Rotary crane systems are widely used to transport heavy loads and hazardous materials from a primary pick-up point to a destination, and therefore smooth and faster trajectories of crane boom motion are required to reduce traveling cost, operating time, and load sway. This paper presents a simple dynamics model for rotary crane systems to generate the time-optimal trajectory of horizontal boom motion under various constraints, which can achieve accurate boom motion and load sway suppression. The proposed trajectory is represented by a polynomial function and it is used to alleviate two-dimensional residual load sway. The generated trajectory is verified using another load dynamics model to confirm the effectiveness of the proposed one. Moreover, a full dynamics model is used to verify boom torque limits and optimized load sway angles, respectively. Computational results confirm the effectiveness of the generated trajectory using the proposed dynamics model for residual load sway suppression.