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Cavitation |
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Cavitation & VentilationCavitation and Ventilation are not interchangeable terms; they refer to two distinct problems encountered during propeller operation. To understand cavitation, you must first understand the relationship between pressure and the boiling point of water. At sea level, water will boil at 212 deg F. As pressure increases, such as within an engines’ closed cooling system, the boiling point of water increases – it will boil at some temperature higher than 212 F. The opposite is true. As pressure decreases, water will boil at temperatures lower than 212 F. If pressure drops low enough, water will boil at typical ambient temperatures of 50-60 F. We have said that, during normal propeller operation, low pressure exists on the blade back. Normally, the pressure does not drop low enough for boiling to occur. However, poor blade design or selection, or blade damage can cause an unusual pressure drop on a small area of the blade. Boiling can occur in this small area. As the water boils, air bubbles form. As the boiling water passes to a higher pressure area of the blade, the boiling stops and the bubbles collapse. The collapsing bubbles release enough energy to erode the surface of the blade. This entire process of pressure drop, boiling, and bubble collapse is called "cavitation". The damage caused by the collapsing bubbles is called a "cavitation burn". It is important to remember that cavitation is caused by a decrease in pressure, not an increase in temperature. Ventilation is not as complex a process as cavitation. Ventilation refers to air entering the blade area, either from above the surface of the water or from a through-hub exhaust system. As the blades meet the air, the propeller momentarily over-revs, losing most of its thrust. An added complication is that the propeller over-revs, pressure on the blade back decreases and massive cavitation can occur. Most pieces of marine equipment have a plate above the propeller area designed to keep surface air from entering the blade area. This plate is correctly called the "anti-ventilation plate", although you will often see it referred to as the "anti-cavitation plate". Through hub exhaust systems also have specially designed hubs to keep exhaust gases from entering the blade area. ----------------------------------------------------------------- Note from author: Apologies for the references to Fahrenheit – I’m quoting from an American text, and am too lazy to do the conversion. As a matter of interest, I participated in
some trials in the Gulf of Mexico that were designed to establish the usefulness
of cavitation as an underwater cleaning tool on offshore oil platforms. The
cavitation bubbles were deliberately induced through a special jet fed by an
1100 psi water pump. The jet was designed to create the necessary low pressure
at the outlet, which in turn produced a stream of collapsing bubbles about 40-60
mm from the jet face. By placing the jet at about that distance from the work
area, we were able to remove around 70-80 mm of hard, conglomerated, calcareous
marine growth from the platform structure. The amount of energy released at the
point of implosion of the cavitation stream was quite spectacular, and the
divers had to wear special hearing protection under their helmets due to the
enormous acoustic energy released as the bubbles collapsed. Regards NOTE; Information supplied by Jim Linden from the technical section of a Clymer Mercury Outboard Shop Manual, published by the Intertec Publishing Corporation, Kansas, USA. |
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