This chapter demonstrates the design and investigation of a mechanical power take-off (PTO) system by including nonlinearities, such as friction, damping losses, and engagement and disengagement scenarios. The model was validated through real-time laboratory-scale experimental tests on the prototype-scale PTO under realistic frequencies. In addition, the feasibility of incorporating a rotary motion rectifier (RMR) into the mechanical transmission of the PTO was determined through experimental validation for optimal energy conversion under real-time-varying wave frequencies. The applicability of a combined PA-WEC, comprising a novel SB-3 floater and the nonlinear mechanical PTO, was investigated through hydrodynamic simulations and laboratory-scale testing. The dynamics and energy conversion of the nonlinear mechanical PTO were analyzed through dry-lab testing. The suitable floater size that could run the real-time PTO under realistic wave conditions was identified, followed by its design validation. Subsequently, the effects of replacing the conventional C-HS floater with the SB-3 floater on the PTO dynamics and power performance were examined. The results demonstrate the superiority of the SB-3 floater over the conventional C-HS floater for designing an optimal PA-WEC.