  Interfacial internal wave excitation in the wake of towed ships is studied
experimentally in a quasi-two layer fluid. At a critical `resonant' towing
velocity, whose value depends on the structure of the vertical density profile,
the amplitude of the internal wave train following the ship reaches a maximum,
in unison with the development of a drag force acting on the vessel, known in
the maritime literature as `dead water'. The amplitudes and wavelengths of the
emerging internal waves are evaluated for various ship speeds, ship lengths and
stratification profiles. The results are compared to linear two- and
three-layer theories of freely propagating waves and lee waves. We find that
despite the fact that the observed internal waves can have considerable
amplitudes, linear theories can still provide a surprisingly adequate
description of subcritical-to-supercritical transition and the associated
amplification of internal waves.
