How much does a horse power?

December 5, 2023 9:28 PM

What is horsepower anyway?
The term was adopted in the late 18th century by Scottish engineer James Watt to compare the output of steam engines with the power of draft horses. This was not science for the sake of science, but an attempt to develop marketing blurb for selling steam engines by talking about how many horses they could replace.
The term is used with abandon in all sorts of ways by manufacturers of all sorts of engines, some of which may even be accurate. But what is a horsepower anyway? There are lots of different measures called horsepower and lots of 'standardised' ways of measuring it.

Jason Fenske, owner of the YouTube channel Engineering Explained talks in depth about the uselessness of the term 'horsepower' and why we shouldn't use it.

Using less engineering-speak and more entertainment value, Donut Media decided to put to the test the question we all really want to know of 'how much horsepower does one horse actually produce?' (inline ad from 0:50 to 2:28).

posted by dg (16 comments total)

somewhat related, I recently burned an Audible credit on Richard Rhodes' Energy (it was ★★★☆☆ but lacking the panache of James Burke or Kathy Loves Physics for that matter). Rhodes spends a lot of pages on the steam era and that's always interesting since steam power is what first got our economy "cooking with gas" so-to-speak.
posted by torokunai at 9:50 PM on December 5, 2023

But what is a horsepower anyway?

In my mind, it's always been about three quarters of a kilowatt.

Is that wrong?
posted by flabdablet at 10:04 PM on December 5, 2023 [1 favorite]

my LEAF can drive just about an hour at 75mph, converting ~30kWh of chemical energy in the battery into physical displacement, for a power output rate of 30kW.

So 30kW / 750W/hp is like driving a carriage with a team of 40 horses I guess.
posted by torokunai at 10:15 PM on December 5, 2023

In my mind, it's always been about three quarters of a kilowatt.
Depends on whether you are referring to imperial or metric horsepower ...
posted by dg at 10:45 PM on December 5, 2023

Nah, those are both about three quarters of a kilowatt.

But TIL that boiler horsepower is a thing, and 1BHP is about ten kilowatts. Where there's an entire order of magnitude between various definitions of a unit, that is indeed (a) an absurd unit and (b) unsurprising to find in use in the US.

Just go metric already. Honestly, it's not that hard. Australia did it when I was a kid and the sky did not fall in.
posted by flabdablet at 11:03 PM on December 5, 2023 [5 favorites]

As somebody points out in the comments - a calculated value of 5.7hp makes James Watt's claim that a 10hp steam engine could replace 2 draft horses, quite accurate.

There would be another approach to this test which would use a horse mill of the type Watt was citing (like one of these surviving or restored mills in Scotland) - probably turned by a pair of Clydesdale horses. The energy expended would be rotational rather than linear and would be calculated over the shift duration of the animals.

Big props to the video makers for trying to actually measure the value however.
posted by rongorongo at 11:06 PM on December 5, 2023

My LEAF . . . is like driving a carriage with a team of 40 horses.
. . . but easier with the parallel parking.
As someone who has shovelled A Lot of dirt and rocks into wheelbarrows, also relevant is the gallon-of-gas = 600 bob-hour equivalence. Neither of these handy units are SI. Tnx for the links dg, my jam.
posted by BobTheScientist at 12:10 AM on December 6, 2023

1BHP is about ten kilowatts
Ah but theres also British Horsepower (BHP)= 745.7 watts
Or Brake Horsepower (BHP) = 735.49 watts

a 10hp steam engine could replace 2 draft horses
Most working horses will work a maximum of two hours per day, so if you have 4 horses working in turns over a working day, each generating 5HP, the average over the day is somewhere around 1 HP per horse.
posted by Lanark at 1:28 AM on December 6, 2023

theres also British Horsepower (BHP)= 745.7 watts
Or Brake Horsepower (BHP) = 735.49 watts

Exactly! Both of those are the usual three quarters of a kilowatt, but there's another unit of power, also denoted BHP, that's fourteen times the size. That's madness.

Per Wikipedia:

Boiler horsepower is still used to measure boiler output in industrial boiler engineering in the US. Boiler horsepower is abbreviated BHP, not to be confused with brake horsepower, below, which is also abbreviated bhp, in lower case.

Oh, OK. Lowercase. So they could never be confused. Check.
posted by flabdablet at 3:13 AM on December 6, 2023 [1 favorite]

For reference - here is a quick tour of a modern horse driven mill. - you will see that the two horses appear to be quite chilled - looking relaxed rather than under a strain (and indeed looking like they could continue this way for a few hours) - but still exerting enough torque to drive an array of machines including mill for grinding flour. The Wikipedia page on horse mills mentions that the Mennonites, who do not approve of using engines, still employ a number of these.
posted by rongorongo at 3:25 AM on December 6, 2023

A user on a homebrewing forum I used to frequent had the signature line "Horsepower sells cars, torque wins races."
posted by under_petticoat_rule at 4:47 AM on December 6, 2023 [1 favorite]

I love horsepower sweeps. Here's one baling hay. They're so calming to watch.

Versus watching a steam powered version of either of the above. Those are loud and look like they'd explode at any moment if someone forgot to touch the right thing at the right moment. Still fun to watch, but not calming.
posted by AzraelBrown at 6:36 AM on December 6, 2023

(much to the chagrin of the French "the cow-power" never caught on as a unit of measure)
posted by rongorongo at 6:47 AM on December 6, 2023

I was taught that (for reciprocating internal combustion engines, anyway) horsepower = torque (in foot-pounds) x rpm / 5252. So far, that's been sufficiently accurate for my purposes.
posted by workerant at 11:25 AM on December 6, 2023

That's correct, workerant and also why horsepower and torque curves on a dyno readout should always cross at 5,252rpm. If they don't, it's because the dyno isn't correctly calibrated. This is most often seen when engine builders want to increase the horsepower figures of their engines artificially, so they calibrate them to read higher.
posted by dg at 2:15 PM on December 6, 2023 [1 favorite]

Where there's an entire order of magnitude between various definitions of a unit, that is indeed (a) an absurd unit and (b) unsurprising to find in use in the US.

Just go metric already. Honestly, it's not that hard. Australia did it when I was a kid and the sky did not fall in.

In Sweden, there's a metric mile ("mil", pronounced "meel'" with a soft L), which is 10 km, used for long travel distances.
posted by alexei at 12:58 PM on December 27, 2023