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| ABOUT PROPELLERS |  |
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Copyright © 2003 |
About propellers… James Watt of Scotland is generally credited with applying the first screw propeller to an engine, an early steam engine, beginning the use of a hydrodynamic screw for propulsion. Mechanical ship propulsion began with the steam ship. The first successful ship of this type is a matter of debate; candidate inventors of the 18th century include William Symington, the Marquis de Jouffroy, John Fitch and Robert Fulton, however William Symington's ship the Charlotte Dundas is regarded as the world's "first practical steamboat". Paddlewheels as the main motive source became standard on these early vessels. Robert Fulton had tested, and rejected, the screw propeller. Sketch of hand-cranked vertical and horizontal screws used in Bushnell's Turtle, 1775. The screw (as opposed to paddlewheels) was introduced in the latter half of the 18th century. David Bushnell's invention of the submarine (Turtle) in 1775 used hand-powered screws for vertical and horizontal propulsion. Josef Ressel designed and patented a screw propeller in 1827. Francis Pettit Smith tested a similar one in 1836. In 1839, John Ericsson introduced the screw propeller design onto a ship which then sailed over the Atlantic Ocean in 40 days. Mixed paddle and propeller designs were still being used at this time (vide the 1858 SS Great Eastern). In 1848 the British Admiralty held a tug of war contest between a propeller driven ship, Rattler, and a paddle wheel ship, Alecto. Rattler won, towing Alecto astern at 2.8 knots (5 km/h), but it was not until the early 20th century paddle propelled vessels were entirely superseded. The screw propeller replaced the paddles owing to its greater efficiency, compactness, less complex power transmission system, and reduced susceptibility to damage (especially in battle) Voith-Schneider propeller Initial designs owed much to the ordinary screw from which their name derived - early propellers consisted of only two blades and matched in profile the length of a single screw rotation. This design was common, but inventors endlessly experimented with different profiles and greater numbers of blades. The propeller screw design stabilized by the 1880s. In the early days of steam power for ships, when both paddle wheels and screws were in use, ships were often characterized by their type of propellers, leading to terms like screw steamer or screw sloop. Propellers are referred to as "lift" devices, while paddles are "drag" devices. Skewback propeller An advanced type of propeller used on German Type 212 submarines is called a skewback propeller. As in the scimitar blades used on some aircraft, the blade tips of a skewback propeller are swept back against the direction of rotation. In addition, the blades are tilted rearward along the longitudinal axis, giving the propeller an overall cup-shaped appearance. This design preserves thrust efficiency while reducing cavitation, and thus makes for a quiet, stealthy design. Propeller, mechanical device that produces a force, or thrust, along the axis of rotation when rotated in a fluid (gas or liquid). Propellers may operate in either air or water, although a propeller designed for efficient operation in one of these media would be extremely inefficient in the other. Virtually all ships are equipped with propellers, and until the development of jet propulsion, virtually all aircraft, except gliders, were also propelled in the same way. The blades of a propeller act as rotating wings (the blades of a propeller are in fact wings or airfoils), and produce force through application of both Bernoulli's principle and Newton's third law, generating a difference in pressure between the forward and rear surfaces of the airfoil-shaped blades. Propellers of all types are referred to as screws, though those on aircraft are usually referred to as airscrews or the abbreviation "prop" or "props"(plural). The distance that a propeller or propeller blade will move forwards when the propeller shaft is given one complete rotation, if there is no slippage, is called the geometric pitch; this corresponds to the pitch, or the distance between adjacent threads, of a simple screw. The distance that the propeller actually moves through the air or water in one rotation is called the effective pitch, and the difference between effective and geometric pitch is called slip. In general, an efficient propeller slips little, and the effective pitch, when operating under design conditions, is almost equal to the geometric pitch; the criterion of propeller efficiency is not slip, however, but the ratio of propulsive energy produced to energy consumed in rotating the propeller shaft. Aircraft propellers are often operated at efficiencies approaching 90 per cent, but marine propellers operate at lower efficiencies. A propeller's efficiency is determined by:
A well-designed propeller typically has an efficiency of around 80% when operating in the best regime. Changes to a propeller's efficiency are produced by a number of factors, notably adjustments to the helix angle (θ), the angle between the resultant relative velocity and the blade rotation direction, and to blade pitch (where θ = Φ + α) . Very small pitch and helix angles give a good performance against resistance but provide little thrust, while larger angles have the opposite effect. The best helix angle is when the blade is acting as wing producing much more lift than drag. A ship propeller operates in much the same way as the aeroplane propeller. In the ship propeller, however, each blade is very broad (from leading to trailing edge) and very thin. The blades are usually built of copper alloys to resist corrosion. The speed of sound in water is much higher than the speed in air, and because of the high frictional resistance of water, the top speed never approaches the speed of sound. Although efficiencies as high as 77 per cent have been achieved with experimental propellers, most ship propellers operate at efficiencies of about 56 per cent. Clearance is also less of a problem on ship propellers, although the diameter and position of the propeller are limited by the loss in efficiency if the propeller blades come anywhere near the surface of the water. The principal problem of ship-propeller design and operation is cavitation, the formation of a vacuum along parts of the propeller blade, which leads to excessive slip, loss of efficiency, and pitting of the blades. It also causes excessive underwater noise, a serious disadvantage on submarines. Cavitation can occur if an attempt is made to transmit too much power through the screw. At high rotating speeds or under heavy load (high blade lift coefficient), the pressure on the inlet side of the blade can drop below the vapour pressure of the water, resulting in the formation of a pocket of vapour, which can no longer effectively transfer force to the water (stretching the analogy to a screw, you might say the water thread 'strips'). This effect wastes energy, makes the propeller "noisy" as the vapour bubbles collapse, and most seriously, erodes the screw's surface due to localized shock waves against the blade surface. Cavitation can, however, be used as an advantage in design of very high performance propellers, in form of the supercavitating propeller. A similar, but quite separate issue is ventilation, which occurs when a propeller operating near the surface draws air into the blades, causing a similar loss of power and shaft vibration, but without the related potential blade surface damage caused by cavitation. Both effects can be mitigated by increasing the submerged depth of the propeller: cavitation is reduced because the hydrostatic pressure increases the margin to the vapor pressure, and ventilation because it is further from surface waves and other air pockets that might be drawn into the slipstream. Cavitation damage evident on the propeller of a personal watercraft.
Kort Knozzles The Kort nozzle is a shrouded, ducted propeller assembly for marine propulsion. The hydrodynamic design of the shroud, which is shaped like a foil, offers advantages for certain conditions over bare propellers. Kort nozzles or ducted propellers can be significantly more efficient than unducted propellers at low speeds, producing greater thrust in a smaller package. For the Bollard Pull it may produce as much as 50% greater thrust per unit power than a propeller without a duct. Tugboats are the most common application for Kort nozzles as highly loaded propellers on slow moving vessels benefit the most. The additional shrouding adds drag, however, and Kort nozzles lose their advantage over propellers at about ten knots (18,52 km/h). Kort nozzles may be fixed, with directional control coming from a rudder set in the water flow, or pivoting, where their flow controls the vessel's steering. Luisa Stipa and later Ludwig Kort (1934) demonstrated that an increase in propulsive efficiency could be achieved by surrounding the propeller with a foil shaped shroud in the case of heavily loaded propellers. A "Kort Nozzle" is referred to as an accelerating nozzle. Type M
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