BR Double blastpipes

The german 05002 had a long and usefull life and single chimney with top at 4550 mm or close to 15 feet.

05002 blastpipe by Hermod posted Aug 5, 2026 at 8:44 AM

 

The Duchess double chimney had two 4 3/8" nozzles, giving 23% more area than the original double blastpipes fitted to the Duke (and the Kings and 9Fs).

Regards,
Alasdair

 

A Duhess needs aproximately 20% more steam to do the job of a Duke.It also put out four exhausts per rev and a Duke 6

 

A lot of 'Ell' related activity seemed to be about sharpening blasts on softer exhausts, which at the expense of a bit of back pressure would at least get locos to steam ( see 'Jimmy') well - which is at the end of the day is a higher priority, so its unsurprising that the Dukes original exhaust nozzles would be smaller than you might expect. Lack of experience with 3 versus 4 cylinders and Caprotti style valve events also - its not really a massive surprise they got the Duke wrong.

 

Is that true? Each cylinder would put out 4 exhausts per rev, surely. Not sure about the DoG as I don't know how the valve events are arranged.

 

Yes, but the two each side were wor

Yes, but the two cylinders each side were working opposite to each other, so so the front end of one would exhaust at the same time as the rear end of the other, so two exhausts from the cylinders would combine as a single beat at the blast pipe.

 

Sort of linked to this and an excuse to not start a new thread on something that's previously crossed my mind, I assume there is a simple answer but in light of the later installation of double chimneys on the Duchesses/Kings/Castle etc is there a reason why same was not looked at for the Princess Royals?

 

In a quick review of six BR reports of tests where sorting out the drafting was a primary purpose, or was needed before testing could be done (for half of which Ell was stated to be on the team, not all list the team members), the V2 and Jubilee blastpipe caps were increased in diameter, it was reduced for the 73xxx and the Crab, and there was no change in blastpipe diameter for the Ivatt Class 2 and 4. Most of the changes were to the dimensions and taper of the chimney / petticoat and distance from the blast pipe top.

The BR reports are generally focused on trying to establish a sweet spot for the locos in terms of load and speed at which pax/freights were most economically handled, with the aim to diagram the train loads/locos accordingly. It was found for instance that a V2 (after mods) and a Britannia could produce well over 30000lb/hr of steam with a single chimney but British conditions rarely required locos to produce that amount, it seems clear that there is an almost straight line fall off in boiler efficiency as steaming rates increase and of course the higher rates of steam production require a higher rate of firing, often beyond the single fireman limit. What prodded the team into action was a Class 4 which produced no more steam than a Class 2, or a Class 6 (Jubilee) or Class 7 (V2) producing less steam than a Class 4.

Amusingly, it is noted that 71000 could not be tested at steam rates more than 31,000lb/hr as the tender did not carry enough coal for a decent length of test at such steaming rates.

 

Because at higher steaming rates the firebed would 'levitate' and any smalls would go straight up the chimney....

 

Possibly the memory that Princess Elizabeth was fitted with an ugly double chimney during the 1930's that was unsuccessful. Perhaps more surprising is that when the Turbomotive was converted to four cylinders the double chimney was replaced by single; surely if there was any thought of trying a double chimney on a Princess this would have been the chance, though admittedly 46202 differed from a standard Princess.

 

Despite the Duchess and Royal Scot, the LM region seemed to have something against double chimneys almost as if it was cheating in some way. For instance they asked Rugby to improve the steaming of Jubilees, doubtless at considerable cost and time, when Connaught had been running successfully with a double chimney for many years and steaming was immaculate according to A.J. Powell. No complaint against Rugby, they did what they were asked to do.

 

The LMS did fit several Black Fives with double chimneys, BR eventually removing that fitted to the Stephenson's geared 4767, presumably for some good reason. The double chimney fitted to the first Ivatt Class 4 2-6-0s were a hindrance to steaming and so also discontinued. 6201's double set-up in 1933 gave equally poor results. So the LMS did try several applications and while some worked, some didn't. If they had 'something against double chimneys' this prejudice might have been based on practical experience?

 

There were two LM Region double-chimney locomotives that actually did demonstrated improved performance, they were the very last of the Black Fives, 44686 and 44687. These two locomotives were built at Horwich in 1951 with the improved British Caprotti valve-gear which set them apart from the previous build of 20 Black Fives with Caprotti valve-gear, not just visually, but also from a performance perspective. They were allocated to Longsight Depot in Manchester and were considered the equivalent of Class 6, being used as such on regional expresses to London and Birmingham. These two locomotives were essentially the test-bed for the Standard Caprotti "Fives", which were fitted with a single chimney; the Standard version did not quite match the performance of the two Black Fives, for lack of a suitably designed exhaust system perhaps...............

Cheerz,

Alan

 

One of the design idioms of the Standard range was that they were over boilered and adequately draughted; the thinking being that better efficiency comes from a loco. configured to work below its maximum. Often said about standards is that if you needed more it was there but you had to thrash them to get it.. Another aspect of that philosophy is that if you needed more in everyday service you could upgrade the draughting to get it ; as done to standard 4's and ( for political reasons) 9f's. You could no doubt get more out of a Britannia, certainly more out of a Clan (the calcs have been done) and probably out of a 5, especially a caprotti . The Double chimney for the 9f would no doubt have been assumed to fit the pacifics, the one for the 5's had already been drawn up ( its in Talbot #2 iirc) by BR.