The Iron Duke class was a group of four dreadnought battleships built for the British Royal Navy before the First World War. The class comprised four ships: , , , and . Launched from October 1912 to November 1913, this was the third class of Royal Navy super-dreadnoughts.
The ships were essentially repeats of the battleships; they retained the same ten 13.5 inch (34.3 cm) guns in five twin gun turrets on the centreline. However, the Iron Dukes had improved armour and a more powerful secondary armament of 6-inch weapons instead of the 4-inch mounted on the earlier ships.
The four ships were the most advanced battleships in the Royal Navy at the outbreak of the First World War, though they were soon surpassed by the five ships of the . They all saw extensive service during the war with the Grand Fleet, where Iron Duke acted as the flagship for the fleet commander, Admiral John Jellicoe. Three of the ships, Iron Duke, Benbow, and Marlborough, were present at the Battle of Jutland; Emperor of India missed the battle by being in dock for periodic refit. The four Iron Duke-class battleships saw limited active duty following the end of the war; they were all demilitarised under the terms of the London Naval Treaty signed in 1930. Iron Duke was reduced to a training and depot ship and lasted in that role until 1946 when she was scrapped. Benbow was scrapped in 1931 and Marlborough followed in 1932. Emperor of India was sunk as a gunnery target in 1931 though was later re-floated to be scrapped in 1932.
Design
thumb|left|Plan and left elevation view of a ship of the Iron Duke class, from Jane's Fighting Ships 1919
General characteristics
The Iron Duke-class ships were 622 feet 9 inches (189.8 metres) long overall, and had a beam of 90 ft (27.4 m) and a draught of 29 ft (8.8 m). This was an increase of 25 ft (7.7 m) in length and 1 ft (.3 m) in width over the preceding King George V-class ships. The Iron Dukes displaced 25,000 long tons (25,400 tonnes).
Armament
Primary battery
thumb||alt=Two large gun turrets seen from the deck of a battleship; each turret has two long guns
The Iron Duke-class ships mounted a main battery of ten 13.5 inch (34.3 cm) Mk V(H) guns in five twin gun turrets, all mounted on the centreline. Two turrets were placed in a superfiring pair forward ("A" and "B" turrets), one turret amidships—the "Q" turret—directly after the two funnels, and two in a superfiring pair aft of the rear superstructure ("X" and "Y" turrets). The gun houses used were Mk II turrets that weighed 600 tons (610 tonnes) and allowed for depression to −5° and elevation to 20°. Despite this, the range dials on the gunsights at the time of construction were graduated to 15 degrees; super-elevating cams and prisms to allow the full elevation of the guns to be used were issued some time after the Battle of Jutland. The forward and aft gun turrets could train 150° in either direction from the centreline, while the "Q" had a much more limited range. It could engage targets on an arc from between 30° to 150° from the centerline on either beam of the ship.
The guns fired at a rate of 1.5–2 rounds per minute. The Mk V "Heavy" gun fired a variety of shells, including high explosive and armour-piercing rounds; they all weighed 1,400 lb (635 kg). The guns were loaded with MD45 propellant charges that weighed 297 lb (135 kg); these were stored in silk bags. This provided a muzzle velocity of 2,491 ft/s (759 meters per second). At maximum elevation of 20°, the guns had a range of 23,740 yards (21,710 m), though at the maximum effective elevation of 15°, the range was somewhat shorter, at approximately 20,000 yd (18,290 m). At a range of 10,000 yd (9,144 m), the gun could penetrate up to 12.5 in (318 mm) of Krupp cemented steel armour, the type used on contemporary German dreadnoughts.
Secondary battery
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The secondary battery consisted of twelve 6-inch (15.2 cm) Mk VII guns mounted in casemates in the hull around the forward superstructure. These guns were chosen because the 4-inch (10.2 cm) guns on earlier battleships were deemed to be too weak and have too short a range to effectively combat torpedo boats with newer, more powerful torpedoes. Admiral Jackie Fisher had opposed the idea of increasing the secondary battery, though he retired from the post of First Sea Lord in 1910. As a result, the Iron Dukes, which were designed in 1911, received the larger 6 inch gun.
There were some significant problems with the casemate guns early on, however. They were equipped with hinged plates that were designed to close off the casemate opening in heavy seas. The plates were easily washed away, though, and without them, water easily entered the ship and caused significant flooding. This problem was compounded by the fact that they had been mounted too low in the hull, which subjected them to heavier pounding from rough seas. The problem was eventually corrected by the addition of dwarf bulkheads in the gun houses and rubber seals to the hinged plates. The guns fired between 12 and 14 rounds per minute, and were expected to fire approximately 1,250 shells before replacement or repair was necessary. The shells fired were 12.5 lb (5.67 kg) with a high-explosive warhead. They were manually operated, and had a maximum effective ceiling of 23,500 ft (7,160 m).
As was customary for capital ships of the period, the Iron Duke-class ships were equipped with submerged torpedo tubes. The ships carried four 21 in (53.3 cm) tubes, two on each beam.
Armour
The Iron Duke-class battleships had an armour belt that was thick in the central area of the ship, where the ammunition magazines, machinery spaces, and other vital parts of the ship were located. The belt tapered down to towards the bow and stern. The barbettes that contained the main gun turrets were on the sides and on the rear, where shells were less likely to hit. The turrets themselves were thick on the sides. The ships' armoured deck was between 25 and 65 mm (1–2.5 in) thick.
Following the end of the war, Iron Duke was transferred to the Mediterranean Fleet. During 1919–20, the ship operated in the Black Sea in support of the White Russians during the Russian Civil War. The Washington Naval Treaty of 1922 mandated that Iron Duke be removed from the active roster. However, she remained with the fleet for a short time, having been transferred to the Atlantic Fleet in 1926. This duty assignment lasted 3 years, after which she was removed from active service. She was later used as a depot ship in Scapa Flow, starting in 1939. After the outbreak of the Second World War that year, her remaining guns were removed to be used in shore defences. Luftwaffe bombers attacked her on 17 October 1939; while they scored no direct hits, several near misses caused significant damage. After repairs were effected, the ship resumed her duties as a depot vessel until the end of the war.