Big changes in maritime propulsion technology haven't come along all too often. First there was muscle power -- humans rowing oars. Then people learned to capture the power of the wind, and the age of the sail lasted for centuries. In the 1800s, sails were supplanted by a new technology, the steam engine. Steam, in turn, gave way to petroleum-powered internal combustion. Marine diesel engines however, while still prevalent, weren't the apex of ship propulsion technology in the 20th century. That honor went to nuclear-powered ships; a modern nuclear aircraft carrier can sail for 20 years before needing to refuel [source: NAVAIR].
Unfortunately, it would be impractical and prohibitively expensive to fit a nuclear power plant to most naval and civilian vessels. Waste disposal issues and the fact that nukes are massively complex mean you won't be attaching an atomic-powered Evinrude to your pleasure boat anytime soon.
So it's perhaps a bit ironic that the next leap in maritime propulsion might come from something as simple as algae. The U.S. Navy, for one, is betting big on the tiny organisms.
In July 2012, the Navy demonstrated its Great Green Fleet carrier strike group composed of the nuclear-powered aircraft carrier USS Nimitz and four support ships. With the exception of the Nimitz, all of the ships in the strike group and all of the attachment's jet planes, support planes and helicopters ran on a blend of renewable (algae and vegetable oil) and petroleum-based fuels.
The effort wasn't cheap to taxpayers. The cost of the 450,000 gallons (1,703,435 liters) of fuel paid by the Navy was about $27 a gallon in 2012. That's close to 9 times more than the cost of normal, petroleum-based fuel.
Those eye-watering early costs are beside the point, according to Navy brass. The key point is developing an alternative fuel source -- any source -- that the military can use when its enemies possess the ability to choke off supplies of oil.
Secretary of the Navy Ray Mabus said, "Simply put, we as a military rely too much on fossil fuels. That dependence creates strategic, operational and tactical vulnerabilities for our forces and makes them susceptible to price and supply shocks caused by either man-made or natural disasters in the volatile areas of the world where most fossil fuels are produced" [source: U.S. Navy].