Cavitation in fluid mechanics and engineering normally refers to the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate shock waves that may damage machinery. These shock waves are strong when they are very close to the imploded bubble, but rapidly weaken as they propagate away from the implosion. Cavitation is a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a metal surface cause cyclic stress through repeated implosion. This results in surface fatigue of the metal, causing a type of wear also called "cavitation". The most common examples of this kind of wear are to pump impellers, and bends where a sudden change in the direction of liquid occurs. Cavitation is usually divided into two classes of behavior: inertial cavitation and non-inertial cavitation.
A go-fast boat is a small, fast powerboat designed with a long narrow platform and a planing hull. Depending on definitions used, it is either a speedboat (synonymously) or a certain type of speedboat.
A hydrofoil is a lifting surface, or foil, that operates in water. They are similar in appearance and purpose to aerofoils used by aeroplanes. Boats that use hydrofoil technology are also simply termed hydrofoils. As a hydrofoil craft gains speed, the hydrofoils lift the boat's hull out of the water, decreasing drag and allowing greater speeds.
A kayak is a small, narrow human-powered watercraft typically propelled by means of a long, double-bladed paddle. The word kayak originates from the Inuktitut word qajaq. In British English, the kayak is also considered to be a kind of canoe.
A propeller is a device with a rotating hub and radiating blades that are set at a pitch to form a helical spiral which, when rotated, exerts linear thrust upon a working fluid such as water or air. Propellers are used to pump fluid through a pipe or duct, or to create thrust to propel a boat through water or an aircraft through air. The blades are shaped so that their rotational motion through the fluid causes a pressure difference between the two surfaces of the blade by Bernoulli's principle which exerts force on the fluid. Most marine propellers are screw propellers with helical blades rotating on a propeller shaft with an approximately horizontal axis.
An outboard motor is a propulsion system for boats, consisting of a self-contained unit that includes engine, gearbox and propeller or jet drive, designed to be affixed to the outside of the transom. They are the most common motorised method of propelling small watercraft. As well as providing propulsion, outboards provide steering control, as they are designed to pivot over their mountings and thus control the direction of thrust. The skeg also acts as a rudder when the engine is not running. Unlike inboard motors, outboard motors can be easily removed for storage or repairs.
Supercavitation is the use of a cavitation bubble to reduce skin friction drag on a submerged object and enable high speeds. Applications include torpedoes and propellers, but in theory, the technique could be extended to an entire underwater vessel.
A jetboat is a boat propelled by a jet of water ejected from the back of the craft. Unlike a powerboat or motorboat that uses an external propeller in the water below or behind the boat, a jetboat draws the water from under the boat through an intake and into a pump-jet inside the boat, before expelling it through a nozzle at the stern.
A watercraft or waterborne vessel is any vehicle designed for travel across or through water bodies, such as a boat, ship, hovercraft, submersible or submarine.
Whitewater kayaking is an outdoor adventure sport where paddlers navigate a river in a specially designed kayak. Whitewater kayaking includes several styles: river running, creeking, slalom, playboating, and squirt boating. Each style offers a different way to experience the thrill and beauty of whitewater environments. Various techniques help paddlers navigate rivers, such as paddle strokes, rolling, and boofing. Whitewater kayaking requires a lot of essential equipment to make a “kayaking kit,” such as a whitewater-specific kayak, spray skirt, paddle, helmet, and PFD.
A motorboat or powerboat is a boat that is exclusively powered by an engine; faster examples may be called "speedboats".
The Kaplan turbine is a propeller-type water turbine which has adjustable blades. It was developed in 1913 by Austrian professor Viktor Kaplan, who combined automatically adjusted propeller blades with automatically adjusted wicket gates to achieve efficiency over a wide range of flow and water level.
A wedge is a triangular shaped tool, a portable inclined plane, and one of the six simple machines. It can be used to separate two objects or portions of an object, lift up an object, or hold an object in place. It functions by converting a force applied to its blunt end into forces perpendicular (normal) to its inclined surfaces. The mechanical advantage of a wedge is given by the ratio of the length of its slope to its width. Although a short wedge with a wide angle may do a job faster, it requires more force than a long wedge with a narrow angle.
In rowing, oars are used to propel the boat. Oars differ from paddles in that they use a fixed or sliding fulcrum, an oarlock or rowlock attached to the side of the boat, to transfer power from the handle to the blade, rather than using the athlete's shoulders or hands as the pivot-point as in canoeing and kayaking.
The Type 22 missile boat is a ship class in the Chinese People's Liberation Army Navy. The first boat was launched in April 2004 by the Hudong-Zhonghua Shipyard at Shanghai. The boats incorporate stealth features and are based on Australian-designed wave-piercing catamaran hulls that are more stable than other fast missile craft in high sea conditions. 82 of these missile boats are currently in service with three flotillas having been produced over a span of seven years, operating in squadrons of eight vessels each.
The Hickman Sea Sled is an inverted vee planing hull invented by Albert Hickman. The Sea Sled is a direct forerunner of the modern high speed catamaran or tunnel hull. The reduced friction is due to a "trapped" gas film between the hull surface and water. A similar effect is seen in super-cavitating torpedoes where a gas generator creates a film separation to water friction.
"A new type of vessel, which promises to revolutionize water craft and which takes the same place on the water that the automobile does on land" - Scientific American 26 September 1914
Wind-powered vehicles derive their power from sails, kites or rotors and ride on wheels—which may be linked to a wind-powered rotor—or runners. Whether powered by sail, kite or rotor, these vehicles share a common trait: As the vehicle increases in speed, the advancing airfoil encounters an increasing apparent wind at an angle of attack that is increasingly smaller. At the same time, such vehicles are subject to relatively low forward resistance, compared with traditional sailing craft. As a result, such vehicles are often capable of speeds exceeding that of the wind.
A thrust block, also known as a thrust box, is a specialised form of thrust bearing used in ships, to resist the thrust of the propeller shaft and transmit it to the hull.
A ducted propeller, also known as a Kort nozzle, is a marine propeller fitted with a non-rotating nozzle. It is used to improve the efficiency of the propeller and is especially used on heavily loaded propellers or propellers with limited diameter. It was developed first by Luigi Stipa (1931) and later by Ludwig Kort (1934). The Kort nozzle is a shrouded propeller assembly for marine propulsion. The cross-section of the shroud has the form of a foil, and the shroud can offer hydrodynamic advantages over bare propellers, under certain conditions.
The Superkavitierender Unterwasserlaufkörper was a German close-range supercavitating torpedo technology demonstrator designed by the Diehl BGT Defence and developed in cooperation with the German Navy. The supercavitating torpedo for a "close-range defense of underwater targets" was presented to the public in 2005 as a prototype, but it never went into development and procurement.
