Reproducibility evaluation of detailed directional spectrum based on mean spreading angle for ship performance estimation in actual seas

Ship response in short-crested irregular waves is calculated as the product of the frequency response function of ship response in regular waves and the directional spectrum of waves. However, the commonly used IACS and JONSWAP spectra may not accurately reproduce detailed spectra by wave hindcasting data and wave radar measurements, leading to over- or underestimation of ship responses. This study focuses on the reproducibility of a detailed directional spectrum by standard directional spectrum based on physical considerations concerning the mean spreading angle. This approach facilitates discussion on the limitations of reproducing a detailed directional spectrum with a standard directional spectrum. Furthermore, a procedure is proposed for the use of a standard directional spectrum based on the mean spreading angle as an indicator. Analysis using 6 example ships demonstrates that employing the procedure enables the extraction of highly reproducible and valuable data for short-term prediction of the added resistance in short-crested irregular waves.