what is cavitation?

[muckypup]

What is cavitation?

as in...

"Thanks to the tenacious bite of the optional DuoProp drive, we experienced no cavitation, no ventilation and were able to straighten our course and accelerate..... etc etc"

S.

 

[Emjaytoo]

Something nasty that MOBOs get?

/forums/images/graemlins/smirk.gif

 

[landlockedpirate]

In simple terms, its when the propeller loses grip in the water. It can happen if you make a too tight turn. Its the marine equivalent of wheel spin on a car.

Duo props tend to have less cavitation, marine equivalent to 4x4.

Mark

 

[cliff]

What you are talking about is air ingestion NOT cavitation. The pressure gradients in the water cause bubbles of vapour to nucleate and almost immediately collapse. This subjects the blade edge to repetitive localised stress and the material erodes away. Hence the failure mechanism is known as cavitation or erosion-corrosion.

A very interesting (and educational) presentation can be found here - scroll through to slide 13.
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[ccscott49]

Ventilation and cavitation, two different things caused by two different things, listen to Cliff!

 

[capnjack]

From the sound of your post you seem to know what ventilation is, however this is often confused with cavitation. The plates on the lower section of sterndrives are often called cavitaion plates, but are in fact ventilation plates.

For those who are not sure, ventilation is when a prop sucks air from the surface and looses grip through a lack of water on the blades, simple really.

Cavitation on the other hand is a little more complicated and involves a little bit of science and techy stuff. Cliff has given a nice explanaton although I think the term nucleate could do with a little more elaboration.

There are generally two forms of cavitation which affect boats and ships. The first usually affect ships or shaft driven boats and is due to the effect of the propellors relation to the hull and is usually known as mechanical cavitation, although I have often argued that they can both be described as mechanical.

The other form of cavitation and the one you are referring to happens when the atmospheric pressure at the inside of the prop blade is lowered dramatically because of defect in the blade (caused by wear or damage etc.) and as Cliff has already mentioned because of the lower pressure the water vapour basically causes the vapour bubbles to implodes and believe it or not the water boils. The scientific principle is if you lower the pressure of water enough at say 12-15c (normal temp of the sea around the UK) it will actually boil. Further damage is caused by the imploding vapour bubbles. Marvelous stuff eh? I luv it!! /forums/images/graemlins/cool.gif

Any more questions? /forums/images/graemlins/smile.gif

 

[landlockedpirate]

Thanks for the science lesson, its a few years since I did any fluid dynamics (about 20 years !)

I think my analogy (if factually incorrect) would have helped the poster put the article he was reading into context.

 

[thefatlady]

Boiling water and imploding bubbles, fine.

Imploding molecules???? You appear to have discovered a new physical phenomenon.

 

[muckypup]

The complete paragraph read...

"Inside Sarasota Bay, the 240 Express cut tight-diameter doughnuts at 25 mph. Thanks to the tenacious bite of the optional DuoProp drive ($1,295), we experienced no cavitation, no ventilation and were able to straighten our course and accelerate from the circuit like a hammer flying from an Olympian's throw, before reaching a spirited 50-mph top end."

Sounds quite dramatic, but I was wondering what this means in real life terms... what would I feel the boat doing if I experienced cavitation and ventilation?

S.

 

[capnjack]

Completely agree with everthing you say matey! You did answer his question. However, I was answering muckeypups question too and did not dream of giving you a science lesson, I think that came from another poster. /forums/images/graemlins/wink.gif

 

[capnjack]

You are completely correct. I am put to shame and my integrity is now in doubt. Please note the very careful editing.

I will of course be beating myself with a Birch branch regularly today in penance for the error.

My excuse for such a mistake is that I wrote the post whilst in bed having a cup of tea and under extreme duress from our Cocker Spaniel and SWMBO subjecting me to the most apauling pressure to get up and take them both to the golf course for a walk. Dog on the lead and SWMBO in my hand of course. My typing rate reached at least 3,000 words per minute. Hmmmm maybe not that fast but my fingures were cavitating under the pressure. /forums/images/graemlins/laugh.gif

 

[capnjack]

In the seventies, we thought we were pushing the boundaries by polishing and sharpening our racing props, adding some cup hear and a little shape there. Surface piercers were newish etc. Now prop technology is really taking shape, with big manufacturers getting involved. I don't see much evidence of cavitation any more. Sometimes it will show on a badly worn ali outboard prop, but stainless steel tends to resist it. Similarly the strength of stainless tends to resist damage therefore protecting against the risk of cavitation.

Your boat would loose grip with either problem, for want of a better way of putting it. You can see cavitation burns or erosion on a prop.

Some props have ventilation holes (exhaust ventilation)built into them so the they spin quicker when hole shotting, then they bite once the water flow over the hole blocks the flow of gases.

I think for your situation mp if you want to know how your boat feels if you had the problem you would first have to know how she feels without it. As I said its basically a lack of grip. /forums/images/graemlins/frown.gif

 

[landlockedpirate]

It feels like-not a lot. Prop spins, doesnt get bite in the water (for whatever reason-exploding molecules, boiling water,wrong type of H2O etc /forums/images/graemlins/wink.gif ). The engine revs as if you are not in gear. Lasts for a couple of seconds if you throttle back or reduce turn radius, if you dont throttle back the engine bounces off the rev limiter for a bit.

 

[Sammo]

[ QUOTE ]
What is cavitation?

[/ QUOTE ]

Cavitation is something given to us by God to see if we have got the prop size right.

 

[pistonbroke]

WTF

Hey CapnJack,

"If "A" is a success in life and A = x+y+z - then x=work, y=play, x = keeping your mouth shut. "

So what does Z = then?

 

[capnjack]

Re: WTF

Now look here, I have already pruved that mi tiipping scills aare a commpleet and utteer noncsensse and these computer thingy wotsits really are beyond my comprehension. Having beaten myself all day yesterday with a Birch branch in penance for my totally inadequate typing I will now proceed to knock some sense into this already decrepit brain.

Don't talk to me about puters....... /forums/images/graemlins/frown.gif

Ask me about boats...... /forums/images/graemlins/grin.gif

 

[cliffordpope]

Re: WTF

Like this:

 

[muckypup]

Re: WTF

So if cavitation is wearing of props, why would the boat reviewer say "we experienced no cavitation and were able to straighten our course and accelerate from the circuit like a hammer flying from an Olympian's throw"

Surely cavitiation happens over time, not half way though a turn?

S.

 

[cliffordpope]

Re: WTF

I suspect people just use the word cavitation, which has a nice descriptive ring to it, when really they mean ventilation.
Whatever the name, I think it describes the gurgling, inefectual noise you get when applying too much power to a small prop at insufficient depth. Like spinning the wheels, really.

 

[capnjack]

Re: WTF

I agree with Clifford, some like to use the word, it sounds kinda special.

However, I suspect that in the article they have used the two together because sometimes they can go together. Cavitation can occur when a prop is venting i.e. prop speeds up in the water lack of pressure on the trailing side and bubbles implode. The big but is that it can be likened to pinking and although valve and piston damage can occur, the effect happens immediately the fault exists. The effect of cavitation as LLP pointed out is that the engine revs, the props spins and not a lot happens. Cavitation under these circumstances will rarely cause much damage as it only occurs for a short time and especially with stainless props.

Hope this helps more.


"Proof readers please note, my speling is mostlie corekt now!"