To measure the power of his steam engines, James Watt estimated that one British horse could move 33,000 pounds, one foot, in one minute, which is the standard we use today for 1 horsepower. In Jim's honor, metric types use kilowatts, with 0.7457kw equal to 1 horsepower. The spec sheets for many engines now show power in both standards.
How much power an engine is said to put out depends on how it's measured. Diesel manufacturers use brake horsepower, which you'll see as "bhp" in our tests. To get this figure, the engine is stripped of all accessories, run at full, screaming wide open throttle, and then braked down by a dynamometer. It's almost-to-destruction testing and frightening to behold.
Shaft horsepower is measured at the prop shaft. It's bhp minus the power lost to gears and bearings. Maximum shaft horsepower is estimated to be 97 percent of maximum bhp for gas and diesel inboards. Since outboards and stern drives have built-in gears and bearings through which they power the prop, these engines are rated for shaft horsepower. In our tests, shaft horsepower is stated as horsepower (hp), because the power at the prop is the horsepower that counts.