HMS Dart (K21) was a River-class frigate of the Royal Navy. Dart was built to the RN's specifications as a Group I River-class frigate. The River class was a class of 151 frigates launched between 1941 and 1944 for use as anti-submarine convoy escorts and were named for rivers in the United Kingdom. The ships were designed by naval engineer William Reed, of Smith's Dock Company of South Bank-on-Tees, to have the endurance and anti-submarine capabilities of the Black Swan-class sloops, while being quick and cheap to build in civil dockyards using the machinery (e.g. reciprocating steam engines instead of turbines) and construction techniques pioneered in the building of the Flower-class corvettes. Its purpose was to improve on the convoy escort classes in service with the Royal Navy at the