boilers. The turbines were rated at and were intended to give the ship a maximum speed of . During her sea trials on 10 April 1911, Indefatigable reached a top speed of from after her propellers were replaced. She carried enough coal and fuel oil to give her a range of at a cruising speed of . The Indefatigable class had a main armament of eight breech-loading BL Mark X guns mounted in four hydraulically powered twin-gun turrets. Two turrets were mounted fore and aft on the centreline, identified as 'A' and 'X' respectively. The other two were wing turrets mounted amidships and staggered diagonally: 'P' was forward and to port of the centre funnel, while 'Q' was situated starboard and aft. 'P' and 'Q' turrets had some limited ability to fire to the opposite side. Their secondary armament consisted of sixteen BL Mark VII guns positioned in the superstructure. They mounted two submerged torpedo tubes, one on each side aft of 'X' barbette, and twelve torpedoes were carried. The Indefatigables were protected by a waterline armoured belt that extended between and covered the end barbettes. Their armoured deck ranged in thickness between with the thickest portions protecting the steering gear in the stern. The turret faces were thick, and the turrets were supported by barbettes of the same thickness. Indefatigable was unique among British battlecruisers in having an armoured spotting and signal tower behind the conning tower, protected by of armour. However, the spotting tower was of limited use, as its view was obscured by the conning tower in front of it and the legs of the foremast and superstructure behind it. During a pre-war refit, a rangefinder was added to the rear of the 'A' turret roof, and this turret was equipped to control the entire main armament as an emergency backup for the normal fire-control positions. Wartime modifications Indefatigable received a single QF 3-inch 20 cwt anti-aircraft gun on a high-angle Mark II mount in March 1915. It was provided with 500 rounds. All of her 4-inch guns were enclosed in casemates and given gun shields during a refit in November 1915 to better protect the gun crews from weather and enemy action, although two aft guns were removed at the same time. She received a fire-control director between mid-1915 and May 1916 that centralised fire control under the director officer who now fired the guns. The