A comprehensive evaluation of free-running Computational Fluid Dynamics (CFD) and System-Based (SB) approaches is presented for simulating transient maneuvers of the Office of Naval Research Tumblehome (ONRT) surface combatant, with a current focus on operations in calm water. The transient maneuvers considered include crashback, crash-ahead, turning from rest [Acceleration Turning (AT)], turning at rest, and full in-behind four-quadrant propulsion operation. Simulations are conducted across Harbor Maneuvering Speed (HMS), Cruising Speed (CS), and the Maximum Design Speed (MDS) conditions using two CFD solvers (CFDShip-Iowa V4.5 and ReFRESCO), and one SB method (aNySim-XMF). The research is performed in support of NATO AVT-399, which aims to assess the predictive capabilities of current tools relative to the STANAG maneuvering performance criteria. According to the CFD and SB predictions for cruising speed, the ONRT will satisfy STANAG criteria regarding stopping and turning from rest abilities. Notable differences between CFD and SB predictions arise in turning at rest scenarios due to complex propeller inflow effects and the omission of propeller side-force modeling in the SB approach. AT simulations exhibit significant differences in maneuvering metrics such as Time to reach 90° (T_90^*) and Advance (AD) compared to previous CFD and experimental results for conventional turning circle maneuvers, attributable to reduced initial forward speed, whereas steady-state conditions show similarities. Yaw rate predictions during turning at rest highlight discrepancies between CFD and SB, emphasizing the need for enhanced modeling and experimental validation. Ongoing and future efforts include the development of scale-effect correlation curves, additional crashback and transient Propeller Open-Water (POW), and targeted experiments in the Iowa Institute of Hydraulic Research (IIHR) wave basin. These efforts aim to improve the understanding of unsteady 3D vortex dynamics, hull-propeller-rudder interactions, and the role of propeller-induced forces in maneuvering, thereby enhancing the predictive fidelity of both CFD and SB approaches in support of mission-specific naval performance requirements.