Corvettes were based on a mercantile design with triple expansion engines, sloops were conventional naval vessels with turbines.

Sloops were larger and faster with a heavy armament of high angle 4-inch guns which had superior anti-aircraft fire control via the Fuze Keeping Clock, while retaining excellent anti-submarine capability.

His sloop Starling became one of the most successful submarine hunters, taking part in the sinking of eleven U-boats.

In April 1949, Amethyst was attacked on the Yangtze River by the Communist People's Liberation Army.

There were incremental improvements as the building developed, and the Woodcock and Wren when completed were practically indistinguishable from the Modified Black Swan class.

The third pair (which were of the Modified Black Swan class – see below) Fourteen sloops for the RN were in the 1940 Supplementary War Programme.