SR Merchant Navy Class Totally Explained

Everything about The Sr Merchant Navy Class totally explained

The SR Merchant Navy Class, also known as Bulleid Pacifics, Spam Cans or Packets, was a class of air-smoothed 4-6-2 Pacific steam locomotives designed for the Southern Railway of the United Kingdom 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 Packets 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. The locomotives featured thermic syphons and Bulleid's innovative, but controversial, chain-driven valve gear. Oliver Bulleid, who had been brought in from the LNER to replace Richard Maunsell as Chief Mechanical Engineer (CME) believed that it was time for the Southern to modernise the locomotive fleet. This was because the company's newest locomotive design was the 0-6-0 Q Class, influenced by Victorian design principles. Bulleid's first suggestion was for an eight-coupled locomotive with a 4-8-2 wheel arrangement, although this was quickly modified to a 2-8-2, equipped with a Helmholz "Bissel-Bogie" – a system already successfully applied on the Continent. The Bissel-Bogie comprised a pivoted frame, similar to that of a bogie, supporting a leading pair of carrying wheels and the front pair of driving wheels. In a curve, the radial movement of the carrying wheel was compensated by the leading coupled wheel sliding laterally over to the opposite side, hence reducing wear and improving ride quality. 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. Classifying a design as 'mixed traffic' neatly circumvented this restriction.

Design features

The Merchant Navy Class design was originally intended to use gear-driven valve gear, though space restrictions within the frames and wartime material shortages led Bulleid to design his novel chain-driven valve gear. It 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. Electric lighting was also provided, supplied by a steam-powered generator fitted below the footplate. The tender could carry of water and featured streamlined panels, or "raves", that gave the top of the tender a similar cross-sectional outline to the carriages hauled by the locomotive. As an aid to the fireman, a steam-operated treadle was provided that used steam pressure to open the firehole doors (where the coal is shovelled into the firebox). 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 redesign of the footplate spectacle plates. These are the small windows on the front face of the cab, which were redesigned to an angled profile, a feature to be seen on all Bulleid-designed locomotives post-nationalisation. 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 ten engines in the class were constructed after the nationalisation of the railways in 1948.
   The Merchant Navy Class spawned the design and construction of the West Country and Battle of Britain Classes, from 1945, a lighter version of the same locomotive with consequently increased route availability. These were required for use on the Southern's "Withered Arm" west of Exeter, as the heavier Merchant Navy Class locomotives were banned from this section due to weight restrictions. This idea resonated in 1941 because the shipping lines were heavily involved in the Atlantic convoys to and from Britain during the Second World War.
   A new design of nameplate was created, featuring a circular plate with a smaller circle in the centre. The inner circle carried the colours of the shipping company on a stylised flag, on an air force blue background. Around the outer circle was the name of the locomotive, picked out in gilt lettering. A horizontal rectangular plate was attached to either side of the circular nameplate, with "Merchant Navy Class" in gilt lettering. The nickname Packets was also adopted by locomotive drivers, as the first member of the class was named Channel Packet. The British Railways standard numbering system placed this batch in the series 35021–35030, with all locomotives completed at Eastleigh between 1948 and 1949.

Rebuilding

Between 1956 and 1960, British Railways rebuilt the entire class to a more conventional design by R. G. Jarvis, adopting many features from the BR 'Standard' locomotive classes that had been introduced. The air-smoothed casing was removed and replaced with conventional boiler cladding, and the chain-driven valve gear was replaced with Walschaerts valve gear, whilst the boiler pressure was reduced from 280 to 250 psi. The first to be released from Eastleigh was 35018 British India Line in 1956. The final example, 35028 Clan Line, was completed in 1960. The rebuilt Merchant Navy Class was also to influence the design of the future rebuild of 60 Light Pacifics.
As a result of the implementation of Walschaerts valve gear, the rebuilts were prone to hammerblow on the track caused by the additional weights incorporated to balance the outside valve gear, an issue that hadn't arisen with the self-balanced arrangement of the original locomotives.

Operational details

The utilisation of welded steel construction and several innovations that hadn't previously been seen in British locomotive design meant that the class earned Bulleid the title 'Last Giant of Steam'. The constant concern for ease of maintenance and utility hadn't previously been seen on locomotives of older design, whilst their highly efficient boilers represented the ultimate in British steam technology, the hallmark of a successful locomotive design. The locomotives were largely successful, though their heavier axle loading meant that they were banned from certain areas of the Southern Railway, and later, the British Railways Southern Region network. The locomotives were often prone to wheelslip, and required very careful driving when starting a heavy train from rest, but once into their stride they were noted for their free running, excellent steam production and being remarkably stable when hauling heavy expresses. The local fire brigade would invariably be called to put the fire out, with cold water coming into contact with the hot boiler, causing stress to the casings. Many photographs show an un-rebuilt with "cockled" (warped) casings, a result of a lagging fire. The rebuilts were provided with LMS-style smoke deflectors and the lack of air-smoothed casing also helped reduce the problem.

Livery and numbering

Southern Railway

Livery was Southern Railway Malachite Green with Sunshine Yellow horizontal lining and lettering.

Post-1948 (nationalisation)

After nationalisation in 1948, the locomotives' initial livery was a slightly modified Southern Malachite Green livery, where "British Railways" replaced "Southern" in Sunshine Yellow lettering on the tender sides. The Bulleid numbering system was temporarily retained, with an additional "S" prefix, such as S21C1. From 1949, the locomotives were renumbered under the British Railways standard system in the series 35001–35030.

Preservation

Many of the class have survived into preservation thanks to the unusual practice at Barry Scrapyard in South Wales, where steam engines bought from British Railways for scrapping were stored rather than immediately cut up. One engine, 35028 Clan Line, was bought by the Merchant Navy Locomotive Preservation Society direct from British Railways in 1967, and has since been regarded as the 'flagship' of the class.

Preserved Merchant Navy Class locomotives

35005 Canadian Pacific, Watercress Line (Mid-Hants Railway)
35006 Peninsular & Oriental S. N. Co., Gloucestershire Warwickshire Railway
35009 Shaw Savill, East Lancashire Railway
35010 Blue Star, British Enginemen's Steam Preservation Society, Colne Valley Railway
35011 General Steam Navigation, Williton, West Somerset Railway
35018 British India Line, South Coast Steam, Portland
35022 Holland-America Line, Southall, London
35025 Brocklebank Line, Sellindge, Kent
35027 Port Line, Stored at Southall, London
35028 Clan Line, Stewarts Lane, London
35029 Ellerman Lines, National Railway Museum (sectioned to show internal workings of a steam locomotive)

� For location details and current status of the preserved locomotives, see: List of SR Merchant Navy Class locomotives

Footnotes

Further Information

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