A screw propeller is a device consisting of a set of angled blades revolving around a hub to provide thrust. Propellers are designed for use in different environments such as water or air, but they all use the same principles. As the blades spin, they corkscrew the air or water away from the propeller, producing thrust. Ships, submarines, planes and helicopters make use of this type of propeller, as do fans, windmills and turbines.
Like its namesake, the screw propeller works much like a screw, twisting through a fluid such as water or air in a similar fashion to the screw’s motion through wood. The blades are fixed to the propeller’s hub at an angle, or pitch, and they function much like the thread of the screw. A steep pitch produces more forward motion but is harder to turn. Most blades are twisted rather than flat, meaning that the pitch varies along the blade’s length, which gives the propeller greater efficiency.
Screw propellers work by pushing air or water away, and true to Newton’s third law, this motion creates thrust in the opposite direction, propelling the vehicle forward. Airplane and helicopter propellers have blades that are long and narrow, and ships use propellers with broader, shorter blades. This is primarily because of the difference in density between air and water. Air is much less dense than water, so more air has to be moved to create thrust. In addition, ships, which are naturally buoyant, don’t need to move at speeds high enough to achieve lift, further reducing the demand on water-based propellers.
The screw propeller's earliest roots lie with Archimedes’ screw, an auger spinning inside a cylinder used for irrigation. It is believed to have been created by Archimedes in the third century B.C. Windmills date to at least the ninth century. Leonardo da Vinci sketched plans for a helicopter in the 16th century, making it the earliest design to use the screw propeller to move a vehicle, but of course, he never built the machine. In 1796, inventor John Fitch of the United States created an auger-shaped propeller for a steam boat.
There is some debate regarding the inventor of the modern screw propeller. In the early 19th century, several inventors were each working independently on the screw propeller as a method of propelling ships. These innovators included Francis Pettit Smith in England; Robert Wilson, James Watt and James Steadman of Scotland; Frédéric Sauvage of France; Richard Jordan Gatling, John Stevens and John Ericsson of the U.S.; and Joseph Ressel of Austria. Smith and Ericsson applied for patents in 1836 and generally are credited with the creation of the screw propeller.