- A fully nonlinear time domain 2D Numerical Wave Tank (NWT) is developed base on the potential theory, MEL/material-node time marching approach, and boundary element method (BEM). A robust and stable 4 th -order Runge-Kutta full-updated time integration scheme is used with regriding (every time step) and smoothing (every 5 steps) process. In front of the wave maker, only the difference between incident and reflected waves is damped out so that the artificial damping may not influence the incident wave field, the performance of which is shown to be satisfactory. The acceleration-potential formulation and mode-decomposition method are used for calculating the time derivative of velocity potential. The frozen-coefficient method is found to be unstable for freely floating body simulation when the body motions are large. The fully nonlinear results are shown to approach the linear case as incident wave heights decrease. It is seen that the nonlinear body force can be significantly different from the linear one when body motions are large. Copyright 2004 by The International Society of Offshore and Polar Engineers.