So far, added resistance in irregular waves is generally evaluated as a mean value in short term by means of the calculation in frequency-domain. The power spectrum of waves and the frequency response of added resistance in regular waves are used in the calculation, and the added resistance in regular waves is assumed to be proportional to square of wave height based on the 2nd-order wave theory. However, it has been found from tank tests that the added resistance in regular waves is not always proportional to the square of wave height. The major reason is considered that the influence of the hull form above the waterline on the added resistance in waves varies depending on a wave height. Therefore, it is not clear if the 2nd-order theory gives a good estimation for the slowly-varying added resistance in irregular waves.

In this paper, higher order components are considered to estimate the slowly-varying added resistance in irregular waves with the wave height effect. As a result, the slowly-varying added resistance in waves with the consideration of wave height effect is mathematically expressed as the 4th-order component. The estimated results are compared with tank tests, and it has been concluded that the 4th-order component greatly contributed to improvement of the accuracy.