The Merchant Navy Class

Introduction

The ‘Merchant Navy’ Class (later informally known as ‘Spam Cans’ – after their appearance, ‘Packets’ – after the first of the class, or simply ‘Bulleid Pacifics’), were originally ‘air-smoothed’ 4-6-2 Pacific locomotives built for the Southern Railway of the United Kingdom to a design by the Company’s Chief Mechanical Engineer, Oliver Bulleid CB. The ‘Pacific’ type wheel arrangement was chosen in preference to several others proposed by Bulleid. The first 20 members of the class were constructed during the Second World War and the remaining 10 in 1948/49 under the aegis of the then newly nationalised ‘British Railways’.

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. The locomotives featured thermic syphons and Bulleid’s innovative, but controversial, 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 rebuilt 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.

Boiler

The boiler was designed for a working pressure of 280lb/sq. in, later reduced to 250. To reduce weight and cost, a welded steel firebox was fitted, as was then common practice in America and Europe. It incorporated two thermic syphons; although their contribution to steam production is not proven, they are a safety aid as they promote circulation of water. The first fireboxes suffered from cracking and had short lives, but the introduction of comprehensive water treatment eliminated the problem. An important advantage was that minor repairs were, and still are, carried out using conventional welding equipment.

Valve gear

Bulleid’s decision to have three cylinders, with separate valve gears, all driving the middle coupled axle, left very little space for the conventional inside set of motion, and prompted the attempt to design a new motion layout compact enough to enclose the whole lot in a casing. All three sets were worked from an auxiliary crankshaft driven by a triple chain from the axle. This shaft drove both eccentric rods and combination levers, giving the desired compact layout. The piston valves had no spindles emerging from the steam chests but were actuated by rocking shafts located between the valve heads.

The motion and the inside connecting rod were contained in a steel box between the main frames. About 2in depth of oil lay in the box, and pumps sprayed it over the various motion joints. None of this was particularly revolutionary, being borrowed from internal-combustion engine practice, and in principle it was superb. There was no daily ‘oiling round’; it was protected from the elements and could run 100,000 miles without attention. In practice, though, it proved unequal to its job, for two main reasons. Firstly, the box was inadequate; water leaked in, causing corrosion, and oil leaked out, causing wheel slip and fire hazards. Secondly, the chain drive was not positive enough for locomotive valve motion; the slack which could develop in the myriad pin joints of even the finest chain made the valve movements highly unpredictable. Moreover, a steam reverser was fitted. If this reverser decided to misbehave, the locomotive seemed to have a will of its own. For example, if it dropped into full gear, the slackness in the motion made the valves over-run their nominal full travel and the locomotive would take off like an unleashed race-horse, putting the fire out of the chimney and running the risk of a violent high-speed slip.

These antics did not worry the drivers unduly, for the engines were so powerful and had such phenomenal steam-raising capacity that as long as the firemen were willing to continue shovelling coal onto the 49sq ft. grate they would keep going. A ‘Merchant Navy’-hauled ‘Atlantic Coast Express’ once arrived right time at Waterloo with a cylinder cover missing! It must be remembered that with modifications and careful servicing the problems were largely brought under control, but the motion was considered to be the cause of the high specific operating cost of the locomotives. We have a legacy from the original arrangement in the form of outside-admission valves on the outside cylinders.

Casing

The Bulleid pacific was instantly recognisable because, instead of the familiar engine shape, it was clad in a straight-sided casing. This was claimed to allow pipe runs to be positioned without regard to appearance, and to enable the engine to be cleaned by putting it through a carriage washer. In service, it was an unmitigated nuisance. The view from the cab was poor, while at speed the flat top generated a vacuum which drew exhaust steam down into the driver’s view ahead. Due to the weight-saving drive it was flimsy, so panels removed by fitters and dumped on the shed floor quickly acquired premature wrinkles. All sorts of things could happen under the casing – not least the oil-soaked boiler lagging catching fire. Once, a driver said that whenever he opened the regulator, the lid of one of the big horn-guide oil boxes flew open! Eventually they found that one of the oil pipes was clamped up alongside the pipe which supplied the steam chest pressure gauge. Vibration had worn both pipes through where they touched, so the steam pressurised the oil pipe and blew open the lid. On rebuilding, the casing was replaced by conventional boiler cladding, but the cab remained in its original form. It is made of much thinner sheeting than other cabs, and by 1980 the cab on Clan Line was rusting through. It was rebuilt with galvanised sheet secured with pop-rivets.

Cab Fittings and Auxiliaries

The ’Merchant Navy’ is the safest steam locomotive to be built for service in this country. The fire door is a patented type with air-holes, intended to be kept closed except while firing is taking place, thus minimising the risk of blow-backs. Originally, it was power operated. The blower is operated by levers, duplicated on both sides of the cab, so that either driver or fireman can knock it full on instantly without having to step in front of the fire hole. The cab is laid out to be convenient for the crew. Mr R. E. L. Maunsell, Bulleid’s predecessor, had adopted the practice of consulting with the enginemen in his cab designs, many years before the term “ergonomics” was coined, and Bulleid followed his example.

Two Davies & Metcalfe ‘Monitor’ type live steam injectors are carried. Clan Line’s present owners have oft-times blessed the performance and reliability of these injectors, but there was one crisis in their early days. Eastleigh Works decided to manufacture some replacement steam cones, instead of buying them, but when these home-made cones were put to use the injectors failed to start at high pressure, resulting in much vituperation and dropping of fires. This gave the injectors a reputation of being assembled with some magic known only to Davies & Metcalfe, a spell which was finally broken in the 1970s when MNLPS Chief Engineer Andy Davies machined up a new cone which worked perfectly. The steam supply to the injectors, brake and other fittings comes from a manifold mounted on the boiler backhead, but the manifold itself can be isolated from outside the cab by a valve whose handle may be seen on the front left-hand corner of the firebox top. All Bulleid pacifies are fitted with a Stone’s steam turbo-generator supplying current at 24 volts for headcode lights, cab illumination and, originally, lights under the casing to facilitate servicing. Electric lights were a novelty on a British steam locomotive; after preservation, Clan Line became the only working locomotive with such a system, and it is much appreciated by her owners and British Railways engine crews.

Tender

The Bulleid tender has a curved side profile to match the standard carriages of its period, thus attempting a visual continuity between locomotive and train. There were two types, carrying 5,000 and 6,000 gallons of water. The coal capacity of 5 tons was adequate for Southern requirements, but on modern-day rail tours patrons are regularly entertained by the MNLPS team producing bags of coal from their support coach and manhandling them into the bunker. The tender is basically a box welded up from 3/16in sheet steel. As built, it was structurally weak and cracks or fractures were common. When called on to do repairs inside the tank, some welders made a practice of cutting a large piece out of the side to give themselves some light and air, and welded it back in afterwards. The frames were also weak and tended to splay outwards, resulting in excessive side-play in the wheels. The 6,000gal tender, No 3342, preserved with Clan Line, showed all the ailments; it received little attention during latter BR years and when lifted in 1981 the axleboxes were seen to have been battered to death by the side-play. New axleboxes (at £1,600 each) and horns were fitted and the frames were reinforced by two substantial stretchers.

For more information please see:-

The Bulleid Society

Southern Railway E-mail Group (SREmG)

Wikipedia

The Vital Statistics of a Merchant Navy Class Locomotive

Liveries

The following drawings illustrate the various liveries that Merchant Navy Class Locomotives wore. They are the work and copyright of Rick Kent and they are reproduced with his kind permission.