A soft hydraulic actuator is presented which uses elastic energy storage for the purpose of pulsed-jet propulsion of soft unmanned underwater vehicles. The actuator consists of a flexible membrane that can be inflated using a micropump and whose elastic potential energy may be released on demand using a controllable valve, in a manner inspired by the swimming of squids and octopuses. It is shown that for equivalent initial elastic energy, the drop in peak thrust is linearly proportional to the decrease in nozzle cross section. Peak hydraulic power amplification of the soft actuator of approximately 75% is achieved with respect to that of the driving pump, confirming that passive elasticity can be exploited in aquatic propulsion to replicate the explosive motion skills of agile sea-dwelling creatures.

Hydrodynamic design of an underwater glider is an act of balancing the requirement for a streamlined hydrodynamically effective shape and the consideration of the practical aspects of the intended operational envelope of the vehicle, such as its ability to deploy a wide range of sensors across the water column. Key challenges in arriving at a successful glider design are discussed and put in the context of existing autonomous underwater vehicles (AUV) of this type. The design cycle of a new vehicle shape is then described. The discussed AUV will operate both as a buoyancy-propelled glider and a flight-style propeller-driven submersible, utilizing its large size to deliver substantial scientific payloads to remote locations to perform environmental monitoring, seabed survey, and exploration for subsea oil, gas, and material deposits. Emphasis is put on using computational fluid dynamic (CFD) methods capable of predicting laminar-turbulent transition of the flow to estimate the performance of candidate designs and thus inform and guide the evolution of the vehicle. A range of considered shapes is therefore described and their hydrodynamic characteristics predicted using CFD are summarized. A final shape for the new glider is then proposed. This is then subject to an in-depth flow-field analysis, which points out how natural laminar flow may be used as a means of drag reduction without compromising the practical aspects of the design, such as its ability to carry sufficient payload. Finally, the obtained data are used to project the expected glide paths, as well as give preliminary estimates of its range. These show the benefits of minimizing the vehicle drag, as well as highlight the possible tradeoffs between maximizing speed and endurance of the AUV.