The development of an accurate digital performance twin of a tug requires a complete understanding of its propulsive capacity and hull-thruster interactions. In this study, the propulsion characteristics of an Azimuth Stern Drive (ASD) tug is investigated using model-scale Reynolds-averaged Navier-Stokes (RANS) simulations. The propulsion plant consists of two counter-rotating thruster units, with each having a Ka4-70 series propeller and 19A duct profile. Comparisons in propulsive performances using the steady-state moving reference frame (MRF) approach and the transient rigid body motion (RBM) models are shown, and validated against data from openwater experiments. The MRF method gives sufficiently accurate predictions of thrust and torque in forward flow and moderate angles-of-attack, while the RBM method performs better at larger inflow angles. The effects of thruster-hull and thruster-thruster interactions on wake characteristics and propulsion performance are also investigated over a range of advance and inflow/azimuth angles. Convergence and mesh independence studies are conducted to determine the optimal spatial and temporal simulation parameters. Results from this study identify flow regimes where hull and thruster interactions are significant.
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