Two Japanese firms are building a solar power system to augment the diesel engines aboard a cargo ship that carries new Toyotas to the United States, a fuel-saving move that makes solar panels on a Prius look like a drop in the bucket.

Slapping photovoltaic cells on a 60,000-ton boat will cut fuel consumption by 6.5 percent, which seems paltry until you realize the average cargo ship burns 120 gallons per mile. The system Nippon Yusen KK is paying Nippon Oil Corp. $1.4 million to develop will save about 46,800 gallons of fuel carrying all those cars 6,000 miles from Japan to Califonia.

To put that in perspective, if you bought a Toyota that arrived on a solar ship with 4,999 other cars, it would have saved nine gallons of fuel before you ever even saw it.

But solar ships do more than save fuel. They also clear the air. Cargo ships burn "bunker fuel," the truly nasty stuff that literally comes from the bottom of the barrel. It's kind of like the Milwaukee's Best of fossil fuels.

A study published in Environmental Science and Technology found emissions from the bunker fuel cargo ships burn may cause 60,000 deaths worldwide. Subsequent research found ship exhaust contributed as much as 44 percent of the sulphate found in fine particulate matter in the atmosphere of coastal California. Anything that cuts the amount of bunker fuel ships burn is nothing to sneeze at.

Nippon Yusen, Japan's largest shipping line, says the solar assist will cut CO2 emissions by 1 to 2 percent per year, or about 20 tons, according to Reuters. Although solar panels have been used at sea to generate power for crews quarters, this is the first time anyone's tried to use them to help propel cargo ships. Beyond seagull-proofing the system, the engineers must figure out how to make it tough enough to withstand salt and vibration. Ikutoshi Matsumura, executive vice president of Nippon Oil, says the first solar ship will be ready to sail by December with "full commercialization of the system in three to five years."

Nippon Yusen could be ahead of the curve, as there is a worldwide push to ban bunker fuel (aka Chanel No. 6) in coastal waters. California, naturally, is among the first, banning its use within 24 miles of shore beginning in July. Similar rules will be in effect worldwide by

2015. Such bans could make shipping cargo more expensive because bunker fuel, as disgusting as it is, is cheap and does the job. Kind of like Milwaukee's Best.

The solar boat isn't the first attempt at cleaning up cargo ships. A much earlier system developed by the Sumerians used wind. More recently, Germany's SkySails has developed a modern interpretation of the sailboat that uses sails the size of football fields to cut fuel use by up to 35 percent. A 2006 design study by Scandanavian shipbuilder Wallenius found ships could use wind and solar power along with fuel cells to create zero-emission shipping.

New technology is another way shipping companies can reduce the cost of transporting cargo in the face of rising fuel prices and stricter emissions standards, all while shortening the "long tailpipe"

and giving car buyers something to feel good about. Still, just like the old "double-nickel" speed limit, the less advanced ships can save some fuel by simply slowing down: Dropping speeds by two knots can increase fuel efficiency by up to 5 percent.

*Photo: tinou bao/Flickr *