If one were to construct say, a replica 4-4-0, it would be somewhat easier and probably cheaper to replicate a locomotive that had a straight boiler than a wagon-top boiler. I've heard, though that the main advantage of the wagon-top design was that it reduced the likelihood of foaming and cylinder damage. Can anyone elaborate on this?

http://storm.simpson.edu/~RITS/structur ... /ckn_1.GIF

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A conical boiler shape reduces sloshing by minimizing the mass of water at the front end of the boiler and by presenting a taper to disrupt the waves of water that originate at the front tube sheet as the locomotive rocks and nods its way down the track. This makes the water under a steam dome calmer, reducing lifting into the steam dome and dry pipe. The end tapers and center spout/dome on a hot metal torpedo ladle car in a steel mill are a slow motion example of the same logic.

The conical boiler course shape also reduces the mass of the water surrounding the tubes as the hot combustion gases travel forward and give up their heat. This keeps the front end of the boiler warmer, reducing front tube sheet stresses and leaks.

Bruce Manwiller

Isn't foaming based more on water quality issues than boiler designs?

Quote by Chris
Chris,
Yes, foaming can be caused by a number of water quality issues, such as high TDS from insufficent blowdown, from boilerwater pH being too high,or from the chemical or thermal reactions from unwanted materials that find thier way into the boiler water. Priming can be caused by foaming from the above,but can also be from high water level, excess load which causes water entrainment, etc.

The traditional defense of the wagon top boiler is better steam collection and defense against bad water. This explanation is not correct based on my operating experience. The crown sheet area is the most turbulent water circulation area, so it is the worst place to collect steam in large volumes. However, as the other posts note, it is the only place to collect steam on a wagon top or conical boiler because of the severe reduction in steam space at the small end of the boiler.

If money were no object and track clearances allowed, I would advise that all boilers be conical. One of the great sources of stress for the engine crew is the fluctuation of the gauge water level as the engine incline changes from uphill to downhill. My memories are 20 years old, but I seem to recall a variation of a third of a gauge on the straight boiler Strasburg engines from uphill of Cherry Hill to the downhill approach to the station. This meant we had to carry a lot more water in the boiler at the station than was convenient for pressure management. Today I have the same problem with my miniature steam locomotive. My engine has a straight boiler with limited crown sheet clearance, and the line I operate on has severe grade variation. My engine can vary from a half glass to an empty glass in 200 feet. My water management has to be very strategic to avoid killing the steam.

A conical boiler would greatly reduce the variation in water level at the gauge glass. As the boiler tilts forward, the available free space at the smaller front is much smaller, so there is less room for the water to flow forward. As the boiler tilts back, the amount of water flowing back is much smaller than the available free space. In both cases, the net change in water level at the wagon top end is much reduced. On a frontier railroad with poor alignment, this would be a great operating benefit to the crew, who would not have to keep adjusting the water level. And of course, on very early engines without sight glasses and with pumps instead of injectors, there was no ability to quickly fill a boiler on demand, such as right before a descending grade.

As locomotives became larger, there was no longer room to allow the expanded shell at the firebox relative to the scale of the locomotive and the track loading gauge. Note the hardships presented by the Wooten firebox or camelback engine. Also, line grade transitions were more gradual on modern, improved lines.

Cheapest and easiest would be to run off another Jupiter / 119 replica using already engineered and constructed patterns, dies, etc. Do these belong to O'Connor or to NPS?

dave