The last four vessels were ordered separately and are sometimes known as the Gadfly class, although they were essentially identical.

Members of the class lingered on as steam lighters, dredgers, boom defence vessels and base ships, lasting in some cases into the 1950s.

The flat-iron gunboats were designed for coastal defence and bombardment, and were constructed from iron.

They were not rigged, and the single 10-inch (18 ton) muzzle-loading rifle was fitted forward on a hydraulic mount that allowed it to be lowered for a sea passage to improve the vessel's seaworthiness, and raised for action.

[1] Power was provided by a pair of two-cylinder horizontal single-expansion steam engines driving twin screws.