Thrust/Horsepower
I've found some info in my P&W Aeronautical Vestpocket Handbook.
For turbojet or turbofans, Thrust Horsepower = Pounds Thrust x MPH / 375
For piston or turboprop, Pounds Thrust = 300 x Shaft Horsepower / MPH (assuming 80% prop efficiency). For static conditions the equation doesn't work, so rule of thumb is 2.5 pounds per SHP.
For turboprops I've heard the term ESHP (equivalent shaft horsepower) used. I understand that there's energy in the exhaust after the turbine, which provides thrust. Adding this to the SHP gives a better indication of the engines output. On some engines this thrust must be significant. I spend a bit of time in Dash 8's (as SLF) and am always impressed by the exhaust size on the PW 123's.
Spec