**5. Conclusions**

This study analyzed the hydrodynamic interaction of a new type of underwater robot, the AUH, in locations near a host ship using the computational fluid dynamic ANSYS-AQWA solver, a potential–based panel method. The simulation results of the investigated AUH–Ship interactions are significant to practical works, including research on automated ship recovery and launch systems to be used in advance for AUH operations in waves. This research can significantly enhance the efficiency of AUH homing automation to host ships in waves. The 6-DOF RAOs of an AUH at various distances from the host ship were predicted, and a suitable location area for AUH launch and recovery was determined.

The host ship exerted significant influence on AUH motion performance in sway and pitch. The influence of the host ship on interactive AUH hydrodynamic disturbances was determined in terms of 6-DOF RAOs. The RAOs of the AUH at different velocities were successfully predicted for enhanced adaptive controllability of homing automation. A specific location for optimizing AUH seakeeping performance during recovery and launch operations in long-crested irregular waves was analyzed in the time domain using the P–M wave-spectrum model to observe good positions. Simulation results showed that the sea condition with a wave frequency of 1 rad/s should be avoided due to the resonance phenomenon. AUH oscillation amplitude was the smallest at X = 3 m, Y = −2.4 m, Z = −2 m, and X = 10 m, Y = −2.4 m, Z = −2 m; these positions should be selected as an appropriate location for launch or recovery.

Experiment validation of the dish-type AUH motion RAO during roll and pitch was carried out. Numerical analysis of motion RAO in roll with a proposed damping model was compared to experiment data in the wave-frequency range of 0.2–1.0 Hz, resulting in average errors being reduced from 21.03% to 9.95%, verifying the accuracy of the adopted method. Comparisons between numerical results and experiment data showed that the tolerance error was acceptable to prove that the numerical results were valid and rational. In the future, AUH behavior induced by sensors and controllability in short-crested irregular waves will be further studied. An experimental study of the designed AUH configurations mounted with di fferent aquatic transducer lengths, to achieve an acceptable trade-o ff between hydrodynamic optimization and acoustic navigation, will be carried out in future research and for practical applications.

**Author Contributions:** Conceptualization, C.-W.C.; Data curation, Y.C.; Formal analysis, C.-W.C. and Q.-W.C.; Funding acquisition, C.-W.C. and Y.C.; Investigation, C.-W.C. and Y.C.; Methodology, C.-W.C., Y.C. and Q.-W.C.; Project administration, Y.C.; Resources, Q.-W.C.; Software, C.-W.C., Y.C. and Q.-W.C.; Visualization, C.-W.C., Y.C. and Q.-W.C.; Writing—original draft, C.-W.C. and Q.-W.C.

**Acknowledgments:** The authors wish to thank the National Key Research and Development Program of China (No. 2017YFC0306100), and the National Natural Science Foundation of China (No. 51409230) for financially supporting the research on the hydrodynamic behavior of the Autonomous Underwater Hovering Vehicle.

**Conflicts of Interest:** The authors declare no conflict of interest.
