The formula is
Power(hp)=torque(ft.lbs)*rotational speed(rpm)/5252
The only thing I can add is that it's the
prop shaft rpm that's of concern here; thus one needs to know the gearbox ratio to find prop rpm. My recollection is that it's about 0.4 gearing (i.e. rpm is about 1200).
(Note for the skeptic: crankshaft torque is reacted within the gearbox; prop torque is reacted in the engine mounts)
