James Watt was ALL WRONG about what a mechanical horsepower is. In fact, I guess he must have been seriously and entirely wrong, and must have been seriously mentally challenged!
I have used a 17 horsepower lawn tractor in absolutely perfect condition. It was actually about as strong as about 3 horses.
My lawn tractor is called 20 horsepower. Now, it has some kind of fuel pump or carburetor problem, and is not quite as strong as one single horse.
Since James Watt's mathematical formula is entirely incorrect, I vote for all math and technology books and manuals that have his formula in it to have a big huge bold lettered disclaimer both before and after it stating that the formula is wrong, and should be treated as a lie.
The right way to calculate horsepower is to hook up the estimated number of horses to the vehicle and have them pull it up some predetermined steepness of a hill and see how many horses it takes to go a certain distance at a certain speed, and then do some simple math. For PTO horsepower, some similar tested measure should be done.
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I truly believe that all these so called 300 horsepower cars will turn out to actually be more like 100 horses, nascar's 800 hp vehicles will be more like 280 horses.
What the motors have is endurance by far compared to any animal. Endurance is another test though.
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I think my good ole gal, she's an '81 ford fairmont futura with a 200 cubic inch straight 6, and closing in on a third of a million miles, I think she has about 25 horses, maybe 30. Going up the hills around here in L, some places she can't quite make 25 mph. On the incline of hiway 20 going to town, I prefer to do it when there is very little traffic. Always some hotrod wants to go faster. (urge to get out and charge the hotrod for the gas money it takes for me to go faster before I stomp the gas pedal just to satisfy some hotrod...)
Anyhow, in short, whenever I see something with a rated horsepower, I always add the following formula that James Watt failed to add:
HP DIVIDED BY 3 = REAL HORSES, maybe, and usually less than that.
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It's similar to alcoholic beverages proof, calling 200 proof 100 percent. It just LOOKS like more. Trust the ole neanderthal. We know these things, and no amount of hooey washes them clean.