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. On trials these units developed between 689ihp and 689ihp, giving speeds of about 11kn. Sylvia and Nassau were completed as survey ships and were powered by 150 nhp Humphreys and Tennant engines. Myrmidon received a more powerful 200 nhp Humphreys and Tennant engine.
The main armament, which was principally intended for shore bombardment, was originally designed with two 68-pounder and two 32-pounder muzzle-loading smoothbore guns. They were finished, however, with a single 7-inch/110-pounder Armstrong breech-loading gun and a 68-pounder muzzle-loading smoothbore gun. A pair of broadside 20-pounder Armstrong breech-loading guns were also fitted. The 68-pounders were later replaced by a pair of 64-pounder muzzle-loading rifled guns.
In common with all other Royal Navy wooden screw gunvessels, the Cormorants were rigged as barques, that is with three masts, with the fore and main masts square rigged, and the mizzen fore-and-aft rigged.
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). A further batch of 4 ships (Sylvia - Myrmidon) were ordered on 5 March 1860 and another batch of 3 (Pegasus - Guernsey) on 25 March 1862. The first completed ships had a draught of 11feet-12feetft (-ft), exceeding the intended 8feet by a considerable margin. Since gunvessels were intended to work in shallow water while bombarding the shore, work on the later two batches was suspended. Sylvia, Nassau and Myrmidon were suspended in 1862 or 1863, but were resumed, with Sylvia and Nassau being finished as survey vessels. Tartarus was broken up on the slipway in 1865, having cost £6,268 and work to Pegasus cost only £339. Guernsey was never laid down.
| Name | Ship Builder | Launched | Fate | ||
|---|---|---|---|---|---|
| Money Wigram & Sons, Blackwall Yard | 9 February 1860 | Sold on 7 June 1870 at Hong Kong | |||
| Money Wigram & Sons, Blackwall Yard | 19 March 1860 | Wrecked near Chefoo, China, on 4 November 1864 | |||
| C J Mare & Company, Leamouth | 23 June 1860 | Sold to Castle for breaking at Charlton on 13 April 1875 | |||
| C J Mare & Company, Leamouth | 15 December 1860 | Completed breaking at Plymouth on 14 March 1877 | |||
| J Scott Russell, Millwall | 18 September 1860 | Broken up at Sheerness in July 1867 | |||
| J Scott Russell, Millwall | 27 February 1861 | Broken up at Sheerness in October 1867 | |||
| Woolwich Dockyard | 20 March 1866 | Survey ship in October 1866. Sold to George Cohen for breaking in August 1889 | |||
| Pembroke Dockyard | 20 February 1866 | Survey ship in July 1866. Broken up at Sheerness April 1880 | |||
| Chatham Dockyard | 5 June 1867 | Survey ship in 1884. Sold at Hong Kong in April 1889 | |||
| Tartarus | Pembroke Dockyard | Cancelled on 16 December 1864 and broken up on 4 November 1865 | |||
| Pegasus | Woolwich Dockyard | Cancelled on 12 December 1863 | |||
| Albatross | Chatham Dockyard | Cancelled on 12 December 1863 | |||
| Guernsey | Pembroke Dockyard | Cancelled on 12 December 1863 (never laid down) |