| Deep condition | |
| Hull: | 6,200 |
| Armour: | 3,460 |
| Machinery: | 3,300 |
| Coal: | 3,000 |
| Armament: | 2,480 |
| General equipment: | 740 |
| Oil: | 700 |
| Reserve feedwater: | 350 |
| Engineers’ stores: | 90 |
| Board margin: | 100 |
| Total: | 20,420 |
1. A nominal displacement increase of 2,650 tons. Length increase was 48ft, with beam and draught increases of 4ft and 6in respectively.
2. Eight 12in and sixteen 4in against four 9.2in and ten 7.5in.
3. Turbine machinery of 41,000shp against 27,000shp with reciprocating machinery.
Construction of the vessels was carried out in great secrecy and, at Fisher’s instigation, erroneous reports were circulated implying that the design was merely a development of the Minotaur class. This misled the German Admiralty into starting their own construction of Blücher, a distinctly inferior type in respect of speed and armament. True particulars of the British ships were not made public until the summer of 1906 when it was too late for the Germans to revise their designs.
Invincible, mid 1909, showing painted funnel bands for identification: one white third funnel Indomitable; one white first funnel Inflexible; one white on each Invincible.
Armament
One of the very early alternative layouts showed ten 9.2in in five twin turrets, arranged as in Dreadnought, but as the Committee had discarded the 9.2in gun, only the designs with 12in armament were considered. The 12in guns were finally adopted on the grounds that the ships were required to run down and destroy any other cruiser then extant. This was the primary role, but they should also be capable of forming a fast division to supplement the battlefleet, and perhaps attack the van of an enemy fleet.
The uniform heavy-calibre armament adopted for these new ships provided maximum destructive effect, together with the greatest possible accuracy and efficiency in long-range fire control. On the basis of the Admiralty requirements, the main armament of eight 12in guns was the maximum commensurate with a speed of 25 knots, protection similar to that of Minotaur and dimensions conforming to existing docks. A primary requirement was the capability for maximum fire ahead unhindered by blast interference from individual turrets. The value of powerful ahead fire for engaging a retreating enemy had been particularly stressed by Lord Fisher and was in contrast to the requirements for Dreadnought where a heavy weight in broadside was the primary objective.
The plans of the cruiser type were very distinctive and the final arrangement was not, in any way, a discarded design for the battleship. The final plan, with the midships turret arrangement in close echelon, and allowing a nominal all-round fire of six guns with a good command for three of the four turrets, was considered the best that could be produced on the available length and beam, these in turn being dependent on the internal capacity of the hull form with proper arrangement of magazines, machinery and boilers. Because of anticipated blast effect, there was never any intention of providing an eight-gun broadside by placing the midships guns en echelon, and the most that was expected was to maintain a six-gun broadside over a limited arc of fire if the other turret became inoperable.
During the spring of 1905 consideration was given to fitting electrically-operated turrets in one of the new cruisers of the following year’s Estimates; this would be on an experimental basis, but it had long been accepted that electrical machinery might have advantages over the hydraulic equipment in use. Invincible was selected for the experiment; her centreline turrets were to be manufactured by Vickers, those on the beam by Armstrong-Whitworth; all were to be electrically-operated. The centreline mountings were Mk B1X, the beam mountings Mk BX.
After completion of the ship, a lengthy series of gun trials revealed that the fittings were something of a failure; training was a little slower than usual and the system was unpopular with turret crews. The machinery showed no marked superiority over the hydraulic equipment, and it was decided to reinstate the standard hydraulic gear during her next refit (it was not refitted until 1914).
Originally it had been intended that these vessels carry 12pdrs as in Dreadnought, but this was abandoned in favour of a new 4in gun introduced early in 1907.
Armour
In accordance with the Admiralty’s instructions to the Committee, protection, which was largely subordinated to armament and speed, was only to the same scale as the immediately preceding Minotaur-class armoured cruisers. Although adequate against attack by medium-calibre, long-range shellfire, such as might be anticipated in the cruiser role for which the ships were primarily intended, it was quite incapable of withstanding the heavy-calibre projectiles to which they would be exposed in their alternative role as part of the battlefleet.
Inflexible at Chatham in October 1908. Note boat stowage arrangements, and 12pdrs located below flying deck level.
Inflexible, as completed and commissioned, Chatham DY in October 1908.
INVINCIBLE
Outboard profile and plan, 1909
The horizontal armouring was especially meagre, in which connection Philip Watts recorded that ‘Admiralty requirements called for protection only against “flat trajectory” fire at about 9,000 yards and that, with a displacement limit of approximately 17,000 tons, no weight was available to increase the thickness of deck armour, although realizing the danger of plunging shellfire from heavy-calibre shells at 15,000 yards and over.’ The probability of future fleet actions being fought at ranges greater than 15,000 yards, with heavy shells falling at steep angles, was not fully appreciated in official circles at that date, although it had been pointed out by, among others, Captain Mark Kerr, when he was in command of Invincible during her first commission in 1909.
The main armoured belt of the class was 6in thick amidships and extended from just outside ‘A’ barbette to abreast of the centre of ‘Y’. The upper edge was at main deck level, some 7ft 5in above the load waterline. The lower edge was 3ft 10in below the waterline in the same load. The belt forward was 4in thick and ran unbroken to the stem over the same height as that for the 6in plates. The forward bulkhead was 7in thick and closed the forward extremities of the 6in armour between the main and lower deck levels. A 6in after bulkhead closed the main belt to the outer face of ‘Y’ in an oblique strake between the main and lower deck levels.
The barbettes for the 12in turrets were 7in thick in ‘A’ ‘P’ and ‘Q’ mountings, and ran down to the protective deck at this thickness. Below deck they were enclosed by 2in rectangular bulkheads extending out to the vessel’s side below ‘P’ and ‘Q’. The outer face of ‘Y’ was 7in to the lower deck and then, as before, reduced to 2in once below.
Indomitable.