SR Merchant Navy Class - Performance of The Unrebuilt Locomotives

Performance of The Unrebuilt Locomotives

The new locomotives demonstrated that they could generate enormous power using mediocre quality fuel, due largely to Bulleid's excellent boiler. They also ran very smoothly at high speed. Partly as a result of having so many novel features, the first few years of service by the Merchant Navy class were beset by a variety of technical problems. Some of these were merely teething troubles, but others remained with the class throughout their working lives. These may be summarised as follows:

• Adhesion problems. The locomotives were often prone to wheelslip, and required very careful driving when starting a heavy train from rest, but once into their stride they were noted for their free running, excellent steam production and being remarkably stable when hauling heavy expresses.
• Maintenance problems. The chain driven valve gear proved to be expensive to maintain and subject to rapid wear. Leaks from the oil bath onto the wheels caused oil to splash onto the boiler lagging in service. Once saturated with oil, the lagging attracted coal dust and ash which provided a combustible material, and as a result of the heavy braking of the locomotives, sparks would set the lagging on fire underneath the air-smoothed casing. The fires were also attributed to oil overflowing from axlebox lubricators onto the wheels when stationary to be flung upwards into the boiler lagging in service. In either case, the local fire brigade would invariably be called to put the fire out, with cold water coming into contact with the hot boiler, causing stress to the casings. Many photographs show an un-rebuilt with "buckled" (warped) casings, the result of a lagging fire.
• High fuel consumption. This became very apparent in the 1948 Locomotive Exchange Trials and at trials at the Rugby locomotive testing plant in 1952.
• Restricted driver visibility due to the air-smoothed casing. The exhaust problem was never adequately resolved, and continued to beat down onto the air-smoothed casing when the engine was on the move, obscuring the driver's vision from the cab.

As a result of these problems serious consideration was given to scrapping the class in 1954, and replacing them with Britannia class locomotives. However, the locomotives had excellent boilers and several other good features and so the decision was taken to rebuild them removing several of Bulleid's less successful ideas.