Prop Cavitation

[oldgit]

A wonderful and esoteric debate.
Nothing in any of those pics would prevent me lowering that boat back in the water and going boating.
Doubt the blades are going to drop off leaving you at the mercy of the elements, lift it out next year and check on progress.
Seen props far worse than that pressed back into service.
Interesting that 4 blade props have been fitted, most boats with that combination of boat class/ power output appear to have fitted with 3 blade props.ie. Princess 35,Princess 330 and of course your Corniche which were fitted with 200HP 41 A or B engines.
Classic case .
Two Turbo 36s, identical boats. One faster than the other.
Boat "A " with 4 blade props faster than boat "B with three blades.
Owner of boat "B" becomes determind to rectify this. Convincing himself, with the help of various "experts" that a set of brand new 4 blade props, all calculations done to modify prop size and pitch by a reputable well known company, will solve the problem.
The result, was a set of spare props, no increase in speed and a £1.500. hole in his bank account.

 

[Dazzajohm]

A wonderful and esoteric debate.
Nothing in any of those pics would prevent me lowering that boat back in the water and going boating.
Doubt the blades are going to drop off leaving you at the mercy of the elements, lift it out next year and check on progress.
Seen props far worse than that pressed back into service.
Interesting that 4 blade props have been fitted, most boats with that combination of boat class/ power output appear to have fitted with 3 blade props.ie. Princess 35,Princess 330 and of course your Corniche which were fitted with 200HP 41 A or B engines.
Classic case .
Two Turbo 36s, identical boats. One faster than the other.
Boat "A " with 4 blade props faster than boat "B with three blades.
Owner of boat "B" becomes determind to rectify this. Convincing himself, with the help of various "experts" that a set of brand new 4 blade props, all calculations done to modify prop size and pitch by a reputable well known company, will solve the problem.
The result, was a set of spare props, no increase in speed and a £1.500. hole in his bank account.

Thanks for the reply.

I have spoken to my engineer about the props and he's said exactly the same thing - really no issue with putting the props back on at this point and seeing what they look like next year and he's seen props far worse than mine still in use. As I mentioned in a previous post, the cavitation damage rubs out with a bit of fine wet and dry so they're obviously not that deep.

I have actually done some looking around on what props people use on their corniches and surprisingly there seems to be a whole host of different size and pitches being used and a combination of three and four bladed. Apparently the 4 blades offer better acceleration where as the three bladed offer higher cruising speeds. With that being the case if I need to buy new ones I will most likely opt for three blades but all the while I can keep using what I have I will... boating is expensive enough as it is without buying things I don't yet need

 

[oldgit]

All noted and my suspicion is that top speed is only of concern to few anal newbie speed freaks and acceleration only to anybody trying to get a skier on the plane.
The rest of us need a nice middle road prop giving economy and minimum stress on engine to ensure long reliable life.
We have 3 Corniches in our club at present, skippers vary from go in any weather/ speed nutcases to careful displacement economy chaps.
All are very pleased and long term owners of Corniches.
Would have been four Corniches but our Commodore, saw sense dumped his and bought a nice Princess 330.
We have a good selection of Corniche/Princess 330/35/DS31 and being weirdly fascinated by the undersides of boats , always wander down to our drying blocks when somebody on there. The variation in prop pitch and diameter on very similar hulls is suprising.
Gearbox ratios in the MS4 range must also have something to do with it.

 

[Dazzajohm]

All noted and my suspicion is that top speed is only of concern to few anal newbie speed freaks and acceleration only to anybody trying to get a skier on the plane.
The rest of us need a nice middle road prop giving economy and minimum stress on engine to ensure long reliable life.
We have 3 Corniches in our club at present, skippers vary from go in any weather/ speed nutcases to careful displacement economy chaps.
All are very pleased and long term owners of Corniches.
Would have been four Corniches but our Commodore, saw sense dumped his and bought a nice Princess 330.
We have a good selection of Corniche/Princess 330/35/DS31 and being weirdly fascinated by the undersides of boats , always wander down to our drying blocks when somebody on there. The variation in prop pitch and diameter on very similar hulls is suprising.
Gearbox ratios in the MS4 range must also have something to do with it.

Indeed - I like to not work the engines too hard and cruise around 3000 - 3200 rpm. At the moment I get around 18 - 20knts at that rpm which is sufficient for me, but if when I need to replace the blades I can get a three blader which will give me a few more knts at that rpm or indeed allow me to reduce my revs and keep the same speed it can only be a good thing economy wise.

When I looked at the Corniche last year, I also looked at a Princess 330 which I probably liked more, but when I started looking in to both it turned out there were problems with the princess so I dropped it and hence here I am with the Corniche... Although so far I'm pretty happy with it!

 

[oldgit]

Corniche a properly built sea boat but a shame to limit with merely coastal stuff .Ideal for trips in non tidal waters such as up the Thames.
You will get as far as Windsor with your radar arch up and much further with it down.
Did manage to get a Fairline Brava underneath Windsor bridge with just riding lights removed.
Chap with the Princess 330 on his first trip up the Thames actually unbolted his arch while moored at Teddington.
Took far less time and less faffying about than anyone expected.
Simply assembled it on return to base.
The trip up into central London from the estuary skippering your own boat equals anything else you will probably ever do with a boat.
Two day trip from Dover with break around Queenborough.

 

[jfm]

I tend to back up the science - see post #22

Surley this link @post #22 adds to the thread knowledge in a “ decent manner “ ?

https://www.iims.org.uk/introduction-propeller-cavitation/ .

But that isn’t science porto. It’s an article written by someone who understands little about fluid mechanics- his mistaken reference to Bernoulli in first para tells you there is no point reading any more. Problem with the internet is you can google and find incorrect stuff (err, qv) just as easily as correct stuff. So I’m saying you linked to carp, not science.
All mapis and I have said is that the pitting in the picture in the opening post happens on the high pressure side, because it’s the high pressure that causes a catastrophic collapse of suspended vapour bubbles. The bubbles that happen to be on or near the metal when they implode chip the metal. This always happens on the hp side. The vapour bubbles might (not always) form on the LP side but they are completely harmless and insignificant there. It’s the implosions on the HP side of the propeller that cause the putting. But I’m content to disagree - we’re done on this.

 

[Portofino]

But that isn’t science porto. It’s an article written by someone who understands little about fluid mechanics- his mistaken reference to Bernoulli in first para tells you there is no point reading any more. Problem with the internet is you can google and find incorrect stuff (err, qv) just as easily as correct stuff. So I’m saying you linked to carp, not science.
All mapis and I have said is that the pitting in the picture in the opening post happens on the high pressure side, because it’s the high pressure that causes a catastrophic collapse of suspended vapour bubbles. The bubbles that happen to be on or near the metal when they implode chip the metal. This always happens on the hp side. The vapour bubbles might (not always) form on the LP side but they are completely harmless and insignificant there. It’s the implosions on the HP side of the propeller that cause the putting. But I’m content to disagree - we’re done on this.

I don’t disagree with any of your second para above , all I did point out it’s a low pressure thing initially to form the vapour in the form of bubbles , heat from localised friction is the energy in so to speak , very quickly as the pressure changes high enough and with such a rate as the bubbles return to liquid so rapidly the energy returned by the implosions damages any surface the actual bubbles come into contact .
A few bubbles here and there for a short time are insignificant , a lot over a longer time then you see the effects even on titanium nether mind softish by comparison prop alloys .

Just wanted to clarify it’s a low pressure albeit localised on the Hp side with is the back or rudder side , we are in agreement and said I thought using Hp for the back side without clarity might lead folks to think it’s caused to high pressure , too much pressure .
Based on no bubbles no cavitation ,

Happy to agree to disagree with the link .

On topic regarding your mods like your HT ( adding kg,s ) have you noticed any cavitation signs , inc aberrant noise ?
Or is it still possible to trim it normally , sos the the trim angle and relative flow now have not significantly altered ?

 

[Dazzajohm]

I don’t disagree with any of your second para above , all I did point out it’s a low pressure thing initially to form the vapour in the form of bubbles , heat from localised friction is the energy in so to speak , very quickly as the pressure changes high enough and with such a rate as the bubbles return to liquid so rapidly the energy returned by the implosions damages any surface the actual bubbles come into contact .
A few bubbles here and there for a short time are insignificant , a lot over a longer time then you see the effects even on titanium nether mind softish by comparison prop alloys .

Just wanted to clarify it’s a low pressure albeit localised on the Hp side with is the back or rudder side , we are in agreement and said I thought using Hp for the back side without clarity might lead folks to think it’s caused to high pressure , too much pressure .
Based on no bubbles no cavitation ,

Happy to agree to disagree with the link .

On topic regarding your mods like your HT ( adding kg,s ) have you noticed any cavitation signs , inc aberrant noise ?
Or is it still possible to trim it normally , sos the the trim angle and relative flow now have not significantly altered ?

Not sure what you mean by HT - but since I've had the boat I haven't added any weight to it. Only purchased late last year. But wouldn't say there any noise which concerned me and yep can trim the boat with the tabs just fine.

 

[jfm]

Hard top is less than a tonne. Completely trivial on a 60 tonne boat, and also it is midships. I can burn a tonne of fuel in just over an hour. My empty to full range on fluids is 8 tonnes and that makes a just noticeable difference. The boat was originally propped with full tanks (even black), tenders, and water bladder tanks representing full load of people and gear, to avoid risk of over propping.

 

[BruceK]

I picked up my drives today from Castle Marine after getting them corrosion repaired. The skegs were looking ropey. They did a wonderful job. The drives look new! I pointed you out to them previously. They take in the bulk of their business from outside of the country on consignment. This just arrived from Ireland. I wouldn't have believed a single one was repairable but he laughed and said these were the easy jobs. Lots of brass props and huge impellers seem to be their main forte. But I was impressed most by their nonchalant "easy job lot" from Ireland