December 10, 2012 - When I set out to answer this question I had no idea just how complicated the answer was going to be. If you do a quick internet search for horsepower you will find some conflicting information about the derivation of the term. As I discovered, the debate over horsepower stems from the fact that:
The term horsepower (hp) originated as more of a marketing term than a scientific form of measurement.
At its foundation, the unit we know today as Horsepower was littler better than estimation.
The term horsepower was coined by James Watt, the inventor of the steam engine. It’s also his last name that is on every light bulb in your home. When Watt invented the steam engine in the late 18th century, he needed a way to market it that the average person could understand. After all, humankind had been using horses to get around for the last 5000 years, so it’s not at all surprising that the steam engine was a confusing concept to most everyone who wasn't James Watt. Obviously, the steam engine was more powerful and efficient than a horse, or even a group of horses, otherwise why should anyone want to use it? Watt decided that in order for the steam engine to be accessible to everyone it had to be easy for everyone to understand. That is why he came up with an equation that explained the power of his engine in terms of horses—horsepower.
The problem for Watt, is that there was not (and there still isn’t) a perfect way to express the power of a mechanical engine in terms of horses. The power of a horse is different from horse to horse, and even day to day within the same horse. Watt did the best he could with what he had to work with.
Horsepower as Watt defined it, is the amount of work done over a certain period of time:
hp = work done/time
Through his calculations (and estimations) Watt determined that:
1hp = 33,000ft-lbs/minute.
That means that if you were to lift 33,000lbs the distance of 1 foot over the span of 1 minute you would be working as hard as 1 horse.
How did Watt come up with those numbers?
Somewhere in rural, colonial America in the late 18th century, a horse attached to a mill, grinding corn, walked in a circle that was 24ft in diameter (75.4 feet in circumference). Watt estimated that while walking, the average horse pulled with a force of 180lbs. On average, Watt found that the same horse horse pulling at a force of 180lbs, made 144 trips around the circle every hour—that is about 2.4 trips every minute. So according to Watt, the average horse goes around a circle, 75.4 feet in circumference, 2.4 times every minute. That means the horse is traveling approximately 181ft per minute.
75.4ft x 2.4/minute = 180.96ft/minute
( In case you are wondering, 181ft per minute is about 2mph.)
To find the power of the horse we take the 180.96ft/minute and multiply that by the force with which the horse is pulling and we will get the equivalent of what Watt called 1 horsepower.
1hp = 180.96ft/minute x 180lbs = 32572.8ft-lbs/minute.
Watt, of course, rounded up to 33,000 because the number was just an estimate for his sales pitch.
That 32,572 number that Watt came up with was already pretty loose, so why not toss a few hundred ft-lbs/minute on top to make his number nice and round right? Maybe Watt was just accounting for the fact that his corn-grinding horse was having an off day? James Watt gave us one of the greatest innovations in this history of the human race, so I guess we’ll let it slide—but just this once! We don’t really have a choice do we? Regardless of its loose beginnings, horsepower has become a standard measurement of power for over 200 years.
It is hard to believe that every engine on every motor vehicle on the road today has been tested to determine its power in terms of a horse grinding corn on a mill in 18th century America.
The pace of life is much quicker in our modern cities than it was in rural America during the 18th century, so today you most commonly see horsepower conveyed in terms of seconds rather than minutes.
The most widely accepted definition of Horsepower today is: A unit of power equal to 550 foot-pounds per second.
(Interesting Fact: It is estimated that a human can sustain only about .1 horsepower. Weak!)
While the term Horsepower might be an estimate, the numbers that the unit stands for are very real. The term Horsepower has lasted as a unit of measurement for over two centuries because modern technology allows us to easily measure what 550ft-lbs/second really is. So what does all of this mean for your car? What does horsepower equate to when it comes to speed, acceleration and the power of your engine?
Check our blog tomorrow for Part 2 of What is Horsepower?