- Fully nonlinear wave interactions with a three-dimensional body are studied using two independent (potential and viscous) Numerical Wave Tanks (NWT). The potential NWT used an indirect Desingularized Boundary Integral Equation Method (DBIEM) and a Mixed Eulerian-Lagrangian (MEL) time marching scheme. The Laplace equation is solved at each time step and the fully nonlinear free surface boundary conditions are integrated with time. A regridding algorithm is devised to eliminate the possible saw-tooth instabilities. The incident waves are generated by a piston-type wavemaker. The outgoing waves are dissipated inside a damping zone by using spatially varying artificial damping on the free surface. The viscous NWT solves a Navier-Stokes (NS) equation by using a finite-difference scheme and a modified marker-and cell (MAC) method in the frame of rectangular-coordinate system. The fully-nonlinear kinematic free-surface condition is satisfied by the density-function technique developed for two fluid layers. Computations are performed for the nonlinear diffractions of steep monochromatic waves by a truncated vertical cylinder. The NWT simulations compared favorably with available experimental results.