This chapter describes the traditional method for seakeeping analyses. 70 years after the pioneering work of Ursell to determine added mass and damping of ship sections oscillating at the water surface, the book describes a highly accurate, mathematically simple and computationally fast method for this. For the sake of brevity, the step from ship sections in forced motion to free motions of a total ship with speed ahead is made by heuristic considerations. A special feature of this chapter is the speed-dependent hydrodynamic interaction between hulls, which is important in catamarans and trimarans.