Designing high-speed boat rudders and appendages is most often based on empirical relationships of appendage and underbody lateral projected area. The empirical approach does not reflect objective evaluation of desired characteristics nor performance in a seaway. This effort was initiated to determine if higher fidelity simulations will provide insights into rudder and appendage sizing in calm water maneuvers and eventually in a seaway. Six Degree-of-Freedom (6-DOF) free-running simulations of high-speed boats conducting standard ship maneuvers are performed using a RANS-based computational fluid dynamics (CFD) tool as described in Wang, Snodgrass, and Ding (2019). The explicit Volume of Fluid (VOF) method is employed to accurately capture the complex free surface dynamics, ensuring precise representation of the interaction between the hull and the water. To accurately model the dynamic propulsive forces during the maneuvers, actual propeller geometries are incorporated into the simulations. The simulation results are validated against experimental data for the USCG 47ft Motor Lifeboat (47 MLB). Multiple maneuvering sea trials were conducted for the 47 MLB under various conditions, including different speeds and appendage configurations. Full scale data used in the included standard tactical diameter and zig-zag maneuvers. By comparing the simulated and experimental results, the accuracy and reliability of the CFD tool are assessed. Key performance metrics, such as resistance, heave/pitch, and maneuvering characteristics, like minimum turning radius, are compared to validate the simulation's predictive capabilities. Furthermore, an improved propeller force model is proposed to enhance the ability to predict propulsive forces during boat maneuvering, especially in dynamic conditions where propeller unsteady flow effects can significantly influence performance. It is anticipated that this force model could be applied during new vessel design when the details of the propeller three-dimensional geometry are not yet available, or it is not computationally feasible to analyse the actual propeller geometry. The comparisons of simulations and measured data provides valuable insights into rudder and appendage design.