Unmanned Systems Technology 012 | AutoNaut USV | Connectors | Unmanned Ground Vehicles | Cobra Aero A33i | Intel Falcon 8+ UAV | Propellers | CES Show report

22 Dossier | AutoNaut Thrust is transferred to the hull through the foil axles attached to vertical struts that in turn attach to stainless steel fixtures at the bow and stern of the hull, which also incorporate lifting hoops. The stern strut also supports the rudder and an auxiliary electric propulsion pod; a second rudder is sometimes fitted to the bow strut. Removable bow and stern cones protect the strut fixtures. While Linden’s Autonaut used flaps much like a diver’s flipper, AutoNaut’s foils are like wings – which are more hydrodynamically efficient – in combination with a hull that is designed for wave propulsion. “Linden used a Naples inshore fishing boat hull design, which happened to have quite a good shape, with lots of rocker,” Poole explains. “Later experimenters such as Hitachi Zosen in Japan and Einar Jacobsen in Norway, as well as the Russians – all in the 1980s – generally used freighters, fishing boats and frigates designed for screw propulsion, or keel boats designed for sail.” The term ‘rocker’ refers to a hull shape that curves upwards from the keel towards the bow and the stern so that if placed on a flat surface it could easily be made to rock back and forth in pitch. Likewise, a hull with sides that curve up from the keel towards the gunwales will naturally roll from side to side in response to any disturbance. Both are important for generating thrust from wave motion. Conventional hulls are generally designed to minimise pitching and rolling or, in the case of fishing boats, to roll comfortably. In contrast, the AutoNaut is designed to provide plenty of movement in both directions to get thrust and stabilisation from the foils, which naturally damp the motion, while also being able to right itself in case of a capsize. The hull form is narrow and round in the bilges, which enables it to be driven easily and puts the foils in the right place while minimising the risk of entanglement, Poole says. Asked whether there is some ideal ratio of wavelength to hull length, he replies that that would apply only to motions in pitch in a test tank. In real seas, he explains, the USV must go reasonably well in all directions relative to the waves, which will usually be of mixed period and direction. “However, the hull does not respond to waves that have a period that is too long or too short for the motions of pitch and roll,” he says. Auxiliary thrust Flat calm conditions pose the only real issue for wave propulsion, he adds, so AutoNaut USVs can use the optional electric propulsion pod that can drive it at up to 3 knots for short periods. In refining the hydrodynamic design of the foils, AutoNaut sought to balance robustness, efficiency and simplicity rather than maximising efficiency, an approach likely to have resulted in foils that were complex and fragile. Foil and hull shapes were developed together as they must work as a single system, with efforts focused on fitness for purpose rather than perfection in any single aspect. Steering is provided by a conventional rudder at the stern, with a bow rudder added for applications in which there is a need to sink a towed sensor to a specific depth by circling the boat very tightly, within a radius of 10 m. That also adds redundancy to the system, as either rudder can be used independently to steer the boat. Consisting of a brushless electric motor and a propeller in a waterproof housing, the propulsion pod attaches to the stern strut, for the sake of simplicity. The device was designed mostly for use in flat calm conditions and at the request of customers who want to ensure they can avoid obstructions such as oil rigs. The use of auxiliary propulsion is entirely the operator’s decision, and the system includes joystick control for use at short ranges, which has proved useful for motoring to and from a slipway and during recovery at sea. Manoeuvrability In terms of manoeuvrability, a 5 m AutoNaut with both rudders achieves that minimum 10 m turn radius in a tank. With one rudder and in real-world sea conditions though, payload and rudder loading have an influence, and the company quotes a larger station-keeping radius of 25-50 m depending on the size of the boat. AutoNaut is very controllable and manoeuvrable, but a little different from February/March 2017 | Unmanned Systems Technology Safety equipment includes a passive radar reflector (Photo: Peter Donaldson)