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Abstract This study investigates dynamic instabilities during turning maneuvers of a high-speed planing hull using computational simulations, including turning circles, quick turn, and avoidance line tests. A Generic Prismatic Planing Hull (GPPH) model is used for the simulations. Relevant performance criteria and dynamic instabilities reported in the literature are summarized and applied to assess the turning maneuvers of the GPPH. Turning circles are analyzed based on circular motion equations and force and moment balances. Instabilities are identified through animation analysis and time histories of motion and acceleration quantities. The maneuvering boat speed is determined in the quick-turn tests, while the performance in the avoidance line test is evaluated under various settings. INTRODUCTION The analysis of maneuvering is essential for documenting vessel performance as mandated by the International Maritime Organization (IMO). While maneuverability standards for displacement ships are well-defined, such standards are not yet established for high-speed planing hulls (Bowles 2012). Specifically, high-speed small craft are prone to dynamic instabilities, which are poorly understood compared to those for displacement hulls (Blount and Codega 1992). Such dynamic instabilities, including spin-out phenomena, often occur during the turning maneuvers of the craft (Katayama and Ohashi 2014; Morabito 2016; Park et al. 2024). Although not required, many researchers use the IMO displacement craft standards for assessing planing craft maneuverability, especially since Annex 4 of the IMO High-Speed Craft Code provides limited guidance on this topic. The American Boat &Yacht Council (ABYC) H-26 standard ("Powering of Boats"), which includes quick-turn and avoidance line performance tests, is also often used (Morabito 2016). It is more challenging to study the maneuvering characteristics of high-speed small craft compared to displacement hulls as the dynamic effects are dominant, and forces and moments are strongly coupled with 6DoF motions. Most maneuvering studies for high-speed planing hulls are focused on experimental tests and theoretical models, restricted to calm water, and struggle to include the effects of coupled horizontal and vertical plane motions (Yasukawa et al. 2016; Tavakoli and Dashtimanesh 2019; Hochbaum et al. 2022). Recently, computational fluid dynamics (CFD) simulations of a high-speed small craft are performed by Park et al. (2024) including many captive, free running, and maneuvering conditions. It is the first known CFD study for the free running of high-speed small craft with 6DoF motions and thus it is exploratory in nature. The simulation results of the steady turning circles are further investigated by Wang et al. (2025) using the circular motion equations and applying force and moment balances with multiple coordinate systems. The horizontal ship-fixed and cylindrical coordinates are used to derive the circular motion equations. The differences in heel angle signs for the displacement and planing hulls are also explained.