The unsteady problems of ships in waves are analyzed by a low order panel method with Rankine source. Considering the basic flow as the uniform incoming flow(so called Kelvin flow) and also the double body flow. the solutions to satisfy the governing equation with the boundary conditions are obtained, and these two results are compared. The hydrodynamic coefficients for the modified Wigley hull and Series 60($C_B=0.7$) are computed and compared with the experimental data available and also other computational results published. It is shown that the computational results by the double body approximation agree well with the experimental results compared with those by the uniform Kelvin flow approximation.