*3.1. Analysis of Hydrodynamic Properties of Unmanned Surface Vehicle*

According to the hydrodynamic model established in Section 2.1, the navigation simulation of the USV is carried out, and the hydrodynamic properties of the USV under nine different working conditions are analyzed. The numerical simulation results are shown in Figures 6–8. The diagrams of navigation status, including heave and pitch motion in time domain under nine working conditions, are detailed in Supplementary Materials Figures S1 and S2. As shown in Figure 7a, the heave motion of the unmanned surface vehicle presents a gentle trend with the increase of speed. As shown in Figure 7b, the maximum heave amplitude is 0.7 m when the speed is 5 kn, and the significant wave height is 1.5 m, and the maximum downward settlement amplitude of the USV is 0.3 m (refer to Figure S1) when the speed is 8 kn, and the significant wave height is 1 m. It can be seen that when the USV navigates at a lower speed at a higher significant wave height, there will be a larger heave motion. In order to

ensure the safety of navigation depth, the maximum downward settlement value is selected as the input parameter to calculate the minimum safe depth of navigation for unmanned surface vehicles.

**Figure 6.** Numerical simulation results of the unmanned vehicle.

(**a**) Heave RMS value of nine cases (**b**) Maximum heave amplitude of nine cases

The pitch of the ship, in waves, can also cause draft changes and induce danger of striking on the sea bed. As shown in Figure 8, the pitch angle of the USV decreases with the increase of speed and significant wave height. As shown in Figure 8, the maximum pitch angle occurs at the speed of 5 kn, and the significant wave height of 1.5 m, reaching 10.5 degrees. The minimum pitch angle occurs at 11 kn, and the significant wave height is 0.5 m, which is 3.7 degrees. The USV has a larger pitch angle at lower speed and higher significant wave height, which causes larger draft change and gives rise to potential hazards to navigation safety of the USV. In order to ensure the safety of navigation depth, the maximum pitch angle is used as the input parameter to calculate the minimum safe depth of navigation for USV.
