The War Emergency Programme destroyers were 112 destroyers built for the BritishRoyal Navy during World War II. They were based on the hull and machinery of the earlier J-, K- and N-class destroyers. Due to supply problems and the persistent failure by the Royal Navy to develop a suitable dual-purpose weapon for destroyers, they were fitted with whatever armament was available. Recent advances in radar and weaponry were incorporated as they came available. As a result they were a relatively heterogeneous class incorporating many wartime advances, but ultimately based on a hull that was too small and with an armament too light to be true first-rate vessels equivalent of their contemporaries. As such they are often described as "utility" destroyers. It was not until the Battle-class destroyer of 1944 that the Royal Navy returned to building larger destroyers. Many vessels were transferred to friendly navies.
The P, and 3 ships of the O, flotilla were fitted with 4-inch guns with a new design of tall gunshield. As a result they carried only the Rangefinder-Director Mark II(W) for fire control.
From the Q and R class onwards a transom stern was incorporated.
From the S and T class onwards the bow was revised to a design based on that of the Tribal-class destroyer, to improve sea-keeping.
From the Q and R class the main gun calibre returned to 4.7 inches.
From the R flotilla onwards the officer's accommodation was forwards, instead of aft as was traditional Royal Navy practice
The S flotilla altered the position of the searchlight between the torpedo tubes with the medium anti-aircraft position abaft the funnel. This more logical arrangement gave the anti-aircraft gun improved arcs of fire in the forward field.
The S class introduced the new mounting CP Mark XXII for the 4.7-inch guns. This could readily be distinguished from the older mounting CP Mark XVIII of the O, Q and R by its sharply raked face, allowing increased elevation.
S-class Savage incorporated the new 4.5-inch gun Mark III, in a prototype twin dual-purpose turret BD Mark IV forward and 4.5-inch gun Mark IV in single mountings CP Mark V aft. The former would be introduced in the Battle-class destroyer.
The T flotilla introduced the lattice foremast, to support the ever-increasing weight of masthead electronics.
The W flotilla introduced the dual-purpose Director Mark III(W), replacing the low-angle Destroyer DCT and High-Angle Rangefinder-Director Mark II(W) in use since the Q and R class.
The Z flotilla introduced the new dual-purpose Director Mark I Type K and the 4.5-inch gun in single mountings CP Mark V as trialled in Savage. These mountings were based on the CP Mark XXII used in the later 4.7-inch gunned ships; there was no obvious difference.
The Ch- flotilla introduced the dual-purpose Director Mark VI with full remote-power control (RPC) for gunlaying. As a result one set of torpedo tubes was removed to counter the increased topweight.