What on earth was Dean thinking of?

In an attempt to get the GWR off the current Webb thread I thought I would ask this question about the infamous 3521 class.

Dean did have a few other disastrous designs such as no 9 and the tandem compounds but they are quite excusable as individual experiments to you out novel ideas. But just looking at one of his 3521s it is hard not to come to the conclusion they will ride badly. I just cannot understand why he produced them. There was a need to replace the old Bogie (Corsair) type 4-4-0STs down in the South West with new convertibles, but he had two years earlier turned out a ten of the double frame 3501 class 2-4-0T which achieved that aim. Ten similar standard gauge locos were built at the same time, initially fitted with condensers for the Severn Tunnel they soon moved onto short main line services such as London to Oxford and Trowbridge to Weymouth.

But then in 1887 he turned out twenty of the unusual sandwich frame 0-4-2Ts with a close together pair of coupled axles at the front end and a trailing axle at the other end held in place only by the spring suspension links. They did have a longer firebox and a very high, for the time, boiler pressure of 180lb. Even while the class was under construction they were found to be unsteady and the tanks were reduced in size to now hold less water than the earlier 2-4-0Ts and after a while the boiler pressure had to be reduced.

But then, with more passenger convertibles needed on the broad gauge he ordered twenty more of the things, shortening the firebox to reduce the wheelbase to the trailing axle, so their boilers were now the same length as the 3501s.

They were still unsteady so the last one was turned out with a suspension bogie, which helped a little.

But I just can't work out why he kept turning the ridiculous things out, especially when he could easily just have turned out more of the 2-4-0Ts, perhaps with a high pressure boiler.

Sir Daniel Gooch was still alive, and the company chairman, when the first batch were produced for the narrow gauge; I wonder what he made of them?

 

I don't think I've ever seen any speculation about the design that seemed to me to have much value. When you look at them the most striking feature seems to me the firebox and ashpan right out in open air, rather than behind wheels. That's a theme also visible in presumably related types like the 0-4-4s 34 & 35 and those strange 0-4-4 conversions of ex Monmouthshire railway 0-6-0ST. That seems to me the most obvious difference from Armstrong types, plus also a long wheelbase. But what the design aims were I couldn't guess. Holcroft has nothing much to say about them other than the riding qualities.

 

Deleted

 

I just formed a wonderful theory for the 3521s, which unfortunately has just been undermined in the cold light of actual evidence... Supposing Dean was trying to get a wider firebox/ashpan/grate in, to get more efficient combustion than a narrow box? Maybe even towards something like his 4-6-0 with outside frames only past the firebox. But sadly the fireboxes appear to be more or less standard P class boiler boxes, so so much for that idea.

 

The first lot had a boiler similar to his original inside framed goods engines but with a longer firebox. The second lot, in order to shorten the long wheelbase had boilers of identical dimensions to the inside framed goods but with higher boiler pressure.

In view of Dean's apparent partiality to tank engines it almost seems that he was trying to create a design which it was impossible to reconstruct as a tender loco. If so he failed.

 

What is everyones thoughts regarding No 36 the Dean Crocodile?

 

No 36 appeared to have achieved its aim, the single locomotive was trialled for a couple of years after which Dean ordered more of the type.

Dean's chief assistant re-worked the design for the production series incorporating his own ideas; smaller diameter cylinders with a very long stroke and a combustion chamber to shorten the barrel length. The results are well known and in retrospect duplicates of 36 with, perhaps, a belpaire firebox world have been wiser.

On trials 36 proved capable of working coal trains through the Severn Tunnel significantly faster than double headed 0-6-0s but even so I wonder if there was a need for quite such a large boiler and in view of the low speeds of coal trains perhaps a similar result might have been achieved by a heavy 0-6-0 with a boiler similar to the Badmintons (had such a loco been built it would no doubt have been seen as the direct precursor to the Aberdares, which eventually filled this requirement nicely).

Edit: Thinking a bit more about it, the fact that Churchward eventually completed the oder for the first batch of Krugers suggests that the something bigger than the no 2 boilers fitted to the early Aberdare was desirable after all (also presumably significant components had been manufactured in readiness) so perhaps 36 was indeed the optimum design by the standards of the mid 1890s. The second order for Kruger's was never issued and the numbers eventually used for further Aberdares with the heavier no 4 boiler which was being used by then.

Apologies for so much mention of the Krugers but it is impossible to consider the context of no 36 without mentioning them.