A mechanical horsepower is independant of it's method of generation (elect, gas, diesel).
Power =
Work ÷ Time
Units of Power:
1 Horsepower (HP)
= 33,000 Foot-Pounds (Ft-Lb) per Minute*, or 550 Ft-Lb Sec
= 745.7 or 746 Watts (or Joules per second)
= 2545 British Thermal Units per Hour (Btu/hr), or 42.42 Btu/min
* James Watt observed that, on average, a mine pony could do 22,000 foot-pounds of
work in a minute. He assumed a horse could do 50 percent more work than a pony, and arbitrarily defined the measurement of horsepower at 33,000 foot-pounds of work in one minute.
Brake Horsepower is measured* at the output shaft of an
engine or
motor; without the loss in power caused by the
gearbox,
alternator,
water pump, and other auxiliaries. The actual horsepower delivered to the
propeller (or driving wheels) is less.
* Measured by a friction brake attached to the drive shaft, and recorded on a dynamometer
Shaft Horsepower is the actual amount of power delivered to the
propeller. The latter will be less than the former because of friction losses in the
transmission and the stern gland, etc.
As a
rule of thumb, shaft horsepower in small craft is usually* between 70 and 90 percent of brake horsepower.
* Nigel Calder quotes “common” shaft horsepowers as about 95-97% of BHP. He probably assumes that BHP includes
water pump &
alternator losses.
Perhaps, someone would be interested in giving us a
basic physics tutorial, describing Force, Work, Acceleration, Power, & Torque, etc.