Iris-class cruiser

The Iris class consisted of two ships, HMS Iris (1877) and HMS Mercury (1878), built for the Royal Navy in the 1870s. They were the first British all-steel warships.

Design and description
The Iris-class ships were designed as dispatch vessels by William White under the direction of Nathaniel Barnaby, Director of Naval Construction, and were later redesignated as second-class protected cruisers. The only visible difference between the sister ships was that HMS Iris (1877) had a clipper bow and was longer than HMS Mercury (1878) with her straight stem. Iris was 331 ft long overall while Mercury was 315 ft long. The sisters had a beam of 46 ft, and a draught of 20 ft. They displaced 3730 LT at normal load and were the first British warships with an all-steel hull. Their crew consisted of 275 officers and ratings. The ships were not armoured but extensive internal subdivision gave them some protection against flooding, as did the 150 ft double bottom under the propulsion machinery compartments.

The Iris class was powered by a pair of horizontal four-cylinder Maudslay, Sons and Field compound-expansion steam engines that were configured with a pair of high-pressure cylinders with a bore of 41 in and a pair of low-pressure cylinders 75 in in diameter. All cylinders had a 36 in stroke. Each engine drove one propeller shaft using steam from eight oval and four cylindrical boilers with a working pressure of 65 psi. The engines were designed to produce a total of 6000 ihp for a speed of 17 kn, which was handily exceeded by the sisters. Iris initially reached a maximum speed of 16.6 kn from 7086 ihp during her sea trials, but after new propellers were fitted, achieved 17.89 kn from 7330 ihp. Mercury became the fastest warship in the world when she made 18.57 kn from 7735 ihp. The ships carried a maximum of 780 LT of coal, enough to steam 4400 - 4950 nmi at 10 knots. They were initially fitted with a barque sailing rig, but this was removed after a few years.

The Iris-class ships were originally armed with ten 64-pounder (6.3 in) rifled muzzle-loading (RML) guns, eight on the main deck and the remaining pair on the upper deck on pivot mounts to serve as chase guns fore and aft.

Construction and career
Iris was launched in 1877 and sold in 1905 while Mercury, launched a year later, was hulked at Chatham in 1914 and sold for scrap in 1919.