Gunboat catamarans are known for their sleek lines and speed. Owners take pride in sailing their boats, and the engines are used primarily for maneuvering and intermittent auxiliary propulsion-; they are rarely used for extended motoring. The resulting combination of high sailing speeds and limited propulsion time provides the ideal scenario for a serial hybrid propulsion system in which a battery-powered electric motor drives the propellers.
Central to the PowerWave hybrid system aboard the Gunboat 60 is a pair of 20 kilowatt (kW) motors that can push the boat at speeds of up to 9 knots, with the initial energy provided by a 29 kilowatt-hour (kWh) lithium-ion battery pack. In practice, this battery pack will support harbor maneuvering for hours at a time and full speed operation for around 30 minutes, after which one, or both of the boat’s two 20 kW Polar Power DC generators will kick in.
A sophisticated energy management system ensures that engine operation is always at, or close to, peak fuel efficiency, while operators maintain control through a touch screen in the navigation station. A generator is held in reserve for longer passages that the batteries cannot support.
Under sail, any time boat speed is above 8 knots substantial amounts of energy can be harvested from the freewheeling propellers-—more than enough to run the house systems and recharge the batteries. Then, when the batteries are charged, the propellers are retracted into the hull for maximum sailing speed. Given the typical operating profile of these boats, it is not unreasonable to think that many owners will enjoy an extravagant lifestyle on board without having to crank any kind of an engine for days, or even weeks at a time.