The design process for the J, K, N, Classes of Destroyer.

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The design process for the replacements for the A-I class started in 1936 and the final half dozen designs being considered were supplied to the DNC in August 1936. In all cases the class leader would be the same as the rest of the class. All of the designs were to be fitted with two quintuple torpedo tube mountings. The new designs had single and twin mountings mixed on the same hull and as seperate fittings.

The main armaments being considered were:
JA 4x4.7" (1x2, 2x1)
JB 4x4.7" (4x1)
JD 6x4.7" (3x2)

The first two designs JA and JB were to be able to be fitted for minelaying operations.

The final working meeting with the DNC and staff in September 1936 at which time the DNC noted that the new destroyers should not be armed with four guns whether in single
mountings or as a mix of twin and singles, to avoid having a large hull with an armament that compared unfavourably with foreign competition. So the JA and JB designs were to
be disregarded and the JD design was to be the working plan that the final design was to be formed from.

Discussion followed which set the following parameters for the new design:
(1) That the design should be worked out with six 4.7" in twin mountings.
(2) One four barrelled 2 pdr pom pom be mounted as the principal AA weapon.
(3) Quintuple torpedo tubes to be fitted.
(4) Two designs showing both two and three boiler room layouts be worked on. (The Engineer in Chief was against two boilers because of the difficulty of maintenance and operational difficulties when the ship was operating on one boiler.)
(5) The 4.7" mountings to be powered by an electro-hydraulic system.
(6) The Leader to be of the same type as the destroyers. (An enlarged aft superstructure was allowed for, for the increased accomodation required for the Leader.)



This is design JA1, 329 feet, two boiler rooms, one engine room, 4x4.7" in one twin aft and two singles forward. The new bridge structure from the Tribal Class was carried through to this class.

The JA3 design was essentially a repeat of the JA1 but with three boiler rooms in place of the two on the JA1. This increased the length to 340 feet.

The JB1 and JB3 designs were a throwback to the I class. Main changes being the larger bridge and the introduction of the quad 2 pounder mounting. Dimensions mirrored the JA designs.

JD1 and JD3 became the final design after the DNC made his wishes known. At 348 and 353 feet, the designs were still smaller than the final design for the J's at 356.5 feet (an extra 8 feet over JD1).

This was an interesting process to follow and draw. The evolution of the design through to the J Class below shows how the designs are living things. Until the DNC signs off on the design and it is put into production, additions and alterations can change the design markedly.

Final production design for HMS Javelin.

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