Investigation on the Shape Effect of Directional Spectrum on the Estimation of Added Resistance in Short Crested Irregular Waves

Abstract The waves that ships encounter in actual seas are short crested irregular waves composed of various frequencies and directions, which are characterized by directional spectrum. While the directional spectrum is defined as two-dimensional tabular data on frequency and direction, mathematical models are often used due to their convenience and simplicity for estimating ship motion and added resistance in actual seas or calculating the strength of ships or marine structures. Although the mathematical models are convenient, there is a risk that the accuracy for estimating the added resistance with the mathematical models will deteriorate if the shape of the mathematical model differs significantly from that of the actual directional spectrum. To ensure the accuracy above, it is necessary to investigate the range of applications of the mathematical models associated with the shape of the directional spectrum. This study investigates the shape effect of the directional spectrum on the estimation of added resistance in short crested irregular waves. Firstly, to consider the shape parameters of the directional spectrum, the author adopted the directional spectrum expressed by the Ochi-Hubble type frequency spectrum and the Mitsuyasu type spreading function and applied to the hindcast directional spectrum on the North Pacific Great Circle Route to investigate the distribution range of the shape parameters. Secondly, the author performed parametric calculations of the added resistance in short crested irregular waves over the distribution range of the shape parameter to investigate the sensitivity of the shape parameter to the added resistance. The author also calculated the added resistance using the IACS mathematical models and investigated the range in which the IACS mathematical model can be used. Finally, the author investigated the correlation between the calculated results of the added resistance derived from the actual directional spectrum and that from the directional model expressed by the mathematical models.