The free surface effects on the hydrodynamics of two-dimensional hydrofoils in tandem

•The lifting force of the upstream hydrofoil increases and that of downstream hydrofoil decreases when the longitudinal distance is limited in unbounded flow.•The lift augment mode is obtained when the downstream hydrofoil is placed below the zero crossings with an upwash wave flow.•The lift attenuation mode can be found when the downstream hydrofoil is placed below the zero crossings with a downwash wave flow.