Armour
A first for any British dreadnought was the use of the all or nothing protection scheme in the G3s. Medium-thickness armour had proven to be useless in stopping heavy-calibre shells during World War I so the vital areas of the ship were protected by the thickest possible armour and the rest of the ship was left unarmoured. Use of this system was pioneered by contemporary U.S. Navy battleship designs starting with the Nevada-class battleship. However, this system of protection required that the armoured citadel should have enough reserve buoyancy to keep the ship stable even if the rest of the hull was riddled by gunfire.
The waterline belt of the G3 had a maximum thickness of 14 inches (356 mm) with the top of the armour angled 18° outwards. This angle increased the armor's relative thickness to horizontal, close-range fire, albeit at the cost of reducing its relative height which increased the chance of plunging shellfire going over or under it. It ran some 522 feet (159.1 m), from the forward edge of 'A' barbette to the rear of the after 6-inch magazine. The belt had a height of 14 feet 3 inches (4.3 m), of which 4 feet 6 inches (1.4 m) was below the designed waterline. Only the forward 259 feet (78.9 m) of the belt had the maximum thickness, it thinned to 12 inches (305 mm) for the rest of its length. The lower edge of the belt abreast the magazines was continued down another 3 feet (0.9 m) by a 4 inches (100 mm) thickness of high-tensile steel inclined at 36° to prevent a shell from reaching the magazines via a wave trough at high speed. The forward and rear ends of the belt terminated in 12-inch and 10-inch (254 mm) tranverse bulkheads respectively. The waterline belt extended forward 46 feet (14.0 m) at a thickness of 6 inches that reduced to 2.25 inches (57 mm) in two steps.
The funnel and boiler room ventilation shafts were surrounded by an armoured box 116 feet (35.4 m) long intended to prevent shells fired from behind the ship reaching 'X' magazine. The box narrowed at an angle of 21° as it rose and had a maximum thickness of 12 inches nearest to the magazine. Aft it reduced in a series of steps to 9 inches (229 mm), 6 inches, 5 inches (127 mm) and 4 inches (102 mm). The armoured deck matched the length of the waterline belt and sloped down 2.5° to meet the upper edge of the belt. It had a maximum thickness of 8 inches (203 mm) from the 'A' barbette to partway over the mid-boiler rooms and thinned to a minimum of 4 inches over the rear engine and boiler rooms. The deck's thickness increased to 7 inches (180 mm) partway over the rear engine room and covered the aft 6-inch magazines. The armoured deck extended forward 46 feet over the torpedo compartment with a maximum thickness of 8 inches, thinning to 6 inches. The armoured deck's rear extension was 106 feet 9 inches (32.5 m) long and was 5 inches thick until the last 27 feet 4 inches (8.3 m) when it thinned to 3 inches (76 mm).
The turret faces were 17.5 inches (445 mm) thick while their sides ranged from 9 to 13 inches (229 to 330 mm) in thickness, and the roof was 8 inches thick. The armour of the barbettes ranged from 11 to 14 inches (279 to 356 mm) in thickness and it was carefully arranged to minimize the likely risk. The conning tower armour was 9 to 12 inches thick and its communications tube to the upper deck was 8 inches thick. The fire-control director atop the conning tower was protected by an armoured hood 3 to 5 inches thick.
The anti-torpedo bulges of the G3 battlecruisers were intended to withstand the explosion of a 750-pound (340 kg) torpedo warhead. They consisted of an outer air space, an inner buoyancy space and the torpedo bulkhead that consisted of two layers of .875-inch (22 mm) high-tensile steel. The bulkhead was situated some 13.5 feet (4.1 m) inboard from the side of the ship. Postwar tests done on a replica of this system showed that filling the buoyancy space with water rather than the sealed steel crushing tubes as used in Hood was just as effective and weighed less. A compressed air system was fitted to blow the water out of the buoyancy spaces and bring the ship upright in 15 minutes after two torpedo hits. The ship's double bottom ranged from 5–7 feet (1.5–2.1 m) in depth.
Read more about this topic: G3 Class Battlecruisers, Design and Description
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