SR Merchant Navy class
The SR Merchant Navy class is a class of air-smoothed 4-6-2 steam locomotives designed for the Southern Railway by Oliver Bulleid. The Pacific design was chosen in preference to several others proposed by Bulleid. The first members of the class were constructed during the Second World War, and the last of the 30 locomotives in 1949.
Incorporating a number of new developments in British steam locomotive technology, the design of the Merchant Navy class was among the first to use welding in the construction process; this enabled easier fabrication of components during the austerity of the war and post-war economies. In addition, the locomotives featured thermic syphons in their boilers and the controversial Bulleid chain-driven valve gear. The engines were named after the Merchant Navy shipping lines involved in the Battle of the Atlantic, and latterly those which used Southampton Docks: a publicity move by the Southern Railway, which operated the docks at the time.
Due to problems with some of the more novel features of Bulleid's design, all members of the class were modified by British Railways during the late 1950s, losing their air-smoothed casings in the process. The Merchant Navy class operated until the end of Southern steam in July, 1967. A third of the class has survived and can be seen on heritage railways throughout Great Britain. They were known for reaching speeds of up to 105 mph ; such speeds were recorded by examples including No. 35003 Royal Mail and Nos. 35005 Canadian Pacific and 35028 Clan Line.
Background
The Southern Railway was the most financially successful of the "Big Four", but this was largely based on investment in suburban and main line electrification. After the successful introduction of the SR Schools class in 1930, the railway had lagged behind the others in terms of modernising its ageing fleet of steam locomotives. Following the retirement of the general manager of the Southern Railway Sir Herbert Walker and Richard Maunsell the Chief Mechanical Engineer in 1937, their successors considered that the time had come to change this situation. In March 1938, the new general manager Gilbert Szlumper authorised Oliver Bulleid, Maunsell's replacement, to prepare designs for 20 express passenger locomotives. The deteriorating international situation prior to the Second World War was an additional factor in this decision.Bulleid's first suggestion was for an eight-coupled locomotive with a 4-8-2 wheel arrangement for the heavily loaded Golden Arrow and Night Ferry Continental express trains, although this was quickly modified to a 2-8-2, a wheel arrangement associated with Nigel Gresley's P2 locomotives; Bulleid himself had worked with Gresley in the past. A second "Mikado" locomotive design was planned to have a Helmholtz pony truck – a system already successfully applied on the Continent. However, both proposals for eight-coupled locomotives were resisted by the Southern Railway's Chief Civil Engineer, so a new 4-6-2 Pacific design was settled upon instead. The new design was intended for express passenger and semi-fast work in Southern England, though it had to be equally adept at freight workings due to the nominal "mixed traffic" classification Bulleid applied to the class for them to be built during wartime. Administrative measures had been put into place by the wartime government, preventing the construction of express passenger locomotives, due to shortages of materials and a need for locomotives with freight-hauling capabilities. Classifying a design as "mixed traffic" neatly circumvented this restriction.
Design
Most of the detailed design for the Merchant Navy class was undertaken by the drawing office at Brighton works, but some work was also undertaken by Ashford and Eastleigh. This division of responsibility was possibly due to Bulleid's wish to restrict knowledge of the new class to a limited number of personnel. The design incorporated a number of novel features, compared to then-current steam locomotive practice in Great Britain.Cylinders, valve gear, wheels and brakes
Three diameter cylinders drove the centre coupled axle. The inside cylinder was steeply inclined at 1:7.5, but the outside cylinders were all horizontal.It was originally intended to use a gear-driven valve gear, but space restrictions within the frames and wartime material shortages led Bulleid to design his novel chain-driven valve gear. This component was unique amongst British locomotive design practices. It later gained a bad reputation, because it could cause highly irregular valve events, a problem compounded by the fast-moving Bulleid steam reverser. The entire system was located in a sealed oil bath, another unique design, providing constant lubrication to the moving parts.
The locomotives were equipped with the unusual 6 ft 2 in Bulleid Firth Brown driving wheels which both lighter and stronger than the spoked equivalent. These proved to be successful and were later used on other Bulleid classes. The leading bogie was based upon that of the SR Lord Nelson class, although it had a 6 ft 3 in wheelbase as opposed to Maunsell's 7 ft 6 in design, and featured 3 ft 1 in BFB wheels. A long coupled driving wheelbase was incorporated into the design, to keep the locomotives within the lineal loading of the Southern Railway's narrower bridges. The supporting rear trailing truck was a one-piece steel casting that gave the smoothest of rides; the design was utilised in the future BR Standard Class 7.
The spaces between driving wheels housed steam-powered clasp brakes, that gripped the wheels by way of a "scissor" action. The two middle brake hangers held two brake blocks each, whilst the two outside hanger on the leading and rear driving wheels held one block each. These were connected together by outside rodding for ease of access, and the whole system was operated from the footplate.
Boiler and welded firebox
The maximum boiler pressure was higher than any other British regular service locomotive at 280 psi.Bulleid decided on cheaper all-welded fireboxes for the boilers as opposed to more common riveted construction, and a steel inner firebox which was lighter than a more usual copper example. Two welded steel thermic syphons were implemented to improve water circulation around the firebox and these were subcontracted to Beyer Peacock.
However he soon discovered that the Southern Railway lacked the facilities to manufacture welded boilers of this size, so the first ten boilers were ordered on outside contract from the North British Locomotive Company.
Air-smoothed casing, smokebox and blastpipe
The boiler was enveloped by Bulleid's air-smoothed sheet-steel casing, which was not for the purposes of streamlining, as demonstrated by the extremely flat front end, but as a way of lifting exhaust gases. The flat sides were also an aid to cleaning the locomotive with mechanical carriage washers, representing an attempt to reduce labour costs. It followed the profile of the Belpaire firebox and extended to a curved profile forward of the smokebox front. Spun glass mattresses were used for boiler lagging. The smokebox was a sheet metal fabrication to the same profile as the firebox, acting as a former to maintain the shape of the air-smoothed casing. In between, the casing was supported by channel-section steel crinolines attached to the frames. The smokebox housed the five-nozzle Lemaître blastpipe arranged in a circle within a large-diameter chimney.Tender
Bulleid designed a new tender which could carry 5 tons of coal on a six-wheel chassis. It featured BFB wheels and streamlined panels, or "raves", that gave the top of the tender a similar cross-sectional outline to the carriages hauled by the locomotive. The water tank was of welded sheet construction to save weight, and the tender was fitted with vacuum braking equipment of a clasp-type similar to that used on the locomotive. Three train-brake vacuum reservoirs of cylindrical construction were grouped on the tank top, behind the coal space. Unusually for a British locomotive, two extra water filler caps were incorporated into the tender front, for access from the footplate. The original tender design proved to be inadequately braced and subject to serious leakage if even slightly damaged, or when water surges caused the welded joints to split. The problem was not solved until 1944 when additional baffling was fitted.Other innovations
lighting was also provided on both the locomotive and the footplate, supplied by a steam-powered generator fitted below the footplate. The gauges had fluorescent dial markings lit by ultra-violet light. This enabled clearer night-time vision of the boiler steam pressure gauge and the brake pipe vacuum gauge, whilst eliminating dazzle, making it easier for the crew to see signals along the track. Close attention was also paid to the ergonomics of the driving cab, which was designed with the controls required for operation grouped according to the needs of both fireman and driver, thus promoting safe operation. As an aid to the fireman, a steam-operated treadle was provided that used steam pressure to open the firehole doors. The footplate was entirely enclosed, improving crew working conditions in winter.Construction
The Southern Railway placed an order for ten of the new locomotives to be built at Eastleigh Works, although the boilers had to be supplied from private industry and the tenders were built at Ashford. The prototype was completed in February 1941, numbered 21C1, and named Channel Packet at a ceremony at Eastleigh works on 10 March 1941. It underwent extensive trials and minor modifications before joining Southern Railway stock 4 June 1941. A second prototype, 21C2 Union Castle was completed in June and named at Victoria railway station 4 July. Both prototypes were found to be seven tons over the specified weight, and, at the insistence of the Southern Railway Civil Engineer, production of the remainder was halted until steps were taken to remedy this. This was achieved by using thinner steel plates for the frame stretchers and covering the boiler cladding, and enlarging the existing lightening holes in the main frames. The remaining eight locomotives in the batch were delivered between September 1941 and July 1942.A second batch of ten followed, beginning in December 1944 and culminating in June 1945. These were entirely constructed at Eastleigh and equipped with tenders. The Merchant Navy class spawned the design and construction of a lighter version of the same locomotive with consequently increased route availability. These were the West Country and Battle of Britain class Light Pacifics, the first of which entered service in 1945.
Just prior to the nationalisation of the railways in 1948, the Southern Railway placed an order for ten more Merchant Navy locomotives, with larger tenders. A shortage of materials meant that delivery was delayed until September 1948, and completed in 1949; the batch never carried Southern Railway numbers. Eastleigh was responsible for the construction of the final batch, which were in the series 35021–35030. Construction was undertaken in-house by Eastleigh works, with the boilers and tenders constructed at Brighton, the frames at Ashford and the rest at Eastleigh. These were equipped with wedge-shaped cab fronts from the outset, and greater use of welding ensured lighter locomotives. The batch was also fitted with the TIA chemical feed-water equipment used on the Light Pacifics. This precipitated scale-forming constituents in the "hard-water" of southern England into a non-adhesive mud that could be cleared from the locomotive using a manual "blow-down" valve. A delay in the construction of the new larger tenders for the new locomotives meant that some were fitted with the smaller examples intended for use with Light Pacifics that were under construction at the time. Two spare boilers for the class were also constructed at Brighton and Eastleigh during 1950/1.