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This article is from ABOUT.com

Propellers - The Basics: Part 2: Ventilation, Cavitation, and more...

Propeller Ventilation

Propeller ventilation is caused by air from the surface or exhaust gases being drawn into the rotating propeller blades. This results in the propeller slipping more than usual due to the reduced water load on the blades. The obvious symptoms of this are a sudden increase in engine RPM as well as a possible loss of speed. This commonly occurs when trying to turn the boat at high planing speeds or if the outboard or sterndrive is trimmed out too high.

In racing conditions this can also occur when following another boat too closely. The small bubbles in the water created by the leading boat can cause ventilation of the propeller of the following boat, with a subsequent loss of speed. This is why you rarely see high speed race boats following directly in the line of the leading boats, even if they are far enough back to be out of the spray and wake.

Propeller Cavitation

Propeller cavitation is less obvious than ventilation, but can be far more damaging. When the propeller blade passes through the water at an increasing speed, the pressure that holds the water to the sides of the blades is lowered. If the water is sufficiently warm, and depending on the speed of the boat, formation of water vapour (boiling) can occur. These bubbles that are produced then immediately collapse, releasing energy that can cause a cavitation burn on the propeller blades. This is one of the great advantages of stainless steel propellers. Due to their superior strength they can withstand cavitation damage better than aluminium and can also be produced with thinner blades to reduce the occurrence of cavitation.

Surface Piercing Propellers

Traditionally, most propellers have been designed to operate fully submerged. Indeed, most of these propellers suffer from severe ventilation if operated too close to the surface. Most race boats and many lighter high performance boats, however, use propellers which are designed for operation at the surface. Many of these propellers offer the best top speed performance when the engine height is adjusted so that the propeller shaft runs at or near the surface of the water at top speed, with only the lower half of the propeller in the water. The most common variations of the surface piercing propeller are the chopper and the cleaver. Though the shape of the blade on these two propellers is very different, both are stainless steel propellers with thin blades, very sharp leading edges, and are usually highly polished.

Author: ABOUT.com
Contact: www.about.com

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