POSEIDON automatically determines the scantlings of all structural components based on rule requirements for vessel parameters; class notation, global bending, cargo loads and external sea pressure, Fig. 2. Transverse and longitudinal member scantlings are determined interactively with static and dynamic loads, structural geometry and allowable stress, fatigue and buckling strength factors automatically taken into account. The designer can easily alter the structural arrangement and scantlings to optimize the weight and the strength. The results of scantling calculations are logically displayed to ensure that the designer has an on-going overview of the design process. Member scantlings are also given in color codes; so the designer can easily determine which members require alteration. For example, red color in Fig. 2 means that scantling of the longitudinal stiffness has to be increased. The calculation method for individual members can be reviewed to permit alteration of key scantling parameters. Last but not least, corrosion margins are explicitly stated for all structural members.