Rating Steam Engine Horsepower

Experienced steam man Gary Yaeger explains

Yaeger_A 12 hp sweep in use on the Mehmke homestead in the Highwood area of Montana

A 12 hp sweep in use on the Mehmke homestead in the Highwood area of Montana.

courtesy Gary Yaeger

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As a long-time reader of Farm Collector ...

there is one question I have that I have never seen an explanation for (or missed somewhere along the way).

I get way behind in reading the magazines during the summer so now I am catching up. I read with great interest the article by Gary Yaeger in the August 2008 issue regarding his favorite steam engines.

My question is this: How did they rate the horsepower of steam engines compared to internal combustion engines? For instance, the picture on page 46 of the August 2008 issue shows a 40 hp Peerless (I assume drawbar-rated) pulling a 20-bottom plow. They are probably 12-inch, but still, to do that today with modern tractors would take a big 4-wheel drive at 250 hp or more.

I know the steam engines were very heavy so traction was not a problem, but only 40 hp? I would find it very interesting if someone could write an article comparing steam engines to modern tractors.

– e-mail from a Farm Collector reader

Steam man Gary Yaeger explains

The earliest threshing machines were generally turned with horse power sweeps. The photo at top shows what appears to be at least a 12 hp sweep in use on the Mehmke homestead in the Highwood area of Montana. If a steam engine were belted to that threshing machine in about 1880, it would have been described as “a 12 hp steam engine.”

Terminology became more sophisticated around the beginning of the 20th century. Later ratings used firebox “heating area” to determine boiler horsepower, which closely followed the ratings used by some companies. The 110 hp Case is a popular example. By 1910, Case went from “nominal” horsepower ratings of 32 hp to the 110 hp rating, which was determined by heating area and brake horsepower. The engines were fired at the factory and tested on a Prony brake (a unit similar in theory to today’s dynamometer), placing a measurable load on the steam engine being tested.

Nominal ratings were loose ratings. Some companies used larger boilers to boost ratings while others used a larger bore and stroke to rate their horsepower higher. They needed to make the boiler capacity and engine’s steam usage compatible, as a too-large engine used more steam than a small boiler could provide. Likewise, a too-large boiler produced more steam than a small engine could use.

One more thing regarding the rating of steam engine horsepower: Combining the horsepower ratings of brake and nominal would make that Case a 32-110 hp engine. Modern thinking always brings in the 110 hp as brake horsepower, but often gets the nominal horsepower distorted by terming it “drawbar” horsepower. No steam engine manufacturer ever used that term. That came later with the gas tractors, perhaps from the Nebraska tests. One last thing about steam power: A steam engine has more torque at 0 rpm than it has at full (or approximately 250 rpm). FC