Why Engines are Commonly Measured in Horsepower

Why Engines are Commonly Measured in Horsepower

We owe the “horsepower” unit of engine power measurement to Scottish engineer James

Watt. In the early 1780s, after making a vastly

superior steam engine to the then classic Newcomen steam engine, Watt was looking for

a way to market his invention, advertising the fact that his engine used about 75% less

fuel than a similarly powered Newcomen, among many other improvements.

At first, he tried selling his engine on a royalty scheme, where the customers would

owe him one-third of the money they saved by using his engine over other steam engines.

Of course, many at the time used horses, not steam engines, so it was difficult to compare

without them actually buying the engine to see how it would perform for their particular

usage. Thus, he scrapped the royalty scheme and decided to try a different tact to convince

people to buy his engine. Ever the inventor, his solution was to come

up with a new unit of measurement that those in need of his engine understood -horse power,

referring to powerful draft horses. Thus, he set about determining how much power

a typical draft horse could generate. It isn’t known exactly how he came up with the numbers

he did, as there are conflicting accounts of the experiments he ran. But after doing

those experiments, he figured out a typical draft horse could do about 32,400 foot-pounds

of work in 60 seconds and maintain that power rate for a good long workday. He then rounded

up, going with 33,000 foot-pounds per minute for 1 horsepower.

So, in other words, by his estimation a good draft horse could lift 33,000 pounds of material

1 foot in 1 minute or 3,300 pounds of material 10 feet in one minute, etc .

In truth, that’s a very generous estimate as very few horses could maintain that kind

of power for a full workday, but getting a perfect figure wasn’t that important to

what Watt was trying to do. Further, by overestimating what a horse could do, whether intentionally

or not, he made sure that his product would always over deliver what he said when trying

to get people to buy it, which is a great word-of-mouth marketing trick.

In the end, Watt’s engine was revolutionary and played a huge role in the Industrial Revolution.

Thanks to this fact, his unit of measure of an engine’s power, horsepower, also became

popular. Funny enough, today the SI unit of power, the Watt, which was named in homage

to James Watt, has widely come to replace “horsepower” in most applications.

Bonus Fact: While Watt came up with the exact measurement

of what would become “horsepower,” he was not the first to propose the idea of equating

a steam engine’s power to a horse’s. The first documented instance of this was suggested

by British inventor Thomas Savery who wrote the following in a letter in 1702:

“So that an engine which will raise as much water as two horses, working together at one

time in such a work, can do, and for which there must be constantly kept ten or twelve

horses for doing the same. Then I say, such an engine may be made large enough to do the

work required in employing eight, ten, fifteen, or twenty horses to be constantly maintained

and kept for doing such

a work…”