Hybrid Unmanned Underwater Vehicles: A Review of Architectures, Enabling Technologies, and Engineering Trade-Offs

Hybrid underwater robotic vehicles integrate features typically found in separate classes of vehicles: autonomous underwater vehicles (AUVs), remotely operated vehicles (ROVs), amphibious robots, and cross-medium robotic vehicles. We review this field from a system-level perspective and identify four types of hybridization: (i) control-mode hybridization (hybrid AUV/ROV, autonomous and remotely operated vehicle (ARV/HROV)), (ii) amphibious mobility (water–land), (iii) cross-medium mobility (water–air), and (iv) propulsion-mode hybridization. We review propulsion, power, sensing and navigation, communications, control and autonomy, structural design, docking, and human–machine interface aspects with particular focus on coupling across subsystems instead of describing each technology individually. We provide a structured comparison of 24 platforms and enabling technology entries based on evidence type, maximum operational depth, application area, and hybridization type. We highlight common trade-offs such as endurance versus intervention capability, autonomy versus operator control, mobility versus mechanical complexity, and growth of capability versus maintenance and reliability. We synthesize qualitative risk factors, informed by our literature review, such as communication loss, navigation failure, energy depletion, transitions or controls not working correctly, water ingress, tether safety issues, and software or human–machine interface problems. The resulting framework elucidates differences among various hybrid concepts, identifies limitations in reporting and evidence, and offers evidence-informed guidance for development of future hybrid underwater robotic systems.