial version of the rotorcraft is the BV 107-II, commonly referred to simply as the "Vertol". The Sea Knight is an amphibious helicopter, able to land directly on calm water and float, but only for a few hours.