Propulsors include propellers and waterjets. Propellers are external marine propulsors that work outside a ship's hull. Waterjets are internal marine propulsors. There are several screw-type propellers, which are distinguished by their abilities to accommodate the effects of cavitation.
Self noise generated by the ship's propellers is known as propeller noise. Propeller noise is mainly caused by cavitation, which occurs when bubbles form on the low pressure side of the propeller blade and grow to full size very quickly (in about 2 microseconds), then collapse. Cavitation is the formation and collapse of vapor-filled bubbles, or cavities, that cause noise, vibration, and often rapid erosion of the propeller material, especially in fast, high-powered vessels. The collapse of these bubbles gives a continuous spectrum of noise, which dominates the higher frequency range of ship's noise and is speed related. That is, cavitation is greater at higher speeds, because the propellers produce more bubbles.
Cavitation is a much more significant factor in surface ships than in submarines. This is because submarine cavitation, which is relatively slight in shallow water, can be almost completely eliminated in deeper water. The deeper the submarine is, the greater the hydrostatic pressure, thus the less cavitation. In addition, improved design in submarine screws (propellers) significantly reduces cavitation.
As long as the rotational and translational speeds of the propeller are not too high, the onset of cavitation, which is underwater noise, can be delayed or limited to an acceptable amount by clever design of blade sections. With the availability of high-speed computers, improved design procedures, and better mathematical models of propeller hydrodynamics, cavitation onset speed is increasing, and one unclassified estimate in 2000 predicated that, within a very few years, cavitation onset speed will improve from 15 knots in the recent past to 25 knots.
Around 1953, William Morgan designed the first supercavitating propeller, perhaps the first one of its kind designed in the US. This propeller worked, using the empirical theory. The propeller worked only because Murphy's law is not absolute. When he checked all of his calculations, Morgan found an error which led to the creation of supercavitating propellers with a correction factor.
Another case involves another supercavitating propeller with very thin leading edges. This particular propeller was designed for the Canadian R -100 hydrofoil craft. After designing the propeller, the Carderock Division built it and sent it up to Canada for testing. The problem was the leading edge. The Canadians tested it. Later they reported that they bent the propeller. There was another propeller designed with double thickness on the leading edges to replace the one that was broken.
Highly skewed propeller development was made possible by lifting surface theory computer programs including thickness effects (Kerwin and Leopold at MIT). Cumming, Morgan and Boswell (1972) significantly reduced propeller-induced vibration and noise resulting in an improvement in cavitation, with no effect on efficiency. Unfortunately, the US Navy was not interested in using these highly skewed propellers. In spite of the many improvements, the Navy wasn't interested because the propeller hadn't yet been on a ship. At that time the Maritime Administration had some research money, which provided the necessary funding, and a highly skewed propeller was designed for a ship being built by National Steel - the first use of that technology on a ship. The British Navy became interested, and it was installed on a British Naval ship before it was installed on a US Navy ship.
While it has been established that open propellers designed with substantial skew-back in the blade shape may exhibit improved cavitation performance relative to more traditional blade forms, in the design of skewed-back propeller blades, it is necessary substantially to reduce the pitch at the outer radii in order to avoid overloading and premature cavitation which would occur if the pitch distribution of an unskewed blade were maintained.
The 1970s saw the development by Mitsui Engineering and Shipbuilding Co., Ltd. of the novel and extremely reliable Mitsui Integrated Duct Propeller (MIDP) for fuel saving in large ships, which effectively eliminated the constant erosion caused by cavitation from the ship's propeller on the conventional propeller duct by moving the duct forward and designing a non-symmetrical duct adapted to the stern flow. This increased propeller efficiency and virtually eliminated the shipyard downtime necessary to correct the erosion problem. It led to substantial fuel savings in over 200 ships, including all of Exxon's Very Large Crude Carriers. Many new ships were built with this design, and older ships were retrofitted.
The screw propellers can also be distinguished by the number of blade rows they contain. Those with one blade row are designated single component subcavitating propellers. Multiple component propellers include two or more rows of blades and often other components as well; they are designed to provide efficient propulsion when a large amount of thrust is required and when the advance ratio (ratio of a vessel's forward speed to the tip speed of the propeller) is large (e.g., when a high-speed vehicle is outfitted with a small-diameter propeller. Cavitation tends to degrade the performance of conventional fully wetted (submerged) propellers at speeds above 30 to 35 knots. To obtain higher speeds, propellers designed for partially or fully cavitating flow must be used. Transcavitating propellers were developed for speeds lower than those for full cavitation (i.e., about 30 to 35 knots). Supercavitating and superventilated propellers enable vehicles to be propelled efficiently where full cavitation exists.
Supercavitating and superventilated propellers are designed to have fully developed blade cavities which spring from the leading edge of the blade, cover the entire back of the blade, and collapse well downstream of the blade trailing edge. The blade of such propellers has unique sections which usually are wedge-shaped with a sharp leading edge, blunt trailing edge, and concave face. Supercavitating and superventilating propellers are distinguished by the nature of the gases in the cavity. Supercavitating propellers have cavities filled with water vapor and small amounts of gases dissolved in the fluid media. Superventilated propellers have cavities filled primarily with air or gases other than water vapor; they may be fully submerged propellers with a gas supply system through the propeller shaft and propeller blades into the cavity, or partially submerged propellers which draw air from the water surface as the blade enters the water. Both of these propeller types are intended for use in high-speed craft (speed greater than 45 knots or 85 km/h) such as hydrofoil boats, surface-effect ships, and the higher speed planing craft with high propeller-shaft rotational speeds.
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