The design was intended as a smaller follow-on from the preceding Tribal class. In a departure from all previous Royal Navy destroyers, the design used a two boiler room layout. This reduced hull length and allowed for a single funnel, both reducing the profile and increasing the arcs of fire of the light
anti-aircraft (AA) weapons. It also increased vulnerability, as there were now two adjacent large compartments with the resultant risk of a single well-placed hit flooding both and resulting in a total loss of boiler power. A three-boiler-room layout was used starting with the
F class in the early 1930s. Early ships also tended to use twin boiler rooms, which are still a great improvement over a single boiler room. As destroyers are lightly armoured and fast vessels meant to survive by avoiding being hit at all, the odds of a hit striking just the right spot to disable both boiler rooms simultaneously were considered remote enough to be worth risking in exchange for the benefits given by a two-room layout. During pre-war trials "...On a light displacement
Jackal attained on the Arran mile in 60 fathoms (360 feet) deep.
Jupiter in 75 fathoms (450 feet) made light, deep displacement". A significant advancement in construction techniques was developed by naval architect Albert Percy Cole. Instead of going for transverse frame sections which were unnecessarily strong, but held together by weak longitudinals, Cole opted for extra strong longitudinals and weaker transverse frames. Another advancement was changes to the bow design. The bow form was also modified from that of the preceding Tribal-class design; the
clipper bow was replaced by a straight stem with increased sheer. This change was not a success and as a consequence, these ships were very wet forwards. This shortcoming was rectified from the
S class onward by returning to the earlier form. Despite the vulnerability of the boiler layout, the design was to prove compact, strong and very successful, forming the basis of all Royal Navy destroyer construction from the
O class up to the last of the of 1943–1945. The armament was based on that of the Tribals, but replaced one twin
QF 4.7-inch (120 mm) Mark XII (L/45) gun mount with an additional bank of torpedo tubes. The gun mountings were capable of 40° elevation and 340° of training. Curiously, 'X' mounting was positioned such that the blind 20° arc was across the stern, rather than the more logical forward position where fire was obscured by the bridge and masts anyway. Though this meant that they were unable to fire dead astern, with a train rate of 10 degrees per second, this did allow for much faster traversing from one beam to the other when manoeuvring & engaging targets off the bow with the full battery. With the tubes now 'pentad', a heavy load of 10
21-inch (533 mm) Mark IX torpedoes could be carried. AA armament remained the same, consisting of a quadruple
QF 2-pounder ( Mark VIII gun mount and a pair of quadruple
0.5-inch (12.7 mm) Vickers Mark III machine gun mounts. Armament was further improved by replacing the quadruple machine guns with
20 mm Oerlikons. These ships, when completed, had a comparatively heavy close range AA armament. Fire control arrangements also differed from the Tribals, and the dedicated high-angle (H/A)
rangefinder director was not fitted, instead only a rangefinder was carried behind the nominally dual purpose
Director Control Tower (DCT). The rangefinder was much modified to allow it to control the main armament for A/A fire, and was known as the "3 man modified rangefinder". These ships used the
Fuze Keeping Clock HA Fire Control Computer. The N class were ordered in 1940 as repeats of the J design, after delays and cost over-runs associated with the larger and more complicated . The only design change was to locate the 'X' 4.7-inch mounting in the more logical position with the 20° training blind spot forward. While building, the same early wartime modifications as the Js and Ks were applied, with a pair of twin power-operated 0.5 in machine gun turrets briefly carried on the
quarterdeck before being replaced by single 20 mm Oerlikons. ==Modifications==