<p>This paper proposes a practical prescribed-time trajectory tracking control scheme for underactuated unmanned surface vessels (USVs) that is based on asymmetric prescribed performance control (PPC), which enables the USV to accurately track the desired trajectory with guaranteed transient and steady-state performance. First, the issue of constraint handling is converted into a stabilization problem with unconstrained variables by combining the error transformation with the barrier Lyapunov function. Second, a novel time-varying scaling transformation function (STF) is employed to provide a more relaxed criterion for the realization of practical prescribed time stabilization. Third, a prescribed-time observer is constructed that achieves disturbance attenuation in a prescribed time. An auxiliary dynamic system (ADS) featuring time-varying gain is subsequently designed to solve the input saturation problem. The proposed controller can achieve convergence of the tracking error in a prescribed time independent of the initial conditions while eliminating the sign limitations of the performance function. Moreover, a hysteresis quantizer is introduced to reduce the data transmission load. Finally, it is demonstrated that all the signals in the closed-loop system are bounded via Lyapunov stability theory, and the effectiveness and feasibility of the proposed control scheme are proven via numerical simulation studies.</p>