The SR Merchant Navy class (originally known as the 21C1 class, and later informally known as Bulleid Pacifics, Spam Cans – which name was also applied to the Light Pacifics – or Packets) 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. 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 (167 km/h); such speeds were recorded by examples including No. 35003 Royal Mail (since scrapped) and Nos. 35005 Canadian Pacific and 35028 Clan Line (both preserved). 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 (CME) 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 twenty express passenger locomotives. The