Unmanned Systems Technology 024 | Wingcopter 178 l 5G focus l UUVs insight l CES report l Stromkind KAT l Intelligent Energy fuel cell l Earthsense TerraSentia l Connectors focus l Advanced Engineering report

59 propeller becoming fouled by waste or seaweed is simply too high. When that happens, the craft and the mission just stops, negating the value of an autonomous system. That could be overcome with jet thrusters, but these are typically used in large craft – over 80 ft (25 m) – that are powered by a diesel engine, and have not been possible on smaller craft that are typically used for autonomous operation. Developing a battery-powered jet thruster that is a third of the size of existing diesel-powered systems opens up a whole new range of design possibilities and applications for much smaller craft. The design of the thruster also allows jets of water to be directed vertically to provide more stability for USV platforms, again opening up new applications. However, these platforms need a more complex vector control system than mainstream USVs. Jet propulsion To achieve the dramatic reduction in size, Stromkind developed a pump and screw to create a scalable, maintenance-free, small electric thruster. Called Sylents, it can be driven by batteries or even solar cells, with a power requirement ranging from 200 W to 420 kW. It is essentially a screw in a tube, driven by an electric motor. There is a key trade-off between the diameter of the tube, the length of the screw, the power requirement and the desired speed and weight of the craft. For example, for surveying applications with a side-scan sonar, the maximum speed is 4-5 knots, which would need a thruster power of 2 kW, says Desch. That then allows the size of the thruster to be optimised for the required speed, maximising the life of the batteries and providing the longest possible mission time. The propulsion system varies in size and power. The smallest version of the USV, all of which are catamarans, has a 15 cm-long thruster with a diameter of 8 cm that generates that 2 kW from 200- 300 W of battery power. For midrange systems, there is a 12 cm-diameter version that is 35 cm long and generates 1 kW. The team has also simulated a 320 kW system in which the thruster is 1.2 m long (1.5 m long with intake and outlet) and is 58 cm in diameter. That weighs 80 kg and delivers a peak power of 600 kW. “We have a lot of torque there,” Desch says. These thrusters can be used to replace more expensive jet thrusters made by companies such as Hamilton, which are powered by diesel engines in larger ships, and to power smaller autonomous craft. “We wanted to use the catamaran concept with a [boat-building] partner in South Africa who have experience with larger hydrofoil craft,” Desch says. “They used a Hamilton 216 120 kW thruster driven by a Volvo diesel engine, and wanted to replace it with our propulsion system. What appealed to the partner was that Stromkind’s proposed largest thruster is a third of the size of the Hamilton but can easily act as a drop-in replacement through the same interface plate. “The only interface [to the craft] is the rear plate where the thruster goes out. It is the same interface that the Hamilton 216 uses, but that’s three times the size of our system,” says Desch. Hydrodynamic simulation software allows the power of all the different thrusters to be optimised by refining the complex Stromkind KAT series | Digest It’s not about the money or the business case – they’re not so important for us – it’s about what we can do as engineers Unmanned Systems Technology | February/March 2019 The new water-jet propulsion design has opened up a surprising range of USV applications