This paper reports on a study into the motions of two vessels when travelling side by side into head seas, a common operating condition for naval vessels when undertaking replenishment at sea activities. A series of model tests was conducted and the influences of various parameters, including transverse and longitudinal separation, on the ships' motions were determined. The data obtained from the experimental study has been used to validate a theoretical ship motion prediction method, FD-Waveload, which is based on a 3-D zero-speed Green function with a forward speed correction in the frequency domain. The results were also used to estimate the expected extreme roll angle of the receiving vessel, and the relative motion between the vessels, during replenishment at sea operations in a typical irregular seaway. An increase in transverse and longitudinal separation between the vessels was found to have a significant influence on the roll motion of the smaller vessel; though only a small reduction in relative motion was found to result from an increase in transverse separation or reduction in longitudinal separation.
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