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The nonlinear hydrodynamics of a three dimensional body beneath the free surface are solved in the time domain by a semi Lagrangian method. The boundary value problem is solved by using the boundary integral method. The geometries of the body and the free surface are represented by the curved panels. The surfaces are discretized into the small surface elements using a bi-cubic B-spline algorithm. The boundary values of phi and partial derivative of phi with respect to partial derivative of n are assumed to be bilinear on the subdivided surface. The singular part proportional to 1/R is subtracted off and is integrated analytically in the calculation of the induced potential by singularities. The far field flow away from the body is represented by a dipole at the origin of the coordinate system. The Runge-Kutta 4th order algorithm is employed in the time stepping scheme. The three dimensional form of the integral equation and the boundary conditions for the time derivative of the potential is derived. By using these formulas, the free surface shape and the forces acting on a body oscillating sinusoidally with large amplitude are calculated and compared with published results. Nonlinear effects on a body near the free surface are investigated.