The War Emergency Programme destroyers were 112 destroyers built for the British Royal 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 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 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, 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 incorporated the new 4.5-inch gunMark 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, replacing the low-angle Destroyer DCT and High-Angle Rangefinder-Director Mark II in use since the Q and R class.
The Z flotilla introduced the new dual-purpose Director Mark IType 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 for gunlaying. As a result, one set of torpedo tubes was removed to counter the increased topweight.
