Screw Propeller
When steam power became practical in the early 1800s, shipbuilders turned to three methods in an attempt to harness this improved power source: paddle wheel s, jet propulsion, and the screw propeller. Jet propulsion was attempted but with little practical success. Paddles were often used at the sides or backs of ships navigating on western rivers because they could take a great deal of punishment and could be easily repaired since half of the paddle wheel sticks out above the waterline. But early steam-powered, oceangoing ships equipped with paddle wheels experienced difficulties in rough seas. The ships would roll, and the wheel would often come completely out of the water, wasting power and making the vessel difficult to handle.
There was a third alternative for transforming power into thrust: the screw. As early as 1785 the Englishman Joseph Bramah patented a sixteen-blade propeller to drive boats. But the first to actually experiment with the concept was American engineer John Stevens (1749-1838), who built a small steamboat in 1804 powered by a high-pressure engine and propelled up to four miles per hour (6.4 kph) by a pair of four-bladed screw propellers. Unfortunately, America did not have the tools or craftsmen to machine the parts the engine required. Progress was made in the 1830s. An English inventor, Francis Pettit Smith (1808-1874), and John Ericsson, who later built the Union's ironclad Monitor, constructed screw-propelled craft that aroused a great deal of interest in England. Smith's propellers drove a large steamship, and Ericsson's device was applied to a tugboat. These vessels impressed Isambard K. Brunel, who was building the Great Britain, the first all-iron oceanliner. He scrapped his plans for paddle wheels and substituted a six-bladed propeller 15.5 feet (4.7 m) in diameter. In 1843 the Great Britain sailed from Liverpool to New York in 14 days.
Two years later, Brunel convinced the British Admiralty to conduct tests to determine the best means of propulsion. One test involved two ships of equal horsepower and weight being attached by their sterns, the only difference being that one used paddle wheels and the other used a propeller designed by Francis Smith. After the tug-of-war commenced, the paddle wheels churned the water, but the propeller-driven vessel ended up towing the other behind it at 2.5 knots (4.6 kph).
This and the other tests helped convince people of the value of propellers, but it was the experience of the British Navy in the Crimean War (1854-1856) that persuaded naval authorities to use only propellers on warships. Russian gunners had an easy time disabling British ships with exposed pa wheels. Soon after the war, screw propulsion was used by nearly all the world's navies and merchant vessels. Paddle wheels were used only for ships sailing coastal or inland waters.
Ever since, there have been modifications in the screw to give it better performance. During the 1970s, for example, the American Navy developed super-cavity propellers, which because of their shape and action in the water, achieved greater speed.
By the mid-1990s, propellers for large commercial vessels as well as for pleasure runabouts were no longer made of once-popular cast iron. Starting in the 1960s, the use of high-tensile brass accounted for the majority of propellers, but by the 1980s, some combination of nickel-aluminum-bronze accounted for the bulk of all propellers. By the 1990s, heavy cruisers, yachts, and sport-fishing boats were usually outfitted with either manganese bronze propellers that were malleable, easy to repair, and cost-efficient, or a new material called Nibral (Nickel, Bronze, and Aluminum). Although more expensive, Nibral has proven to be an especially good combination for high-power use. It was also in the 1990s that the three-blade propeller finally became the one most commonly used.
A propeller is a deceptively simple device that is in fact, a highly complex piece of equipment when it is in action. In addition to a propeller's seven characteristics (diameter, pitch, bore, number of blades, rotation, style, and material), its performance can be influenced by such variables as hull shape and even water temperature. Propellers of the 21st century will be computer-machined to exact specifications, and their manufacture will be digitally controlled, improving their precision and specialization of design and use.
This is the complete article, containing 693 words
(approx. 2 pages at 300 words per page).
