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7 Platform one Unmanned Systems Technology | April/May 2017 The Sylents system addresses many of the drawbacks of conventional water jet drives Water jet thrusters have grown in popularity for powering unmanned surface vehicles such as small catamarans, and now Austrian company Stromkind has developed a water jet system using a brushless dc motor for such USVs (writes Rory Jackson). The Sylents system is based broadly on conventional axial water jet drive designs that consist of an intake, a compressor containing an impeller, and a drive nozzle. In most systems, the impeller is driven by an engine shaft that is guided through the intake or drive nozzle. That though has the disadvantage of needing a sealed shaft passage – a technically complex, high-friction and maintenance- intensive component. The Sylents system therefore has the impeller driveshaft mounted to the rear wall of its housing, and the outer shaft connects to the motor’s outrunner via a belt running above the nozzle. The drive can be made secure by using a toothed belt, a chain drive or a positive-locking mechanism. The drive is used in a 5.5 x 3 m KAT 500 catamaran USV developed by Stromkind, and will also be used to power a future 17 m hybrid catamaran design. Water jet drives have lower efficiency at low speeds compared with propellers, so Stromkind used CFD simulation software from Numeca to improve the internal dynamic flow of its drive to allow for low-rpm operations and the use of an electric drive. The result is lower emissions and noise, particularly compared with gasoline-powered systems, which are often heavily regulated or even prohibited from being used in stretches of water such as recreational lakes or nature reserves. They are also considered unsuitable for marine scientific observations and military reconnaissance. “Simulating this hydrodynamic flow during the design phase was challenging. We found that very few tools were accurate enough to get simulation results that matched our measured values,” said the company’s Andreas Desch. The Sylents drive also uses a single- blade helical-screw impeller with a diameter that runs at an incline at greatest width in the middle of the shaft length, giving the impeller an elliptical planform rather than a rectangular one. During testing, this arrangement was found to be the best at avoiding the issue of conventional systems being clogged by debris such as seaweed and sandy water; even small fish are expelled uninjured from the drive nozzle. The design also eliminates the cavitation that can occur with underwater propeller systems. Operating at relatively low rpm and low power reduces the formation of vapour bubbles that result from cavitation, and the conical shape of the compressor section further inhibits it. To prevent the excessive thermal signature occasionally caused by power system electronics, there is a cooling surface for the motor control unit, next to the outer portion of the intake area. The Sylents will be available in a range of configurations, from 25 to 348 kW, with power inputs ranging from 48 V to 400 V three-phase. “High voltage for smaller platforms in maritime applications are not common, and so parts are not available,” said Desch. “Here we gained a lot of experience and ended up with a 400 V top operational voltage.” The nozzle comes with optional reverse control on drive-by-wire installations, and integrated vector control for additional manoeuvrability when required. Direct current thinking Marine craft