An improved Schools – building a Roedean
Nick turns to the boiler of his 3¹₂-inch gauge locomotive project, starting by deciding which of two distinct published designs to use for his model.
Imentioned in earlier editions of this series that the part-built model locomotive that I acquired came with both sets of published drawings for the Schools class in 3½-inch gauge. After carefully studying the details it became clear that neither design of boiler would be satisfactory. However the LBSC version was closest to a design that could be constructed to comply with modern boiler standards.
The Jackson design not only had a number of features that I did not like but in my opinion was inadequately stayed to provide sufficient margin to withstand a 160psi hydraulic test pressure. Although the dimensions of the fireboxes are similar on both designs, using girder stays for the firebox crown, LBSC employs a total of 127 1/8-inch stays for the sides and ends. The Jackson design uses 29
3⁄16-inch stays for I assume the same working pressure and a similar surface area, with spacing of up to 11⁄16-inch. On the LBSC design the spacing is ⅝-inch or less.
Clearly there is a big difference in strength between the two designs. Neither drawing actually states the operating or test pressure relevant to these designs; although the drawings I received appeared to be new they may be some years old. As there are no dates or revision numbers on them it is difficult to know if they are the same as currently on sale. This is an often-criticized aspect of model engineering drawings.
The Jackson design also has no provision for a blowdown valve, while the firehole is too high being right at the top of the inner firebox plate, the bottom nut of the water gauge is very close to the firebox crown and so on.
Best elements
My ‘hybrid’ boiler uses a 4-inch outside-diameter main tube. This allows room for lagging and cladding to match up with the 4⅜-inch outside diameter of the smokebox. There are 13 7⁄16-inch flue tubes and a pair of ⅞-inch superheater tubes, which is what I have used on my Q1 boilers in the past.
The firebox is fitted with 118 ⅛-inch side stays while 20 3⁄16-inch phosphor-bronze rod stays replace the girder firebox roof stays. I am not keen on girder stays as it is very difficult to know if the silver solder has reached everywhere it needs to be to make a strong joint.
Construction of my boiler generally followed the principles set down in 1998 by Alec Farmer in his excellent book on the subject (Model Locomotive Boilermaking, currently published by TEE Publishing, www. teepublishing.co.uk). I have a few of my own tips, which as I recall were not emphasized in the book.
Firstly, if you live in a hard-water area as I do, then do not under any circumstances allow tap water to come into contact with your boiler during the construction. This includes mixing up the flux. As Alec lived in the Birmingham area, with relatively soft water, this will not have been an issue for him. The reason for this is of course the calcium salts in solution found in hard water can form deposits in your joints which no amount of flux will shift, resulting in poor joints and leaks.
The solution to this is to use boiled water, or condensate from a dehumidifier, which is a useful addition to any workshop.
Fresh air not hot air
Secondly if your workshop is of bijou dimensions (estate agent speak for cramped) then I strongly recommend that you do your boiler making outside. This becomes even more important if your workshop is wooden and lined with polyurethane foam insulation panels as mine is! The manufacture of even a 3½-inch gauge silver-soldered copper boiler requires a large amount heat that will soon have the lids popping off your paint tins and dead spiders falling from the roof – the risk of an accidental fire is very real!