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Martin Johnson 106/02/2016 15:44:29
135 forum posts
1 photos


I am currently analysing past IMLEC results to try and improve the thermal design methods for small boilers. Please can anyone help with the following dimensions of the Martin Evans “Nigel Gresley” design:

Cyliner bore

Cylinder stroke

Wheel diameter

Grate length

Grate width

Firebox height

Firehole diameter

Smoketube diameter and number

Superheater flue diameter and number

Length of tubes

Working pressure psi

Safety valve bore

Many thanks in advance


oldvelo06/02/2016 18:02:13
280 forum posts
54 photos


The full size specifications can be found at

Scroll down for all Technical Details.


duncan webster06/02/2016 18:31:20
3710 forum posts
69 photos



Edited By duncan webster on 06/02/2016 18:33:22

duncan webster06/02/2016 18:33:48
3710 forum posts
69 photos

try again!

Cyliner bore 1.5625

Cylinder stroke 2.25

Wheel diameter 4.875

Grate length 8.0625

Grate width 2.8125

Firebox height 6.625 at front, 5.125 at back

Firehole diameter 1.625

Smoketube diameter and number 19 off 1/2 * 20 swg, 14.9375 long

Superheater flue diameter and number 2 off 1.125 OD, 16swg

Length of tubes

Working pressure psi 100 psi

Safety valve bore 2 off 1/4

Martin Johnson 107/02/2016 09:04:58
135 forum posts
1 photos

Absolutely brilliant.

Thank you Duncan and Eric.


duncan webster07/02/2016 15:56:35
3710 forum posts
69 photos

And the dimensions you didn't ask for but might need

release holes in top of safety valve, 4 off #40 (0.098" which might mean these are the limiting factor on steam release, not the main bore

Superheater elements, 2 per flue, 3/8" OD 20 swg

blastpipe diametert 1/4"

julian atkins08/02/2016 00:12:51
1246 forum posts
353 photos

i thought there were 2 boiler designs for Nigel Gresley?

i must admit i find the posts by martin rather bemusing.

there are so many variables that i think his task is extremely difficult. i cant find any common parameters re loco design.

the late Percy Wood won IMLEC twice with a bog standard LBSC Maid of Kent with inside cylinders and Joy Valve Gear.

a Stirling Single in 5"g won IMLEC once.

Phill Haines was disqualified for reasons that are still not clear, except i was told why in total confidence!

quite a few people apart from Percy Wood have won IMLEC more than once.

Jim Ewins spent much effort building at least 2 locos designed to win IMLEC but never won!

Laurie Lawrence had his own view on IMLEC with which i agree. it is something i have studied for 30 plus years.

i could expand, but perhaps best to PM martin!



Edited By julian atkins on 08/02/2016 00:25:30

Neil Wyatt08/02/2016 11:20:44
18899 forum posts
734 photos
80 articles

I won't try and second guess Julian's enticing web of intrigue, but my own impression from having read and considered many IMLEC reports in depth is that, given a well-matched load, driver skill and familiarity has a far greater influence over a loco's performance than any physical aspect of the loco's design.


Martin Johnson 108/02/2016 15:21:39
135 forum posts
1 photos

Thank you all for your contribution.


I agree that a good driver has a huge influence on efficiency. I also think experimental accuracy (or lack of) has a huge influence on just who wins - just tell me again that you can weigh a coal consumption of 2 lbs. to 3 decimal places of accuracy...........


I agree that there are other factors in the safety valve that can ruin it's performance. But if the 'ole aint big enough to start with, then even Gordon Smith's clever designs can't put it right. I ignored other factors to try and keep my survey within bounds. On that subject though, does anybody have trouble getting Torquay Manor and Les Warnett's 9F through the 10% pressure rise test? My correlations show that they both have rather small areas compared to grate size.


I agree that the whole subject is fiendishly complex. However, my initial goal is rather more modest - what is the grate loading in a miniature Stephenson engine? I do have a statistical answer beginning to emerge, but the scatter is large. I do have some ideas on narrowing that down a little, but in any case a rough guess is better than no guess.

My reason for wanting to know is to develop a boiler performance calculator. Until one knows that grate loading figure, then you cannot calculate heat input, mass flow, flow velocity, Reynolds numbers, Nusselt numbers or very much else. You will be pleased to know that the boiler calculation module is done and working and has been correlated against some full size experimental results. The superheater calculation module is proving rather "challenging".

Such boiler thermal design work that I have been able to trace in ME and similar is very waffly speculation and of no use in designing anything a bit off the usual.

Thanks to everyone again, I am off to put in some more numbers on the computer..................


julian atkins08/02/2016 22:31:21
1246 forum posts
353 photos

hi martin,

can i suggest you read Jos Koopmans' book on locomotive draughting, the last few issues of LLAS by Don Young where he stated (just before his death) the principles he tried to adhere to in boiler and smokebox design, and the BR Rugby test reports and the work of Sam Ell. Jos has completely overturned the traditional Greenly formula used for miniature loco draughting.

a few more potential factors:-

constant steaming rate, free steaming boiler, well designed draughting. even firing 'little and often' with a fire that suits the loco and firebox. driving so as to try and achieve a constant steaming rate and as near as possible a constant pull on the drawbar for the dynamometer car readings. listening to the loco for any sign of slipping and how it is behaving. watching the colour of the exhaust.

probably cleaning all oil off the driving wheel treads before an IMLEC run also has a lot of effect!

a steam locomotive is most efficient when you have a constant steaming rate at 'optimum' performance for spead and load. finding that 'optimum' level of speed and load also depends on the track - gradients, curves, and rail head conditions.

when i entered IMLEC in 1995 i was told afterwards by the guy on the dynamometer car i had almost a constant drawbar pull throughout the whole run of 30 lbs.



Bob Brown 108/02/2016 23:10:41
1021 forum posts
127 photos

There are two designs for the boiler one with a thermic syphon and a simpler one with out and I'm part way through building “Nigel Gresley” so I have all the drawings and articles


Martin Johnson 109/02/2016 09:54:30
135 forum posts
1 photos


Thanks for the clarification on the boiler design. I am going to keep it simple and consider the one without thermic siphon. From a thermal point of view, the siphon has to be a good idea, it transfers more heat to the water by radiant firebox heat, but reduces the heat transfer in the tubes (by lowering the entry temperature). Overall, it will gain more than it loses.


I am aware of some of the work you mention on draughting. However, I think this where it is easy to get bogged down in detail. I come at that problem from a jet pump point of view (having been a pump designer in a previous incarnation), and all the various "design guides" are aimed at optimising the efficiency. However, you cannot get much beyond 35% Effy. for absolute optimum geometry, and typical chimneys with restrictions of scale, height etc. will be considerably less. The major factor here is that the blast nozzle area is tuned to the whole engine. I should state here that my interest is in road engines which have a tall chimney, offering much better jet pump geometry.

I agree with your comments on driving style, but again it is a bit of side issue - I know that anything I do will be through a thick fog of statistical scatter.


Bob Brown 109/02/2016 10:44:44
1021 forum posts
127 photos


I've put some pictures/drawings of the two types of boiler which may help, for a 5" loco it is a big boiler.

Bob Brown 109/02/2016 15:16:58
1021 forum posts
127 photos

boiler3.jpgboiler 1.jpg

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