For a given specific energy in hydraulic machines, there is a peak efficiency at a certain flow rate, known as the Best Efficiency Point (BEP). Beyond this point, efficiency decreases in an almost quadratic manner as flow rate deviates. This study aims to establish a connection between this phenomenon and the flow stability properties at two different operating points. The base flow vortex considered here is based on the experimental data by [1, 2]. Based on experimental measurement at a location of turbine outlet (upstream of draft tube), Susan-Resiga et al. [1] proposed a mathematical fitting of the turbine vortex into one rigid body motion and two batchelor vortices with two different core radii Ub = [0,Vθ,Vz]: