Underway replenishment is an important naval topic which means a lot for the deep ocean stratergy of every country. From a hydrodynamics point of view it brings a lot of difference to the conventional ship seakeeping and manuvering problem. The interaction between ships is contributing a lot of new elements to the already complex topic. But this means challenge and opportunity. If the hydrodynamics of two ships can be predicted sucessfully, the manuvering can become more under controlled, thus better simulator and controlor will be assistted.