Since 1986, MARIN has used in-house software SUBSIM for predicting the manoeuvring behaviour of submarines. Using the hull geometry as input, this time domain simulation applies slender body theory in all six degrees of freedom. In addition, the program contains nonlinear models for hydrostatics, cross-flow drag, fins, sail, masts and propellers. Each lifting surface generates its own trailing vortex, causing interaction effects. Given the modest computational complexity, SUBSIM is highly suitable for real-time interactive simulation, as well as the early design stage.