A short review is presented of available instrumentation systems designed to assist the operation of ships in heavy weather by warning against dangerous wave loads. Some systems also give guidance to the master by predicting the outcome of evasive actions, and the bases on which such predictions are made, such as visual observations of the wave system, are questioned. A method is presented in which the motions of the ship are used to determine the sea state in the form of an "equivalent" wave spectrum. Two investigations of the possibility of improving the guidance capability of warning instruments are described, in which the predictions are based on the equivalent wave spectrum. For this purpose, recorded full-scale data from a container ship and a tanker have been analysed and the two methods, spectrum analysis and a statistical method, are described. Using the equivalent spectrum, predictions of the effect of a change of course and estimates of one response from another have been made and compared to measured values. The results of these comparisons, which are presented graphically and in the form of correlations between measured and predicted values, are discussed with respect to error sources and factors which limit the method's applicability. The accuracy in predicting one response from another was found to be higher the closer the correlation between the responses, and correct estimations of the relative heading and the angular energy distribution of the wave system were found to be of importance. Theoretical calculations of ship responses to irregular waves have been made by linear superposition of transfer functions and wave spectra and a new way of extrapolating the transfer functions is described.

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