In the initial design of container ships or car-ferries which are strictly required to keep the time schedule, it is important to evaluate their sea margin accurately. Up to the present day, the value of 10-20% has been conventionally used for sea margin from experience without regard to ship's dimensions, cruising range, season etc. In this paper, the author tries to evaluate the power increase due to waves ,which seems to occupy a main part of the sea margin, and to investigate on approximate formula of Response Amplitude Operator(R.A.0.) and the quantity of thrust increase in irregular head waves. The formula of R.A.O.is derived from analyzing R.A.O.obtained from model tests in regular head waves, to make it possible to calculate mathematically the thrust increase. Values of the thrust increase calculated by this method are compared with values measured on board or obtained from model test, and it has been confirmed that this approximate method is useful to estimate thrust and power increase in the initial stage of design.
