Many people know cavitation as the dreaded enemy of submarines and other vessels. The small water bubbles which often form on propellors are what is known as initial cavitation. If these bubbles expand to cover a part of an object this becomes partial cavitation. When this bubble covers the entire object moving through the water, then it is supercavitation which occurs.
The reason it is of great benefit for the object to be encased in air is that air has roughly 1/1000th of the density of water, giving tremendous opportunities for high speed travel underwater, since less drag equals more speed!
Supercavitation can occur in two ways.
- Vaporized Supercavitation
Supercavitation naturally occurs to a correctly shaped object traveling at over 100 knots (115 mph, 185 km/h). Water at the tip of the projectile is vaporized and forms a supercavitational bubble along the entire length of the projectile.
- Artificial Supercavitation
Artificial supercavitation occurs when vapor is passed through the nose of the supercavitating projectile (for example from the exhaust).
In either case traditional propulsion methods become obsolete, as a supercavitating vessel travels in air and not water, which makes propellors rather useless! This has led to the situation where supercavitating torpedoes are now rocket powered!
More on the science of supercaviation coming soon