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CAVITATION |
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The
phenomenon of cavitation becomes present when a propeller rotates its blades
and expels the water backwards; leaving emptiness that is immediately filled
by new liquid molecules. The blades create such depression (reduction of pressure or vacuum) in its front side, that water boils at room temperature; the bubbles that come out of the propeller aren’t air bubbles, but strictly water steam. These bubbles are displaced very quickly to the backside until they find a higher-pressure zone in which they will become imploding (opposite to explode) water against the propeller blades and pulling out a microscopic particle of metal in each clash (Fig 1). |
From the desktop of
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If
the speed of the propeller blade tips (tangential speed) surpasses certain
limits: 150 feet per second (fps) in 5 bladed propellers, 175 fps in 2-4
bladed propellers, and 100 fps for propellers in nozzles; the expelled
water will carry such strength that it’ll prevent that any other water
molecules occupy the formed vacuum (Fig.2), therefore, cavitation is
produced by tangential velocity of excess of RPM’s.
This erosion can always be seen in the tips of the blades.
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 Fig. 1 |
 Fig. 2 |
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CAVITATION
BY LACK OF BLADE AREA If
the pressure on the blade is higher than 7 psi in open propellers, and 8.5
psi in ducted propellers, the cavitation is produced by lack of area
(DAR). The
origin of the bubbles is in the propeller blades leading edge, but the
damage is shown on the backside with the aspect of corrosion that goes
backwards in its destructive process to the center of the blade. The
erosion produced by any type of cavitation is shown with more intensity
when the cathodic protection is not adequate. And in some extreme cases
the propeller is completely consumed within days or weeks. When a propeller is being designed, it is very important to observe the relation Diameter vs. RPM’s, in order to avoid reaching the above mentioned tangential velocities where the cavitation by excess of RPM’s is produced, and at the same time the Relation Diameter-DAR to avoid cavitation by lack of blade area, that finally any type of cavitation will reduce the efficiency of the ship by increasing fuel and reparation costs, or have to replace the eroded propeller.
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