An existing computer program of rule scantling, which is being used by ship structural designers, has been reformulated as a design expert system by utilizing an expert system development shell - Nexpert. It is observed that the system for design support is useful in three aspects. The trace-back of rule firing shows what portion of the 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 revised. Finally, the GUI part allows visualization of fired rules, object hierarchies, and design results. This user interface enhances user understanding of the process usually hidden behind the computer screen.