Abstract:  Wave resonance in the narrow gap between two side-by-side non-identical ships is investigated by employing a two-dimensional numerical wave flume based on OpenFOAM package. The focus of this study is to examine the influence of free surface nonlinearity and fluid viscosity on the hydrodynamic behavior of wave response at around resonant conditions. Numerical simulations show that the unrealistic resonant amplitudes in the narrow gap by linear potential model are not only from the energy dissipation due to vortical motion, but also from the nonlinearity due to free surface. With the increase of incident wave steepness, more relative energy is reflected, leading to the decrease of relative resonant amplitude and relative energy dissipation in the narrow gap at resonant frequency. When the incident wave frequency is outside a little band to resonant frequency, relative energy dissipation becomes the dominant factor for the decrease of relative wave amplitude in the narrow gap with the increase of incident wave amplitudes. In a word, both the free surface nonlinearity and fluid viscosity play the important, but different, roles on wave resonances in the narrow gap.