There are different methods to reduce drag in high speed hulls. One of these methods is a change in the shape of the body by adding longitudinal side tunnels. In this paper it has been attempted to determine the influence of the tunnel height on hydrodynamic characteristics of the hull to achieving an optimum shape for the tunnel. To achieve this purpose, numerical simulation of the problem has been done using finite volume method considering moving mesh. For turbulence modeling, k-ε model and to simulate free surface, the Volume of Fluid (VOF) two phase model has been employed. The results show that creating a tunnel in the base mono-hull would lead to reduction of the total drag at high speed as well as decreasing the vessel draft of the hull over the whole range. Furthermore, in the Volumetric Froude number around 4, a reduction in the height of the tunnel could decrease the hull drag. In higher Volumetric Froude numbers, reduction of the tunnel height, to some extent, increases the drag with a slight slope.