Lexington Class Battle Cruiser - Design Development

Design Development

In their original 1916 configuration, the battlecruisers were to carry ten 14"/50 caliber guns in four turrets, with two triple superfiring over two dual because there was not enough beam to accommodate the larger barbettes of the triple turrets slight further forward and aft. They would have a secondary armament of eighteen 5"/51 caliber guns on a displacement of 34,300 long tons (34,900 t) and steam at 35 knots. All of these specifications were tempered by their sparse armor compared with contemporary battleships. C&R estimated 180,000 shaft horsepower would be needed to obtain this planned speed. This would require 24 boilers, which became problematic. Normally, they would be placed under the armored deck; however, because the designers were dealing with a long, comparatively narrow hull, there was simply not enough room to house them all there. The solution decided upon was to place half of the boilers above the deck on the centerline with armored boxes fitted around each one. There was also the challenge of the many exhaust uptakes needed. The Lexington's were given "no less than" seven, four of them side-by-side.

Also, with such a long, narrow hull came a consequent penchant for bending, which led to the challenge of adequate hull strength, especially longitudinal strength. This challenge was complicated in a capital ship by the heavy weight of main turrets and guns. This was an area in which British battlecruisers were notably deficient. Structural members on HMS Invincible were so weak that her double-bottom frames distorted. HMS Renown had to go into drydock immediately following her preliminary gunnery tests because the hull structure could not withstand the bending stresses from firing her forward main guns. When the "large light cruiser" HMS Courageous weathered a heavy gale during her initial trial run, a number of her outer hull plates were so distorted that they had to be removed, sent back to the foundry and renewed.

In the Lexingtons longitudinal strength was challenged further by the large amount of freeboard required at the forward section of the hull to keep the ships dry and maintain a high speed in various types of weather. Also, while the 8 inches (200 mm) of belt armor being considered was not an impressive amount in itself, the belt's running potentially along 80 percent of the waterline and covering the entire side amidships made the amount of armor protection impressive by European standards. Because of the difference in ultimate tensile strength between armor steel and hull steel, severe stresses on the hull were expected. These factors plus the ships' unusual length prompted Naval Constructor R. H. Robinson, who led the design group for the Lexingtons, to make careful analyses of strength, buoyancy and stresses expected in service. For instance, designers assumed customarily that a ship needed to withstand stresses caused by a wave of the ship's length with a ratio of height to length of 1:20. Robinson found a more reasonable ratio at 1:26 for the Lexingtons, which also promised considerable savings in weight.

One suggestion from C&R was to make the belt armor a load-bearing member by connecting plates end to end. This was found inordinately difficult to be practical and, while it would have added girder strength where most badly needed, was considered too radical a proposition to be truly safe. Another idea, subsequently adopted, was to design the forecastle to break abaft the turrets. The challenge then became to continue the longitudinal strength contributed by the armored deck past this point to the end of the stern. This became a difficult design problem, especially with the need to save weight wherever possible and the fact that light structural members combined with heavy armament weight had become a source of grief for the British. One proposed solution was to use a combination of three decks—a strength deck at the top of the hull, a protective deck which would rest atop the belt armor, 10 feet (3.0 m) above the waterline, and a splinter deck below that, just above the waterline. A third idea, also adopted, was to continue the longitudinal bulkhead between the protective and splinter decks down to the bottom of the ship to add strength. The severity of the strength and weight challenges necessitated a larger displacement of 33,000 tons and a hull of 850 feet (260 m) instead of 800 feet (240 m) to give enough internal volume to accommodate all the needed machinery. Even so, the size of the power plant meant pushing the main turrets further toward the ends of the ships, which increased hull stress. This was why the idea was adopted to place half the boilers above the armored deck.

Eventually, a then-unorthodox approach was adapted to framing these ships. Traditionally, a series of transverse frames would be fitted together closely from the keel upwards. Longitudinal framing, also known as the Isherwood system after British naval architect Sir Joseph Isherwood, who patented it, used large, widely spaced transverse frames in conjunction with light, closely spaced longitudinal members. This method, which was adopted for the Lexingtons, was felt by Isherwood to lend a ship much greater longitudinal strength than in ships built in the traditional method. Ships built with longitudinal framing proved to be exceptionally durable. In one case, the longitudinally-built tanker F.D. Asche survived grounding on two reefs in the Bahamas in 1921 and was towed to New York for repairs despite having virtually her entire bottom ripped out. Divers investigating the Asche while it was grounded found that, apart from the bottom being gone, the hull structure remained intact and therefore worth salvage. In another instance, the superstructure of the longitudinally-built steamer Curaca was blown away entirely and the frame buckled by a dynamite explosion in Halifax harbor during World War I but the ship stayed afloat.