Objectives  This study seeks to correct the distortion of results caused by the inviscid-flow assumption when potential flow theory is used to calculate a two-ship floating system with a small gap, and analyze the motion response characteristics of the gap water.

  Methods  A CFD numerical model of a ship-to-ship transfer system is established and the water response characteristics and mechanism in the gap are analyzed. The CFD calculation results are then compared with those of potential flow theory in order to obtain the accurate damping coefficient of the two-ship gap and correct the results of potential flow theory.

  Results  The results show that the trend of wave elevation is different when waves of different frequencies pass through the gap. For low frequency waves, the incident wave cannot pass through the gap between the two ships, so the gap wave elevation is smaller than the wave amplitude of the incident wave. For high frequency waves, the incident wave can pass through the gap, so the gap wave elevation is greater than the wave amplitude of the incident wave. There is a high-speed area in the gap between the two ships which decreases the hydrodynamic pressure on the inner sides of the two vessels, resulting in greater suction which may adversely affect transfer operations.

  Conclusions  The correction method proposed herein can obtain the damping coefficient of gap water more accurately, providing guidance for the high-precision and rapid hydrodynamic and motion calculation of ship-to-ship transfer systems.