Understanding and investigation of a high-speed craft dynamics, their longitudinal dynamic instabilities in calm water and behavior in waves are of a great importance. Determination of motion equation coefficients will help to analyze the dynamics of these kinds of vessels and the factors affecting their dynamic stabilities. Therefore, it could be useful in controlling the vessel instabilities. The main purpose of this research is to determine the coefficients of longitudinal motions of a planing catamaran using computational fluid dynamics (CFD) and evaluating the interceptor effect on hydrodynamic coefficients of that, which is widely utilized in controlling the motions of a high-speed craft. The forced pure heave and pitch motions are simulated in a numerical domain with RANS and continuity governing equations to determine the hydrodynamic coefficients. Determination of these coefficients using experimental approaches is relatively expensive, and requires precise laboratory equipment; therefore, utilizing numerical methods and develop a virtual laboratory seem highly efficient. In the beginning, the hydrodynamic coefficients of a high-speed craft obtained using CFD simulation and then the numerical results were verified by Troesch (J Ship Res 36(4):317–331, 1992) experimental tests. At the following, after determination of hydrodynamic coefficients of a planing catamaran with and without Interceptor, the interceptor effects on its hydrodynamic coefficients will be evaluated. The results indicate that most of coefficients are frequency independent especially at high frequencies. Also, the interceptor effect on hydrodynamic coefficients in different frequencies is negligible except for a few cases. © 2014, The Brazilian Society of Mechanical Sciences and Engineering.