The present study aims at the quantification of the effect of uncertainty of the weather conditions on the fuel consumption of a vessel. More specifically, the uncertainty of the seaway conditions, namely spectral wave height, period and heading, on ship's added resistance will be introduced by an innovative probabilistic approach known from reliability theory. The ultimate goal of this study is to provide a rational methodology that will allow the estimation the fuel consumption of a vessel on a certain route, while considering the effect of weather conditions and their related uncertainties.