page_title: SR Merchant Navy class
section_title: Air-smoothed casing, smokebox and blastpipe
hierarchical_section_title: SR Merchant Navy class / Design / Air-smoothed casing, smokebox and blastpipe
caption_attribution_description: English: 'Bournemouth Belle' leaving Waterloo View towards bufferstops: ex-LSW London terminus of main lines to the West. The 'Bournemouth Belle' (12.30 Waterloo - Bournemouth West) was the first Pullman express to be restored after the War and this is its fourth run. Bulleid Pacific No. 21C18 'British India Line' was built 5/45, rebuilt 2/56 and withdrawn 8/64.
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 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 (strengtheners used to maintain the shape) attached to the frames.  The smokebox housed the five-nozzle Lemaître blastpipe arranged in a circle within a large-diameter chimney.
caption: 21C18 British India Line hauling the Bournemouth Belle in 1946
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