• Outside outside diameter

  • Discussion of steam locomotives from all manufacturers and railroads
Discussion of steam locomotives from all manufacturers and railroads

Moderators: slide rules, Typewriters

  by AllenHazen
This is an idiot question, and I should have tried to find out the answer years ago.
Steam locomotive specifications typically include "BMOD": boiler maximum outside diameter. (Given the slightly conical design of typical boilers, this is there outside diameter just in front of the fire box.) But the diameter given is that of the steel boiler shell. This would have been covered by "boiler lagging" (= insulation-- typically asbestos in the period of 20th century steam production), and an outer layer of sheet metal covering this. How thick would these outer layers (typically) have been?
In other words, if a locomotive "officially" had a BMOD of, say, 100 inches (like the New York Central Niagara), what was the VISIBLE outside diameter?
Or to look at it another way... Suppose a locomotive (like PRR Decapod 4483 in its current location at a museum in western New York State) is preserved with a "stripped" boiler: if you look for recent photos, you can see the rivets, which would have been hidden under lagging and exterior sheet metal in service.(1) How much "skinnier" does it appear than it would have "in life"?
(1) When I last visited 4483, at its previous location outside the Westinghouse plant in Turtle Creek PA, in the mid 1970s, some of the sheet metal had deteriorated, even splitting open in places. And, of course, modern safety regulations would necessitate removal of asbestos lagging. (I think there is a modern substitute, involving fibreglass.). The long-term hope, of course, is to restore the locomotive to its original appearance, but railroad museums are under-funded, so for the moment the best that can be done is to give the boiler a good coat of (I hope weatherproof!) paint to protect it from rusting.
  by Cowford
The lagging and jacket does not add significantly to the girth of a boiler : ~3" of insulation (which I believe was typically a magnesia-asbestos blend) covered with a 1/32" (+/-) jacket. I believe the PRR decapods had so-called "extended wagon top" boilers, not conical.
  by AllenHazen
Thank you! ... The sheet-metal jacket sounds negligible, but the lagging... Did you mean typically (about) 3 inches thick? In which case it contributes 6" to the total diameter, which might be enough to be perceptible in a photograph or model.
As for "coned". I probably used the wrong word, but even in "extended wagon top" boilers the rear end of the boiler barrel seems typically a bit larger in diameter than the front end: in a riveted boiler, there is some overlap between boiler courses, and the in-front course is fitted inside the in-behind course, I think. Anyway, in diagrams which show maximum outside diameter and also minimum inside diameter, the arrow for m.i.d. is usually drawn right behind the smokebox, and the arrow for m.o.d. right in front of the firebox.
Thanks again! I have, of course, seen photographs of locomotives under construction or repair with some lagging on and other lagging off, but can't really estimate how thick the lagging is.
(Question, of course, was intended for American practice: I suspect railways in other countries may have had different practices about how much, and what kind of, insulation to "wrap" locomotive boilers in!)
  by AllenHazen
Diagram (accessed through "Fallen Flags") of a PRR I1:
http://prr.railfan.net/diagrams/PRRdiag ... z=sm&fr=ge
The vertical double-headed arrow I take to be"boiler maximum outside diameter" is the one labeled "93 inches" just in from of the Belpaire firebox, and the one I take to be "boiler minimum inside diameter" is the one just behind the firebox labeled "84 and 1/2 inches".
I take it (please correct me if I am still wrong!) that in what is properly called a "coned" boiler, the individual boiler courses are slightly conical: here, each course is cylindrical, but there is a (slightly, approximately) conical overall effect since the middle course fits inside the rear course and the front course inside the middle.

(Posted for the benefit of ignorant people like me. Thanks Cowford for answering my original question!)
  by Pneudyne
Boiler cladding does not seem to be a subject that had garner much attention in the railfan press.

The attached few pages from the 1938 Locomotive Cyclopedia give some background information, although not addressing your specific question. One supplier’s advertisement indicates that the cladding blocks were available in thickness from one to four inches.

I recall seeing an “unclad” boiler on a NZ Railways J or Ja class 4-8-2 many moons ago. These locomotives had “long and thin” boilers to start with. Without cladding the boiler looked extremely thin.

Locomotive Cyclopedia 1938 p.368.png
Locomotive Cyclopedia 1938 p.369.png
Locomotive Cyclopedia 1938 p.370.png
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