Conventional computer programs developed and used by practicing engineers can be considered to contain expert knowledge and design experience. If these conventional programs are converted into expert systems, the difficult and time consuming process of knowledge acquisition can be simplified. Also the constructed knowledge-base can have higher confidence level than that constructed by the usual knowledge acquisition method of interviews. An existing computer program which is being used by ship structural designers has been reformulated as a design expert system by applying an expert system development shell-Nexpert. Utilizing the callable interface provided by the development shell, external design tools have also been integrated. The interfaced external functions are a graphical user interface (GUI) for the design process control, and graphics functions for the visualization of design results. It is observed that the developed system for design support is useful in two aspects. The trace-back function shows what portion of design rules are applied in arriving at certain design decisions. Also the knowledge-base can be conveniently updated as design rules of the classification societies are updated.