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Hank Bloomer 06-05-2006 02:14 PM

Can someone enlighten me about how chimneys were/are sized? The diameter must be related to the flue area, but in what ratio? How does diameter interact with the height required to achieve an optimum draft? Does the fuel being burned (and the grate design) affect the proportions of the stacks? Old photos show such a diversity of diameter/height ratios that I've gotten curious.

R. Dale Flick 06-05-2006 04:30 PM

HI, Hank:
Interesting topic you have Re: 'chimneys' on steamboats. I'm not technically minded like Alan Bates, Bob Reynolds, Tom Schiffer, Shipyard Sam etc. but recall some months back a discussion on this beginning with how high some of the old steamboat stacks were. I clipped and saved but can't find my folder containing all of that now. Perhaps some of the gang reading this will refresh our memories. You could click and look on the Archives for as it may be there.

We also talked back then about how the stacks were wired up and also using iron links. Pretty interesting stuff and you wonder how those old boat builders accomplished it with the limited tools and equipment they had back then. Alan, Tom and others also gave great insights on boiler grates etc. between steamboats burning wood and those burning coal.

R. Dale Flick

Shipyard Sam 06-06-2006 01:07 PM

Try this Link

Tom Schiffer 06-06-2006 08:31 PM

Hank: Shipyard Sam is onta sumpin'. However, first, you need to know the size of the grates required to release enough heat to provide the horsepower to give the speed desired. I do not have the numbers near at hand but using 1.3 X the square root of the water line length is the formula to determine what the natural "hull speed" of a displacement hull might be. Anything over hull speed is gotten at an increasingly severe tax on horsepower required. Usually only the military and ships desiring high speed to attract pax can afford such an extravagance. All of the info that I have relates to small steam launches...not big steamboats. Having determined the horspower required, there are tables giving square footage of grate required to produce enough heat release to provide that horsepower (depending upon fuel). Then there is the expansion due to the oxygen combining with the carbon in the fuel to produce carbonic acid gas plus the thermal expansion due to the increased temperature of the gasses. Stack temperature has to be greater than the temperature of the water at the pressure of the boiler and/or superheat of the steam desired...likely over 600 degrees F. in an old fashioned steamer. If the temperature is too low, draught suffers and you are likely to get condensation on the inside of the stack of sulfurous acid (from the sulfur in the coal when coal is burned) which will destroy the stack in short order. On the other hand, high stack temp is waste of heat. I'd say that the stack height was dictated by the bridges that must be negotiated; the diameter determined by the aggregate area of the flues plus a bit of a fudge factor. Not mentioned is the effect of air passing over the tops of the chimneys, inducing further draft when under way. I would suppose that the fancy "feathers" on the stacks would tend to negate this effect. I really don't know, but I would speculate that designing chimneys for steamboats is akin to designing propellors for displacement hull boats in that it is really hard to design a really bad propellor. I suspect that the reason that you observe so many ratios is that each yard had their own ideas about it and exercised their ideas accordingly. Since it was hard to measure the real efficiency, they did'nt and were able to live with whatever they produced. The Ohio/Mississippi river steamers that we all know and love are pretty much a study in inefficiency from stem to stern, but profits were great enough in their day to ignore efficiency (and often safety, years ago) in favor of convenience, cheap first cost and expediency. Freely translated: I don't know the answer either!

Linda Fisher 06-06-2006 08:44 PM

Somewhere in my museum research it seems to me that I read that the height of the stacks was at least in part determined to be as tall as possible and go under bridges. This was because one of the greatest enemies of the steamboats was fire as they were made of wood. Therefore the taller the stack the less chance of hot embers and/or sparks from the burning wood would have to land on the boat and start a fire. This is said to be one of the reason for the crown or feathering on top of the stacks. I admit this isn't very scientifid, but makes for a reasonable story.

Tom Schiffer 06-06-2006 09:41 PM

Linda: Dunno about the tops of the stacks doing anything for the embers etc. but I think you are absolutely right about the height factor getting the sparks/embers away from the flammable freight and superstructure. I think many if not most boats had tar paper roofs. Steamboats were often raised from sinkings, but it was hard to bring them back from a fire...although it was done. If those stacks were tall enough and skinny enough to give the flue gas some velocity, the gases would loft the embers so that they would burn out before coming in contact with the boat. Those embers could not burn when within the oxygen deficient flue gas, but when they contacted the oxygen in the air, business picked up, but, hopefully, they burned out before alighting. I forgot to mention that the flue gas also contained the 80% of the air that is nitrogen.

Hank Bloomer 06-07-2006 09:44 AM

Thanks to both of you. I strongly suspected that the old time builders used mostly rule of thumb, or it worked last time, lets try it again. Most of them weren't really engineers in the modern sense of the term, just solid practical mechanics of a type that almost doesn't exist anymore. Before there were bridges across 'most every river the only limit on chimney height was what the owner wanted. Sitll, I would guess that you could get them too short or too tall. I was hoping that some of the old rule of thumb ratios were still known. Just for curiosity does anyone know the ratio of flue area to stack area on Julia Belle Swain or Natchez?

Lexie Palmore 06-07-2006 01:50 PM

As steamboats evolved, so did the stacks. Wood burning boats with no forced draft required very tall, large diameter flues. With the advent of coal and oil as fuel, along with forced draft, smokestacks got noticeably shorter and skinnier. To get under bridges, they hinged the things. We greatly improved the draft on our steamboat by lenthening the stack 4 ft. With a wood fire like we have, there are so many variables, but draft is really, really important. Our boiler has better draft than our wood stove. Go figure.

Hank Bloomer 06-07-2006 03:16 PM

What is the 'Ghost's ratio of stack area to flue area and her ratio of height to diameter for the stacks? Do you have a vertical or horizontal boiler?

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