This work studies the effects of inertia (added mass) on horizontal axis tidal-stream turbines in unsteady flow. Experiments have been conducted using both an instrumented model rotor and a porous disc as an analogue subjected to unsteady planar (axial) motion in a steady flow water flume. Measurements are presented of the instantaneous axial thrust of the rotor operating at constant torque. Drag and inertia coefficients are calculated for the rotor over a range amplitude parameters (Keulegan-Carpenter number) and rotor operating points (tip speed ratio). These results are compared to Morison’s equation for axial force on a body in unsteady flow and also to model predicting dynamic inflow effects. The results provide an insight into the axial added mass and wake effects and they will help our conceptual understanding of these phenomena which overlap, for a rotor operating in unsteady flow.