In this paper, the authors proposed a ship weather-routing algorithm based on the composite influence of dynamic forces, i.e. wind, wave and current forces, for determining the optimized transoceanic voyages. Our developed routing algorithm, three-dimensional modified isochrones (3DMI) method, utilizes the recursive forward technique and floating grid system for both the east- and west-bound ship routes in the North Pacific Ocean. In order to achieve the goals of minimized fuel-consumption or the maximized-safety routes for the transoceanic voyages, two sailing methods are applied as the prerequisite routes in the earth coordinate systems. The illustrative analysis of ship routes has been presented and discussed based on the realistic constraints, such as the presence of land boundaries, non-navigable sea, external forces, parametric roll responses as well as ship speed loss. As a result, the proposed calculation is verified to be effective for the optimized sailings by adjusting the weighting parameters in the objective functions.