Ironclad Warship - Propulsion: Steam and Sail

Propulsion: Steam and Sail

The first ocean-going ironclads carried masts and sails like their wooden predecessors, and these features were only gradually abandoned. Early steam engines were inefficient; the wooden steam fleet of the Royal Navy could only carry "5 to 9 days coal", and the situation was similar with the early ironclads. Warrior also illustrates two design features which aided hybrid propulsion; she had retractable screws to reduce drag while under sail (though in practice the steam engine was run at a low throttle), and a telescopic funnel which could be folded down to the deck level.

Ships designed for coastal warfare, like the floating batteries of the Crimea, or USS Monitor and her sisters, dispensed with masts from the beginning. The British HMS Devastation, started in 1869, was the first large, ocean-going ironclad to dispense with masts. Her principal role was for combat in the English Channel and other European waters; and while her coal supplies gave her enough range to cross the Atlantic, she would have had little endurance on the other side of the ocean. The Devastation and the similar ships commissioned by the British and Russian navies in the 1870s were the exception rather than the rule. Most ironclads of the 1870s retained masts, and only the Italian navy, which during that decade was focused on short-range operations in the Adriatic, built consistently mastless ironclads.

During the 1860s, steam engines improved with the adoption of double-expansion steam engines, which used 30–40% less coal than earlier models. The Royal Navy decided to switch to the double-expansion engine in 1871, and by 1875 they were widespread. However, this development alone was not enough to herald the end of the mast. Whether this was due to a conservative desire to retain sails, or was a rational response to the operational and strategic situation, is a matter of debate. A steam-only fleet would require a network of coaling stations worldwide, which would need to be fortified at great expense to stop them falling into enemy hands. Just as significantly, because of unsolved problems with the technology of the boilers which provided steam for the engines, the performance of double-expansion engines was rarely as good in practice as it was in theory.

During the 1870s the distinction grew between 'first-class ironclads' or 'battleships' on the one hand, and 'cruising ironclads' designed for long-range work on the other. The demands on first-class ironclads for very heavy armor and armament meant increasing displacement, which reduced speed under sail; and the fashion for turrets and barbettes made a sailing rig increasingly inconvenient. HMS Inflexible, launched in 1876 but not commissioned until 1881, was the last British battleship to carry masts, and these were widely seen as a mistake. The start of the 1880s saw the end of sailing rig on ironclad battleships.

Sails persisted on 'cruising ironclads' for much longer. During the 1860s, the French navy had produced the Alma and La Galissoniere classes as small, long-range ironclads as overseas cruisers and the British had responded with ships like Swiftsure of 1870. The Russian ship General Admiral, laid down in 1870 and completed in 1875, was a model of a fast, long-range ironclad which was likely to be able to outrun and outfight ships like Swiftsure. Even the later HMS Shannon, often described as the first British armored cruiser, would have been too slow to outrun General Admiral. While Shannon was the last British ship with a retractable propellor, later armored cruisers of the 1870s retained sailing rig, sacrificing speed under steam in consequence. It took until 1881 for the Royal Navy to lay down a long-range armored warship capable of catching enemy commerce raiders, Warspite, which was completed in 1888. While sailing rigs were obsolescent for all purposes by the end of the 1880s, rigged ships were in service until the early years of the 20th century.

The final evolution of ironclad propulsion was the adoption of the triple-expansion steam engine, a further refinement which was first adopted in HMS Sans Pareil, laid down in 1885 and commissioned in 1891. Many ships also used a forced draught to get additional power from their engines, and this system was widely used until the introduction of the steam turbine in the mid-1900s (decade).