High speed craft have longitudinal instabilities in waves and calm water, so, it is very important to know the dynamics and behavior of these vessels. In recent years, there has been more attention paid to the study of hull form optimization, movement improvement and instability control for planing marine vessels. The problem of increasing movement and acceleration of marine structures, especially in high speed craft, has a negative effect on the performance of the structure, its crew, passengers and equipment. So, estimation of vessel movement is an important part of its design. In this regard, wide studies have been carried out, experimentally and numerically, on the motion of these vessels in regular waters.But, in high speed craft, due to the complex and nonlinear behavior of the motion equation, the numerical solution of the equations of motion in the time domain is highly complex and time-consuming in calculating irregular waves. Therefore, researchers prefer to use experimental testing. Due to a limitation of equipment in marine laboratories, the use of irregular wave makers and expensive tests for real vessels in open seas, it is necessary to obtain an analytical solution for estimation of motion in irregular waves. The purpose of this study is to illustrate how we can achieve an analytical method, based on initial conditions, for planing vessels in irregular waters. In this study, an analytical model, with low computational cost, will be provided for the longitudinal movement of a catamaran. This model is based in consideration of analytical and numerical studies, and during this research, other published findings will beproduced. The proposed method uses the principle of a superposition of forces, which has no antonym with the nonlinear motion of high speed craft. In this method, the hydrodynamic coefficients must be constant, relative to movement frequency, so, numerical evaluation of the hydrodynamic coefficients has been discussed in every motion frequency. Various experiments have shown the hydrodynamic coefficient tendency to be constant at high collision frequency. By examining the various hydrodynamic coefficients in collision frequencies, an increase in the frequency of collisions, as well as fixed coefficients, were observed. The method used in this research has acceptable answers for high wave lengths, with respect to vessel length.