cavitation on aluminium props.

BruceK

BruceK

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Guys, what is normal and what isnt. I had B5 that within 10 hours had blade root cavitation erosion. Now I have the same on B4 at mid blade within 6 hours.

Is below normal? Most of my time has been on Stainless C series and I find this a bit unacceptable

https://imgur.com/02xQi1a
 
Ok so first principles .Cavitation is the friction creating heat which momentarily vaporises the water , turns it into gas .
It ( because it’s moving and the boats moving ) then meets cooler water and recondenses back to solid water .
As it does this conversion back from gas to more liquid water the gas bubbles implode , it all happens in milliseconds and the energy released as it goes from a gas back to liquid is expressed as little indentations in the surfaces the bubbles happen to be adjacent .In this case the alloy , so tiny chunks are nibbled away = cavitation scars .

So to fix it get rid of the heat generation characteristics.
water density is a constant .
variables are
- Less pitch reduces drag and heat , slower boat
- smooth surface ....the AF might be microscopically a rough surface = polish the props properly polish I mean .
- harder surface accepting the cavitation .....SS will just take longer hrs to reach the same position , just takes more bubbles that’s all .
- lighter boat = less prop slipping, its the extra weight = more slipping = less cooling fwd water across the blades = more localised friction = more heat = more cavitation .

I feel along with other threads that’s the weights exceeded the Hp available to achieve a set historical speed and you are trying to hang on to theses historical performance numbers .I know from previous threads you disagree with me on the ageing reduction of Hp in marine diesels due to overloading and mushrooming of the exhaust valves .

This is arguably a manifestation of that , a new thing to your boat ?



Ideally and I know you can not do this with your set up prop it using EGT and load readings , find a prop set the engines are comfortable with at said weight of boat .The resultant speed is what it is .

Summary loose weight and or lower the pitch lower the performance expectations and accept the boats growing old gracefully .

Or remove the cylinder heads and undergo a top end rebuild .
 
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Sorry Portofino, its nothing to do with heat. Its the low pressure on the blade front surface which cause air molecules in the water to expand into bubbles which then implode causing the erosion. The rest of your explanation is correct. You can easily see this phenomenom if you half fill a syringe with water place a finger over the end and pull the plunger further out, the water will appear to boil which is what is happening at lower than atmospheric pressure.
 
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spannerman said:
Sorry Portofino, its nothing to do with heat. Its the low pressure on the blade front surface which cause air molecules in the water to expand into bubbles which then implode causing the erosion. The rest of your explanation is correct. You can easily see this phenomenom if you half fill a syringe with water place a finger over the end and pull the plunger further out, the water will appear to boil which is what is happening at lower than atmospheric pressure.
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Yes agree Lower pressure is a better explanation .
The lower P means it boils at a lower T or higher T at higher P on the other side .Semantics but as you infer the principle of imploding gas rapidly turning to liquid releasing back energy is the same and the it does not change the outcome which way the bubble formation is described .
 
I see Tweedledum and Tweedledee are on a cross forum raid again. Very 90's chaps

Portofino said:
feel along with other threads that’s the weights exceeded the Hp available to achieve a set historical speed and you are trying to hang on to theses historical performance numbers .I know from previous threads you disagree with me on the ageing reduction of Hp in marine diesels due to overloading and mushrooming of the exhaust valves .

This is arguably a manifestation of that , a new thing to your boat ?
Click to expand...

I know what cavitation is, however you constantly warp my words to fit your argument and that is unacceptable. I have never set out to achieve OEM advertised performance. Please show me where I have ever stated that! I have always maintained I am looking for a prop set most suitable to the boat.
Nor have I ever said there is no reduction in HP with aging. That would be bollox. I said if maintained and everything is in good working order there is no reason for HP loss.

BTW my engines were fully rebuilt the year before I bought the boat by the Volvo dealer in Pwllheli. That is six years ago. I'd hardly call them aged.

I agree my boat is heavily loaded which is why I am looking for a suitable prop. I have tried many. You will notice that each one is a reduction. Your single minded focus has not only twisted my words to your end but is somewhat insulting that you then use them to make me look an idiot. My last thread on the matter was I found it extremely odd and perplexing that the B4 gave better performance figures at the same revs to the B5. Perplexing because it means I cannot predict what kind of performance I will get from any one prop set. The B5 is supposed to be less pitched than the C4 but it proved not to be as well. That was it. How you manage to create this fantastical world of yours is beyond me.
 
I'd also like to point out that the cavitation is not happening on the rear of the blade and is highly unlikely therefore to be a low pressure bubble. That happens on the rear face of the blade. However there is no corresponding cavitation scarring on the rear face of the front props. The rear props are picking up bubbles from somewhere and I'd expect it's front prop leading edge vortices. This should not happen though with duo props. I thought that was the point.
What I am leading with here is the rear props blade front have negative incidence. That would cause cavitation and if the props are defective may explain why their performance is unexpected. Too high a camber or curvature on a blade would also have mid chord cavitation but this is the smaller prop in the set so that is also perplexing
 
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BruceK said:
I see Tweedledum and Tweedledee are on a cross forum raid again. Very 90's chaps



I know what cavitation is, however you constantly warp my words to fit your argument and that is unacceptable. I have never set out to achieve OEM advertised performance. Please show me where I have ever stated that! I have always maintained I am looking for a prop set most suitable to the boat.
Nor have I ever said there is no reduction in HP with aging. That would be bollox. I said if maintained and everything is in good working order there is no reason for HP loss.

BTW my engines were fully rebuilt the year before I bought the boat by the Volvo dealer in Pwllheli. That is six years ago. I'd hardly call them aged.

I agree my boat is heavily loaded which is why I am looking for a suitable prop. I have tried many. You will notice that each one is a reduction. Your single minded focus has not only twisted my words to your end but is somewhat insulting that you then use them to make me look an idiot. My last thread on the matter was I found it extremely odd and perplexing that the B4 gave better performance figures at the same revs to the B5. Perplexing because it means I cannot predict what kind of performance I will get from any one prop set. The B5 is supposed to be less pitched than the C4 but it proved not to be as well. That was it. How you manage to create this fantastical world of yours is beyond me.
Click to expand...

No word twisting just analysis of your recent postings related to this eg .
Post #5 on here ?
Engine longevity

Nevermind I am genuinely trying to assist .

Which ever side it needs sorting , Good luck and good bye from me on this thread .

Anybody else dare help him ?
 
Portofino said:
No word twisting just analysis of your recent postings related to this eg .
Post #5 on here ?
Engine longevity
Click to expand...

And my response was.....

BruceK said:
If maintained then the short answer is no. You'll lose power on compression, injectors, pump, boost etc. But if maintained, serviced and replaced there is no reason to loose power.
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Portofino said:
Nevermind I am genuinely trying to assist .

Which ever side it needs sorting , Good luck and good bye from me on this thread .

Anybody else dare help him ?
Click to expand...

Porto, I appreciate your effort but you have a fixation. It's not just me telling you this, the whole forum is. You are liable to warp everything to fit your own sense of order.

Volvo = crap
Deadrise has to be >24 degree
EGT. EGT EGT!

Nothing else ever enters your head. Now you are not wrong in some of your assertions but they are not the be all and end all of every goddamn thing.
 
BruceK said:
I see Tweedledum and Tweedledee are on a cross forum raid again. Very 90's chaps
Click to expand...
Not a raid old man, we have free entry. Anyway, I was stating that between the two options Porto suggested, I reckoned the rebuild would be your preference as opposed to loosing weight by either leaving the wife, kids or wine on the shore. Not every comment is a pop. Don’t be paranoid.
 
Doesn't look right does it. Another one for the mix....Trim?

If the boat v leg trim means that the prop is slightly off axis and unevenly loaded?
 
seastoke said:
You need a fly bridge at your age , and rodders knows about trimming to stop his prop hitting tarmac.
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He's taken to stalking. Sending pictures from his boat in the yard of me transiting Beaumarris. Probably has a radar gun!

Anyway, returning to the boat Thursday for 5 more days of fun. I queried the props from the supplier who is querying the manufacturer and Volvo. So far the response seems to be..."they all do that sir"... which suits me fine because if they shred further I have a comeback.
 
BruceK said:
He's taken to stalking. Sending pictures from his boat in the yard of me transiting Beaumarris. Probably has a radar gun!

Anyway, returning to the boat Thursday for 5 more days of fun. I queried the props from the supplier who is querying the manufacturer and Volvo. So far the response seems to be..."they all do that sir"... which suits me fine because if they shred further I have a comeback.
Click to expand...
Do you fancy tycock
 
BruceK said:
Guys, what is normal and what isnt. I had B5 that within 10 hours had blade root cavitation erosion. Now I have the same on B4 at mid blade within 6 hours.

Is below normal? Most of my time has been on Stainless C series and I find this a bit unacceptable

https://imgur.com/02xQi1a
Click to expand...

Have to say and maybe it's the pic through water but it looks like a not so good paint job and generally uneven prop surface and edges which might make it worse. Also IIRC my experience with different props was very different to yours so maybe there is something else going on.

That said I swapped C4 to J series (VP suggestion did not work great so had to go up one size) but remember some paint coming of quite quickly but then stopped.
 
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