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Old 05 July 2007, 11:01   #1
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Cavitation

Ever wondered what cavitation actually looks like?



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Old 05 July 2007, 12:24   #2
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Liz,

Is that cavitation or the normal picture you would get with a running prop ?

I would have thought cavitation would be masses of bubbles.

I found a couple of piccies showing cavitation and they aren't as dramatic as I thought either.
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Old 05 July 2007, 12:59   #3
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Looks like a blender in action!!!

http://www.joecartoon.com/cartoons/67-frog_in_a_blender
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Old 05 July 2007, 13:02   #4
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What all those pictures show is that you don't need lots of air bubbles to start losing grip.
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Old 05 July 2007, 13:27   #5
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I find that a lot of people get confused between cavitation and ventilation. What most boats suffer when put into a tight turn is ventilation rather than cavitation.

Cavitation is what you suffer due to poor propellor design which causes the water to affectively boil and turn from liquid to a gas causing little explosions on your propellor blade which result in premature erosion of your propellor.

Ventilation however is when you get air entrailment from the propellor being shallow enough in the water that it becomes easier to pull in air from the air/water interface rather than spin around in the water turning rotational movement in one axis into thrust in another axis known as forward thrust. When you lose the grip on the water due to this air entrailment (ventilation), the boat tends to slowslightly and as a result put the propellor lower in the water and thus make the prop grip the water again (and stop ventilation due to the increased distance between the prop and the air/water interface).

Hopefully this makes sense - i can't help being an engineer - Univeristy has done this to me!
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Old 05 July 2007, 13:58   #6
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Cool photos and explanation!

Merlin doesn't like ventilation...

K & P
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Old 05 July 2007, 14:04   #7
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Funny how Zerstorer suffered quite bad ventilation if trimmed out at all with its original prop but a different prop improved this dramatically.
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Old 05 July 2007, 15:11   #8
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The pics are actually taken in VT Halmatic's test tank in Portsmouth in stationary water.
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Old 05 July 2007, 17:52   #9
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And if you want to know what each of those little bubbles is doing when it collapses then try this image. You can see from this high-speed camera that the middle of the bubble jets down through the middle of the bubble. It is as this jet hits the solid surface underneath that the characteristic pit marks are made as this jet can be traveling at incredible speeds. This frame is taken just before the bubble collapses completely.

Incredibly the collapse can be so violent with enough heat generated to cause a pulse of light to be emitted - this is sonoluminescence.

Now we know why those bubbles can be such destructive little b*ggers
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