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Original horse power[edit]

The original horsepower unit was selected to show potential buyers of the new steam engines how much they would save by switching from horses to steam. The horsepower unit represented the rate of work a horse could do on a continous basis (all day long). Thats why its only a small fraction of the maximum capability of a horse.

Why is this entry flagged?[edit]

"This article or section may contain original research or unverified claims." How so?

Horsepower from a horse[edit]

The section "Horsepower from a horse" is very incomplete: the most pressing issues here are:

  • What kind of draft horse are we talking about here ? Searching a little on the net we find:

"What type of horse was a brewery horse? In England at the time a work horse most likely would have been one of the three British "heavy breeds" – the Suffolk punch, the shire horse, and the Clydesdale. The Clydesdale is said to have originated in the latter 1700s, perhaps too late to be a common work horse at the time Watt was doing his horsepower calculations. So it seems likely the horse in question was either a Suffolk punch or a shire horse." --> so it would appear that it's either a Suffolk punch or a shire horse; however at we find: "I've gotta say that I think James Watt was a bit off on his calculations, or perhaps he was using a mini-pony. My team of Belgian draft horses (that's two horses) is roughly capable of doing half the work of my old Ferguson 30hp tractor, which would mean that each horse can crank out about 7hp on a sustained basis. Assuming a relatively constant ratio of work potential to weight, Mr. Watt's horse would weigh less than 300lbs. But since he was in the business of replacing horses, it was to his advantage to minimize their stated capabilities."

and at we find:

"David, I think Watt used an average horse, not a draught horse, and it was a rate of work that could be sustained all day by said horse. I think you will find the horsepower of an old steam tractor to be more accurate. A 5hp steam tractor would probably come pretty close to five of your horses. The old Fergies are great little tractors, but I doubt you woud be getting your 30hp out of it today. Also, by the time you get to drawbar power, after gearbox etc, and wheel slip, you only get about 80% of what engine puts out. The horse was measured by what it put, but it's lungs ("engine") can probably operate at 1.5 hp."

so, either it's a shire horse of suffolk punch (none of them are even the strongest horses I believe), or it's even an average horse. Also, it seems that some estimate regular draft horses to have 1.5 HP of power for "sustained" work. That leaves allot of room of suggestion, ...

  • Another issue is the "sustained work"; what does this mean ? Is that 8 hours of work, more like 12, ... (750W x 8 = 6000W, 12 x 750 = 9000 W, quite the difference).

Finally, perhaps that we can provide a small comparison between the strongest and weakest draft horse breeds, ie would say a Russian Draft Horse be able to pull much more than ie a Suffolk punch/Shire ? (ie 1,9 HP vs 1,5 HP ) ? Also, I wonder how much HP say an ox can pull (see Plough. (talk) 12:31, 26 November 2010 (UTC)

British horsepower[edit]

The abbreviation bhp may also be used for British horsepower[15] (though the usual use is brake horsepower), defined as 33,000 lb·ft/min. The description of the unit "British horsepower" is used when differentiation from "metric horsepower" is required.

I am removing this section because it seems to me to be highly suspect. I don't think there is any such thing, and don't think this (which I have left in the article) can be regarded as a reliable source, at least not for this bit of nomenclature. BHP is usually brake and sometimes boiler but not British, surely? If anyone can provide better references I will be happy for the section to be reinstated. Globbet (talk) 09:04, 26 August 2011 (UTC)

AIUI, this was a genuine name for the unit, but it was horribly obscure and is long fallen from use - however the unit is still around and is in fact the source of the main hp definition. "Horsepower" in the sense of brake horsepower was first defined as sensible round-number units of either 550 foot-pounds/second (in the Imperial world) or 75 mkgf/s (in the metric world, usually the French CV or German PS). All of these are close enough to be considered equal, within the measuring accuracy of anything pre-electric.
For a while afterwards (WW2 era?) they were separate units of slightly different quantity and needed to be identified separately, as they were now within the precision that could be distinguished by measurement.
With the move to SI base units for British measurements ('70s?), the horsepower in Britain was redefined as an arbitrary integer number of SI units and so the hp was rounded up to the well-known 746W figure of today. "British horsepower" then created a new name for an existing legacy unit, with no purpose other than this legacy, and remained based on 550 foot-pounds, with an actual value slightly less than 746W. Its only real function was in updating textbooks - the "550 hp" and "746W hp" were (deliberately) both within the measuring precision of the time, for nearly all physical measurements. Andy Dingley (talk) 09:53, 26 August 2011 (UTC)
It would be nice to have a citation for the change in definition of the horsepower in Britain from 550 ft lb/s to exactly 746 watts. I'm not sure what "time" you're referring to; the first definition of the metre was valid to about 5 significant figures, in 1795. I'm sure metrologists of the time wouldn't whack off a trailing digit just for fun. Watt couldn't have defined his unit as exactly 746 watts because the SI system didn't exist yet; I'm not sure why "33000" was considered a preferred number, since the precision of mesaurements of actual horses evidently could have justfied, oh, say, 30000, an even "rounder" number. All the tables I've seen call a horsepower 745.699 watts except for the rating of electric motors when it is rounded out to 746 watts (good enough for slide-rule calculations), in such publications as NEMA standard MG-1. If you're converting to SI anyway, why make up a new unit based on a non-power-of-10 multiple of an SI unit and give it the name of an old unit? It'd be like re-defining the inch as exactly 2.5 cm because you don't like dealing with 3 significant digits. Even British love of tradition must have limits? --Wtshymanski (talk) 14:20, 26 August 2011 (UTC)
Sorry, Wtshymanski, it is not clear to me exactly what you are saying. Globbet (talk) 19:59, 26 August 2011 (UTC)
I'm saying a British horsepower is a horsepower is 550 ft lb/s is 745 and change watts, and that the only reason the term gets used is to distinguish between 550 ft lb/s and 75 kg m /s definitions. --Wtshymanski (talk) 21:06, 26 August 2011 (UTC)

Nominal horsepower[edit]

There is a sort-of-relevant discussion going on at Template_talk:Convert#Nominal_horsepower? - Globbet (talk) 21:27, 26 August 2011 (UTC)

If templates were smart enough to figure outthis sort of thing for themselves,we wouldn't need editors. Anyone remember User:Bobblewik? --Wtshymanski (talk) 17:52, 31 August 2011 (UTC)
That is pretty much in line with the conclusion reached there. I don't. Globbet (talk) 22:27, 31 August 2011 (UTC)

"Metric Horsepower"[edit]

There is no official measure of horsepower called "Metric Horsepower", only Americans call it this way. it is properly called "PS", and is based, obviously, on the German "Pferdestärke". This equals to ca. 0,735 kW, therefore 1PS = 1.36 kW (ca.). Also, the American Horsepower measurement (or is it the British one, I believe its called "SAE hp", and/or "bhp", which is 1.34 kW, is missing a detailed description here, how come?.--Daondo (talk) 22:36, 17 October 2011 (UTC)

Well, we've got a whole section titled "Metric Horsepower" which gives all these different metric horsepower units, ( those Europeans have different words for everything), and the rest of the article is talking mostly about 550 ft lbs/sec which is a British or Mechanical or SAE horsepower. You do know that SAE stands for Society of Automotive Engineers which isn't particularly British? --Wtshymanski (talk) 01:23, 18 October 2011 (UTC)
To make things more "interesting" in {{Infobox German Railway Vehicle}} and in {{DRG locomotives}} I found PSi which probably means indicated PS. One more conversion problem. Peter Horn User talk 20:43, 17 October 2012 (UTC)

Mechanical horsepower conversion procedure - numerical precision[edit]

From the standpoint of error propagation I found silly that the original 2 significant digits (1 hp ≡ 33,000 ft·lbf/min) expand through the conversion into SI units to final 17 significant digits (= 745.69987158227022 W). Ignoring the precision of the conversion factors themselves and assuming 1% relative error, the final conversion should be something like 1 hp≡ 33,000 ± 330 ft·lbf/min ≡ 745.70 ± 7.46 W. Even taking the original number defined by Watt as a number of 5 significant digits, this brings an error in the order of 0.00746 W (relative error of 10^-5) which renders the tail of digits into a meaning less nonsense. — Preceding unsigned comment added by (talk) 22:55, 15 April 2012 (UTC)

(months later) Well, definitions are (by defninition) exact, so Watt's "33,000" has as many sig figs as you need. As long as we're talking about "definitions" of units and interconversions between them the precision is as high as ever needed. But it is absurd to give the final result to more significant figures than could ever be possibly resolved in an actual measurement. Anything more than 6 figures is going to make the reader's eyes glaze over and doesn't really improve the effect of the presentation. --Wtshymanski (talk) 13:36, 21 August 2012 (UTC)
The value used for the pound-mass has an erroneous extra digit. It should be .45359237 kg exactly, not .453592376 kg. And a full 5 digits of the final result, 745.699881448 W, are wrong. The exact value denigrated above, 745.69987158227022 W, is correct. Exact calculation in the definition of units avoids this sort of error. — Preceding unsigned comment added by (talk) 14:15, 17 July 2013 (UTC)

Contradiction in bhp definition[edit]

At the top of the Measurement section it says bhp is "power delivered directly to and measured at the engine's crankshaft", and if you subtract "frictional losses in the transmission" you get shp. Later it says bhp is power before subtracting the auxiliaries such as alternator and hydraulic pumps.

So shouldn't that first section say something like this? "Brake / net / crankshaft horsepower (power delivered directly to and measured at the engine's crankshaft) minus frictional losses in the transmission (bearings, gears, oil drag, windage, etc.), minus auxiliaries such as alternators and pumps, equals Shaft horsepower." Kendall-K1 (talk) 16:48, 21 February 2013 (UTC)


Why is there a globalize template on the "Current definitions" section? Isn't horsepower a US unit? Kendall-K1 (talk) 02:15, 22 February 2013 (UTC)

Historical symbol[edit]

A ligature-style symbol/glyph/character \mathrm{H\!\!P} is used to denote 'horsepower units' in the book Aiba, S., A. E.Humphrey, and N. F.Millis, Biochemical Engineering. 1965, New York, U.S.A.: Academic Press. 333 pp.. (e.g. pages 167ff.). —DIV ( (talk) 02:28, 22 March 2013 (UTC))

There is a unicode character ㏋ (U+33CB "square hp") but I can't find anything that says what it means or what it's used for. It's in the CJK compatibility block with other units of measure like mV, Hz, and gal, so it's probably horsepower. But it's not a ligature, it's two separate letters crammed into one glyph. Kendall-K1 (talk) 18:41, 4 April 2013 (UTC)

I welcome the article. My only comment is that I believe units of measurement named after those who helped define them have their first letter capitalized. (e.g. Watts, Amps, Joules, Faradays, Volts, etc.). If this is correct, the frequent reference to 'watts' should be corrected to 'Watts'. — Preceding unsigned comment added by (talk) 22:39, 6 August 2013 (UTC)