This paper discusses the harbour calmness index and factors concerned with the calculation of the harbour calmness index. Motions of model ships were measured simultaneously during the wave diffraction test. The results of the model tests indicate the inadequacy of calculating the harbour calmness index based solely on the wave height in front of a berth when ship motions are influenced by long period waves. A calculation method for the wharf operation effeciency based on the revised definition of the harbour calmness index is proposed.
　The relation between ship motions and factors affecting these motions, such as the load-deflection characteristics of the mooring systems, the irregularity of waves and the fluctuation of wind is also examined. Numerical simulation of ship motions was carried out in order to achieve an incease of the wharf operation efficiency and it was found that htis might be obtained from an improvement of mooring systems.