Mr. Watt is Pretty close I'd say,,, on the coining of HP
Mercury has been rating their outboards in HP/Kw for years,,,, @ the Prop 250/189
Well, a lot have been using the term.. simply because it was a common and known measurement.. Does not mean it is relative to what a horse can actually do.
Anybody who has ever worked around horses knows a horse can outperform a small gasoline engine.. the question has always been debated on how long they can sustain it.. the other obvious question related to relationship of a horse and say a tractor is related to traction ability, the horse with the same power will almost always win over tires in traction ability. If you have ever seen a team of large draft horses in a pulling contest, they can move impressive loads!
However, looking up the term.. trying to determine if anybody had done any research.. here is what I found.
A horse can apparently produce nearly 15 HP.. on continous basis..!
The horsepower measurement is the amount of work that a horse can produce for a continuous time period at a slow haul.
However in a gallop for short periods the power level for that same horse shoots up dramatically..
A healthy person can peak at about 1 HP . But the continuous power output for a human is about 1/8th HP.
But back to horsepower.. what is the record or aveage?
The unverifiable record is apparently a two horse team of clydsdales, pulling a sled with 52 tons of logs across snow. To me seems a little sketchy ..
But it depends on traction, surface, etc.. as well.. so if the snow was packed .. and the surface smooth enough.. assuming they could get traction you can pull a lot on packed snow once moving.
On other hand a horse can easily pull more than its own weight all day long.. and a draft horse can easily weight over a ton. (that is a long way from 21 tons I will admit
.. but this is on any ground, up hill, down hill, etc..)
But.. taking a locomotive.. a friend of mine who is now retired from CN.. says they used to figure around 1HP of locomotove per ton of weight pulled as a minimum. They had charts.. and used to run more than minimum on trains many times apparently - and that went up depending on where in the country you were headed (hills or mountains impact this greatly). (he said the locomotives ran around 4,500HP each)
Given that.. maybe a HP on a train is close to being what a real horse can do (but maybe a little lower than real horse ability) .. my dad, who had a lot more experience around them than I always said a horse could easily do more than a HP of work continously.. it has always interested me to find out.. but can not ever get solid answers..
So.. I give up..