Is it possible and practical to consider using existing automotive hybrid technology as a means of propulsion for a boat.

Asked by Cornell Dawson, MS ’68, Hyde Park, New York.

Humanity has thousands of years of experience with sustainable sailing. Before the invention of the steamboat in the late 18th century, all water transport was powered by muscle or wind. Even today, despite running almost entirely on fossil fuels, maritime transportation has relatively low energy intensity. Shipping a Toyota Prius from Japan to San Francisco would use the energy equivalent of 40 gallons of gasoline. In comparison, the Prius would use 110 gallons of gasoline while driving the same distance on its own four wheels.* Nevertheless, many efficiency improvements and fuel shifts are possible.

High-Performance Hydrodynamics

Just as rolling resistance and air resistance are the main sources of power demand for ground transportation, water resistance drives fuel consumption on the sea. Although ship designers, like airplane designers, have been more mindful of dynamics in their designs than car manufacturers, there are still many unrealized efficiencies. In the past, designers have turned to nature for inspiration, increasing hydrodynamic efficiency by replicating the streamlining effect of a dolphin's nose on the bow of modern vessels, reducing fuel consumption by 25 percent.

Almost 10 percent of the energy the U.S. Navy uses to power its ships goes to offsetting hydrodynamic drag caused by marine growth on the hulls of their vessels. An experimental coating for ship hulls based on the roughness of a shark's skin could deter algae, barnacles and other sea creatures from latching onto the surface of ships—decreasing drag and saving time, energy and money. It would also eliminate the need for toxic paints and chemical cleaners used to fight marine growth on hulls today.

In addition, future ships could travel the seas at greater speeds while using less energy by floating on a thin layer of air. Because the resistance offered by air is about one percent that presented by water, trading hull-water contact for hull-air contact could also significantly reduce drag on ships. In ideal test conditions, frictional drag can be reduced up to 80 percent by continuously generating microbubbles of air along the underwater surface. So far, researchers have been able to create only a three percent reduction in drag on a real ship, but in theory, shielding a ship's hull in air offers the most dramatic drag reductions imaginable. Other proposals for lubricating ships with air include trapping a film of air between the water and a hydrophobic hull coating, and creating stable cavities of air along the hull. This approach has been extensively tested by the Russians since the 1980s, and at least 50 commercial vessels have been delivered that reduce drag up to 40 percent using the technology. In some cases, air cavity technology could even be retrofitted onto existing ships.

Petroleum-Free Propulsion

While high-performance hydrodynamics can make shipping more efficient, a shift in fuel will be required before we achieve true sustainability. In the Essential Answer we explored the hybrid options, but let's look at some other possibilities.

At least one company is going all in on wind-powered ships, heading back to the future with complete wind power and launching a modern slow freight movement. The Compagnie de Transport Maritime à la Voile (CTMV) is reviving the sail with a fleet of wind-powered merchant vessels, transporting Fair Wind Wine from France to Ireland, and soon to Canada. The Irish journey takes twice as long by sail, varying between four and eight days, and is slightly more expensive, but wine sellers like the "carried by sailing ship" label, and consumers gain insight into the effort involved in bringing products to market.

The accomplishments of eco-sailing legend Kenichi Horie demonstrate some of the other propulsion possibilities. In 1962, Horie gained fame as the first person to sail alone across the Pacific Ocean. In 1993, he set the record for longest distance traveled in a pedal-powered boat by pedaling more than 4,000 miles from Hawaii to Japan. In 1996, he set the record for the fastest crossing of the Pacific in a solar-powered boat. In 1999, he sailed across the Pacific on a craft made from reused beer barrels. In 2008, he became the first person to sail a boat propelled solely by wave power. While human power, solar power and wave power may seem laughable as serious sources of propulsion, each could provide new tools for our future toolbox.

On paper, Wallenius Wilhelmsen Logistics has designed a next-generation zero-emission ship—the solar-, wind- and wave-powered Orcelle. A hydrodynamic pentamaran made from lightweight materials, the Orcelle is designed to capture wind and solar energy with three rigid solar sails, and wave energy with fins on the hull, generating hydrogen to power the ship's fuel cells. Additional hydrogen could be generated by storing onshore renewables onboard. While some of the Orcelle's innovations could be put to use immediately, it will probably be years before a shipping company commits to building a modern zero-emissions vessel sails the seas.

For now, diesel-wind hybrids are the most promising technology for increasing efficiency, wind-powered slow-freight vessels with solar-powered auxiliaries are the innovative present of sustainable shipping, and advanced biofuel-wind hybrids or hydrogen or battery-electric boats might sail us to a sustainable future.

*Waterborne commerce energy intensity (2006): 571 BTU/ton-mile. Prius weight: 1,400 kg (1.5 tons). Japan to California distance: 5,000 miles. Shipping a Prius requires 1.5 tons * 5,000 miles * 571 BTU/ton-mile = 4,282,500 BTU. Gallon of gasoline contains 115,400 BTU. 4,282,500 BTU = 37 gallons. Prius efficiency = 46 mpg. 5000 miles/46 mpg = 109 gallons.

Nick Enge plans to receive his bachelor's degree in atmosphere/energy engineering and master's degree in earth systems in 2011.