As leisure activities go, owning a luxury yacht doesn't have the greenest of reputations. Motor yachts in particular are not fuel-efficient vessels and, in addition to all the energy used up and pollutants generated by their engines, there's the power required to provide air-conditioning and all the other electrical functions needed to make them a home on the water. Then there are the materials they're made from: non-recyclable fibreglass for the hull, for example, and all kinds of resins and plastics in the interior. However, driven by consumer interest in all things green, the yachting industry is trying gradually to get its environmental act together.
Of course, that's easier to do with sailing yachts, powered as they are mainly by a completely renewable source. As Luca Bassani Antivari, founder and president of ultra high-end yachtmaker Wally, puts it: "Sailing is probably the most sustainable and renewable way to move around the world." In addition to wind, wave and solar power are also becoming more popular ways of reducing yachts' carbon footprint.
Bassani's company is best known for motor yachts such as the extraordinary angular shark's fin that is the 118 WallyPower, but its roots lie in sailing yachts. And its latest creation is quite a sailing yacht: the graceful giant Better Place, a 164-footer that is the first sailing yacht to be granted the coveted Registro Italiano Navale (RINA) Green Star environmental certification.
Better Place uses a diesel electric system, consisting of three generators, fitted with anti-particulate filters, to produce electricity for the boat. It's not perfect, but it's more efficient than fuel-burning engines. Very low displacement means the boat is more efficient than most. Various environmentally friendly technologies and principles are also used in its construction, from a system to eliminate gases from resins, to water-based paints, to a commitment to using local suppliers to diminish energy use through transportation - yacht miles, if you like.
The yacht's size, says Bassani, is the biggest challenge in maintaining sustainability. "When we build racing boats, by definition they're more ecological. But on a boat like Better Place, a cruising boat with much higher displacement, you have to adopt new systems and technologies.
"Our customers are more and more aware about this issue, and they're looking for systems to make their boats more sustainable."
It was this growing demand from boat buyers that inspired Mattia Massola to form his company, Green Yachts, after he had worked in various boatyards, including working on the 118 WallyPower. "I thought: it looks like we're going to start seeing something different," he says. "People wanted boats with less consumption, so I built a company focusing on green yachts.
"A yacht is a less green thing than almost anything else you can have. Maybe it can never be totally green, but if we don't discuss it, we will never investigate the possibilities.
"It's important to think about the construction process and the aim of the boat. You can't recycle fibreglass, but you can aluminium - it's absolutely the best material." In fact, the aluminium used in Massola's boats comes entirely from recycled sources.
But the biggest environmental impact, he says, comes from the energy used by the engines and the generators: "It needs a lot of electricity for the air-conditioning and so on. And the trend is wider windows; it's nice, but it means more electricity."
Massola says that where environmental technologies are concerned, boat builders largely take their cues from developments in the car market, but are behind "because emissions are far higher". One technology that's finding its way onto boats is hybrid engines; not particularly ambitious, but easier to sell to customers because, when you're far from land, it's nice to have more than one method of propulsion. "Captains are scared of the new technology," says Massola. "We understand that - they're responsible for taking guests safely around the world. So what we do is hybrid, and captains are fine with that."
More ambitious is Fred Berry of US-based Independence Green Yachts, who's trying to create the world's first motor yacht that uses no fossil fuels. The idea, he says, "came from the fact that a lot of boats on Chesapeake Bay weren't going anywhere. The price of fuel kept going up. If it's high here, it's higher elsewhere.
"What I thought was: boats sit empty. They're only used at the weekend, and then not always. There's so much sun bouncing off them that we can use." A boat of 60ft or above, he says, has sufficient roof space to generate enough solar power to move it through the water. His Independence 60 motor yacht has a range of more than 1,900km at six knots in darkness.
Gradually, then, a range of boat builders are proving that it's possible to build yachts - even motor ones - that are green and glam. The industry is nowhere near there yet, but with more customers demanding it, and with the limitless resources of the wind, waves and sun available to them, the path to sustainability is in sight.