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.
is measured* at the output shaft of an engine
; without the loss in power caused by the gearbox
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
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
Perhaps, someone would be interested in giving us a basic physics tutorial
, describing Force, Work, Acceleration, Power, & Torque, etc.