J Class Destroyers

Briefing
Written by
James Davies

Key Information

Overview

Destroyers are small warships (defined in the London Treaty of 1930 as being no more than 1,850 tons), and armed with light weapons (guns of calibre no more than 5.1 inches (130 mm)). In the British Royal Navy during the Second World War a ship must also be fitted with torpedo tubes to be classed as a destroyer. They are usually equipped for anti-submarine work, although some may alternatively be equipped for minelaying operations.

Before the Second World War, new British destroyers were generally designated as fleet destroyers for work in support of the main fleet, which includes cruisers and capital ships. Experience during the war lead to older destroyers often being refitted and re-designated as escort destroyers (principally because older destroyers lacked the speed of modern warships) and used for less glamorous tasks such as convoy escort. The need for new specialist escort destroyers was recognised, however, and these were also built during the war. The J class ships are fleet destroyers, equipped for anti-submarine work.

The naval treaties between the wars had a significant effect on the design of all warships. Weight limitations forced all naval powers to focus on weight saving designs, bringing significant advances in several areas, such as advanced propulsion systems and dual purpose weaponry (to be used in both surface and air actions). The previous destroyer class - the Tribal class - were very much larger than all other existing British destroyers, with a correspondingly greater cost to build. The J class was intended to be a compromise, smaller and cheaper than the Tribal class and yet with comparable speed and armament.

The J class were fitted with a total of six main guns and 10 torpedo tubes, compared to the Tribal class with eight main guns and four torpedo tubes. This increase in torpedo tubes represented a significant improvement in the capabilities of the ship over the Tribal class, giving them a bigger punch against capital ships. The reduction in main guns from eight to six was in practice not significant, and as most British destroyers were only armed with four guns the J class were considered to be heavily armed.

For air defence a four barrelled pom-pom was fitted. This at last gave British destroyers some defence against dive-bombers, as they fired 115 rounds per minute per barrel, and were effective out to 1100 metres (1200 yards). Finally, eight 0.5 inch (13 mm) machineguns were installed in two mounts of four guns each, although these were found to be prone to jamming.

For the anti-submarine role the ships were fitted with asdic and depth charges. Asdic was effective out to about 2500 yards (2286 metres), although high speed and bad weather both adversely affected its operation. Generally, asdic was ineffective if the ship was travelling above about 18 knots, and was unable to detect anything within about 200 yards (183 metres) of the ship. Two racks of depth charges were provided at the stern of the ships, as well as two depth charge throwers on either side.

To protect the fleet from mines provision for a two speed destroyer sweep (TSDS) was fitted to all but HMS Jervis, who as the flotilla leader had increased accommodation for staff which reduced the space available at the stern of the vessel.

There was some discontent when it was found that the J class were only slightly smaller than the Tribal class (356.5 feet compared to 377 feet for the Tribals), had fewer main guns (six compared to eight), and yet cost more to build (£390,000 each compared to £340,000). Their performance silenced the critics, and the basic hull form was adopted for the 'emergency' programme.

Eight ships formed the J class (HMS Jervis as the flotilla leader, with the others being HMS Jersey, HMS Jaguar, HMS Juno, HMS Jupiter, HMS Janus, HMS Jackal and HMS Javelin). They mainly served in the Mediterranean, and (as with all small ships) they were extremely vulnerable to air attack. Later fleet destroyers tended to have anti-aircraft armament that was better still. Only two ships survived the war, with five being sunk before the end of 1942.

Despite the heavy losses of J class it was generally accepted that the design was sound, and formed the basis for the 'emergency' designs that followed. A total of 124 ships entered service as variations on the J class design, although 28 of these were completed after the end of hostilities.

Units

There is a tradition in shipbuilding of finding a design that works, then modifying it for other newer ships so that the best of the old can be combined with new knowledge. Evolution rather than revolution is the watchword. The reasons for this are complex, but given the unpredictable nature of the sea, the time and cost of construction, the difficulty of predicting performance for a new design (still a problem today), the need for more destroyers quickly, the inability to build a prototype for testing, the difficulty of future modifications and the potential consequences of failure it can perhaps be understood why many subsequent fleet destroyers adopted a slightly simplified version of the J class hull form.

Although the variants were all different in detail from the J class, with later models incorporating hard-won war experience, they were essentially J class ships. They all had the same power plant and basic hull form, the same speed and similar main weapons (generally with slightly reduced guns to speed construction and reduce cost). Often anti-submarine capability was increased, along with anti-aircraft armament, and the greater weight made them sit lower in the water (with increased displacement and reduced freeboard).

The J class variants were the K class, N class, Q class, R class (new), S class (new), T class, U class, V class (new), W class (new), Z class, CA class, CH class, CO class, CR class and the Weapon class. These variants were operated mainly by the Royal Navy, however some units were also operated by Polish Navy (1), Royal Australian Navy (12), Royal Canadian Navy (2), Royal Netherlands Navy (2) and the Royal Norwegian Navy (2).

Specifications

Note 2: Modifications were made during the war, with the addition of radar and increased anti-aircraft armament in 1940 / 41. Changes were also made after this, generally improving air defence capability and providing better radar.

Note 3: The CH class, CO class, CR class and the Weapon class data are not included as the units were commissioned after the end of hostilities.

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