Earthblog

Here are several items from around the Net on Cavitation:

Cavitation

A propeller moves by creating lower pressure on the back sides of its blades. As the propeller is drawn toward the lower pressure, the boat is carried with it. Water boils at 212°F at normal (sea level) pressure. At much lower pressures, water can boil at low temperatures, even 50° or 60°F. If a propeller blade creates sufficiently lower pressures, water can boil and create small bubbles of water vapor along part of the blade. As these bubbles collapse, the can release enough energy to burn the blade material. These burns look like small pits on a propeller blade. This process is called cavitation.

When cavitation occurs, the boat’s performance will decline significantly. The propeller will also be damaged. Cavitation can be caused by using a damaged, improperly pitched, or poorly designed propeller.

What is propeller ventilation?
Ventilation occurs when air from the water's surface or exhaust gases from the exhaust outlet are drawn into the propeller blades. The normal water load is reduced and the propeller over-revs, losing much of its thrust. This action most often occurs in turns, particularly when trying to plane in a sharp turn or with an excessively trimmed-out engine or drive unit. Ventilation can also be caused by aerated water from step bottom hulls.

What is propeller cavitation?
As a shape passes through water at an increasing speed, the pressure that holds the water to the sides and back of the shape is lowered. Depending upon the water temperature, when the pressure reaches
a sufficiently low level, boiling (i.e., the formation of water vapor) will begin. The collapsing action, or implosion, of the bubbles releases energy that chips away at the blades, causing a "cavitation burn" or erosion of the metal. 

 A Lesson-Cavitation

Cavitation is probably one of the most misunderstood and used words in boating. Here is a crash course in the basics. When a propeller “breaks free” in a sharp turn it is actually ventilating, not cavitating.  The prop breaks the surface of the water and sucks air in and loosens the grip on the water. The plate above your prop is actually an anti-ventilation plate, not a cavitation plate.
 
Cavitation actually transpires when a fully submerged prop creates a low-pressure area on the backside of the blade. The prop is turning so fast that the “hole” it cuts in the water has not had a chance to fill. Low-pressure in this area allows the water to actually boil at the lower temperature and bubbles are formed that can create a feeling of an unbalanced prop. Those bubbles also eventually implode under the increasing water pressure and can actually “eat” away the paint and metal on the prop. This not only makes it look bad but can weaken the blades. Having a prop with too much cup, nicks or a sharpened leading edge caused by filing the surface in the blades, can increases the possibility of the condition. The next time you see a swath of removed paint form the blades of your propeller, this is likely what you are seeing.

 The initial cause of the low pressure may be nicks in the leading edge, too much cup, sharp leading edge corners, improper polishing, or, sometimes, poor blade design. Massive cavitation by itself is rare, and it usually is caused by a propeller that is severely bent or has had its blade tips broken off resulting in a propeller that is far too small in diameter for the

How does propeller torque produce boat roll?
When observing from behind a boat, the propeller turns clockwise when underway with a normal right-hand propeller. As water resists the clockwise rotating propeller, it causes the boat to roll slightly in the opposite direction (counterclockwise) or down on the left (port) side and up on the right (starboard) side. To offset this slight imbalance, the driver's seat is placed on the starboard (right) side. Boats differ significantly in the degree of their reaction to prop torque.engine.