A Coarse Guide to the Steam Locomotive for ‘N’ Gauge Modellers

[Train Waiting]

A Coarse Guide to the Steam Locomotive for 'N' Gauge Modellers - Part 93


Hello Chums

The Final, Exhausting Section of Our Gloriously Brief Mini-Series

'The Exhausting Problem'

After the live steam has done its work in the cylinders (or cylinder in some of the earliest examples) of a steam locomotive, what can one do with it? Simply letting it escape from the valve chest to atmosphere would be a nuisance, with clouds of steam enveloping the engine. No doubt frightening horses as well. I disapprove strongly of frightening horses.

The earliest locomotives, with their single-flue boilers, had a long chimney to disperse the smoke and create a draught on the fire. Just like factory chimneys - a well-known technology at the time.

Richard Trevithick dealt with the problem of exhaust steam by taking it from the valves of the cylinder/s into the base of the chimney. According to contemporary accounts these pipes were turned through 90^o in the chimney so the exhaust steam travelled straight up and out. This pipe was called an 'exhausting' or 'eduction' pipe. ['Eduction' - what a SuperSpiffing word.]

At the time his Pen-y-darran locomotive was undergoing trials in February 1804, Mr Trevithick was in correspondence with David Giddy, later President of the Royal Society. On 20 February 1804, Mr Trevithick wrote:

'The fire burns much better when the steam goes up the chimney than when the engine is idle.'

And, on 4 March:

'[Steam] makes the draught much stronger by going up the chimney.'

Giving evidence before a Committee of the House of Commons on 'Steam Carriages' in 1831, Mr Trevithick stated that he used a 'forced draught', created by the steam, for the purpose of working on the roads. This meant a high chimney was not required, helpful for a road vehicle.

Mr Trevithick's exhaust system came into general use for the early 'steam dinosaurs'. It gave a gentle exhaust blast, ideal for the single-flue boilers of the time where a sharper exhaust blast would have pulled the fire up the chimney.

'Having a Blast'

There has been much argument over the years regarding who invented the blast pipe. From the 'battle of the blast pipe' in the pages of The Engineer in 1857 right up to BackTrack Volume 38, No. 12, December 2024¹. Incidentally, the 1857 hare was set running by John W Hackworth, Timothy Hackworth's son.

What is known is Mr Trevithick led the exhaust steam into the chimney and then discovered the effect it had on the draught. This, like penicillin over a century later, was an accidental discovery of great significance.

It is not known if any of these early locomotives had the exhausting pipe placed centrally in the chimney. What we can say for certain is a typical locomotive of the time, Lancashire Witch, built in 1828 by Robert Stephenson & Co for the Bolton & Leigh Railway had two upturned exhausting pipes, one for each cylinder, at opposite sides of the chimney. The picturingham shows a plan view of this engine:





The next development was combining the exhaust pipes in a central pipe, coned or tapered to form a contracted orifice. The blast pipe. Credit for this development belongs to Timothy Hackworth who used it on his Royal George of 1827 for the Stockton & Darlington Railway. This locomotive had a 'U'-shaped return-flue boiler, 2ft 2in diameter at the furnace end and reducing to 1ft 6in at the chimney.




[Timothy Hackworth's Royal George]

The Stephensons adopted the coned blast pipe and it permitted the success of Rocket's multi-flue boiler. All these small diameter tubes offer a resistance to the hot gasses resulting from combustion and a much stronger blast is required than for an engine with a single-flue boiler.

Why the fuss? My view is Timothy Hackworth felt hard-done-to by the results of the Rainhill Trials and the subsequent fame of George Stephenson - 'The Father of the Railways'. Robert Stephenson deserved but did not achieve similar recognition as I've mentioned several times in our amazingly brief mini-series.

But, in the public mind, just as James Watt invented the steam engine, Geordie Stephenson invented the steam locomotive. Utter tosh, of course, but who says 'dumbing down' is a new phenomenon.

It was probably a widespread view that the Stephensons 'invented' the coned blastpipe that prompted John W Hackworth to write to The Engineer. My argument is the Stephensons accepted Timothy Hackworth's development of the coned blast pipe and letters from George Stephenson to Timothy Hackworth in 1828 and from Robert Stephenson to Samuel Smiles, George Stephenson's biographer, in 1858 are, in my view, evidence of this.

Commenting on George Stephenson's letter, which the Hackworth family had recently donated to the National Railway Museum, the Railway Magazine of October 2005 stated:

'The letter is controversial for Stephenson has until now been credited with inventing the blast pipe.'

Please contrast this with EH Ahrons writing in his The British Steam Locomotive 1825-1925:

'The credit for coning or tapering the blast pipe to form a constricted orifice is undoubtedly due to Timothy Hackworth in 1827, and was acknowledged by Robert Stephenson himself.'² [My emphasis]

*

The remarkable Rocket/Northumbrian/Planet progression of 1829-1830 saw the multi-tube boiler and coned blast pipe established as fundamental parts of the British steam locomotive and this remained the case until Evening Star was completed at Swindon in 1960.

In the next part, we'll discuss blastpipes and suchlike in some more detail.


¹ Mike Norman, 'Only in America', BackTrack, Volume 38, No. 12, December 2024, Page 711. This is an interesting but rather strange article. It seems to start from the premise that Timothy Hackworth has been denied his place in history and that the Stephensons were to blame. My view is that, although others might have claimed George Stephenson invented the coned central blastpipe, the Stephensons didn't. I swithered about indulging in correspondence with the magazine, but Andrew McLean, Assistant Director and Head Curator of the National Railway Museum (NRM), fortunately got there first. He made a splendid job of demolishing Mr Norman's argument, quoting from original sources held in the NRM. Please see page 126 of the February 2025 BackTrack. (Vol 39. No. 2) for impressive scholarship from an NRM employee.

Mr Norman responded in the April 2025 BackTrack at page 253. (Vol 39. No. 4) In my view this letter adds nothing of value to the debate and attempts to continue his 'hatchet job' on Robert Stephenson.

² EH Ahrons, The British Steam Locomotive 1825-1925, Locomotive Publishing Company, London, 1927, Page 8.

*

Very many thanks to @martyn for reviewing the draft of this postington and identifying what would have been an embarrassing typo.


'N' Gauge is Such Fun!

Many thanks for looking and all best wishes.

Cheerie-bye

John

[Train Waiting]

A Coarse Guide to the Steam Locomotive for 'N' Gauge Modellers - Part 94


Hello Chums

The Final, Exhausting Section of Our Gloriously Brief Mini-Series

'Blastpipes - Some Theory-Type Stuff, but with Stories'

The conical blastpipe, pioneered by Timothy Hackworth and taken up by Robert Stephenson was essential to provide sufficient draught to enable a multi-tube boiler to steam well. As the engine worked harder, the exhaust sharpened and increased the draught. Magic!

But, there's a but. Why is there so often a but? Restricting the surface area of the blastpipe will help steaming, at the risk of pulling the fire to bits - we'll have a Saintly little story about this in, probably, the next part - but it causes increased back pressure in the cylinders. Put simply, the constricted blast pipe means exhaust steam will be tempted to linger longer in the cylinders rather than making a sharp exit through the valves and away out to the sky. Increased back pressure causes increased coal consumption. Which meant locomotive engineers had to think carefully about how much to constrict the blastpipe.

Robert Stephenson recognised this and mentioned it in correspondence. His point was sadly misrepresented in the BackTrack article mentioned in Part 93. Let's turn to the invaluable Dr Tuplin for a succinct explanation:

'The ideal was the largest [blastpipe] nozzle that would enable the engine to steam at full power in conditions somewhat inferior to normal ones. If average conditions deteriorated over the years, what was originally satisfactory might cease to be so and should be changed.'¹

Very nicely put, Dr T.

However, in practice, things could be a bit more complicated and the enginemen might end up with a locomotive that was steaming badly. Their traditional answer was to restrict the blastpipe nozzle by means of a home-made (or shed blacksmith-made) device. These had several names - 'Jimmy' and 'razor' being familiar to me. Please feel free to add any other in the discussion. This improved the engine's steaming but it was important not to get caught using one. Authority would not be happy.

But here's a nice little story. When Mr Stanier, later, Sir William, introduced his '5XP' three-cylinder 4-6-0s on the LMS in 1935, they were poor steamers. Possibly even terrible steamers. As a Great Western man, Mr Stanier had no experience of three-cylinder engines. Amongst a series of modifications, guess what they did. Yes, reduced the area of the blastpipe's nozzle. Of course, they used a nice new cap rather than a home-made 'razor' but the effect was the same.




[An LMS '5XP' or 'Jubilee' 4-6-0, made by Rivarossi for Peco. Is this the most influential British 'N' gauge locomotive of all time?]


Then, nearly 20 years later, BR had a steaming problem with its brand new 'Standard' '5MT' 4-6-0s. Whoops! The LMS 'Black Fives, on which the BR design was based, on tended to be good steamers as, of course, were the GWR 'Halls'. Whatever could they do? Yes, they reduced the blastpipe cap from 5 1/8in. to 4 7/8in. Evaporation of water increased from 19,000 lb./hr to 24,000 lb./hr. ES Cox, who had responsibility for the design of the BR 'Standards', helpfully included details of this in an interesting book.² Mr Cox's description is worth reading if you have an interest in such things. He discusses smokebox vacuum and blastpipe pressures which are, I think, beyond the scope of our Coarse Guide. Important stuff, though, and please feel free to mention them in the discussion if you wish.




[A BR 'Standard '5MT', No. 73072, with high-sided tender of 9 tons coal and 4725 gallons water capacity.  Built at Derby for the LMR as part of the 1953 Building Programme. Entered service on 11 December 1954, allocated to Chester. Transferred to Polmadie on 22 November 1958 and withdrawn from there on 27 October 1966. Not even a dozen years' service - what a waste!  My late Father-in Law was an engineman at inverness and thought highly of these locomotives. He preferred them to 'Black Fives'. Photograph from my non-collection. I think it's an official BR one.]


But, there we were, in the era of the BR 'Standard' locomotives and we were still working out important design features by trial and error. In the BR example, a 26% increase in evaporation rate (a good metric for 'steaming') was achieved by reducing the blastpipe diameter by 1/4 in. after the class had entered service. By the way, before they went down to 4 7/8 in., they tried 5 in. which gave a water evaporation of 22,000 lb./hr.

This rather supports my contention, made earlier, that this exhausting matter was the last major area of unfinished business at the end of steam locomotive development in Great Britain. A policy decision, made at the start, was that the new BR 'Standards' would have conventional single blastpipes and chimneys. So, why did some locomotives of Class 9 and 4 have double chimneys?³ More of this later.

One last point, a very sharp blast could cause damage to railway infrastructure such as footbridges (Poppingham's now has smoke plates - thank you @icairns) and station canopies.

¹ WA Tuplin, British Steam Since 1900, David & Charles, Newton Abbot, 1969, Page 44

² ES Cox, British Railways Standard Steam Locomotives, Ian Allan. London, 1966, Page 132.

³ As did the '8P' 4-6-2 No 71000, Duke of Gloucester. This engine was not in the original plans for the range of 'Standards'.

Special thanks, once again, to @martyn for reviewing the draft and making helpful comments thereon.


'N' Gauge is Such Fun!

Many thanks for looking and all best wishes.

Tickety-tonk.

John

[Bealman]

To answer your question about the Peco Jubilee, it is my belief, yes.

Fantastic series, John.  

[chrism]

To answer your question about the Peco Jubilee, it is my belief, yes.

Fantastic series, John.  

It's amazing how good it still looks, despite the complete absence of the parts of the motion that control the valve travel lead - i.e the drop link, union link and combination lever.

[martyn]

Thanks again, John.

I'll add  a few comments after your next post, as I think they will be more appropriate then.

Martyn

[Train Waiting]

A Coarse Guide to the Steam Locomotive for 'N' Gauge Modellers - Part 95


Hello Chums

The Final, Exhausting Section of Our Gloriously Brief Mini-Series

'Lang may yer Lum Reek'

Lum, chimney, funnel or smokestack - this is an important part of a locomotive. And not just for aesthetics, although many engineers went to considerable lengths to make the chimney pretty. The view at the time was a chimney is to a locomotive what a hat is to a lady.




[Mr Dean certainly made sure his eponymous goods engine had a pretty chimney.]


As model railway folk, we are interested in what a chimney looks like on the outside. A nicely-shaped one is a delight.

But, for steam locomotive engineers, what goes on inside the chimney is of vital importance to the performance of a locomotive. I don't intend to go into a lot of detail, but here are some basics.

The centrally-mounted restricted-nozzle blastpipe causes the exhaust to form a cone shape. I've been given to understand that the sides of the cone are at around 10^o.  Generally speaking, the lining of the chimney will also be cone-shaped, with the narrowest part called the choke.

You know how we hear so much about the importance of smokebox vacuum? All that stuff about a tightly-fitting smokebox door and the integrity of the wrapper plate, if the smokebox has one (Mr Churchward did us all a favour with his US-inspired round smokebox mounted on a saddle - no wrapper plate rivets to work loose allowing the smokebox to draw air). The 'Royal Scots' in original parallel boiler form needed regular attention to keep maintain the integrity of their smokeboxes.

But this notion of vacuum begs a question - how can one have a vacuum in something that has a big hole cut in top, with a chimney surrounding it? The answer is the exhaust cone , where it leaves the chimney, ought to be the same diameter as the opening of the chimney. That way, when the engine is working, the chimney is effectively sealed to atmosphere by the exhaust.

Getting all this right requires clever design and, often, trial and error. The diameter of the blastpipe and the blastpipe's height in relation to the chimney affect the shape of the exhaust cone. Also, it's worth remembering the chimney isn't simply plonked on top of the smokebox, its lining will likely be continued into the smoke box.

As an aside, and a SuperSpiffing excuse for another picturingham, a few locomotive engineers didn't bother with elegantly-shaped chimneys and the exterior shape followed that of the conical interior. Mr Adams on the GER, then LSWR, was famous for his 'stovepipe' chimney.

The Uries, father at Eastleigh and son at St Rollox, also went in for severe stovepipes as this picturingham of a Drummond 'T9' 4-4-0 shows. Mr Urie, senior, rebuilt the class with superheaters - this made them better engines but, unfortunately, his chimney wasn't an improvement.





The original 'N15' 4-6-0s on the LSWR, later, after Mr Maunsell's attention, included in the 'King Arthur' class (the 'Urie Arthurs') had a stovepipe of remarkable hideousness. The reason why one sees so many long-lived ex-Caledonian 0-6-0s with plain stovepipes is Mr Urie, junior, used these to replace the more elegant offerings from Messrs Drummond and McIntosh.

For the majority of the time of steam locomotive development in Great Britain, draughting was the subject of ad hoc methods. Some designs had excellent steaming characteristics, others were wanting. Then, just after the War, a clever chap called Sam Ell began a thorough and painstaking analysis of draughting at Swindon, using the stationary test plant and controlled road testing. His work demonstrated that some locomotive types were performing at a fraction of their potential capacity.

His most important insight was that quite minor modifications to the blastpipe, chimney shape or chimney size could produce dramatic results. The GWR 'Manor' class had been something of an enigma. Fitted, due to weight restrictions, with a smaller boiler than other 4-6-0s on the railway, they were uncertain steamers. Most un-GWR-like. Mr Ell and his people got to work and the result was amazing.  Prior to modification, a 'Manor' couldn't sustain more than 10,000 lb. of steam per hour continuously. Not a good figure. After modification, 22,000 lb. of steam per hour could be achieved. Golly gosh - how was that achieved?

By a reduction in the diameter of the blastpipe and a narrowing of the chimney proportions.

The LMS '4MT' 2-6-0s, designed by Mr Ivatt the Younger, ought to have been great engines. But, initially, they were poor steamers. At first this was believed to be due to the double chimney (sorry - I'm getting ahead of myself, we'll discuss double chimneys later in this encouraging brief mini-series) and they were fitted with single chimneys.¹ Better, but there was still something not quite right. An engine was taken to Swindon for the Ell treatment. The single blastpipe and boiler proportions were unchanged, but the maximum continuous steam production was improved by 89%. How?

A reduction in the size of the choke of the chimney from 1ft 2 1/4in. to 1ft 0 3/4in. and a reduction in the amount of taper in the chimney.²

Incidentally, Mr Ell's excellent work had, in my view, a bit of a sting in the tale later. We'll come to that ... later.

Let's finish with a story which might seem far-fetched but it comes from an impeccable source. Effective from 1 November 1932, Mr Ivatt the Younger was appointed Mechanical Engineer, Scotland for the LMS, based at St Rollox Works. JE Anderson had retired and Mr Urie, junior, had been appointed to succeed him as Chief Motive Power Superintendent, causing the vacancy at St. Rollox. Let's now hear from HAV Bulleid, a locomotive engineer and Mr Ivatt's nephew:

'Ivatt also found McIntosh's instruction still in force, that the centre line of the blastpipe should be 1/16in. forward of the chimney centre line, in order to allow for the effect of the engine moving forward! His mind boggled.'³

That's enough, I think, on chimneys. In the next part we'll return to blastpipes.

Once again. thanks to @martyn for reviewing the drafts for this postington.


¹ The first 50 '4MT' 2-6-0s, built from December 1947 to November 1949, were fitted with double chimneys. After trials with 43027, they were all fitted with single chimneys by August 1956. The 112 engines of the class, built between July 1950 and November 1952, had single chimneys from new. 

² Fortunately, Mr Ell wrote up his work and this can be found in the Proceedings of the Institute of Locomotive Engineers, No. 235, 1953.

³ HAV Bulleid, Master Builders of Steam, Ian Allan, London, 1963, Page 174.


'N' Gauge is Such Fun!

Many thanks for looking and all best wishes.

Cheerie-B

John