Wave drift forces on a tanker in shallow water have been measured in regular and irregular waves. The results of measurements are compared with computations based on three-dimensional potential theory. Results of computations and measurements show the effect of the second order part of the wave potential associated with the 'set-down' phenomenon. In order to measure the low frequency part of the second order wave forces, a special system for restraining the model of the vessel was developed. This system incorporated active position control devices working partly on the basis of 'feedback' of the position error of the model and partly on the basis of the application of a 'wave-feed-forward' control signal generated from real-time measurements of the relative wave elevation around the model. The time records of the mean and low frequency forces on the vessel were analysed by bispectral analysis techniques to obtain frequency domain quadratic transfer functions of the second order forces. Attention was also paid to low frequency phenomena in the undisturbed irregular waves such as wave grouping and wave setdown. Measured data on both phenomena were compared with theoretical predictions based on the random wave model and on potential theory respectively.