Several constrained problems with a number of design factors are to be solved in the design stage of a ship equipment. Generally, the design factors connected with a ship equipment have a tendency of fuzziness and randomness. In spite of this, the actual design is usually based upon the typical values of these factors. A more practical choice for analyzing these indeterminable problems is to use the theory of fuzzy sets and extended operations. This paper presents an analytical method which is covenient to deal with these indeterminable problems by a combinations of many small scatters in the design factors, using the Cartesian products of fuzzy sets. As an example, the proposed method is applied to the system that collects oil and water through the bottom structures in cargo oil tanks or water ballast tanks, to calculate the pumping hours and the quantity of residual fluids. Further, the drain collecting system is evaluated from the viewpoint of determining an optimal location for the bellmouth and a pattern of the drain course, using the fuzzy multi-criteria analysis.