Editing Invincible-class battlecruiser (section)

===Propulsion===
Early in the design process the "Committee on Designs" had thought to power these ships with the traditional reciprocating vertical [[Marine steam engine#Triple or multiple expansion|triple-expansion steam engines]], but were persuaded to adopt [[Parsons Marine Steam Turbine Company|Parsons]] [[steam turbine]]s as they required fewer boilers for the same amount of power, were easier to protect from damage as they were more compact than reciprocating engines and could be kept below the waterline. In addition they were significantly lighter and more reliable than the older design. The [[Direct-drive mechanism|direct-drive]] turbines then in use did have one significant drawback in that they ran at a relatively high speed which required small-diameter, fine-pitch propellers of a large blade area which adversely affected manoeuvrability at low speeds. [[Charles Algernon Parsons|Parsons]] alleviated this problem by his suggestion of fitting more powerful astern turbines on all four shafts, which could increase manoeuvrability by reversing the turbines as needed.<ref>Roberts, pp. 68–69</ref>

An additional solution was to fit twin balanced rudders behind each inner shaft, in contrast to the single central rudder used on earlier ships. This greatly increased the effectiveness of the rudder and substantially decreased the turning circle of the ''Invincible''s in comparison to earlier ships of their size.<ref>Roberts, pp. 69–70</ref>

The ''Invincible''s had two paired sets of Parsons turbines housed in separate engine-rooms. Each set consisted of a high-pressure ahead and astern turbine driving an outboard shaft, and a low-pressure ahead and astern turbine driving an inner shaft. A cruising turbine was also coupled to each inner shaft, although these were not used often and were eventually disconnected. Each shaft drove a propeller {{convert|11|ft|m}} in diameter. The turbines were designed to produce a total of {{convert|41000|shp|lk=on}}, but reached nearly {{convert|47000|shp|abbr=on}} during trials in 1908. Designed speed was {{convert|25|kn|1}}, but all three bettered {{convert|26|kn|1}} during trials.<ref>Roberts, pp. 76, 80</ref> {{HMS|Indomitable|1907|2}} maintained an average speed of {{convert|25.3|kn|1}} for three days during a passage of the [[Atlantic Ocean|North Atlantic]] in August 1908.<ref>Roberts, p. 75</ref>

The steam plant consisted of 31 [[Yarrow Shipbuilders|Yarrow]] (''Invincible'' and ''Inflexible'') or [[Babcock & Wilcox]] (''Indomitable'') [[water-tube boiler|large-tube boiler]]s, arranged in four boiler rooms.<ref>Roberts, pp. 70–75</ref> Maximum bunkerage was approximately {{convert|3000|LT|MT|-1}} of coal, with an additional {{convert|725|LT|MT}} of [[fuel oil]] to be sprayed on the coal to increase its burn rate.<ref>Roberts, p. 76</ref> At full fuel capacity, the ships could steam for {{convert|3090|nmi|lk=in}} at a speed of {{convert|10|kn}}.<ref name=c24>Preston, p. 24</ref>