Definition
Shaft horsepower (shp) is the power an engine delivers to its output shaft, measured before that power is converted into thrust by a propeller or other device. In a turboprop, it represents the mechanical power the gas turbine sends through the reduction gearbox to the propeller shaft, and does not include the small additional thrust produced by the engine's exhaust gases.
Plain English
It's how much usable power the engine puts out at the spinning shaft that turns the propeller — the work the engine is actually handing over, before the propeller turns it into pulling force.
Context Anchor
Seen in turboprop engine discussions, performance charts, and power settings where the pilot needs to know how much engine power is being delivered to the propeller.
Derivation
Horsepower' was coined by James Watt to compare steam engine output to the work of draft horses. 'Shaft' specifies where the power is measured — at the rotating output shaft itself — to distinguish it from power measured elsewhere, such as at the propeller (thrust horsepower) or inside the cylinders (indicated horsepower).
Why Pilots Care
It directly indicates how much power is available to drive the propeller, affecting takeoff distance, climb performance, and cruise speed.
Intuition Check
Do not read shaft horsepower as the total power created inside the engine. It means the usable turning power delivered at the shaft to drive the propeller.
Example Sentence 1
The PT6A-67 is rated at 1,200 shaft horsepower at takeoff.
Example Sentence 2
The pilot monitored shaft horsepower during the climb to stay within engine limits.