All completed just pre-war, the "J" class returned new life into RN destroyer designs. A large sweeping ser was carried aft, and, crazilly, the after gun could not train aft, and when trained 5 degrees either side of forwards, could not fire due to superstructure in the way! © Andrew Arthur.

Completed one month before WWs, Javelin, is seen here with addtion of a 4" AA gun in place of after tubes and a gaudy paint scheme. © Andrew Arthur.

This class, a trend setter like the "A" class, whose succesors where just copies with a few modifications, the J class also set the pattern for all destroyers up to the "O" class.
The requirements for this class were a smaller, cheaper and better armed for fleet work - torpedoes increased - version of the "Tribal" class. A 4.7" DP gun was originally carried, but before plans were completed, the complexity of this mount gave way to a simple twin 4.7" LA mounting, limited to 40° elevation. To increase bottom weight, lower the centre of gravity and increase stability, heavier, more powerful machinery was installed, but boiler power reduced, so they had no real increase in speed over the "Tribals". The loss of one of the "Tribals" 4.7" twin mountings was offset by adding an extra pentuple torpedo bank, and thus 20 ft. could be removed from the length. Unfortunately, what they lacked in length they made up for in complexity and cost, being more expensive that a "Tribal", and only of same performance, if on a smaller hull.
Several flaws, such as mounting the aft twin 4.7" mount to train forward left it with large blind arcs aft, and arcs masked by the superstructure forward (!) were found in the design, but they were none-the-less pouolar, due to the powerful armament in the small hull.
The loss of one boiler over the "Tribals" allowed only one funnel to be needed, and this was a change from all revious ships, one that would remain for some time.
During the war, AA armament was augmented, and, because of additional equipment needed, they were now about 70 tins overweight, and saw feirce fighting, losing six of their number wartime.
The remaining units stepped a lattice mast, and increased light AA armament was again added, in the form of 20mm AA cannons, and modern RDF was installed, requiring a mainmast to be stepped for the first time in this class.

Pictures of "J&K" class Destroyers

Javelin showing herself off well, especially twin 4.7" mounts and the characteristic bridgework of destroyers of the time

Javelin in 1941. Note 4" AA amidships and single funnel