Alecto-class sloop
The "Alecto"-class steam vessels (SV3) later reclassed as Third Class Sloops, were designed by Sir William Symonds, the Surveyor of the Navy. The design was approved on 12 April 1839. Initially four vessels were ordered and completed. A fifth unit was ordered (Rattler), but was reordered as a screw propelled vessel. And a sixth unit had been ordered but was requested to be reordered to a different design. The screw driven "Rattler" participated in the first of three 'tug o- wars with the "Alecto". The vessels all participated during the Russian War. All had gone to the  breakers by the mid-1860s. "Alecto" was the second named vessel since it was used for a 12-gun fireship, launched by King of Dover on 26 May 1781 and sold in 1802. "Ardent" was the fifth named vessel since it was used for a 64-gun third rate, launched at Blaydes of Hull on 13 August 1764, captured by the French off Plymouth on 17 August 1779, recaptured in April 1782 and renamed "Tiger" then sold in June 1794. "Prometheus" was the second named vessel since it was used for an 18-gun fireship, Launched by Thompson of Southampton on  27 March 1807, assigned to harbour service in May 1819, then renamed "Veteran" on 2 May 1839 and broken in August 1852. "Polyphemus" was the second named vessel since it was used for a 64-gun third rate, launched at Sheerness Dockyard on 27 April 1782, converted to a powder hulk in September 1813 and her breaking was completed at Chatham on 15 September 1827. Design and specifications. The vessels were ordered in February 1839. The first two were laid down in July 1839 at Chatham and Sheerness Dockyards. The second pair were laid down at Chatham Dockyard in February  1840. The first two vessels ("Alecto" and "Prometheus") were launched in July 1839 with "Polyphemus" launched in September 1840 and "Ardent" in February 1841. The gundeck was with the keel length of reported for tonnage. The maximum beam was with reported for tonnage. The depth of hold was . The builder's measure calculated at tons whereas the vessels displaced 878 tons. Seaward and Capel of Millwall supplied the machinery for all vessels. They were equipped with two fire-tube rectangular boilers. The engines were 2-cylinder vertical single expansion (VSE) direct acting steam engines rated at 200 nominal horsepower (NHP ). All four ships were armed with two 32-pounder 42 hundredweight (cwt) muzzle loading smooth bore ( MLSB) guns on pivot mounts and two 32-pounder (25 cwt) MLSB carronades on broadside trucks.