the handrails on the smokebox sides. The experiments produced mixed results, and Maunsell requested the assistance of the University of London in staging wind tunnel tests . These resulted in a standard plate design (illustrated in the infobox), which was gradually fitted to the class from late 1927 onwards. 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. 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. A series of trial runs changed this assumption, and demonstrated that steam pressure gradually decreased on the flat. The trials also revealed that the robust construction of the motion produced the heaviest hammerblow of any British locomotive class, and had caused cracked frames on the test locomotive. 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. The 1921 Coal Miners’ strike meant that two class members (Nos. 737 and 739) were converted to oil-burning. One of the modified locomotives subsequently caught fire at Salisbury shed, and both were reverted to coal firing by the end of the year. 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. When Maunsell inherited the design as CME of the Southern Railway, he began trials using the weakest N15 (No. 742) in 1924. 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. Eight extra King Arthur-type boilers were ordered from North British and fitted to N15s Nos. 737–742 by December 1925 in an effort to improve steaming. The remaining Urie boilers were fitted with standard Ross pop safety valves to ease maintenance. Maunsell also addressed draughting problems caused by the narrow Urie "stovepipe" chimney. The exhaust arrangements were modified on No. 737 using the King Arthur chimney design and reduced-diameter blastpipes. This proved successful, and all "Urie N15s" were modified over the period 1925–1929.