Parallel hybrid propulsion

Parallel hybrid propulsion

The parallel hybrid propulsion system of a new 135-passenger excursion boat can be switched to electric mode to explore a nature reserve’s waters quietly and ecologically.

Hybrid propulsion systems that can be switched between an internal combustion engine and an electric motor are proving increasingly popular with operators frequenting environmentally sensitive waters. A case in point is Corsica-based Nave Va, an excursion operator that takes tourists on boat tours of the island’s spectacular Scandola Nature Reserve. Nave Va asked electric motor manufacturer Leroy-Somer and Mayday Electronics to develop a parallel hybrid propulsion system for its new boat, a CNB Pro20 designed by naval architecture firm Bureau Mauric and built by Bordeaux-based CNB, a subsidiary of French boatbuilding group Bénéteau. The aluminium-hulled Chjara Stella is 19.95 metres in length for a beam of 5.8m and can carry up to 135 passengers. She was commissioned for the 2016 summer tourism season.

Two electric, two diesel

The parallel hybrid system’s electric mode is used at low speeds and diesel propulsion at higher speeds. Silent, pollution-free electric propulsion is used for docking and other port manoeuvres and, more importantly, in the nature reserve’s protected areas. This mode offers a maximum speed of 8 knots and an endurance of 6 to 8 hours at 6 knots. Battery recharging takes 6 hours using either a three-phase quay-side connector or the boat’s onboard generator set. The two 800-hp diesels take over at higher speeds up to a top of 20 knots in open water. Note that the diesels are shut down completely during electric propulsion. “Parallel hybrid systems differ from serial hybrid arrangements in that the electric motors can be matched more closely to the application; in this case cruising at 6–8 knots. Efficiency is also higher as the electric motors are used in their optimal operating range.

The overall configuration is lighter and more compact for a given endurance,” says Eric Brun of Mayday Electronics. The permanent magnet synchronous motors are from Leroy-Somer’s LSRPM-Dyneo range and run at speeds up to 3000 rpm. “We chose this model for its efficiency and small size. There are smaller motors on the market, but they require separate cooling equipment for which we had little space in the confined engine room. In addition, Leroy-Somer offered to marinise the motors by supplying aluminium enclosures tailored to the engine room environment,” says Eric, the propulsion system’s chief designer. “The reliability of Leroy-Somer solutions is also a plus. The first boat designed by Mayday Electronics with Leroy-Somer motors has been in continuous service since April 2012. The vessel, the Marseille-based Hélios, was designed in conjunction with operator Icard Maritime and, by late 2016, had logged over 2000 excursions without incident. In both cases, we chose industrial motors for the simple reason that they are designed for more intensive use than most marine electric motors. Given that industrial applications call for around-the-clock operation whereas ours call for just a few hours a day, the choice offers comfortable margins.”


Ecological and economical

“By using electric propulsion for low-speed manoeuvring and cruising, the diesel engines see far less use when they are least efficient which means lower fuel consumption and reduced maintenance. The saving is around 70 litres of fuel per engine and per hour of electric propulsion, or 140 l/h. Taking into account the length of the tourist season, Nave Va estimates that it will recover the additional cost of electric propulsion — between €350,000 and €400,000, depending on battery capacity — in four seasons or less while increasing the diesel engines’ service interval and life,” says CNB’s Pierre Lompech. “The cost of this type of electric propulsion may well be lower in future. The batteries take up a lot of space and are still relatively expensive at around €1 per watt-hour. The batteries, with an installed power of 125 kWh, represent about half the cost of the system, while the two electric motors, their variable speed drives and the power train represent the other half. Only a few modifications were required to accommodate marine conditions, primarily stainless steel fasteners and additional paintwork,” says Eric Brun. “Despite the many benefits of parallel hybrid systems, they have not yet found many applications. In France in particular, there are very few indeed. Nevertheless, here at Mayday Electronics, we’re working on several projects, including pleasure craft, superyachts and naval vessels in addition to passenger boats like the Chjara Stella.”

Original, in French, by Caroline Britz, translated and adapted by Steve Dyson