This experimental study investigated the hydrodynamic performance of the first free-surface semicircular breakwater supported on piles under regular waves. The research focused on SCB models with porosity levels of 0, 9, 18, and 27. Experimental tests were conducted in a wave flume to evaluate the transmission (CT), reflection (CR), and energy dissipation (CL) coefficients of the SCB models. Wave disturbance coefficients (CF) in front of the breakwater and within the breakwater chamber (CC) were also examined. Horizontal wave loading was measured using normalized force coefficients (Fn), including force coefficients of wave crests (Fn,c) and wave troughs (Fn,t). Empirical formulas were proposed to provide a quick estimate of the hydrodynamic performance, showing good agreement with the measured data. The findings highlight the impact of varying porosity levels on wave attenuation, with the impermeable SCB model (0 porosity) exhibiting superior performance compared to the perforated SCB models. This research contributes valuable insights into optimizing SCB model design and enables efficient estimation of its hydrodynamic performance under regular wave conditions. The results provide valuable guidance for the design and implementation of SCB structures, enhancing their effectiveness in wave attenuation applications. © 2023 by the authors.