A numerical study of a semi-circular twin-hull section under heave oscillation is presented. Two different time domain simulation methods were used: Firstly, a boundary element method based on potential theory which incorporates fully nonlinear free surface boundary conditions by using a mixed Eulerian Lagrangian scheme. Secondly, a finite volume method in combination with a volume of fluid scheme for capturing the free surface. In order to evaluate the effects of viscosity, simulations with the finite volume method were carried out using inviscid as well as viscous flow assumptions. Hydrodynamic mass and damping coefficients were derived from first order Fourier coefficients and validated against results from linear theory and experiments. A detailed comparison of nonlinear results from both simulation methods was carried out in vicinity of the piston-mode resonance frequency and a very good agreement was found. The phase angles corresponding to the Fourier coefficients varied strongly with frequency, which went along with a phase shift of the fluid motion in the gap. Influence of viscosity on the flow was found to be present but had relatively low impact on the hydrodynamic forces.