They were somewhat unsuccessful; intended for shore bombardment in shallow water, they exceeded their design draft by 50%.

The first 6 ships had a 2-cylinder horizontal single-expansion steam engine provided by Robert Napier and Sons and rated at 200 nominal horsepower, driving a single screw.

[2] Sylvia and Nassau were completed as survey ships and were powered by 150 nhp Humphreys and Tennant engines.

[2] The main armament, which was principally intended for shore bombardment,[3] was originally designed with two 68-pounder and two 32-pounder muzzle-loading smoothbore guns.

[3] The first 6 ships were ordered from commercial yards (Money Wigram & Sons, C J Mare & Co and J Scott Russell), with fitting out to be done in the Royal Dockyards at Chatham (first pair) and Woolwich (last 4).