Supercavitating propeller
The supercavitating propeller is a variant of a propeller for propulsion in water, where supercavitation is actively employed to gain increased speed by reducing friction. They are being used for military purposes and for high performance racing boats as well as model racing boats.
This article distinguishes a supercavitating propeller from a subcavitating propeller running under supercavitating conditions. In general, subcavitating propellers become less efficient when they are running under supercavitating conditions.
The supercavitating propeller operates submerged with the entire diameter of the blade below the water line. Its blades are wedge-shaped to force cavitation at the leading edge and to avoid water skin friction along the whole forward face. As the cavity collapses well behind the blade, the supercavitating propeller avoids the spalling damage due to cavitation that is a problem with conventional propellers.
An alternative to the supercavitating propeller is the surface piercing, or ventilated propeller. These propellers are designed to intentionally cleave the water and entrain atmospheric air to fill the void, which means that the resulting gas layer on the forward face of the propeller blade consists of air instead of water vapour. Less energy is thus used, and the surface-piercing propeller generally enjoys lower drag than the supercavitating principle. The surface-piercing propeller also has wedge-shaped blades, and propellers may be designed that can operate in both supercavitating and surface-piercing mode.
Supercavitating propellers were developed to usefulness for very fast military vessels by Vosper & Company
The pioneer of this technology and other high speed offshore boating technologies was Albert Hickman (1877–1957), early in the 20th century. His Sea Sled designs used a surface piercing propeller.
Correction: propellers used in racing are never supercavitating. A cavity of any kind covering the low pressure side of a blade reduces the lift/thrust coefficient by 75%! Surface-piercing propellers are not in any supercavitating mode. They simply have a tip vortex that ends on the water surface, air is sucked into the cavity but does not extend past the trailing edge of the blade. Hence, propeller efficiency is normal and high (60% or more). A fully or partially supercavitating propeller will not even get a race boat off the start dock.
See also
References
- Damned by Faint Praise, article in Wooden Boat about Albert Hickman
- Albert Hickman biography