Both vessels were ordered 12 March 1838 and laid down in September at Portsmouth and Sheerness. The vessels were launched in July (
Vesuvius) and August (
Stromboli) 1839. 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 light draught forward was and aft. The builder's measure was calculated at 965 79/94 tons whereas the vessels displaced 1,283 tons.
Robert Napier & Sons of
Govan supplied the machinery for both vessels. They were equipped with two fire-tube rectangular boilers. The engines were 2-cylinder vertical single expansion (VSE)
side-lever steam engines rated at 280 nominal horsepower (NHP). This gave the ships a speed under power of about . Pictures show
Vesuvius with a
barque rig. All four ships were initially armed with two 10-inch 84 hundredweight (cwt) shell Millar's original guns on pivot mounts and two 68-pounder 64 cwt muzzle-loading smooth bore (MLSB) carronades and two 42-pounder (22 cwt) MLSB carronades on broadside trucks. In 1856 the 10-inch guns were replaced with a Dundas 1853 68-pounder 84 cwt MLSB gun and the carronades were replaced with four 32-pounder 42 cwt MLSB guns on broadside trucks. In the 1860s the 68-pounder was replaced with an Armstrong 7-inch rifled breechloader (RBL) gun. This weapon is also known as the 100/110-pounder gun depending on the weight of shell fired. They had a complement of approximately 149 men and grew to 160 men with the change in armament. ==Initial cost of vessels==