Smoke Deflectors

Apologies if this has been done before but just wondered why there was never a standard approach to the size, style or even presence of smoke deflectors? Given that the size shape and tragectory of loco boilers on all engines capable of running at speed will all impart a similar effect in drawing exhaust back towards the surface due to the lower air pressure I am surprised that the size and design did not also standardise across the regions over time. The lack of smoke deflectors perhaps more than anything gave GWR locos a unique "family" style that makes me wonder whether GWR independence and GWR marketing overruled GWR engineers in choosing to do without them entirely but their absence did not lead to any greater incidence of driver sighting errors. Even within a region we see LNER A1s and A2s carrying large style deflectors whilst the A3s carried an almost cosmetic German style or none at all. Personally I like them, having grown up with Bulleid's giant eared engines, and on the Southern, predecessors like the King Arthurs and Nelsons had half sized deflectors, suggesting that driver experience suggested larger models were required - in direct contrast to GWR practice. They can't all be right!

 

Suggest you peruse Holcroft's paper to the ILocoE.

 

There were a lot of variables: size of boiler; exhaust pressure at the blastpipe; height of chimney rim above the top of the boiler. All these, and probably other factors, played apart. You also have to bear in mind that while deflectors might improve the driver's view forward by clearing smoke, they also physically restricted that vision by creating blind spots, especially if handrails were attached to them. The derailment at Milton (also sometimes called Didcot) 20/11/58 is a case in point.

 

What happens is that with the loco moving forward at speed the near flat front of the smokebox forces air out sideways. This causes a vacuum around the sides and top of the smokebox, and as nature abhors a vacuum, air, often containing smoke and assisted by a suitable wind direction, fills the void. No doubt the best deflectors are the German Witte ones as seen on LNER A3s in their Kylchap double chimney days, they not only extend far enough forward to capture the 'splatted out' air, but are also set out as far enough away (within the load gauge) to give plenty of space to channel the air past the smokebox. For what it is worth my feeling is that deflectors of the type fitted to LNER Thompson and Peppercorn Pacifics, also LMS 'Duchesses' fail on both these requirements. It is notable that in post-war times in Germany virtually all locos fitted with the earlier Wagner type (say up to about 1941) were converted to the later Witte type. Personally I feel that the Witte type suits an A3 as they fill in an empty space when viewed from the side - but that's a different subject to be argued about!

 

now that really would be a neat trick, way to go nature

Colin

 

The GWR favoured what one might describe a "cannon blast" type exhaust and tended to throw their exhaust high in the air , obviating the need for smoke deflectors-(except under footbridges which otherwise suffered floorboard lifting).

 

Maybe, but they had somewhat smaller diameter smokeboxes plus with their rather generous loading gauge, somewhat taller chimneys.

 

The Southern engines, Schools, N,U, etc were not originally fitted with smoke deflectors. They were fitted to cure a problem of steam hugging the boiler in operational use. The soft blast of Bulleid engines made the problem worse so smoke deflectors became common on the southern

 

The thing with them was that, on the LMS at least, they seemed to be a bit random as to which locos were fitted with them. The Princesses never had them, Duchesses had them added later (btw I think a Duchess just looks wrong without them) Scots and Patriots had them, Jubilees didn't.

 

Yes, they did, presumably as they had the same boiler as the rebuilt Scot and Patriot

 

Is the whole of that paper available on line?

 

Good point. The question is whether the LMS had good reasons for those choices or whether it really was random. A possible reason for the difference between the Jubilees and the Patriots could be the considerably different sizes of smokebox. The same might perhaps apply between the two sizes of Pacifics, but there is less difference between their smokebox diameters.

 

Princesses had single chimneys (with one short-lived exception I think). The Duchesses only got deflectors after receiving double chimneys. Maybe that's a factor.

 

I think the Duchess looks odd because of its unusual steam-pipe casing. The deflectors hid this from view.

 

As a small child I was always more excited by an engine with Blinkers. Somehow they looked more important than the rest.

 

Double chimneys generally means a softer blast, the main reason the A3's got their deflectors in their last days. Generally, the softer the blast, the bigger the problem. GW locos have a notoriously(?!) strong blast which throws the exhaust well clear.

 

Not as far as I am aware (in a free form that is). There was a window of a few weeks earlier this year when the IMechE was changing its systems and all the papers were freely available.