One horsepower is supposed to be the amount of work a horse can do. So why is it that a horse can run up to 50 miles per hour. Or pull a car out of a ditch. Things that a 1 hp car couldn’t have a hope of doing. Basically, we saw a video of a horse pulling a Focus out of a ditch which led to an office talk about how many horses are really powering a horse. So here’s a look at how horsepower works, and why it’s not exactly what it appears to be.
The unit “horsepower” was the creation of James Watt. And yes, that’s the same Watt for whom the watt was named. And while we associate the watt with electricity, both the watt and the hp are ways to measure power. That’s right. Horsepower isn’t the force. It’s not even the work. It’s how much work you can do over time. That’s power.
Watt was an engineer who happened to be one of the pioneers of using steam to do work. Since the possibilities of steam were as yet unknown, Watt needed a way to compare his steam engines to something people were familiar with. Since the workhorse of the time was, yup, the horse, that was Watt’s target.
The unit horsepower was conceived as a sales tool. To show you how a steam engine’s output compared to that of a horse. But since horses come great and small, Watt needed to make a standard horse.
Watt started measuring horses doing work. Starting with a horse that was walking in circles to turn a grindstone at a mill. He determined that the horse could turn the wheel 144 times an hour. Based on the diameter of the wheel, it meant the horse was walking just under 181 feet every minute. He then estimated that the horse was pulling with a force of 180 pounds. That meant that the horse could exert 32,572 feet times lbs per minute. He rounded that to 33,000. Another story has it that he determined a pony could lift 220 lbs 100 feet a minute during a four-hour shift and then decided a horse was 50 percent more powerful. Again getting 33,000.
You’ll notice a few assumptions and shortcuts in there, though. Like the estimates of the weights and forces involved. And a lack of variety in horses. In short, they weren’t exactly as precise as one might expect for what became a nearly universal unit.
Because the hp was a unit of power, it was what Watt estimated the horse could do over a normal horse working day. Not an instant measure. An average. Later tests showed that horses could make much more power.
Measurements of draft horses in the 1920s showed power of up to 14.9 hp from a single horse when measured over just a few seconds. People, as a reference, can make around 1 hp for short times and about 0.1 over an extended period.
That makes horsepower a calculation, not a direct measurement. You don’t measure hp, you measure the amount of force, the distance, and the time it took to do it. When it comes to measuring engines instead of horses, time is swapped with rotational speed. And it adds a constant you may have seen in dyno equations, 5252. Because horses don’t spin and engines do. On a car engine (or electric motor) power is equal to the amount of torque times the RPM. Divided by 5252 because that’s the number that turns RPM back into time. And yes, that’s a simplified explanation, but unless you really want to get into radians, it’s enough.
So how does a horse pull that car out of the ditch? It’s not really about gearing, either. Horses have legs, not gears. Let’s do some more math. A draft horse, like a Clydesdale, can make peak power of just under 15 hp. That means it can exert 8,000 lbs of force for a second. And nearly that rate for a few seconds longer. A 3,000 lb car, even if slightly stuck in the grass, is nothing compared with that force. Remember that a 3,000 lb car doesn’t need 3,000 lbs of force to move it, either. After all, on flat ground, you can probably push it.
What about when a team of horses pulls an 80,000 lb tractor trailer out of the snow? Well, adding a second horse doesn’t double your pulling power. It triples it. And, again, they’re not dragging that weight. It’s on wheels, so once it starts to roll, then things get easier.
The important thing is that they’re only doing that pulling for a very short time. Horsepower is a measurement of force and distance over time. When Watt defined it, he was thinking about replacing the work a horse could do in a workday. You’re thinking about spinning tires and launching to 60 mph as quickly as possible. Could a horse tug a car out of a ditch constantly for hours? Not a chance. Then again, try doing a burnout all day and see how much your engine’s ponies like that move.