Unmanned Systems Technology 010 | nuTonomy driverless taxi | Embedded computing | HFE International marine powertrain | Space vehicles | Performance monitoring | Commercial UAV Show Asia report

50 output shaft instead of the drive going out straight down, in the manner of a normal outboard engine (as per the stock engine). The H70 power unit as a whole consists of the HFE-modified engine, a starter motor/generator unit and a water jet impeller. The starter motor/generator unit charges a lithium-polymer battery, which is contained in a power module. That module contains all the electronics including the starter motor/generator controller, the fuel pump, the fuel injection driver, the CDI box, the engine management system control box and the power management circuit boards. The power module provides electrical power to all the buoy’s sensors. Since it contains the electric fuel pump, the fuel supply has to go in and out of it. Quick connect-disconnect couplings ease switching between all-electric and IC power options. Battlefield International supplies the necessary couplings and associated fluid lines, which are all to Mil-Spec. The drive As in the stock outboard engine arrangement, the crankshaft is vertically mounted in the craft, with the single cylinder then horizontal (and facing forward). In the stock arrangement a driveshaft extends down from the lower end of the crankshaft, its vertical alignment taking it down into the water. Underwater there is then a 90 º drive to a conventional propeller. HFE maintains the vertical orientation of the crankshaft, but at the lower end its power take-off is in the form of a 90 º gear drive to a horizontal shaft. That in turn powers an impeller within a water jet drive. A water jet drive (sometimes referred to as a pump) consists of an impeller inside a narrowing tube that draws water in the front and ‘pumps’ it out the back, increasing velocity as the passage narrows. The propelling force is generated by adding momentum to the water by accelerating a flow of water sternward. Water inflow from under the vessel is fed through the impeller, adding head to it. It then passes through an integrated stator that removes the swirl generated by the impeller and thus converts all the pressurised energy into speed, discharging a straight, high-speed jet much higher than the craft’s speed. This water acceleration generates the propulsive thrust; steering thrust is achieved by deflecting the water flow by means of a steering nozzle. The H70’s water jet impeller driveshaft is run through 90 º by a pair of helical mitre gears. That term refers to mating bevel (as in conically shaped teeth) gears having proportional numbers of teeth and with shaft axes at right angles. West explains that straight-cut gears would have had to have been larger in diameter to handle the same amount of torque; the mitre gears were to the benefit of the packaging of the H70 unit within the tight confines of the hull. “And by the way,” he adds, “those helical mitre gears are virtually impossible to have made!” A circular plate on top of the engine, at the upper end of the crankshaft, acts as a flywheel. That keeps the inertia going, since when the engine is running there often isn’t a great load on the crankshaft; the aim is to maintain low- speed operation. The stock flywheel is integrated with the stock ignition system, which is also replaced. That flywheel is replaced with a redesigned smaller diameter stainless steel flywheel, which is lighter. It provides the minimum weight required for smooth running. On the underside of HFE’s flywheel October/November 2016 | Unmanned Systems Technology Dossier | HFE H70 marine powertrain Elements of the buoy’s power module to which the H70 engine system is married