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As midnight approached last July 4, Kenichi Horie, a 69-year-old Japanese environmentalist and redoubtable solo yachtsman, eased the catamaran Suntory Mermaid II into Wakayama, southern Japan.

After 111 days at sea, nearly double the 60 days estimated for the Mermaid's 7000km voyage across the Pacific from Hawaii, Horie - who survived on rice, curry and fish caught from the ocean - had concluded the world's longest wave-powered odyssey.

Despite the delay, the trip was judged a success. The 9.4m, 3 tonne Mermaid, which was designed by Japan's Tokai University School of Marine Science and Technology, was propelled by wave energy, harnessed by two independent fins mounted at the cat's bow. As water flowed past the twin hulls, the fins moved up and down, delivering a dolphin-like kick that drove the ultra-light boat, made from recycled aluminium. Calm seas and clement weather had limited speeds to 1.5 knots, hence the leisurely crossing.

"The time has come for us to shift from fossil fuels," Horie told the New York Times. Horie, who holds the record - 148 days - for a solar-powered Pacific crossing, and who also pedalled a boat from Hawaii to Okinawa in 1993, hoped to "increase awareness and interest in natural energy".

In fact, concerns about fuel costs and the impact of carbon emissions from commercial ships on climate change are already roiling the maritime industry. Sulphur emissions from "bunker fuel", low-grade diesel, also pose health risks, with an estimated 60,000 deaths worldwide in 2007. Shipping is excluded from the Kyoto Protocol to counter global warming.

But with worldwide CO2 emissions up 3 per cent in 2007, according to the UN, pressure is mounting for the shipping industry - which pumped out some 847 million tonnes that year, or 2.7 per cent of the world total according to the International Maritime Organisation (other estimates are much higher) - to clean up its act.

While Horie's feat echoes an era - typified by Thor Heyerdahl and Jacques Cousteau - of derring-do that is fast slipping below the horizon, his advocacy of clean-tech is making waves in the shipping world.

Last year, the MS Beluga SkySails, a 10,000 tonne container ship, left the German port of Bremerhaven bound for Guanta, Venezuela. The 132m diesel-powered Beluga was the world's first commercial ship to use a 160sq m kite, deployed 300m above the bow, to boost power, trim fuel costs, and cut CO2 emissions.

Whereas conventional sailing vessels use masts, spars and lines to manipulate sails, the kite sail, stored below deck, was launched and recovered from a retractable mast at the Beluga's bow. The kite was tethered by a synthetic hawser and controlled from the bridge by computers.

The kite sail can be used in wind speeds of 12 to 74km/h, or 7 to 40 knots. Unlike traditional sailing ships, which had to cope with the more turbulent winds nearer the water, the Beluga's kite traced figure-of-eight arcs across the sky, exploiting stronger, more stable winds to harness extra power.

An aerial control pod, positioned between the hawser and lines attached to the kite, automatically aligned the sail in response to changes in wind direction and velocity.

The Beluga Group, the ship's parent company, wants to use kites to slash fuel bills by 20 per cent. The company says it cut its diesel fuel bill by 10-15 per cent - between US$1000 ($1924) and US$1500 per day - on the two-week passage, during which the kite was launched in trade winds south of the Azores.

"Everything has worked out as we had planned," said Beluga's project manager, Verena Frank. "There's still a lot of testing, adjusting and experimenting taking place."

The sail was used again "for periods of a few minutes and up to eight consecutive hours", before the Beluga's 19,000km maiden voyage concluded in Norway in March.

As of January 2008, according to Lloyd's Register, the world's merchant fleet numbered 50,525 vessels, including general cargo ships, bulk carriers, container ships, tankers and passenger ships. SkySails believes its kite sails could cut CO2 emissions by 146 million tonnes, about 0.5 per cent of the total from all human-generated sources. It hopes to debut 320sq m and 600sq m kites on 1500 ships by 2015.

The use of such high-tech sails would mark the first time that wind has been harnessed by large commercial ships since the demise of windjammers, most of which had disappeared by the 1950s.

Beluga faces potential competition from KiteShip, a California company whose kite sail pulled a 25-tonne America's Cup yacht around Auckland Harbour for 45 minutes at 7 knots in December 2002. But KiteShip, which says its rival has German government aid, is scrambling for funds.

The Climate Action Network, a global alliance of 430 non-governmental organisations, says kite sails might reduce fuel costs by 10-15 per cent or, at slower cruising speeds, by 40 per cent on some routes.

"Anything that can cut a double-digit percentage off a ship's fuel costs is an enormous wedge in the bigger fight [to cut emissions that cause climate change]," says Kert Davies, research director at Greenpeace USA. "That's a big deal. Kite sails might be used in tandem with old sailing ship routes, using the trade winds and currents."

Thus, fuel-conscious ships will search for the most efficient routes in the same way that planes use the jet stream and real-time satellite weather reports.

"We've been looking at alternative propulsion systems for the Greenpeace fleet," says Davies, "like Cousteau's cylindrical sail system, a vertical wind turbine. We've looked at kite sails and traditional sails. The truth is, there are hundreds of solutions, including ship and hull design."

The bottom line for ship owners, says Eric Dawicki, president of the Northeast Maritime Institute, is that "you have to get a ship from A to B in the fastest and safest time available". He favours liquid natural gas, cleaner than bunker diesel, as the "wave of the near future", and says alternatives must match the power from diesel engines.

Whereas kite sails evoke the age of sail, more futuristic propulsion ideas include huge solar wing sails - solar panes mounted on masts - an "air rotor system" that harnesses wind power, wave energy, collected by fins similar to those used by Horie, and combinations of each.

One design, envisaged by the Scandinavian shipping giant Wallenius Wilhelmsen, is the E/S Orcelle. Touted as the first "zero-emissions ship", it would derive energy from the sun, wind, waves and hydrogen fuel cells.

Solar and wind would be harnessed via photovoltaic panels located in three sails - which can be tilted or laid down on the deck - made from lightweight composites, while 12 fins would transmit wave energy.

Conceptual work on the futuristic vessel, intended as a car carrier, began in 2004. The design will exploit any clean-tech leaps, such as fuel cell and battery innovation driven by the automobile industry. It could set sail in 2025. The Orcelle would carry more cargo than today's carriers, says Wilhelmsen, using deck space equal to 14 football pitches.

The dramatic "pentamaran" hull - a main hull and four sponsons for stability and speed - built from aluminium and thermoplastic composites, would eliminate ballast water - a crucial innovation as invasive species flushed from ballast tanks can wreck ecosystems.

"This will be the first truly environmentally friendly ship, protecting the atmosphere and marine species," says the company, which believes sustainable energy is the wave of the future. Intriguingly, Wilhemsen predicts Orcelle might one day be part of an environmentally friendly supply chain - from factory to customer and ultimately disposal - as pressure grows to reduce or banish carbon footprints.

Others are gambling on liquefied natural gas, a "clean" fossil fuel that emits less CO2 than diesel. Finland's marine engine supplier Wrtsil, anticipating EU rules for CO2 cuts, has designed a 310m, 10,000 tonne, LNG-propelled cruise ship, reportedly able to cut CO2 emissions by 20 per cent. Given that there are almost 6000 passenger vessels in service, this is potentially a huge market.

Despite an operational problem - a global scarcity of LNG supply stations - Wrtsil argues that "better environmental performance is always a competitive asset".

Australia's Solar Sailor has patented technology, using retractable and adjustable wings, to harvest solar and wind power. Urban ferries are seen as a potential market. The company has signed a contract with the US Navy to supply its clean-tech systems to unmanned vessels and also has deals with ferry operators in the US, Asia and the UAE.

But most shippers have yet to invest in clean-tech propulsion systems. Even the Beluga used its kite sail as an auxiliary. If the kite had failed, she would still have kept to schedule.

"The low-hanging fruit - improving hull design, redesigning propellers - has been picked," says John Konrad, who has explored clean-tech vessels for maritime website gCaptain, although the Orcelle tests this assumption. "A lot of companies are looking at these new technologies. The problem is that fuel is still relatively inexpensive."

Because fuel costs are passed to customers via cargo rates, Konrad says shippers lack an incentive to change. Prices for low-grade, highly polluting bunker diesel (which spews out sulphur as well as carbon emissions) burned by the world's fleet have yet to reach a "tipping point". This is happening in the more PR-conscious aviation industry, hammered by soaring jet fuel costs, but shipping is only now sailing on to the radar.

To appreciate the challenge facing clean-tech advocates, it is necessary to understand the key role shipping plays in globalisation. Maritime freight - containers, oil, grain, iron ore, coal, and other products - accounts for 90 per cent of the world's trade and counting. Since 1968, says Fearnleys Review, the shippers' bible, seaborne trade has quadrupled.

In 2007 it is estimated that over 7.7 billion tonnes of cargo were transported by sea, worth US$380 billion in freight rates.

As a vital cog in the world economy, it is crucial that vessels, especially container ships, keep to tight schedules. If time is lost at sea, costs quickly mount up.

Take Wal-Mart, the world's largest corporation. Like other companies, Wal-Mart seeks to cut expensive warehousing costs by reducing static inventory.

"They've transferred to a JIT - just-in-time - delivery system," says Konrad. "They predict sales of a product, manufacture items in China, then ship them to the US. But if a vessel is held up, then their US warehouse doesn't have supplies to meet need."

Even so, says Konrad, echoing Bunker Bulletin, a trade magazine, "there's also a big push to adjust speed, to go slower and burn less fuel. A container ship isn't going to adjust speed too much. But a bulker, carrying iron ore or grain, can go a little slower and still make a profit from the voyage."

Shipping companies such as Hapag-Lloyd and Moller-Maersk have trimmed speeds, saved fuel and cut CO2 emissions. Getting the right balance is tricky, as extra operating costs during longer voyages offset fuel savings.

Konrad believes the big problem with new propulsion technologies like solar, waves and wind - beyond funding - is scaleability: they might work on yachts and ferries but, Beluga aside, haven't really been proven on large ships.

Instead, he says reducing frictional drag - where a ship pulls water along with it as the vessel moves, cutting speed - may be a clean-tech winner. The Dutch Maritime Research Institute believes "slippery ships" could enhance efficiency by 20 per cent. This can be done by using polymers (already used to lubricate the inside of oil pipelines), or by creating an air cushion, formed by emitting micro-bubbles or filling cavities with air, around the ship's hull.

A Super Eco Ship envisaged by Japan's National Maritime Research Institute, which has conducted sea trials with modified vessels, would emit tiny bubbles from slots near the bow. The bubbles create a sheet of air along the hull, reducing turbulence caused by drag to enhance speed.

The US and Russia have used computer models and small craft to reduce drag, while the Dutch DK Group envisages a slippery-hulled freighter able to cross the Atlantic in two-and-a-half days, a quarter of the present time.

Alternatively, as the International Maritime Organisation (IMO) moves to ban toxic antifouling paints, hulls might be covered with "sharkskin" coatings - minute scales tipped with spines that mimic a shark's skin - to curb algae and barnacle growth, increase speed and save fuel.

Which still leaves the fleet's CO2 emissions. "Bunker fuel is the filthiest of fuels," says Davies, "the bottom rung of diesel." It has also been linked to illnesses such as cancer, driving a worldwide move to force docked ships to use shore-to-ship electric power and switch to cleaner diesel as they near coastlines. The conservation organisation WWF says shipping emissions have "roughly doubled" since 1990.

And as industry is increasingly forced to calculate, and declare, the amount of carbon emitted during both upstream and downstream operations - the production, distribution, sale and disposal of products - this trend will inevitably affect the shipping world.

But how to make ship owners pay, thus giving them an incentive to switch from bunker fuel to clean-tech?

"It's a very important subject," says Davies. "They were excluded from Kyoto because no one could figure out who would get the bill. If you have a container ship contracted by Wal-Mart to carry goods from China to the US, but it's a Liberian-flagged vessel, operated by a Japanese shipping company, who pays for the voyage's carbon footprint?"

And while Wal-Mart is a US company, do all its worldwide emissions - from ships and factories - count as US emissions? "The basic thing," says Davies, as corporations ponder their total carbon footprint, "is finding an entry point for shipping into a broader accounting scheme."

Climate campaigners who want to include shipping (and aviation) in a new regulatory framework under the Kyoto Protocol are focused on the next UN Climate Change Conference, to be held in Copenhagen late this year.

"The shipping industry is quite receptive to coming into the global climate agreement," says Peter Lockley, transport policy head at WWF UK. He says the industry is "waking up to the fact that aviation has a big PR problem [with CO2 emissions] which they don't want themselves."

The IMO's Marine Environment Protection Committee is looking at a CO2 design index for new ships, an operational CO2 index for existing ships and cleaner practices.

Creating a regulatory framework within which to cut emissions is harder. The problem is reconciling the IMO principle that all ships are equal with the UN belief that developed nations must cut greenhouse gas emissions while developing nations are exempt, even though ships registered to the latter - most are flagged by Panama, Liberia, the Bahamas and the Marshall Islands - create 77 per cent of emissions.

But any approach - billing flag nations, ship owners or end-users - may be gamed. Ships can be reflagged or reregistered. And if end-users are charged and a vessel visits Mexico en route from China, for example, then the US only pays for the last leg. The WWF wants all ships to pay, with fees used to help developing nations cope with climate change.

Lockley warns that developing nations such as China, India, Brazil and South Africa are "digging in their heels" about efforts to widen the burden of CO2 cuts.

Any switch to clean-tech propulsion must also deal with some practical issues. One of the possible drawbacks of, say, deck-mounted solar arrays is that the structures might interfere with cargo handling, a very precise affair.

"It's nonsensical if you look at the practical side," says marine consultant Jean-Pierre De Lutz, when asked how ships will cope with add-ons, like solar sails, that would generate clean energy for propulsion. "On a freighter you'd have to take everything down to load and unload cargo."

Instead, De Lutz roots for methanol fuel cells. He cites an EU project, METHAPU, where fuel cells are providing auxiliary power, but not propulsion, for a car transport ship in sea trials that began in March last year. Unlike hydrogen, methanol doesn't need to be compressed, comes in liquid form and its by-product is water.

De Lutz cautions that everything that "went into METHAPU as proof of principle is obsolete", due to advances in fuel cell technology. The revolution is driven by US, Japanese and South Korean automakers, but there are cross benefits.

Take batteries. At sea, any energy harnessed from clean-tech sources must be kept in batteries. But existing batteries have very limited storage capacity.

"The revolution going on right now is storing electricity," says De Lutz. "This is what is driving the paradigm."

The key to clean-tech may be ultracapacitors, a quantum leap from lithium-ion batteries. EEStor, a secretive Texan company, says its "electrical energy storage unit" will dramatically increase existing battery storage, as well as allowing rapid and unlimited charging.

Despite industry scepticism, the US defence contractor Lockheed-Martin signed an exclusive deal with EEStor last year for military and homeland security applications. If EEStor's prototype holds up, says De Lutz, it will be a "phenomenal breakthrough".

Meanwhile, shipping's tipping point has yet to arrive. Several forces are lining up to push: fuel costs, corporate concerns about carbon footprints, the IMO, environmentalists, and the possibility that a new US administration will take climate change seriously. "The big question is, when do people care?" says Davies. "Like the shift from SUVs to hybrids. No one can guess when that will come. But those who are out ahead will be the winners."