John C Wrote:
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> Exactly why steam locomotives are not rated in
> horsepower, because it is such a variable number.
> This makes it easier to explain to people though
> who come along and ask "How much horsepower this
> thing got?".
I agree, though there's a little more to it than that, too. During most of the steam era, horsepower of a locomotive didn't matter all that much except in an academic sense. What mattered was how much a locomotive could pull--tractive effort. Horsepower, meaning speed potential, wasn't the major factor when freight trains were content making ten or fifteen miles an hour. In addition, those were the days of journal bearings with high starting resistance; if a locomotive could start a freight train, it could pull it at sufficient speed. This of course began to change in the late steam era, hence the eventual focus on steam generating ability/horsepower and "100% boilers" and such. With increased speed expectations and roller bearing equipment today, tractive effort isn't quite as much of a limiting factor as it used to be outside of grades. In short, horsepower is tricky to compute for a steam locomotive, and for most of the steam era wasn't terribly important either.
It should be noted for folks here who might not realize it that comparing maximum horsepower of a diesel and a steam locomotive is utterly useless because they have totally different power curves. If they're to be compared, then they must be compared at some specific speed--figures which are much tougher to obtain.
The formula posted is a rough approximation of indicated horsepower, as it includes the work done to move the locomotive itself and is in fact based on tractive effort as calculated at the rim of the driving wheels.
While many here will know this, a note about horsepower as calculated above: The figure entered for tractive effort must correspond to the speed entered. A steam locomotive's tractive effort declines with speed at some rate depending on the locomotive's design. The Baldwin catalogue that this formula was pulled from should also provide a chart to figure approximate tractive effort at a given speed for a given boiler capacity. That chart, in turn, requires knowing a given locomotive's heating surface. The catalogue, depending on what year, may also provide some important data regarding piston speed and efficiency loss at various piston speeds.
It should also be noted that this formula dates to the days before superheaters. Baldwin gives a recommended modifier for locomotives so equipped.
with that many ifs, the Baldwin calculation is obviously a ballpark figure, not even accounting for normal steam locomotive variables (how's the fireman feeling today, how good is the water, etc). It's still a useful calculation because all locomotives are subject to those same variables, and because it's the only calculation we have for locomotives which no longer exist. I've compared it to a few indicator cards I found posted online and the Baldwin calculation wasn't really that far off, close enough to be considered useful for what it was meant for.
