The effects of steady state conditions (e.g. blade pitch, rotor yaw angle, blockage, velocity shear profile) on turbine performance has received much attention over the past few years. However, tidal flows are highly dynamic and their effects on turbine performance and reliability unknown. Previous work has shown how waves and uniform unsteady inflows increase the loadings acting on a turbine, but further investigation is required over a wider range of conditions. This work used static grids to generate turbulence with different turbulence intensities and length scales. Two different turbines were operated in these turbulent flows and their performance monitored and compared. It was found that increasing turbulence intensity reduces power and thrust coefficients by up to 10%, and increases the fluctuations in thrust and torque. Increasing the turbulent length scale showed an increase in power and thrust coefficient, for each turbine, with a significant increase in their fluctuations. Overall, it has been found that turbulence has a significant effect on turbine performance and work is ongoing to consider a broader range of conditions with real site data.