Editing LSWR N15 class (section)

===Performance of the Urie batch and modifications===
Under LSWR ownership, the N15s were initially well received by crews, though the batch soon gained a reputation for poor steaming on long runs.<ref name=Clarke-49 /> Through running of the class into Exeter was stopped in favour of engine changes at Salisbury, and Urie attributed the problem to poor driving technique.<ref name=Bradley44 /> A series of trial runs changed this assumption, and demonstrated that steam pressure gradually decreased on the flat.<ref name=Clarke-49 /> The trials also revealed that the robust construction of the motion produced the heaviest [[hammerblow (railway)|hammerblow]] of any British locomotive class, and had caused cracked frames on the test locomotive.<ref name=Bradley43-44>Bradley (1987), pp. 43–44</ref>

Another criticism from locomotive crews concerned the exposed cab in bad weather, which necessitated the installation of a [[tarpaulin]] sheet over the rear of the cab and the front of the tender, restricting rearward vision.<ref name=Chadwick439-442>Chadwick (2005), pp. 439–442</ref> The [[Black Friday (1921)#The mining crisis, March 1921|1921 Coal Miners’ strike]] meant that two class members (Nos.&nbsp;737 and 739) were converted to oil-burning.<ref name=Bradley44 /> One of the modified locomotives subsequently caught fire at Salisbury shed, and both were reverted to coal firing by the end of the year.<ref name=Bradley44 /> When the LSWR was amalgamated into the Southern Railway in 1923, Urie had done little to remedy the shortcomings of the N15s, and it fell to his successor to improve the class.<ref name=Bradley44 />

When Maunsell inherited the design as CME of the Southern Railway, he began trials using the weakest N15 (No.&nbsp;742) in 1924.<ref name=Bradley46 /> The results indicated that better performance could be obtained by altering the steam circuit, valve travel and draughting arrangements, although the first two recommendations were deemed too costly for immediate implementation by the Locomotive Committee.<ref name=Bradley46 /> Eight extra King Arthur-type boilers were ordered from North British and fitted to N15s Nos.&nbsp;737–742 by December 1925 in an effort to improve steaming.<ref name=Bradley48 /> The remaining Urie boilers were fitted with standard Ross pop safety valves to ease maintenance.<ref name="Haresnape54" /> Maunsell also addressed draughting problems caused by the narrow Urie "stovepipe" chimney.<ref name=Clarke-49 /> The exhaust arrangements were modified on No.&nbsp;737 using the King Arthur chimney design and reduced-diameter [[blastpipe]]s.<ref name="Haresnape54" /> This proved successful, and all "Urie N15s" were modified over the period 1925–1929.<ref name=Swift-38>Swift (2006), p. 38</ref> The oil-burning equipment was refitted to Nos.&nbsp;737 and 739 during the 1926 [[1926 United Kingdom general strike|General Strike]] and removed in December of that year.<ref name=Bradley48 />

Beginning in 1928, all but No.&nbsp;755 had their [[Cylinder (locomotive)|cylinder]] diameter reduced from {{convert|22|in|mm}} to {{convert|21|in|mm}} when renewals were due, improving speed on flat sections of railway, but affecting their performance on the gradients west of Salisbury.<ref name=Clarke-50 /> No.&nbsp;755 ''The Red Knight'' was modified in 1940 by Maunsell's successor, [[Oliver Bulleid]] with his own design of {{convert|21|in|mm|adj=on}} cylinders and streamlined steam passages. This was married to a [[Lemaître exhaust|Lemaître]] multiple-jet [[blastpipe]] and wide-diameter chimney, allowing the locomotive to produce performances akin to the more powerful Lord Nelson class.<ref name=Bradley53>Bradley (1987), p. 53</ref> Four other N15s were so modified with four more on order, though the latter were cancelled due to wartime shortages of metal.<ref name="Haresnape60" />  The soft exhaust of the Lemaître multiple-jet blastpipe precipitated an adjustment to the smoke deflectors on three converted locomotives, with the tops angled to the vertical in an attempt to improve air-flow along the boiler cladding.<ref name="Haresnape60" /> This failed to achieve the desired effect, and the final two modified locomotives retained the Maunsell-style deflectors.<ref name=Bradley53 />

The final modifications to the "Urie N15s" involved the conversion of five locomotives (Nos.&nbsp;740,&nbsp;745,&nbsp;748,&nbsp;749 and 752) to oil-firing in 1946–1947.<ref name=Bradley57>Bradley (1987), p. 57</ref> This was in response to a government scheme to address a post-war coal shortage.<ref name=Bradley57 /> The oil tanks were fabricated from welded steel and fitted within the tender coal space.<ref name=Bradley57 /> After initial problems with No.&nbsp;740 ''Merlin'' were rectified, the oil-fired locomotives proved good performers on Bournemouth services.<ref name=Bradley57 /> A further addition to the oil-fired locomotives was electric headcode and cab lighting, which was retained when the engines reverted to coal-firing in 1948.<ref name=Bradley57 />