N3 - class battleship The N3 class was dreadnought battleship class designed for the Royal Navy after World War I , incorporating all the lessons learned from that conflict . They were very similar in design to the s , but had larger guns and thicker armour . They were never ordered due to signing of the Washington Naval Treaty in 1922 , which limited the size and armament of battleships to and no gun bigger than . Background . In 1916 the US had declared its intention to create a Navy " second to none " ; Congress had authorized the building of a large number of battleships and battlecruisers . In response , the Japanese government also began a large programme of warship building ( the 8 - 8 fleet ) . Two improved hulls were rebuilt into the two s by the Royal Navy during the war . The only new capital ships laid down during the war were the s . Their design had been called into question after the Battle of Jutland in 1916 and three ships of this class were cancelled , leaving only to be completed to a modified design . The US plan had been delayed by the wartime need to build smaller vessels . Nevertheless , estimates by the Admiralty were that by the early 1920s the Royal Navy would be behind in ships . By the beginning of 1920 , the Americans had completed one battleship since the end of World War I and had five more building . Seven more were intended to be laid down in 1920 – 21 , six of these were the very large and powerful , armed with twelve 16 - inch guns . The Japanese had finished one battleship since the end of the war and had three more under construction . To correct this state of affairs , the Admiralty initially planned to build three battleships and one battlecruiser in Fiscal Year ( FY ) 1921 – 22 and again in FY 1922 – 23 , but this was changed to four s to be built first , presumably to be followed by the same number of battleships the following year . A pair of designs were prepared in June 1920 , derived from the " U - 4 " battleship design of 1914 , of ships with displacements of about and armed with eight or nine guns , in four twin or three triple gun turret s mounting a new gun then under development . The only limitation of the design was the inability to use British dockyards and pass through the Suez Canal . The most unusual feature of these designs was that none of the turrets were superfiring , presumably to keep the centre of gravity as low as possible and avoid the extra weight required for tall , superfiring barbette s . The designs were revised in October and split into separate battleship and battlecruiser designs . The battleship designs were given letters of the alphabet from L through N , with the use of triple or double gun turrets shown by 3 or 2 respectively . Both ' L2 ' and ' L3 ' had superfiring guns and the armour was reduced to a inclined waterline belt while the main armoured deck was thick ( where it sloped to meet the belt ) . They both had a designed speed of and had transom sterns . ' L2 ' displaced , but ' L3 ' was a thousand tons lighter . ' M2 ' and ' M3 ' followed in November and December and were very different from the earlier designs . ' M2 ' and ' M3 ' sacrificed fire directly astern by moving the rear turret ( s ) amidships in order to save weight by shortening the length of the armoured citadel . Compared to the earlier , more conventional , designs , ' M2 ' saved and ' M3 ' . More weight was saved by reducing the designed speed to and using only two propeller shafts , although it was thought that this would improve manoeuvering power over four smaller propellers . These changes saved for ' M2 ' and for ' M3 ' over their predecessors . A lengthened version of ' M3 ' was chosen for further development as N3 and approved in November 1921 . Description . Most noticeable of the N3 design was the concentration of the main battery forward of the bridge and machinery spaces . A related feature of the design was the tower bridge structure behind the first two gun turrets . This provided a better and more stable foundation for fire - control equipment , greatly improved accommodation and protection from the weather . The N3 battleships were significantly larger than their predecessors of the . They had an overall length of , a beam of , and a draught of at deep load . They would have displaced about , nearly double the displacement of the older ships . They had a complete double bottom deep . The ships would have had two geared steam turbine sets , each of which drove one propeller shaft , in two engine rooms forward of the boiler rooms . This allowed the funnel to be placed further aft and increased the ability of the rear turret to fire to the rear . The turbines would have been powered by small - tube boilers intended to produce a total of . The ships ' maximum speed would have been about 23 knots . Housing the main armament in triple turrets was new to the Royal Navy though British companies had been involved in the production of triple gun turret designs for other navies . The choice of a high muzzle velocity with a relatively lighter shell was taken from the German practice ; it ran counter to previous British guns such as the BL 15 - inch Mark I gun of 42 - calibre length which were lower - muzzle - velocity weapons firing heavy shells . Armament . The N3 design mounted nine 45 - calibre BL 18 - inch guns in three triple gun turrets , designated ' A ' , ' B ' , and ' X ' from front to rear . The guns had a maximum elevation of 40° . As none of these guns was ever completed and test - fired , sources differ on their exact specifications . Naval historian John Campbell quotes the projectile weight as fired at a muzzle velocity of , but Alan Raven and John Roberts cite a fired at a muzzle velocity of . The N3s carried a secondary armament of sixteen BL 6 - inch Mk XXII guns in superfiring twin turrets . Four turrets were sited around the forward superstructure and four at the stern . The guns could elevate between – 5° and + 60° . They fired projectiles at a muzzle velocity of . Their maximum range was at 45° elevation . Their rate of fire was five rounds per minute . An anti - aircraft battery of six QF 4 . 7 - inch Mk VIII guns was included . They had a maximum depression of - 5° and a maximum elevation of 90° . They fired a high explosive shell at a muzzle velocity of at a rate of eight to twelve rounds per minute . The guns had a maximum ceiling of , but an effective range of much less . The ships were intended to carry four 10 - barreled mountings for the QF 2 - pounder Mk VIII gun ( commonly known as a " pom - pom " ) , two abaft the funnels and two at the stern . Each barrel was provided with 1300 rounds of ammunition . The gun fired a shell at a muzzle velocity of to a distance of . The gun ' s rate of fire was approximately 96 – 98 rounds per minute . Like previous classes of British battlecruisers , a pair of submerged , broadside - firing torpedo tube s were planned for these ships . Their compartment was located just forward of the ' A ' shell room on the platform deck . Six torpedoes per tube were to be carried in peace - time , but this would increase to eight in wartime . These Mark I torpedoes had a warhead of of TNT and were powered by oxygen - enriched air . They had two speed settings which governed their range : either at , or at . Fire - control . The main guns of the battleships could be controlled from any of the two director - control tower s ( DCT ) . The primary DCT was mounted at the top of the forward superstructure . Another was mounted on the roof of the conning tower in an armoured hood . Each main gun turret was provided with a coincidence rangefinder in an armoured housing on the turret roof . The secondary armament was primarily controlled by two DCTs mounted on each side of the bridge . The anti - aircraft guns were controlled by a high - angle control system mounted on the mizzenmast . Each " pom - pom " mount had its own director and there was also a height - finder aft . Two torpedo rangefinders were located on the sides of the funnels . Armour . A first for any British dreadnought was the use of the all or nothing protection scheme in the N3s and 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 . 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 N3 had a maximum thickness of thick with the top of the armour angled 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 , from forward of ' A ' barbette to the rear of the after 6 - inch magazine . For about , it reduced to over the engine and boiler rooms . The belt had a height of , of which was below the designed waterline . The lower edge of the belt abreast the magazines was continued down another by a thickness of high - tensile steel inclined at 36° to prevent a shell from reaching the magazines via a wave trough at high speed . The ends of the belt terminated in tranverse bulkheads . The armoured deck matched the length of the waterline belt and sloped down to meet the upper edge of the belt . It extended forward over the torpedo compartment which had a separate transverse bulkhead protecting it that was thick . The steering gear was protected by a deck and bulkhead thick . The turret faces were thick while their sides were probably in thickness , and the roof was 8 inches thick . The armour of the barbettes and the conning tower was 15 inches thick and the conning tower ' s communications tube to the upper deck was 8 inches thick . The fire - control director atop the conning tower was protected by an armoured hood 4 to 6 inches thick . The anti - torpedo bulge s of the N3 were internal to the hull and were intended to withstand the explosion of a torpedo warhead . They consisted of an outer air space , an inner buoyancy space and the thick torpedo bulkhead . The bulkhead was situated about 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 was just as effective and weighed less . Cancellation . The four N3 battleships were never ordered because the Washington Naval Treaty , an arms limitation treaty under negotiation at the time , forbade construction of any ship larger than 35 , 000 tons . Many of the aspects of their design ultimately were incorporated into the two s , and they are often described as being a cut - down N3 . Indeed , the " Nelson " s received the design designation ' O3 ' , marking them as next in the design sequence , although they used the guns intended for the G3 battlecruisers for cost reasons and to comply with the Treaty ' s 16 - inch limitation on main armament .