The nonlinearity on the ship motion problem can be separated to two main characteristics. The first one is freesurface nonlinearity, and the other is body nonlinearity. The analytic or numerical analysis of the free-surface nonlinearity is not an easy task. On the other hand, the body nonlinearity, which is mostly due to the body geometry, is relatively easier to consider in numerical analysis than the free-surface nonlinearity. Fortunately, when the floating body is slender like ship, the disturbance due to body motion may not be very significant. When this is the case, more effort can be made to consider the effect of body nonlinearity. Many recent researches on nonlinear seakeeping analysis are based on such hypothesis.