Indefatigable follows a year of battlecruiser experience

By 1909 the Royal Navy had commissioned all three Invincible-class battlecruisers and had between roughly six months and a year of experience with them. It then ordered Indefatigable as an evolved version of that first generation. She retained eight 12-inch guns and the offset wing-turret arrangement, despite the restricted firing arcs already apparent in British dreadnought layouts. Her machinery used thirty-one Babcock & Wilcox boilers and four direct-drive Parsons turbines, rated at 43,000 indicated horsepower. A speed of 25.8 knots gave her nearly five knots over contemporary British battleships, a margin the navy could judge from operating both types itself.