Abstract: A marine vehicle, which may comprise a sidewall gas-cushion vehicle, is provided with a pair of steering rudders, a sensor for detecting incipient undesirable motions of the vehicle, and an electrical, mechanical, and/or hydraulic system, or a hybrid thereof, for operating the rudders so as to apply restoring forces to the vehicle. The steering rudders are mounted beneath the vehicle and on opposite sides of the longitudinal axis thereof, being disposed in planes which are inclined to the vertical and which intersect above the rudders. The sensor for detecting incipient undesirable motions of the vehicle may comprise an angular rate and/or an accelerometer. The signal output of the sensor is applied to a band pass filter of pre-determined transfer function and the filtered sensing signal is applied to a summing point, to which is also applied a steering command signal.