The project of a modern underwater weapon system can not be separated from the use of FEM simulations. The costs associated with the development, combined with the high performance and short lead time, require the execution of design activities to minimize design change iterations and reducing the risks associated with the test, often destructive. The use of FEM simulations can also help the designer to understand some physical phenomena difficult to observe during test in the operative environment. In this paper an overview of the uses of FEM simulations for the development of WASS products will be shown.