In the present paper, wind tunnel experiments with horizontal axis propellerhydrokineticturbine models are carried out. The turbine performance is assessed for different diffuser configurations and operational ranges. The present experiments evaluate the use of two different geometries of rear diffusers to enhance the power output of the free flow machine. The present paper reports an increase of the power coefficient of about 48%–79%. A discussion about the use of the diffuser technology is pointed out in a framework of a concept of the minimization of the volume of material contained in the turbine parts. It represents an important result that has to be taken into account in the design of hydrokinetic turbines.