page_title: SR Merchant Navy class
section_title: Performance of the unmodified locomotives
hierarchical_section_title: SR Merchant Navy class / Performance of the unmodified locomotives
caption_attribution_description: English: Down 'Bournemouth Belle' passing Farnborough. View eastward, towards London: ex-LSW Waterloo - Basingstoke and the West main line. The 'Bournemouth Belle' was the first Pullman train to be reintroduced after the War, leaving Waterloo at 12.30 it reached Bournemouth West at 14.52. Here Bulleid 'Merchant Navy' No. 35012 'United States Lines' (built 1/45, rebuilt 2/57, withdrawn 4/67) is at its head.
context_page_description: The SR Merchant Navy class is a class of air-smoothed 4-6-2 Pacific 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 class members were named after the Merchant Navy shipping lines involved in the Battle of the Atlantic, and latterly those which used Southampton Docks, a publicity masterstroke by the Southern Railway, which operated Southampton Docks during the period.
context_section_description: The new locomotives demonstrated that they could generate enormous power using mediocre quality fuel, due largely to Bulleid's excellent boiler. They also ran very smoothly at high speed. Partly as a result of having so many novel features, the first few years of service by the Merchant Navy class were beset by a variety of technical problems. Some of these were merely teething troubles, but others remained with the class throughout their working lives. These may be summarised as follows:
  Adhesion problems. 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.
  Maintenance problems. The chain driven valve gear proved to be expensive to maintain and subject to rapid wear. Leaks from the oil bath onto the wheels caused oil to splash onto the boiler lagging in service. Once saturated with oil, the lagging attracted coal dust and ash which provided a combustible material, and as a result of the heavy braking of the locomotives, sparks would set the lagging on fire underneath the air-smoothed casing. The fires were also attributed to oil overflowing from axlebox lubricators onto the wheels when stationary to be flung upwards into the boiler lagging in service. In either case, 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 casing. Many photographs show an unmodified locomotive with a 'buckled' (warped) casing, the result of a lagging fire.
  High fuel consumption. This became very apparent in the 1948 Locomotive Exchange Trials and at trials at the Rugby locomotive testing plant in 1952.
  Restricted driver visibility due to the air-smoothed casing. The exhaust problem was never adequately resolved, and continued to beat down onto the air-smoothed casing when the engine was on the move, obscuring the driver's vision from the cab.
  As a result of these problems, in 1954 serious consideration was given to scrapping the class and replacing them with Britannia class locomotives. However, the locomotives had excellent boilers and several other good features and so the decision was taken to rebuild them, removing several of Bulleid's less successful ideas.
caption: 35012 United States Lines hauling the Bournemouth Belle in 1950
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