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[Isambard]

Apart from external cosmetic features, 20th century steam locomotives had a wide variety of smokestack designs, some short andsquat, others tall and slim, some cylindrical in diameter, others tapered (such as on the "Russian" Decapods or the UP's MK Special USRA Mikados with Sweeney smokestacks).
How sophisticated was the effort in coming up with the smokestack design for a new locomotive, was it based largely on practical experience and past history or did it also involve significant design studies and analysis? What sort of factors would be considered?

[tree68]

The stack consisted of much more than what you see from the outside. The flow of the exhaust gasses and steam is the primary consideration, and the stuff inside the smokebox took care of most of that.

That said, ATSF had stack extenders on some locos. Others may have, too.

[Anonymous]

I'm not an design engineer but the stack had to be of sufficient diameter to get rid of the steam exhaust from the cylinders, which together with the nozzles at the base of the smokebox, had to fill the stack enough to create a vacuum inside the smokebox. This in turn gave a good draft for the fire-- the harder the engine was working the better the draft. A long stack gave better efficiency up to a point but vertical clearances really limited the height. Passenger engines needed to get the smoke above the engine so it wouldn't trail back into the cab or the cars at high speeds. On freight engines this usually wasn't a problem. The UP tried the huge Sweeney stacks on some of their engines in the '20's but I've never heard if they really were any better than the usual size.

I hope this helps a little. Shafter