A theoretical prediction method for calculating the resistance increase of a ship in oblique regular waves is investigated in this paper. Two kinds of approximate formula on second-order steady-state forces are derived by using the momentum principle, and two kinds of approximate expressions on resistance increase are obtained from these approximate formula. Furthermore, numerical calculations on resistance increase are carried out using a container ship (SL175) and a bulk carrier. The results of the calculations are compared with the results of model experiments which were done at the Tsu Ship Model Basin. It is confirmed from the comparison that the resistance increase in a regular following or quarter wave can be predicted accurately enough for practical use. This is accomplished by the first approximate expression. Also, the resistance increase in a regular head or bow wave can be predicted accurately enough for practical use by the second approximate expression.
