The vessel was initially envisaged as having a displacement of approximately 500 tons, a speed of at least , and a range of The opportunity to build a prototype in the place of a cancelled Bar-class boom defence vessel saw the proposed design increased to a 680-ton vessel, with a top speed, and a range of , armed with a 4-inch gun, equipped with asdic, and able to fitted with either depth charges or minesweeping equipment depending on the planned operations: although closer in size to a sloop than a local defence vessel, the resulting increased capabilities were accepted due to advantages over British-designed mine warfare and anti-submarine vessels.