An online procedure for estimating directional wave spectra for marine vessels is proposed. The procedure is developed as a subsystem of onboard marine guidance equipment. To simplify the system, the hull of a ship is regarded as a wave sensor and time history data of ship motions is analyzed to estimate directional wave spectra. Using the estimated spectra, ship response to all possible course and speed changes can be simulated based on short-term prediction techniques from the field of seakeeping research. Information about ship response can then be predicted before a decision to change course or speed is made.
With the obligation to equip voyage data recorders (VDR), there is increasing pressure from mariners for this data to be used for safe navigation. In addition, with the development of on-board guidance systems, statistical analysis of this data and prediction of ship response are also becoming important. Many on-board monitoring systems have been developed in the recent past. Most of these systems give shortterm predictions about ship response based on knowledge from seakeeping research under the assumption that the seaway can be modeled by stationary stochastic processes. Iseki and Terada (2002) also developed a guidance system for ship operators, with the system based on three calculating procedures, cross spectrum analysis of measured time history data of ship motions, Bayesian estimation of directional wave spectra using cross spectra of ship motions, and prediction of future ship response using estimated directional wave spectrum. Thus, mariners can obtain future information about ship response before deciding to change course or speed. In these procedures, the assumption of stationary stochastic processes is applied to the seaway, but not to ship response because ship response also depends on ship maneuvers. Ship response is strongly affected by changes in the encounter angle and frequency of waves.