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Everything you need to know about Because propellers are vital to engine performance, Mercury Marine manufactures over 400 propellers for Mercury, Mariner, and Force outboards and MerCruiser sterndrives. These range from the simplest plastic propellers for electric trolling motors to the blueprinted, custom-tuned Mercury Hi-Performance/Racing propellers. Each propeller is designed to maximize the performance return available from a specific engine or group of engines. Mercury can assure this only through designing, engineering, and manufacturing their own propellers - over half a million a year - at dedicated facilities. Mercury's engineering staff is the industry's most experienced in propeller design and innovation.

Propeller History

History and Development of Propellers The concept of a propulsion device resembling what is now called the screw propeller is certainly not new. The experience of ancients with sculling oars, coupled with the later development of rotary engines, obviously suggested a combination of a series of inclined plates secured to a rotary hub. In 945 BC, the Egyptians used a screw-like device for irrigation purposes. Archimedes (287-212 BC), the first scientist whose work had a lasting effect on the history of naval architecture and ship propulsion, has been credited with the invention of the screw, which he created to pump out flooded ships. His screw pump, also designed for supplying water to irrigation ditches, was the forerunner of the screw propeller. Drawings done by Leonardo da Vinci (1452-1519) contain pictures of water screws for pumping (see figure above). However, his famous helicopter rotor more nearly resembles a marine screw (see figure below). Despite this knowledge, application of screw propulsion to boats and ships didn't take place until the advent of steam power. Due to greater suitability with the slow-turning, early steam engines, the first powered boats used paddle wheels for a form of water propulsion. In 1661, Toogood and Hays adopted the Archimedian screw as a ship propeller, although their boat design appears to have involved a type of water jet propulsion. At the beginning of the 19th century, screw propulsion was considered a strictly second-rate means of moving a ship through the water. However, it was during this century that screw propulsion development got underway. In 1802, Colonel John Stevens built and experimented with a single screw, and later a twin-screw, steam driven boat. Unfortunately, due to a lack of interest, his ideas were not accepted in America. The Invention of the Screw Propeller The credit for the invention of the screw propeller narrows down to two men, Francis Petit Smith and John Ericsson. In 1836, Smith and Ericsson obtained patents for screw propellers, marking the start of modern development. Ericsson's patent covered a contra-rotating bladed wheel, as well as twin-screw and single-screw installations. Ericsson's propeller design took advantage of many of the unique benefits of the bladed wheel. With the wheel, it was possible to obtain the increased thrust of a large number of blades in a small diameter without cluttering up the area adjacent to the hub. Yet, both the inner and outer elements supplied propulsion thrust. The wheel design was inherently strong, without much unnecessary material to interfere with its basic action. The outer ring also served to keep lines, ice, and debris away from the blades. There is no clear-cut evolution of the bladed wheel into the modern screw propeller, although the bladed wheel possessed most of the elements of a successful propulsive device. It seems to have been used in the original Ericsson form and then dropped in favor of the conventional screw. The Fortunate Accident Most of these Archimedian screw inventors suggested little to improve the configuration of the screw for use as a propulsion device. Their main variations consisted of changing the number of convolutions or altering the diameter along the length of the screw. Francis Petit Smith accidentally discovered the advantages of a shortened Archimedian screw. Originally, his wooden propeller design had two complete turns. But, following the collision on the Paddington Canal in which half of his blade was carried away, his boat immediately gained speed. Smith capitalized on his observation by increasing the number of blades and decreasing the blade width -- for a design not unlike modern propellers. In 1839, impressed by the superior performance of Petit Smith's screw, I.K. Brunel changed the design of the Great Britain, an iron ship under construction, to screw propulsion. The Great Britain had 1500 indicated horsepower and achieved a speed of 11 knots. Despite this success, it was many years before screw propellers overwhelmingly displaced paddle wheels for seagoing applications. The Next Step Although the Archimedian screw in a wide variety of forms continued to be proposed for ship propulsion, the final transition of this type of propulsion device to what is now recognized as a screw propeller was made by George Rennie's conoidal screw. Rennie combined the ideas of increased pitch, multiple threads, and minimum convolutions in what he called a Conoidal propeller, which was patented in 1839. Despite the successes of Smith and Ericsson, there were still many problems to be solved in the design, construction, and operation of screw-propelled ships. The early wooden-hulled ships were subjected to heavy vibration, and iron hulls were needed to resist the vibratory forces. With shaft and machinery below the waterline, stuffing boxes had to be developed to prevent leakage without damaging the rotating shaft. Thrust bearings were required to transmit the forward force exerted by the propeller to the hull. Higher speed engines had to be developed in order to realize the inherent efficiency of the screw, and techniques were needed for casting and machining strong, tough metals. As many problems were gradually overcome, and as higher speed engines were developed, more and more screw propellers were installed to supplement or replace paddle wheels. In 1869, C. Sharp, of Philadelphia, Penn., patented a partially submerged propeller for shallow-draft boat propulsion. It employed a large yaw angle to offset the transverse force generated by the propeller, as well as high pitch and cambered or cupped blades. Sir Charles Parsons inadvertently discovered the phenomenon of propeller supercavitation when his first turbine ship, the Turbinia, initially failed to achieve its predicted speed of 30 knots due to the envelopment of the propeller blades in air cavities. This problem was solved by fitting three propellers to each of three shafts. The invention of the marine reduction gear soon rendered multiple propellers per shaft unnecessary. The End of the Paddle Wheel Screw propellers installed in the 1860 era lacked refinement, but their performance exceeded all other devices conceived up to that time. The paddle wheel was gradually rendered obsolete in seagoing ships, as the screw propeller became practically the only type of propulsive device installed in seagoing ships (see figure below). During the twentieth century, the art and science of marine propeller technology has steadily advanced in the direction of greater efficiency, more reliable design and performance prediction, improved materials, and cavitation resistance. Special note: The figures used in this section are adapted from "Propellers for High-Performance Craft" by John L. Allison, Marine Technology, vol. 15, no. 4 (October 1978), with permission of the Society of Navel Architects & Marine Engineers.

Propeller Parts