There are a lot of numerical analysis for solving hydrodynamic responses of a floating body in the time domain. Most of them can give a theoretical solution in given irregular waves. It means, however, that the solution can be obtained only if the accurate irregular waves represented by the wave spectrum should be given. As we consider the actual operation, we know it is difficult to detect the accurate irregular waves instantaneously as needed accuracy in the most of the time domain analysis for feed backing the control force to the system.
This paper proposes a new method to predict the practical wave force from the displacement of waves at a floating body in time domain analysis almost instantaneously. The method, that can apply to predict forces in wave energy converter with linear electric generator, helps us to choose the control force for convert more electric power in irregular waves. We confirm the algorithm and examine its effectiveness.