Restoration
Ashpan Airspaces
A surprise came early in the restoration of No 71000 when a new ashpan was being constructed. The old ashpan was of a through compartment hopper type, but advanced corrosion meant that it was only fit for scrap. No drawings were available at the time so, in order to keep the work moving as planned, team leader Chris Ball fabricated a new ashpan assembly using the dimensions of the original as reference. On completion, the BR drawings had come to hand so Chris checked his new ashpan against them and was amazed to find that the damper door air spaces (through which air passes to the firegate), were too small by 72sq in! The old ashpan was immediately compared and, as it had been the source of the measurements, was also deficient in air space. Quite clearly, the mistake had been made in construction. The new ashpan was quickly and easily corrected.
Certain factors were taken into account when assessing the importance of the deficiency in the damper door air space. The deficiency was all on the front and at the bottom, i.e. nearest the track, which would give it more significance. In 1961 Crewe enginemen had complained that that there was not enough air through the firebed and slots were cut in the sides of the old ashpan in an attempt to rectify this, with a degree of success. However, considered opinion was that this might have been a contributory factor to poor steaming, but not the main cause. Detailed research was put under way in an endeavour to establish if other deep seated flaws were present in the boiler design.
The Chimney & Blastpipe
The Duke's steaming performance was often measured against that of the rebuilt Bulleid 'Merchant Navy' and Peppercorn 'A1/A2' Pacifics, the boilers of which were not so different dimensionally, so it made sense to compare the three types. Also, it seemed worthwhile to look at aspects of other big contemporary British steam engines to put the comparisons in perspective.
This research showed that in theory, No 71000 was at least as good as the other two designs except for the chimney and blastpipe arrangement.The total area of the plain blastpipe nozzles of No 71000, at 25sq in, was less than either the five-nozzle unit of the 'Merchant Navy' at 27.5 sq in, or the twin-cloverleaf pattern of the 'A2' Kylchap blastpipe at 29.5 sq in. Both of the others, because of their different designs, had a surface area of steam that was greater still. Mr Daniels had always considered that not only was the Swindon double chimney design unsuitable for 'The Duke', but it actually caused 'throttling' of the steam at the exhaust ports. Ample evidence is to hand to show that Swindon's principles of draughting, while producing superb results in modifying the 'Castles' and 'Kings', were not necessarily suited to engines other than those of the first-named GWR class. Experiments on LNER Pacifics had proved disastrous.
Comparison of the chimneys produced an equally disturbing result. The total choke area of No 71000's double chimney was less than half that of the 'Merchant Navy' large single chimney. Because the five nozzles of the Bulleid blastpipe were angled outwards it was felt that this might not be typical, but the 'A2' Kylchap choke area was found to be half as big again as that of 'The Duke'. Further checks showed that the 'Britannia' single chimney choke was 5/6 the size of No 71000's total choke area despite the latter's extra cylinder and its potentially fiercer blast resulting from the use of the Caprotti valve gear. Also odd was the fact that the '9F' double chimney would fit straight into position on 'The Duke'. Quite clearly, the double chimney and blastpipe assembly fitted to No 71000 was unusually small. In view of these findings, there was no sense in wasting hard earned funds on duplicating the original blastpipe and chimney assembly. A better proportioned alternative had to be found and the comparative merits of several designs were considered.
Mr Daniels stated that he had asked for a Kylchap type chimney to be fitted to 'The Duke' when under construction and he reiterated this preference which was acceded to. As André Chapelon, the inventor of the Kylchap blastpipe, was dead, the only solution was to produce one using existing designs as a foundation. Mr Daniels outlined the important criteria and the starting point was the Kylchap assembly on preserved 'A2' Pacific No 532 Blue Peter.
No 71000 differed from the LNER design Pacifics in that the exhaust ports in the smokebox ran from 'port to starboard' and not 'bow to stern' - something which meant substantial redesign of the blastpipe casting. The characteristics of the new shape did not affect the smoothness of the exhaust in any way. Blastpipe and nozzles were cast by Fletcher and Stewart of Derby. Two attempts were made at producing drawings for the chimney before the desired BR Standard appearance was pronounced acceptable and K. C. Patterns of Peterborough made it. The two pairs of cowls were relatively simple to produce. Despite the design changes, perfect Kylchap proportions were maintained in both blastpipe and chimney - something that was crucial for effective steaming. Following the manufacture of the new Kylchap chimney assembly for 71000, a rumour had persisted that originally, prior to Swindon taking charge of chimney design, something similar to a Merchant Navy assembly had been proposed.
However, a drawing produced from Crewe Works proved differently. The first proposal for the three cylinder Pacific, with rapid exhaust from poppet valves, was no more than a marginal modification of the assembly fitted to the two cylinder Britannia's. Not only was the blastpipe nozzle the same size at 5 3/8 inches diameter, but the chimney flue choke was actually smaller at 15 inches diameter, The Britannia being 15 5/8 inches! The other main differences were in the distance between blastpipe nozzle and chimney flue choke, and chimney taper. The successful Kylchap, fitted to the restored Duke, has two blastpipe nozzles each of 5 1/2 inches diameter and two chimney flue chokes.
Many years later other information came to light from Crewe Works. It would seem that the Works had been frequently asked to deal with a problem of the cylinder relief valves operating unnecessarily but nothing was ever found to be wrong with them or their settings.The fact was that the valves were operating because of a throttled exhaust causing excessive back pressure when the engine was working hard! The steam chest pressure was noted to rise above boiler pressure at these times, indicating excessive compression in the cylinders. It might be logically reasoned that, with relief valves set at 265 lb/sq.in and continually operating, the pressure in the cylinders would have been greater than boiler pressure (250 lb/sq.in) and would have affected steam admission. This would seem to totally support Mr Daniels original theory.
Mainline Running
Concrete Arch
The concrete arch in the firebox, installed during restoration, reached the end of its satisfactory life in 1991. It had been cast without an official drawing, at an angle obtained by 'rule of thumb' - taking a projected line from where the arch would meet the lower part of the combustion chamber towards the rear top corner of the firebox. The profile from centre to sides was tailored to suit the studs in the sides of the box which support the arch. This produced a slightly unusual shape, being fairly "flat" against the throatplate, but with greater curvature towards the rear nearer the firehole door. 'The Duke' steamed well with it.
When a new arch was cast in 1991, the BR arrangement drawing was to hand, but difficulty was experienced in making formers to achieve the correct upward angle and correct arch shape. As time was limited in the preparation for a new season a compromise was reached, although the resulting arch was neither as high nor as flat in shape as it should have been. It appeared to be satisfactory, but in practice this did not prove to be quite the case. Unhappily, whilst 'The Duke' still performed well in the hands of skilled firemen, it did not seem to have the same margin, in terms of steaming capability, to compensate for poor quality coal or less skilled firing, that it had shown previously.
At the end of 1992, there was more time to look into this matter. The arch was compared carefully with the BR drawing and was found to be significantly lower than it should have been! The subsequent demolition revealed that the (original) studs in the sides of the firebox, which support the arch, were not only lower than on the drawing but also at the wrong angle! This discovery prompted two questions - did 'The Duke' run with an unsuitable arch in BR days' and if so, did this compound the problem with the original front end? Given the experience outlined above, the only answer to both questions is "yes".
Later another arch was cast, higher in the box, using formers to the 'designed' profile, but incorporating flat vertical side sections extending down to, and covering, the studs. The angle was adjusted by raising the height at the rear. This appears to be completely satisfactory.
Headlamps and Other Equipment
'The Duke' has purpose built electric headlamps and sidelamps, made from original oil lamp cases by John Beesley. These present an appearance as authentic as possible. Most importantly, the headlamps meet the specifications necessary to have a maximum permitted speed of 90 MPH.
'The Duke' is fitted with a steam turbo-generator supplying 24 volt A.C. electricity and an alternative 24 volt A.C. supply is provided by a Diesel generator underneath the Support Coach, which also supplies 240 volt A.C. electricity for coach services. A radiotelephone, on the locomotive, connected to the Railway Network. A B.R. Railtrack radiotelephone is fitted in The Duke's cab and there is a "hard line" intercom system, from locomotive to support coach, to enable communication between footplate staff and support crew.
