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Old 05-12-2010, 18:48   #1
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Cavitation - What Is it ?

The title says it all.
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Old 05-12-2010, 18:56   #2
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if you're talking about prop cavitation, it's when the prop loses its grip on...

...the water.

Any underwater appendage can cavitate. Rudders. Skegs.
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Old 05-12-2010, 19:02   #3
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The prop moves faster than its design, creating vacuum pockets which pings on your prop causing rapid pitting. I'm sure someone can give a better scientific explanation...
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Old 05-12-2010, 19:04   #4
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The title says it all.
If you're speaking of propeller cavitation, cavitation occurs when the low pressure side of the blade reduces the local pressure to a point that water can boil - which it does. When the air bubbles collapse (pressure has risen), the inrush of water will bang against the propeller - this is what is typically referred to as cavitation.

It's generally not a good thing.

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Old 05-12-2010, 19:35   #5
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It can happen to rudders and centerboards on dinghies etc as well as propellers. from memory its caused by the loss of laminar flow over the foil shape and bubbles occur causing vibration. I note that wiki has a slightly different answer to what I was taught.

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Old 05-12-2010, 19:53   #6
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Quote:
Originally Posted by beetle View Post
If you're speaking of propeller cavitation, cavitation occurs when the low pressure side of the blade reduces the local pressure to a point that water can boil - which it does. When the air bubbles collapse (pressure has risen), the inrush of water will bang against the propeller - this is what is typically referred to as cavitation.

It's generally not a good thing.

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Old 05-12-2010, 20:31   #7
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It can happen to rudders and centerboards on dinghies etc as well as propellers. from memory its caused by the loss of laminar flow over the foil shape and bubbles occur causing vibration. I note that wiki has a slightly different answer to what I was taught.

Cheers
Oz
Loss of laminar flow leads to 'stalled' - not cavitating.
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Old 06-12-2010, 18:36   #8
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What can one do to combat it? We just pulled the prop off of a boat that until now had never had cavitation problems. When we pulled it was all pitted from cavitation. This prop has been used and inspected many times before and only now is showing symptoms. What could have changed?
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Old 07-12-2010, 08:31   #9
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unbusted, how do you know the pitting was from cavitation?
Are we talking of a sailboat wheel here or a planing OB prop?
" Ventilation " is what happens to an OB jacked up too high, not cavitation.
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Old 07-12-2010, 09:10   #10
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What can one do to combat it? We just pulled the prop off of a boat that until now had never had cavitation problems. When we pulled it was all pitted from cavitation. This prop has been used and inspected many times before and only now is showing symptoms. What could have changed?
Is most of the pitting confined to the edges, or at least really concentrated there? That's where the bubbles normally pop up. If it's pitted all over it might be electrical. Hard to imagine that all of a sudden you'd have cavitation that never happened before, unless you've been motoring around a lot really fast.
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Old 07-12-2010, 09:59   #11
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Cavitation - Wikipedia, the free encyclopedia

The link says it all....
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Old 07-12-2010, 10:10   #12
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I'll take a wild stab that it's electrolysis that is causing the pitting....not having seen your prop...or know what you mean by "pitted"
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Old 07-12-2010, 11:25   #13
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I'll take a wild stab that it's electrolysis that is causing the pitting....not having seen your prop...or know what you mean by "pitted"
I'd second that. Do you leave the outboard sitting in the water. And what about it's zinc's?
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Old 07-12-2010, 12:12   #14
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From Wikipedia

"As an impeller's (in a pump) or propeller's (as in the case of a ship or submarine) blades move through a fluid, low-pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. As it reaches vapor pressure, the fluid vaporizes and forms small bubbles of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may be audible and may even damage the blades."

So we know what it is. The question now is, what can be done to fix it: increase or decrease propeller size and/or pitch?
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Old 07-12-2010, 14:32   #15
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Is most of the pitting confined to the edges, or at least really concentrated there? That's where the bubbles normally pop up. If it's pitted all over it might be electrical. Hard to imagine that all of a sudden you'd have cavitation that never happened before, unless you've been motoring around a lot really fast.
We thought that it might be electrolysis too but the zincs seem fine and there is a definite pattern to the pitting. It is localized to the outer edges of the prop. We have been "motoring around really fast" because this is referring to the power vessel we bought in Maine not the sailboat right next to you! Stan is flying your way now as we speak.
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