The power of a steam engine is commonly designated as horse-power. One horse-power is a force strong enough to raise 33,000 lbs. one foot high in one minute; this has been found to be about what a very strong horse could do working 8 hrs. a day. An engine of 100 H. P. would be, of course, able to do a hundred times as much as this. A steamboat of 1000 tons generally has an engine of 360 H. P. A man-of-war usually has one horse-power for every ton.
There are several kinds of horse-power referred to in the discussion of a steam engine; nominal, indicated, and actual or net.
Nominal horse-power was a term used during the invention of the steam engine to express the amount of work an engine could perform during a given time.
Indicated horse-power is obtained by multiplying the mean effective pressure in the cylinder in pounds per square inch, by the speed in feet per minute, and dividing the product by 33,000.
Actual or net horse-power is the difference between the indicated horse-power and the amount of horse-power expended in overcoming friction.
Example. - What is the horse-power of an engine that can pump 68 cu. ft. of water from a depth of 108 ft.?
1 cu. ft. of water = 62 1/2 lbs. 68 X 62 1/2 = 4250 lbs. 4250 X 108 = 459,000 ft.-lbs.
459,000 / 33,000 = 153 / 11 = 13 10/11H.P. = 13.9 H.P.