After many years at Doncaster under the steadying influence of Nigel Gresley it is perhaps no surprise that the Merchant Navy pacifics, Oliver Bulleid's first design, bristled with innovation. The performance target he set was to haul a 600 ton boat train from London to Dover at a start-to-stop average of 60mph. In fact the traffic department had no such speed requirement and no platforms to accomodate such a length of train! The locomotive he produced was unlike anything seen before on a British railway.
The boiler, of conventional design, was married to an all steel welded firebox incorporating two thermic syphons. Later testing at the Rugby stationary test plant indicated a steam-producing capacity of at least 42,000 lbs per hour. The three cylinders all drove the centre coupled axle. Steam distribution was by three sets of miniaturised Walchaerts valve gear running enclosed in an oil bath set between the main frames. These were driven by chain from a sprocket on the centre coupled axle. The exhaust was of the LeMaitre multiple jet type which had so successfully rejuvenated the Lord Nelson's.
The locomotive wheels were a specially designed patented by Bulleid and steel makers Firth Brown generally refered to as BFB. They were in fact very simiilar in appearance to the American 'Box-Pox' wheels. Conventional boiler cladding was discarded in favour of a casing in the shape of an inverted 'U'. It didn't take long for some wag to point out the similarity to the shape of a can of Spam thereby giving rise to the nickname 'Spam Can' or simply 'Can'. Bulleid's intention was that this casing would allow the locomotives to be washed in a carriage washing plant, an event for which no records appear to exist.
Another first for a British engine was the use of electric headcode and inspection lamps supplied by a small steam powered generator. Inside the cab an ultra-violet lamp was used for nightime illumination of various gauges which incorporated flourescent markings. Eschewing the traditional British symetric layout of controls and piping, Bulleid grouped all those required by the driver to the left of the cab, those required by the fireman on the right. The firedoors could be operated either manually or by a small steam cylinder which the fireman actuated with a foot pedal. After all this innovation the tender was completely conventional.
Bulleid adopted a continental system of numbering, the first of the class being 21C1 ie 2 leading axles, 1 trailing axle, six coupled wheels, the second 21C2 and so on. The names derived from shipping lines using the Southern Railways port at Southampton.
With so many (for British practise) unconventional features it is perhaps not surprising that that many teething problems were encountered. While most of these were addressed in time the problem of smoke clearance was never really successfully tackled despite various redesigns of the front end casing. Another lingering problem was the high consumption of oil which leaked profusely from the oil bath. However, there was no doubt about the haulage capacity - with all forms of transport stretched to the limit during the war a Merchant Navy could handle a 20 coach train apparently with ease.
The 1948 interchange trials really told everyone what they already new about these engines - seemingly unlimited steaming capacity but very uneconomical compared to other contemporary locos. Stationary test confirmed this. The valve gear and steam reverser came in for much criticism apparently subject to completely random changes in cutoff. With nationalisation in place and Bulleid now departed for Ireland pressure grew to address the problem areas. The result was a radical redesign. The valve gear was replaced by a conventional Walschaerts setup. The "spam can" disappeared in favour of conventional boiler cladding to ease maintenance access and the unsatisfactory steam reverser was discarded in favour of a manual version - harder work for the driver to work but at least it stayed put. The Brighton drawing office had the very good sense to leave the boiler, firebox and pistons untouched.
The result was a conventionally handsome loco. Some operating economies were achieved though perhaps less than had been hoped for. The drivers, not surprisingly, were less enthusiastic about the changes but nevertheless were able to extract equally good performances from the rebuilt locos.
All of Bulleid's designs have produced a great partisan following amongst British steam railfans. It is no surprise then that many of this class survived into preservation. The National Railway museum in York has one, Ellerman Lines, which will certainly never run again - it has been sectioned to show the internal workings of a steam loco.
Photos to go here.