Unmanned Systems Technology 033 l SubSeaSail Gen6 USSV l Servo actuators focus l UAVs insight l Farnborough 2020 update l Transforma XDBOT l Strange Development REVolution l Radio telemetry focus

28 to charge the batteries and power the electrical loads directly. The power transfer efficiency of solar cells varies with the light falling on them and the electrical characteristics of the load, so MPPT controllers sample the cells’ output and apply the proper resistance to obtain the maximum power available in the conditions. Todter notes that such controllers have been used in large solar power installations for years, but have only recently become available off the shelf in packages compact enough for small boats. “We just chose a 40 mm square circuit board that has everything we need on it,” he says. “We actually made our own at one point, but it wasn’t worth the effort.” The electrical harness is made up of polyurethane jacketed multicore wiring with waterproof connectors from Blue Trail Engineering. The Gen6’s battery pack capacity is around 450 W/h. NiMH chemistry was chosen because of the shipping limitations imposed on lithium-ion batteries and armed forces’ dislike of them. Operating speeds of between 1 and 2.5 knots are typical, Todter says, adding that theoretically the boat can remain at sea indefinitely, while the practical limits have yet to be found. Also yet to be determined are its operational and survival limits in terms of wind speed and wave height, but it has been calculated that it will be able to operate successfully in winds of up to 30 knots and waves at least 3 m high, and survive in waves up to 5 m. With its 1.3 m draft, the vessel needs a minimum of about 1.5 m of water to sail safely. While the submersible option is intended principally to avoid detection and rough weather, the vessel can move slowly under water when propelled by its thruster, but the intent is simply to loiter until the threat has passed. Buoyancy and righting moment The Gen6’s configuration presents a challenge when it comes to generating a righting moment, which is the force that resists the tendency of the wind acting on the sail and the action of waves to push the boat over. To create a righting moment, the vessel has to have positive buoyancy (rather than the neutral buoyancy that would be appropriate for a submerged craft) and the centre of buoyancy has to be above the centre of gravity. “In our case, the centre of gravity is as far down as we can get it because we put all the heavy stuff in the streamlined hull at the bottom,” Todter says. “But since you have to be able to support the wing just above the surface you’ve got to have something that pierces the surface, and you can’t balance the weight and the flotation exactly, so there has to be a little residual flotation.” The challenge is to minimise that residual buoyancy. “To do that you have August/September 2020 | Unmanned Systems Technology Dossier | SubSeaSail Gen6 USSV Height: 152 cm for the wingsail, 152 cm for the vessel, 304 cm total Length: 150 cm Width: 25 cm Weight: 28 kg Endurance: theoretically unlimited Depth rating: 10 m Propulsion: sail, primary; thruster, back-up and station-keeping Speed: 0.5-3.0 knots Battery pack: 450 W/h nickel metal hydride Solar panels: 30 W peak on wingsail, 10 W peak on deck System operating load: 0.6 W/h Maximum system power: 5 A @ 13.2 V DC Computer system: PIC CPU, XBEE wi-fi, 9603 Iridium, serial, Micro SD, IMU IMU: MEMS three-axis accelerometers, gyros, magnetometers GPS: 12-channel, accurate to 3 m horizontal dilution of precision Station-keeping accuracy: 30 m radius in a 0.5 knot current Auxiliary power and comms port: for external sensors and payloads Maximum payload: 20 kg (neutrally buoyant) Payload power: 20 W peak, 5 W maximum continuous Ports: 12 V, RS-232, battery charge Comms: Iridium 9603, wi-fi, cellular FreeWave option User interface: chart-based GUI for pre-programmed missions, full manual control via PC or tablet Options: Conductivity, temperature and depth, turbidity and salinity sensing, water speed measurement, AIS receiver, magnetometry, mast camera, AIS receiver, custom comms, sonar, hydrophones, swarming behaviours, Iridium RUDICS data service Some key suppliers Sonar arrays: Applied Ocean Sciences Waterproof connectors: Blue Trail Engineering Auxiliary thruster: Blue Robotics Satellite comms: Iridium Component and subsystem manufacture: Kroova Water quality sensor package: Sontek/Xylem Specifications Bolted to the rear of the hull is the T200 electric thruster from Blue Robotics, which has a fully flooded brushless motor with encapsulated motor windings