SR Merchant Navy Class - Subsequent Modifications

Subsequent Modifications

As mentioned, the main production batch of Southern-built locomotives differed from the two prototypes, Channel Packet and Union Castle. The steam-operated firehole door treadle was removed, and a new type of boiler cladding was utilised in response to the worsening supply situation during the Second World War. Modification was also made to the air-smoothed casing surrounding the smokebox after reports were made of drifting smoke obscuring the locomotive crew's vision ahead. Initially, the only form of smoke deflection was a narrow slot in front of the chimney, intended to enable air to lift the smoke when the locomotive was travelling. This proved inadequate because of the relatively soft exhaust blast that came from the multiple-jet blastpipe, which failed to be caught by the air flow. After several trials, the air flow was increased by extending the casing roof over the front of the smokebox to form a cowling whilst side smoke deflector plates were also incorporated into the front of the air-smoothed casing. The latter added to the poor visibility from the footplate and the expedients combined never fully solved the smoke drift problem.

During the brief time they operated under the Southern Railway, further modifications were applied to the class, such as the reduction in boiler pressure to 250 psi (1.72 MPa) and the redesign of the footplate spectacle plates. These are the small windows on the front face of the cab, which were redesigned to a wedge-shaped profile, a feature to be seen on all Bulleid-designed locomotives post-nationalisation. They had been introduced in Britain in 1934 with the Gresley-designed Cock o' the North. Originally, the spectacle plates of the Bulleid Pacifics were at the conventional right-angle to the direction of the locomotive, and offered limited vision ahead along the air-smoothed casing. The Southern-built batches also had variations in the material used for the air-smoothed casing with a change from sheet steel to an asbestos compound, forced upon the manufacturer by wartime expediency. This resulted in several class members having a horizontal strengthening rib running down the length of the casing. The final Southern Railway-initiated experiment involved equipping 21C5 Canadian Pacific with a Berkeley mechanical stoker imported from Canada. Little improvement in performance was seen when trialled under British Railways auspices in 1948 and the locomotive was re-converted to hand-firing.

As mentioned, the British Railways batch had detail differences to previous versions. The most significant modification was the reduction of weight using lighter materials unavailable during wartime. From 1952 the air-smoothed casing ahead of the cylinders was removed to ease maintenance and lubrication. This coincided with the removal of the tender "raves" on all locomotives, as they obstructed the packing of coal into the bunker and restricted the driver's view when reversing the locomotive. The resultant "cut-down" tender included new, enclosed storage for fire-irons, revised step ladders and glass spectacle plates to protect the crew from flying coal dust when running tender-first.