Despite a large amount of work so far, it is said that the prediction accuracy in the added resistance is not enough particularly in a range of short waves. For engineering purposes, application of the enhanced unified theory (EUT) seems promising for evaluating the wave-amplitude function which is the most important term in Maruo's formula for the added resistance. To confirm applicability of the EUT, measurements of the wave-induced ship motions and added resistance are carried out using a modified Wigley model at several Froude numbers, and obtained results are compared with computed ones. Discrepancy in the added resistance is observed at short wavelengths when the forward speed is present, and the amount of this discrepancy tends to increase and then become constant as the forward speed increases. This discrepancy may be attributed to hydrodynamic nonlinear effects in the wave diffraction at the bow, which may be intensified in the presence of forward speed. A practical factor for correcting this discrepancy, which is to be applied only to the component due to diffraction of incident wave, is proposed in a form of mathematical function in the Froude number and the ratio of wavelength to ship length.
When a ship navigates in waves, the ship's forward speed decreases compared to that in calm sea because the resistance increases in waves. This increase in resistance is called the added resistance, which is caused due mainly to unsteady wave making, specifically the wave radiation by ship oscillations and the diffraction of incident wave on the ship hull. The added resistance caused by unsteady wave-making phenomenon can be exactly estimated by Maruo's formula (1960) which is based on the principle of momentum and energy conservation. The wave-amplitude function (which is referred to as Kochin function) included in this formula influences greatly the prediction accuracy of the added resistance.
