pro•pul•sion: the force that moves something forward
Ships transport people and cargo from one place to another.
In many cases, it is easier and cheaper to move them in ships than by any other form of transportation. Getting a cargo of cars, for example, from Japan to New York is easiest to do by ship—a shipment of thousands of cars is too heavy to fly them in an airplane, and you can’t drive a truck or train across the ocean. A big car carrier called a “RO/RO”—short for “Roll On/Roll Off”— can take more than 5,000 automobiles in one trip.
Ships all move through the water by some form of propulsion. Today, the biggest ships are usually powered by gas turbine or diesel-electric engines, which burn hundreds of tons of fuel per day to travel across the open ocean.
The first engine-powered ships used steam engines, which either turned large paddlewheels or a shaft and propeller (oftentimes called a “screw”). Before the invention of the steam engine, however, sailors relied on wind to move large vessels. Once mariners figured out how to harness the wind to move their ships, they built larger and larger ships with more and more sails. And, before the invention of the sailing rig, ships called galleys were rowed, with hundreds of men working in sync to row multiple decks of huge oars.
Today, people are experimenting with new technology, and with old. There is a solar-powered ship going around the world right now, and in Vermont and New York, a new company called the Vermont Sail Freight Project has started a business carrying cargo under sail…just as people did for hundreds of years before the marine engine was invented. Both the sun and the wind are in plentiful supply and are not likely to pollute the environment the way fossil fuels do.
Did You Know?
Today, shipyards have a number of ways to get a ship out of water, either by hauling it out or by floating it into a basin and the water pumped out.
Historically though, sea captains would careen their vessels in shallow water by either heaving it over on its side while it was still afloat or by anchoring in shallow water at high tide and then waiting for the tide to go out. The vessel would touch bottom, and, as the tide went out, lay over on its side.
How does one go about getting a ship, especially a big ship, high and dry out of the water today?