Sarathlal
New Member
Can anyone tell me..... why a stainless steel prop found corroded but the drive aluminium found verymuch new.....is it a material defect?????
Pitting on Stainless steel duoprops
- Thread starter Sarathlal
- Start date
PCUK
Well-Known Member
Cavitation? Stainless props are used to reduce it.
jfm
Well-Known Member
Yup, it may well be cavitation. Esp if the pitting is close to the hub rather than the tips. Needs fixing. Can you post pics?
Sarathlal
New Member
Pics
PFA for the pics.....
PFA for the pics.....
Portofino
Well-Known Member
Cavitation is the introduction of air adjacent to a foil or other body moving through water. It can come from the surface - or in this case from low pressure zones caused by the movement of the prop blades through the water. The water will boil, just like in a kettle if the pressure is reduced far enough. The continuous exploding of gas into the water against the surface of the prop is very powerful, and erosion is severe and fast. The props may be turning too fast for their design, they may have turbulence issues or any number of things going on. Prop design is seriously complicated stuff - even the best designers in the world will admit that they are working in a very grey area of physics that confound the best super-computers in the world. The fact is that the actual way subsurface propellers work is only partly known - there are forums out there with some of the best naval architects in the world debating incredible concepts and writing new software every month to try and get better performance and life from propellers. The calculations fall apart in most cases because you can't go too big or too slow - the bigger you go and the slower you turn the better the prop, but this quickly runs out of reality if the software tells you a 200 meter blade is most efficient at 0.3 rpm!!
In practice a lot of work is needed to design the props for this modified boat. Tell me, the engines and rpm were increased right? The stuff that was working before would still work - these brand new props sound like they are unable to get the boat up to the new design speed, or are certainly exceeding tip speed limitations in this application, and are suffering from either not enough, or too much slip.
Most cavitation signs are at the leading edge.
These pic look like a metalogical fault ,some sort of galvanic corrosion
Sarathlal
New Member
This is a duoprop of Volvo D3 AQ & its hardly 1.5 year old......the prop came as a complete assembly...its manufacture supplied with engine & drive.
ARA
Well-Known Member
Is this the new generation D3?
Sarathlal
New Member
Its Volvo Penta D3 170 new model....
ARA
Well-Known Member
Ok, I have a pair of D3-170 which are coming up to 1 year old. I will be down on the boat in about 1 weeks time & have a look at mine! I would also point the engine provider towards the pitting as you should be in warranty still & I would alert them to this sooner rather than later. Do you have shore power plugged in all the time & how much usage has it had over the 1.5 years & finally what boat are you talking about ?
jfm
Well-Known Member
Tricky one. The localised pitting is normally cavitation (and, incidentally, I woudn't fully agree Portofino's description of it from a fluid mechanics pov but that's neither here nor there) but in this case
1. The pitting on the inside of the rim of the hub isn't intuitively a logical location for cavitation to occur
2. There is an oxidised/rusted patch surrounding each pit
3. Slightly but not certianly, pic 2 is too near the tip to indicate cavitation
I can't tell what side of the blade the [pic2 pitting is on. That would be another big clue.
Therefore I'd expect electrolytic activity. That raises questiosn like do you have galvanic isolation? Diodes or transformers? Are you plugged into shorepower? How are your anodes? How are your neighbours?
How are your anodes? is the first question. Anodes should prevent this. But not if they've fizzed away 2 months before you lifted the boat.
MapisM
Well-Known Member
I suspect it's as simple as that.
VP has a reputation for paying peanuts to their OEM props suppliers.
And as the old saying goes, if you pay peanuts, you get monkeys...
MapisM
Well-Known Member
Why not, J?
He obviously has a deep knowledge of the subject, garnered from an extremely reliable source (scroll down to "best answer")...
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jfm
Well-Known Member
Why not, J?
He obviously has a deep knowledge of the subject, garnered from an extremely reliable source (scroll down to "best answer")...
Tee hee. Your specialism must be finding needles in haystacks!
I agree material defect is possible. Or material defect combined with electrolysis. Also, if material defect is suspected I'd like to know if the prop hub rim is a separate component or part of the same casting
Of course, as we all know, getting VP's warranty department to agree will be a pointless waste of OP's life; better to send props to www.SteelDevelopments.net for a £250 repair.
MapisM
Well-Known Member
LOL, that's actually not so difficult, nowadays.
Whenever some comments make you wonder whether they come from some webpages, it's sufficient to google one sentence and see the results...
I fully agree with all your other comments, including the (sad, sort of) conclusion.
ARA
Well-Known Member
Of course, as we all know, getting VP's warranty department to agree will be a pointless waste of OP's life; better to send props to www.SteelDevelopments.net for a £250 repair.
I would disagree JFM, based on my experience. I had a minor (ho ho) problem where the computer decided that I only had one engine, & 1 fuel tank where I was sure last time I looked I had two ( I had the bills to prove to Volvo in case they asked me !) An engineer came down, spent an hour checking fuses etc & reckoned that it was a faulty throttle quadrant, they ordered & replaced the £3k worth of quadrant all FOC. I am now probably going to eat my words as it will be the start of year 2 of my warranty shortly
GrahamHR
Well-Known Member
These DPS props are not protected by the anodes, the stainless part is isolated by a plastic bush. It's crevice corrosion, If you poke at the holes you'll find a track to the plastic bush between the aluminium hub and the prop.amd probably a lot of other hole in the vicinity of the bush.
I've had this on my DPS props; never on the blades wihich are always exposed to oxygen, In the first case, after one season a small rust spot was found which when poked, a thin stainless skin collapsed revealing a large hole filled with black liquid. The forward prop also had such rusty surface spots and associated holes. The holes went deep into the prob hubs towards the inner machined surface that is in contact with the plastic bush..
I did get a replacement set off VP under warranty. After one season they too started to show the problem.
I filled the holes on the original props with "molecular metal", after cleaning out and drying the holes. They've now just done their 6th season since I did that, they don't seem to be getting any worse !
I've had this on my DPS props; never on the blades wihich are always exposed to oxygen, In the first case, after one season a small rust spot was found which when poked, a thin stainless skin collapsed revealing a large hole filled with black liquid. The forward prop also had such rusty surface spots and associated holes. The holes went deep into the prob hubs towards the inner machined surface that is in contact with the plastic bush..
I did get a replacement set off VP under warranty. After one season they too started to show the problem.
I filled the holes on the original props with "molecular metal", after cleaning out and drying the holes. They've now just done their 6th season since I did that, they don't seem to be getting any worse !
jfm
Well-Known Member
Ah, ok, good stuff, if you know the machinery and you're sure the prop metal isn't connected (physically by a conductor) to anything then yes it can't be electrolysis with another bit of metal on the boat, and must be crevice corrosion or cavitation (but I doubt cavitation for reasons given above)
Good also to hear volvo replaced under warranty. That's OP's next port of call then? If unsuccessful, then get it weld repaired by steel developments
jfm
Well-Known Member
I would disagree JFM, based on my experience. I had a minor (ho ho) problem where the computer decided that I only had one engine, & 1 fuel tank where I was sure last time I looked I had two ( I had the bills to prove to Volvo in case they asked me !) An engineer came down, spent an hour checking fuses etc & reckoned that it was a faulty throttle quadrant, they ordered & replaced the £3k worth of quadrant all FOC. I am now probably going to eat my words as it will be the start of year 2 of my warranty shortly
OK, thanks, good to hear. One swallow does not a summer make, however
. I think you're in a minority in having nice things to say about volvo warranty!GrahamHR
Well-Known Member
The problem seems to be associated with the machined surfaces, suggesting porosity in those sections. Porosity allows crevice corrosion. Basically voids that fill with water but are deprived of oxygen; the phases in the stainless steel alloy form little electrolytic cells. Those holes that appear on the "as cast" surface are probably the result of corrosion that initiated elsewhere.
I cleaned the props today that I referred to earlier; there has been marginal further corrosion; at the edges of where the filler is. Suggesting that all the porosity exposed by machining that was going to cause problems has been "consumed" and there are no new primary corrosion sites.
I cleaned the props today that I referred to earlier; there has been marginal further corrosion; at the edges of where the filler is. Suggesting that all the porosity exposed by machining that was going to cause problems has been "consumed" and there are no new primary corrosion sites.
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Portofino
Well-Known Member
Daily I work with stainless steel and devices that produce cavitation .
I used the " copy " in the earlier post to save typing , re prop cavitation .
It self explanatory .
SS ( not Sunseeker - stainless steel ) comes in various compositions
Austentitic ,used in seawater application - high chrome that gives its hardness + anti corrosive properties + ,molybdenum flexibility + % carbon all have to be " mixed " at the right temp for the the correct time and then cooled down correctly ,then cast/ machined within correct tolerance .Its easy to get this wrong ,over work harden it = it becomes brittle and susceptible to corrosion .
It's a difficult metal to cast ,and get it right all the time ?
In my view bronze or Alluminium are Preffered .More room for casting error ?
SS is harder - stiffer distorts less ie keeps it profile under load more ( if you think that's more important ) than Al props and bronze .
OP need to exchange under warranty ,it's a casting defect looking at Pics the composition is not properly mixed ,cast,and cooled .
I used the " copy " in the earlier post to save typing , re prop cavitation .
It self explanatory .
SS ( not Sunseeker - stainless steel ) comes in various compositions
Austentitic ,used in seawater application - high chrome that gives its hardness + anti corrosive properties + ,molybdenum flexibility + % carbon all have to be " mixed " at the right temp for the the correct time and then cooled down correctly ,then cast/ machined within correct tolerance .Its easy to get this wrong ,over work harden it = it becomes brittle and susceptible to corrosion .
It's a difficult metal to cast ,and get it right all the time ?
In my view bronze or Alluminium are Preffered .More room for casting error ?
SS is harder - stiffer distorts less ie keeps it profile under load more ( if you think that's more important ) than Al props and bronze .
OP need to exchange under warranty ,it's a casting defect looking at Pics the composition is not properly mixed ,cast,and cooled .
