This paper proposes a methodology to overcome failures on Mamdani rule based fuzzy inferences in navigation. The fuzzy inference failures are observed in three situations: Intersected contradictory decision boundaries, improper transition region between the inference boundaries of non-intersected contradictory decisions and contradictory decision accumulation. In order to solve the problems, it is proposed to insert a smooth transition region between intersected contradictory decision inference boundaries, to determine the size limitations between non-intersected contradictory decisions inference regions, and finally to integrate a “multi-level” process. The proposed solutions are applied to a decision making process of ocean navigation and successful simulation results are shown in this study.