A perturbation analysis of the nonlinear coupling between the pitch and roll modes is used to illustrate that an energy approach can be used to advantage in developing the nonlinear equations governing the motion of ships. It is shown that employing Taylor series expansions to determine the loads on the hull of a ship can lead to the physically unrealistic prediction of self-sustained oscillations, unless certain relationships among the nonlinear coefficients are satisfied. It is shown that the simplified equations of motion which result after imposing these relationships can be found directly from an energy formulation of the problem. The energy approach is used to develop the nonlinear equations governing the roll and pitch modes to third order and the equations governing motions having six degrees-of-freedom to second order.