Model tests are carried out with two side by side rectangular barge models rigidly linked to the carriage. Free surface elevations are measured at 9 locations along the gap. The experiments are run for two gap widths and two bilge shapes (square and rounded). RAOs of the free surface elevations derived from tests in irregular waves are compared with numerical RAOs from a linear potential flow software. In the sharp bilge case the calculated values overpredict the measurements. Numerical calculations are then performed with the gap covered by a row of massless plates, each plate being assigned a quadratic damping force with respect to its vertical velocity. With a proper choice of the drag coefficient, good agreement is obtained with the experimental RAOs.