Editing Battlecruiser (section)

===Construction===
For most of the combatants, capital ship construction was very limited during the war. Germany finished the ''Derfflinger'' class and began work on the {{sclass|Mackensen|battlecruiser|4}}. The ''Mackensen''s were a development of the ''Derfflinger'' class, with 13.8-inch guns and a broadly similar armour scheme, designed for {{convert|28|kn}}.<ref>Breyer, pp. 283–84</ref>

In Britain, Jackie Fisher returned to the office of First Sea Lord in October 1914. His enthusiasm for big, fast ships was unabated, and he set designers to producing a design for a battlecruiser with 15-inch guns. Because Fisher expected the next German battlecruiser to steam at 28 knots, he required the new British design to be capable of 32 knots. He planned to reorder two {{sclass|Revenge|battleship}}s, which had been approved but not yet laid down, to a new design. Fisher finally received approval for this project on 28 December 1914 and they became the {{sclass|Renown|battlecruiser|4}}. With six [[BL 15 inch Mk I naval gun|15-inch guns]] but only 6-inch armour they were a further step forward from ''Tiger'' in firepower and speed, but returned to the level of protection of the first British battlecruisers.<ref>Roberts, pp. 46–47</ref>

At the same time, Fisher resorted to subterfuge to obtain another three fast, lightly armoured ships that could use several spare {{convert|15|in|adj=on|0}} gun turrets left over from battleship construction. These ships were essentially light battlecruisers, and Fisher occasionally referred to them as such, but officially they were classified as ''large light cruisers''. This unusual designation was required because construction of new capital ships had been placed on hold, while there were no limits on [[light cruiser]] construction. They became {{HMS|Courageous|50|2}} and her sisters {{HMS|Glorious||2}} and {{HMS|Furious|47|2}}, and there was a bizarre imbalance between their main guns of 15 inches (or {{convert|18|in|0}} in ''Furious'') and their armour, which at {{convert|3|in|spell=in}} thickness was on the scale of a light cruiser. The design was generally regarded as a failure (nicknamed in the Fleet ''Outrageous'', ''Uproarious'' and ''Spurious''), though the later conversion of the ships to [[aircraft carrier]]s was very successful.<ref>Roberts, pp. 50–52</ref> Fisher also speculated about a new mammoth, but lightly built battlecruiser, that would carry {{convert|20|in|adj=on|0}} guns, which he termed {{HMS|Incomparable}}; this never got beyond the concept stage.<ref>Breyer, p. 172</ref>

It is often held that the ''Renown'' and ''Courageous'' classes were designed for Fisher's plan to land troops (possibly Russian) on the German Baltic coast. Specifically, they were designed with a reduced [[draft (hull)|draught]], which might be important in the shallow Baltic. This is not clear-cut evidence that the ships were designed for the Baltic: it was considered that earlier ships had too much draught and not enough [[Freeboard (nautical)|freeboard]] under operational conditions. Roberts argues that the focus on the Baltic was probably unimportant at the time the ships were designed, but was inflated later, after the disastrous [[Dardanelles Campaign]].<ref>Roberts, p. 51</ref>

The final British battlecruiser design of the war was the {{sclass2|Admiral|battlecruiser|4}}, which was born from a requirement for an improved version of the ''Queen Elizabeth'' battleship. The project began at the end of 1915, after Fisher's final departure from the Admiralty. While initially envisaged as a battleship, senior sea officers felt that Britain had enough battleships, but that new battlecruisers might be required to combat German ships being built (the British overestimated German progress on the ''Mackensen'' class as well as their likely capabilities). A battlecruiser design with eight 15-inch guns, 8&nbsp;inches of armour and capable of 32 knots was decided on. The experience of battlecruisers at the [[Battle of Jutland]] meant that the design was radically revised and transformed again into a fast battleship with armour up to 12 inches thick, but still capable of {{convert|31.5|kn}}. The first ship in the class, {{HMS|Hood|51|2}}, was built according to this design to counter the possible completion of any of the Mackensen-class ship. The plans for her three sisters, on which little work had been done, were revised once more later in 1916 and in 1917 to improve protection.<ref>Roberts, pp. 55–61</ref>

The Admiral class would have been the only British ships capable of taking on the German ''Mackensen'' class; nevertheless, German shipbuilding was drastically slowed by the war, and while two ''Mackensen''s were launched, none were ever completed.<ref name=R601>Roberts, pp. 60–61</ref> The Germans also worked briefly on a further three ships, of the {{sclass|Ersatz Yorck|battlecruiser|4}}, which were modified versions of the ''Mackensen''s with 15-inch guns.<ref>Gröner, pp. 58–59</ref> Work on the three additional Admirals was suspended in March 1917 to enable more escorts and merchant ships to be built to deal with the new threat from U-boats to trade. They were finally cancelled in February 1919.<ref name=R601/>