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Rating Steam Engine Horsepower

Experienced steam man Gary Yaeger explains

| November 2009

  • 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
  • Yaeger_A 40 hp Peerless pulling a 20-bottom plow
    A 40 hp Peerless pulling a 20-bottom plow.
    courtesy Gary Yaeger
  • Yaeger_Mike and John Schrock operating a 40 hp Peerless pulling a 20-bottom plow
    Mike and John Schrock operating a 40 hp Peerless pulling a 20-bottom plow.
    courtesy Gary Yaeger

  • Yaeger_A 12 hp sweep in use on the Mehmke homestead in the Highwood area of Montana
  • Yaeger_A 40 hp Peerless pulling a 20-bottom plow
  • Yaeger_Mike and John Schrock operating a 40 hp Peerless pulling a 20-bottom plow

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.

10/15/2019 9:25:55 AM

Good Morning... I am a piano technician doing research on old piano factories with a specific focus on the Chickering & Sons factory in Boston built in 1853/1854. Power was provided to all of the belt driven machinery by a 125 hp Stationary steam engine built by Otis Tufts in East Boston at the Boston Steam Engine works. There were 6 custom made boilers built by Chubbuck . later reference states the power as 145 hp but I cant find any reference how 125 became 145. There was some changes done in the factory to handle increased workload so I am wondering if the increase was done by adding more boilers . On another note I am interested in knowing more about Otis Tufts

3/25/2019 4:59:19 AM

The subject of rating the horse power produced by a steam engine, on the railway or from a stationary engine, has been confused by people not really understanding what they are talking about, by rating the boiler output rather than the power delivered to the wheels, and using a term called nominal horse power which has been derived from indicator diagrams which are obtained by measuring the cylinder pressure at different engine loads. As a marine engineer by training, I have seen many times people making quotes that a steam traction engine produces only 6 nominal horsepower at the steam cylinder, but this bears no relation to the fact that the steam engine can drive a 15 tonne machine at a considerable speed along a roadway which would indicate much more power is being produced like 50-60 bhp. Today we have become used to quoting the brake horse power of piston engines measured at the flywheel, which is a much better way to rate the power of an engine. This still doesn't answer this question because as I understand it, very few people/manufacturers of steam engines are prepared to measure the actual brake horsepower of their engine against a brake drum at different crankshaft speeds, and normally rely on just quoting a nominal horse power which at the end of the day is a fairly meaningless concept. It is my view that the twin cylinder engines on large traction engines are producing power of at least 50bhp when running near max boiler pressure, as I have worked with such machinery when at sea using similar boiler pressures and engine rpm's. Single cylinder engines will be producing 25-30 bhp which would agree more readily with the power required to move that piece of machinery along a roadway or across a field.


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