Winch Calculations Winch horsepower can be approximated by the following formula: HP =     (L) (S)     (E) (33,000) Where: HP is the horsepower L is the load (pounds) S is the cable speed (feet/minute) E is the overall drive efficiency As cable builds up on a drum the radius changes thereby altering the speed and horse power for a constant load and RPM drive system. Therefore, the conditions of which load and speed are specified must be defined. It is common practice to specify conditions at mid drum unless other consideratons dominate in which case they should be used as design criteria. Drive efficiencies vary greatly on the type of drive, as a guide: Electric Drive worm gears TYP 60 - 75% Electric Drive Bevel Box TYP 85 - 95% Electro Hydraulic TYP 60 - 70% Diesel Hydraulic TYP 60 - 70% Exact figures depend on individual designs but duty factor and heat dissipation have a large impact on winch capability. The following formula gives a close approximation of the length of wire rope that can be spooled onto a drum: L =  w x h    4 Dd ² Where: L = the length of line which will fit drum (feet) h = height of wire rope on drum when full (in) d = diameter of drum (in) w = width of the drum (in) D = diameter of wire rope (in)