A hydraulic shaft coupling apparatus transfers power from a rotating drive shaft to a driven load shaft, where the apparatus has a self-contained internal fluid flow path having a fluid control valve that controls the flow of hydraulic fluid through the flow path to allow fluid flow when there is a substantial difference between drive shaft and driven shaft rotational speeds, and to gradually restrict the fluid flow as the difference between drive shaft and driven shaft speeds decrease to the point where fluid flow through the fluid passage is substantially stopped by the control valve resulting in the one-to-one transfer of rotational power from the drive shaft to the driven shaft.