"Watts" Horsepower

by F. G. Rohm, Chief Qualification Engineer (Ret..)

This interesting article was written for us by Fred Rohm, who was our Chief Qualification Engineer when he retired. Fred had a career of 44 years in the industry, with a majority of those years spent at Textron Lycoming. Most of his career at Lycoming was as Chief Experimental Engineer, which establishes his qualifications to author this kind of article.
James Watt, Scottish physicist, had an engine problem even in 1769. Although steam engines had been invented before he was born, they were crude, inefficient machines and only a few were in use. So he had, after much experimental work, developed a relatively efficient condensing steam engine, the forerunner of the present day type.

Being a good business man, Watt tried to sell his engine to coal mine operators who were then using draft horses to supply power to drive the pumps which kept the mines free of water. But the mine owners had sales resistance! They insisted on knowing exactly how many horses each engine would replace, or, in other words, the horsepower of the engine. How much work would his steam engine do? This, then, was James Watt’s problem.

Although simple machines such as sailing vessels, windmills, and waterwheels had been used for centuries, Watt realized that for the most part, the majority of work in the world had been done by man and his domesticated animals. Work was measured and paid for by the day, from "sun to sun". With the advent of reliable clocks, work was then accounted and paid for by the hour. Evaluating work by this time method, it was assumed that all men and animals could and did perform the same amount of work. This was far from being true.

Watt realized that in order to have his steam engine used by the coal mine operators he would have to answer their questions - "how much work will it do, and how many men and horses will it replace?" Since the "power" of one horse was a generally known and a constant quantity, he would have to determine the "power" of his engine in order to compare it with the horses which it was to replace. His problem then was to define "power."

Power did not mean force. The mine owners cared nothing about the force Watt’s engine might exert. They wanted to know how fast the engine would pump water out of the mine; in other words, how fast will the engine do the work? Simply, that was the definition of "power."

The methodical physicist experimenting with draft horses used to operate mine pumps found that, on an average, a horse pulling with a force equal to a weight of 150 pounds walked 2 ½ miles per hour. Since work is force exerted through a given distance, it is measured in terms of feet pounds. Thus, on an average, one horse could do work at the rate of 33,000 feet pounds per minute or 550 feet pounds per second.

Watt’s definition for one horsepower, which has now become universal, was, therefore, the doing of work at the rate of 33,000 feet pounds per minute. Today, all conventional power producing units are rated on this basis.

The 250-horsepower engine in the modern light plane is capable of doing work at the same rate as that of 250 average horses. From an interested engineer’s point of view, it is capable of moving 137,500 pounds of weight one foot in one second. Yet, what a difference there is in its size and weight (approximately 400 pounds) when compared to the horses it replaces!