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In 2012 Acciona, the Spanish alternative energy group, sponsored what it called the world’s first zero-emission sailing yacht for the single-handed Vendée Globe race. It was a lightweight racing machine plastered with solar panels, and without the usual auxiliary diesel engine.
Since then, electrical energy and propulsion systems for boats, cars and even aircraft have continued to proliferate. Hybrid cars such as the Toyota Prius — which can be propelled by a battery-powered electric motor or by an internal combustion engine, or both together — are routinely used by commuters and for city taxi fleets around the world.
Solar Impulse 2, the experimental sun-powered aircraft created by Bertrand Piccard and André Borschberg, is halfway through its pioneering flight around the world (albeit grounded for now in Hawaii after its batteries overheated).
Among owners of large, heavy motoryachts, however, initial enthusiasm for hybrid propulsion systems (and for the tried and tested diesel-electric, in which a generator powers an electric motor) is waning in the face of complaints about cost and complexity.
While lightweight cars and aircraft can roll along the ground or fly through the air with relatively little friction, it takes a lot of energy to push a ship or a large boat through the water.
Yachtmakers say that — for the time being, at least — the best way of doing this for long periods at a manageable cost and with a power source of moderate weight remains the trusty internal combustion engine and its diesel fuel.
“Most power requirements for boats are quite high,” says Perry van Oossanen, director of Van Oossanen naval architects. “Water is quite heavy — 1,000 times heavier than air.”
Among the drawbacks of hybrid propulsion for superyachts, therefore, are the huge power requirements (500kW or even a megawatt to drive a ship at 10 knots) and the cost of lithium-ion batteries, plus uncertainty over how to dispose of them at the end of their lives.
One of the supposed advantages of a system in which a diesel engine works consistently at its most efficient speed to charge batteries has not yet been found in the real world.
There is only a 10-15 per cent gap between the most and least efficient engine speeds, and any gain is typically cancelled out by losses in converting diesel power to electricity and transmitting that to an electric motor.
Hans Konings, head of design at Amels, sees little financial benefit from a hybrid system for a motoryacht that travels the typical 8,000 to 10,000 sea miles a year. He says: “In the superyacht industry, you don’t see these systems much at all. They are not so efficient,” he says. “The cost of ownership of such a system is tremendously high.”
That is not to say that there is no merit at all in hybrid power or in other innovations of the energy systems of superyachts — sometimes so large that they are equivalent to floating hotels with gas-guzzling air conditioners and roll stabilisers that also use plenty of electricity.
True, it is no secret that many yacht-owners are rich enough not to care about the cost of fuel, and that only a handful even of those who do talk about sustainability and environmental issues will make a serious investment in green technology.
But owners do care about comfort: hybrid propulsion systems give a skipper the option of cruising for short distances without noise, at slow speed with just the electric motors.
Sufficient battery power also allows a yacht to spend the night silently at anchor in a beauty spot — without the offensive throbbing of the diesel generators to disturb.
So the 50-metre Project Nova yachts being launched at Monaco by Heesen Yachts, advised by Van Oossanen, will not only be of lightweight aluminium construction with a fast-displacement hull form offering high fuel efficiency, but will also come with the option of a hybrid package.
Amels offers a method of recovering waste heat from the engines — for swimming pools and hot water, for example — and what Mr Konings calls a “hybrid switchboard”.
This is essentially an electricity management system including a battery pack, a small generator and a large generator, to ensure that each piece of equipment is used as efficiently as possible.
The hunt for innovative ways of generating and saving energy is unending on land, at sea and in the air.
The 2009 sailing yacht Ethereal, a 58-metre ketch built by Royal Huisman, can recharge its batteries from its propellers when under sail, just as a Toyota Prius car can harvest power from braking in traffic, and incorporates numerous energy-saving appliances and design features.
However, Ethereal and Acciona are, in fact, useful reminders that there exists an age-old method of propelling a vessel swiftly across the oceans, without using a single drop of fuel. It is known as sailing.