HMS Mercury (1878): One of the Royal Navy's First All-Steel Warships

A Genuine First for the Royal Navy

HMS Mercury was laid down at Pembroke Dockyard on 16 March 1876, launched on 17 April 1878 and completed in September 1879, an Iris-class dispatch vessel, later redesignated a second-class protected cruiser, and one of only two ships of her class.

Her significant claim to naval history lies in her construction. Her sister ship HMS Iris, laid down and launched a full year ahead of Mercury, usually gets the credit as the Royal Navy's actual first steel-built warship, with Mercury following close on her heels; between them the two ships were built from steel produced by the Siemens-Martin process at the Landore Siemens Steel Company in Swansea, a quietly revolutionary piece of engineering that helped pave the way for the entirely steel-built fleets that would follow in the decades ahead. Contemporary naval opinion took the class seriously enough that The Times devoted a seven-page technical analysis to it in 1882, and naval historians have since ranked its significance alongside genuine watershed ships like HMS Warrior in 1860 and HMS Dreadnought in 1905.

It is worth saying what steel actually bought, and why it took so long. Steel had been available cheaply since Bessemer's converter of 1856, and the Admiralty had spent twenty years declining to build warships out of it, which was not obstinacy. Bessemer steel was made in minutes and was very hard to control; a bad heat came out brittle, and brittle is the single worst property a hull plate can have. Wrought iron was heavier and duller and entirely predictable, which in a ship is a virtue.

The Siemens-Martin open-hearth process changed the calculation. It took about fourteen hours to make a heat instead of twenty minutes, which sounds like a step backwards until you notice what those fourteen hours allow: the melt can be sampled and analysed as it goes, and adjusted, so the steel that comes out is the steel you specified rather than the steel you happened to get. Guaranteed mild steel is stronger than wrought iron for the same weight. Build a hull of it and you can either save the weight or spend it, and Iris and Mercury spent it on machinery.

That is the whole of the connection between the steel and the speed record, and it is why the class gets mentioned in the same breath as Warrior and Dreadnought despite never having done anything in particular. Everything above her waterline was ordinary. What was new was that she existed at all, in that shape, at that weight.

Fast for Her Time

HMS Mercury displaced 3,730 tons, measured 315 feet in length and was powered by twin compound-expansion steam engines producing 6,000 indicated horsepower, giving her a designed speed of 17 knots that she comfortably exceeded on trials, reaching 18.57 knots and, in doing so, beating the 17.89-knot mark her sister Iris had set as the fastest speed recorded by any warship up to that point, an impressive turn of speed for a vessel of her era, armed with ten 64-pounder rifled muzzle-loading guns.

What she was for is worth spelling out too, because the type has vanished so completely. A dispatch vessel existed to carry a message faster than anything else afloat. Admirals could not talk to each other, or to London, except by sending somebody, and how fast a fleet could react was therefore a function of how fast one ship could steam. Mercury carried twelve boilers in two compartments, about 7,300 horsepower on twin screws, and 780 tons of coal, which gave her six thousand miles at ten knots and roughly two thousand at full speed. She could go a very long way slowly or a fairly short way very fast indeed, and the second of those was the job.

The category did not survive her. Within a quarter of a century of her launch a wireless set was doing the same work at the speed of light, as described in the article on HMS Magicienne (1890), which was sitting at anchor in Delagoa Bay in 1900 relaying signals from the blockading squadron to Simon's Town by wireless and landline. Nobody needed the fastest ship in the navy to carry a piece of paper any more.

Her guns tell their own story about how fast everything was changing. She went to sea with ten 64-pounder rifled muzzle-loaders, was rearmed with four 6-inch and four 5-inch breech-loaders, and by 1886 or 1887 was carrying thirteen 5-inch breech-loaders. Three complete outfits inside a decade, on a ship that never fired any of them in anger. The argument between muzzle-loading and breech-loading that produced all this churn is set out in the article on HMS Imperieuse (1883).

Reserve, China, and Reserve Again

HMS Mercury spent her early career largely in the Portsmouth Reserve from 1879 until 1890, before deploying to the China Station between 1890 and 1895, a substantial overseas commission befitting a fast cruiser built to protect British interests across a global empire. She returned to Portsmouth Reserve from 1895 to 1903, before taking on a training role as a navigation school ship between 1903 and 1905.

From Submarines to Accommodation

HMS Mercury served as a submarine depot ship at Portsmouth between 1906 and 1913, and briefly in the same role at Harwich in 1913, supporting the Royal Navy's still relatively young and rapidly developing submarine service. There was a passing plan in 1912 to rename her Columbine, quietly dropped before it ever came to anything, so she remained Mercury through to the very end.

She was hulked at Rosyth in 1914 as the First World War began, and from 7 January 1918 served as an accommodation ship at Chatham, a quiet, unglamorous final duty for a ship that had once represented the cutting edge of naval construction.

Scrapped at Bo'ness

HMS Mercury was paid off in March 1919 and sold for scrap to the Forth Shipbreaking Company at Bo'ness on 9 July that year, closing out the career of a ship whose real legacy lay not in any single dramatic action, but in the simple fact of having been built from steel at a moment when most of the fleet around her was still made of iron.