Making a prior evaluation of a power increase and a speed loss of ships in actual seas is necessary to estimate an added resistance in regular waves. With its result and a wave spectrum, a mean added resistance in irregular waves can be estimated. When this method is used in actual seas where wind waves and swells coexist, a way to evaluate the wave spectrum must be studied more. Such the wave field is usually expressed by overlapping two wave systems having different generation process and mutual independent property.

Usually, a wind wave is expressed by Pierson-Moskowitz spectrum and swell by JONSWAP spectrum. A composite wave spectrum of wave field is generally expressed by simply adding two spectra mentioned above. But, such handling on the composite wave field is insufficient. Therefore, examining probabilistic characteristics of the wave field is required.

Based on the above-mentioned discussion, authors show the theoretical method which evaluates short-term responses of ships in the composite wave field by using probability. As examples, prediction results of statistical values of probabilistic distributions of a mean added resistance are shown. Furthermore, this idea is applicable to the maximum value's prediction as well as an expected value's prediction. Compared with a conventional method, these results indicate a significant difference to us.