IN THE world of mega-yachts, with their multiple sun decks, staterooms, jacuzzis, saunas, cinemas and helicopter-landing pads, there might seem little room left for one-upmanship beyond fitting yet more gold-plated taps and a mini-submarine berth. But the modern fashion for greenness has enabled a number of designers to find something new, in the shape of vessels so clean and self-sufficient they might never need refuelling.

Despite the wind-powered image projected by the word, not all modern luxury yachts have sails and those that do rely on their engines during calm weather or when manoeuvring in harbour. Even a medium-sized luxury motor yacht can burn through several hundred litres of diesel an hour while it is at sea. And the generators needed to produce the electricity required by accommodation akin to a small five-star hotel gulp down many more. As John Pierpont Morgan, a financier who was keen on such craft, is said to have remarked, if you have to ask how much a large yacht costs you cannot afford one.

The secret to avoiding gas-guzzling at sea is to go back to the roots of the word “sailing” and extract power from the wind. The difference between the new, green mega-yachts and the sailing vessels of old is that modern batteries mean a proportion of the power can be squirrelled away for future use, and that can reduce running costs in a very welcome way.

The design proposed by Paracas Yachts, a new Miami-based company set up by a group of yachting enthusiasts, collects its energy using traditional sails. Each hull of its catamarans has an underwater duct which serves two functions (see illustration). When the boat is under sail, water forced through the duct turns the blades of a small hydroelectric generator that charges a bank of lithium-ion batteries. When engines are required an electrically powered water jet can be fired from below the stern to propel the boat forward.

Sail power

The company's largest design, the 48-metre-long, three-masted Paracas 158, should be capable of running all of its refrigeration, air-conditioning, lights and other electronics for a week or more without having to start one of its two backup diesel generators or plug into a dockside power source. Once the power begins to run down, an afternoon or so at sea under sail will recharge the batteries, says William Ray, Paracas's director of engineering. If the weather is sunny (and when is it not, where mega-yachts sail?) the batteries can also be topped up by an array of gallium-arsenide solar cells.

There is, of course, a penalty to be paid for diverting some of the water that the boat is passing through into its turbines. But once the vessel is in motion the additional drag results in only a 3-5% reduction in speed, says Mr Ray. Unless you are in a race, this hardly matters. The Paracas 158, with six double cabins for guests and four double cabins for crew, would cost about $22m for the basic boat.

Sails are not the only way of collecting power from the wind, though. Richard Sauter, of Sauter Carbon Offset Design, based in Bali, has worked out a way to do it from the waves the wind whips up using devices similar to the tuned mass dampers (in essence, giant pendulums) mounted inside some tall buildings to stop them swaying. When the pendulum swings in one of Mr Sauter's designs its movement is used to generate electricity, which is then stored in a lithium-ion battery pack. In a cunning economy, Mr Sauter uses the battery pack itself as the bob of the pendulum.

The motion-damping system that is used in Mr Sauter's 78-metre Ark Angel is said to be capable of producing 100kW. A further 100kW would come from solar panels built into the carbon-fibre structure of the yacht. In addition, and for extra greenness, Ark Angel and the 44-metre Ocean Empire (picture below) would be fitted with giant automated kites, to tow them. SkySails, a Hamburg-based company that makes such kites, reckons they can cut a vessel's fuel consumption by half in ideal conditions.

The engines used by Ark Angel are electric motors driving counter-rotating propellers, mounted in pods below the hull. This avoids the use of a traditional propeller shaft and puts the propellers in a clear flow of water, which greatly improves efficiency. Cunard's new liner, Queen Mary 2, also has propellers mounted in pods under the hull. And, as in a hybrid electric car with a petrol-powered range extender, there is an internal-combustion option. Depending on the cruising speed required, one or more of four low-emission Daimler BluTec diesel engines can be started to generate additional electrical power. Even at 28 knots (52kph), Mr Sauter says Ark Angel's fuel consumption and greenhouse-gas emissions would be cut by about half—as they should be, for building such a craft is likely to cost more than $100m.

Yet all this is not just for those with Morganesque fortunes. Many commercial vessels, such as refrigerated cargo ships, also have a huge demand placed on their generators. Mr Ray, for one, believes this sort of technology could reduce the fuel consumption and exhaust emissions of quotidian boats, as well as luxury ones, and has set up a company called Hydro-Kinetic Designs to develop the Paracas generating system for working vessels. And SkySails sees broader uses, too. In 2012 it aims to have its biggest kite to date towing a 30,000 tonne cargo ship operated by Cargill, a large food company.

So, the next time you see a party in full swing on a floating gin palace remember this. It might look like everyone is whooping it up in search of a good time, but they could be conducting an important experiment in naval technology.