France-headquartered Leroy-Somer and UK-headquartered Control Techniques and Mayday Electronics have joined forces to offer a hybrid propulsion system for a new Corsican excursion vessel.

Leroy-Somer and Control Techniques have been suppliers of marine applications for many years. In association with Mayday Electronics they have now produced a high-performance propulsion system which the partnership claims is at the leading edge of hybrid technology. The system was developed for CNB PRO, a trademark of Construction Naval Bordeaux (Bénéteau Group), and its client, NAVE VA.

Trusting in CNB PRO's know-how in the field of passenger transport vessels and the expertise of the naval architects, Bureau Mauric, the Corsican boat excursion company, NAVE VA, recently acquired a new model CNB PRO 20 Hybrid. As specialists in organising visits to protected sites, NAVE VA do not consider this solution to be an option, but rather an environmental duty.

In low speed mode, for manoeuvring in port or in protected areas, the vessel switches to electric power. "The silence is incredible," explains Eric Brun of the Mayday company, which designs and supplies the propulsion system for these hybrid vessels. It has a speed of 6 to 8 knots, which is quite sufficient.

Parallel hybrid differs from serial hybrid in that the size of the electric motor can be designed to match the desired application, which in the case of NAVE VA is for a 6 to 8-knot low-speed mode. Optimum efficiency is therefore achieved: at low speed, the electric motor is used in its optimum operational range. In addition, the weight of the installation is much less for equivalent endurance. Another advantage is that the internal combustion engines can be stopped when the electric mode is activated. The CNB PRO shipyard, situated in Bordeaux (western France) and part of the Bénéteau Group, has identified a decisive advantage of hybrid propulsion systems: "Diesel engines do not run at optimum efficiency at low speeds. They exhibit low levels of efficiency and are prone to fouling. The fact that the diesel engine can be shut down in order to use the electric propulsion system helps to preserve it. Its service life can therefore be doubled, depending on the applications," explains Pierre Lompech of CNB PRO.

For the NAVE VA vessel, this would mean changing the engine after 16 years, instead of just 8. In real terms, the replacement of the two diesel engines represents a budget equivalent to the price of the electric propulsion system.

"The use of an electric propulsion system instead of internal combustion for manoeuvring in port, for berthing and for low-speed travel reduces the length of time that the internal combustion engines are in use during the phases when they are least efficient. In these operating situations, significant reductions in fuel and maintenance costs are achieved. 70 litres of fuel per hour can be saved for each diesel engine, therefore totalling 140 litres per hour. Taking account of the overall added costs of the electrical propulsion installation (about €350k to €400k depending on the capacity of the battery set) and the seasonal use specific to our client, the latter expects to be able to write off this extra cost in less than four seasons, and at the same time increase the service life of the diesel engines while extending their maintenance intervals," says Pierre Lompech.

The electric propulsion system also provides redundancy, which is essential for passenger vessels.

"Legislation is changing. Electric power is becoming mandatory for an increasing number of applications, and not just in protected nature reserves. We are developing the hybrid system, in particular the parallel hybrid system, which is ideally suited to ships of less than 30 metres. Its use could be extended to regular ferry services, links between two ports, and river or pleasure navigation," Pierre Lompech concluded.

The passenger vessel supplied to NAVE VA is equipped with two 800 hp motors, allowing it to achieve a speed of 20 knots in diesel mode. In electric mode it has a maximum speed of 8 knots and an endurance of 6 - 8 hours at a speed of 6 knots. Recharging takes a maximum of 6 hours, and uses a harbour three-phase supply or the on-board electrical generator.

"The price of electric propulsion could come down even more in the future. The batteries take up a lot of space and are still quite expensive, at about 1 euro per Watt-hour. Since the installed electrical capacity is 125 kWh, the batteries represent about half the cost of the propulsion system. The remainder is made up of the mechanical drive system, the two electric motors and the variable speed drive units. Ensuring compliance with marine conditions is straightforward. The motors and variable speed drives are installed in dry dock, the fastenings are made of stainless steel and often only minimal environmental adaptations are required, such as adding coats of paint", explained Eric Brun.

"Despite the system's advantages there are still very few parallel hybrid systems in use, especially in France. Apart from passenger ferry, we have projects in the field of pleasure boats for 'super yachts' and in military applications," said Eric Brun.

"We chose the Dyneo range LSRPM series permanent magnet 750 - 3000 rpm synchronous motor because of its high output and exceptionally compact design. We did find more compact motors, but these used separate cooling systems, which complicates installation in the limited space available in the engine room. Furthermore, Leroy-Somer offered to make the motor suitable for use at sea, in particular by supplying an aluminium housing appropriate to the environmental constraints", added Brun.

The first vessel equipped with Leroy-Somer motors on a Mayday propulsion system has been in continuous operation since April 2012. This is the Hélios ferry in Marseilles (southern France), designed with the operator, ICARD Maritime.

By Jake Frith