Who's eaten the prop blades?

[Chris_Robb]

I have been monitoring this thread and did wander if the damage was not due to but damage. A friend wid a sail drive lost several props when the rubber insulating bush failed not unlike as shown in pic 2. I also agree with VyV_Cox in that the prop in pics 3 & 4 also looks like the blade broke off possibly due to hitting an underwater object and not corrosion.

Could some of the damage be caused by extreme cavitation?

 

[Tranona]

>The fresh and salt water mixing paper is interesting but I have yet to see a suggestion that it could lead to corrosion in anything.

In Lagos, Portugal I've seen two GRP yachts taken by the marina boat to the hard to have their props replaced because corrosion caused by the current caused by the mix of salt and fresh water that destroyed their props. Both boat had been left in the water so long that the anodes were badly eaten away and then covered in calcium. The same can happen with a bad earth in a marina 240v supply.

So, are you telling us that the mixed water was responsible - but at the same time the anodes were not active? Seems to me that if the anodes have eroded or calcified that is the reason for the props corroding - nothing to do with the "electricity" in the water.

You are confusing two effects. The mixture of salt and fresh water makes the anodes ineffective so the props which they were protecting corrode.

 

[brianhumber]

Interesting mix of opinions on this thread.

Can only add what I have found on Ronhilda.

Ronhilda is kept in Chichester Marina at the far end of the marina so sits in more fresh water in winter and more brackesh in summer. Couple of years ago I started leaving the shore power on 24/7 and found the anodes disappeared pronto. Last year trialled keeping the shore power off unless I was going out when I would put it on couple of days before hand.
Have just toddled around to Bosham for a scrub ( normal slime and small tuffs of green weed, the mullet always seem to do a good job of nibbling the weed down).
I was expecting to have change all hull, shaft and Gori prop anodes as per previous year but no, anodes were less than 50% gone - changed the prop shaft one just in case. Rate of anode corrosion now seems to be back to what it was when Yacht was kept on mooring in Itchenor Reach.
Concluded it must be due to better paths for the electricity through my hull than return cables on Premiers electrical system. If I wanted to keep the shore power on all the time then I would be looking at fitting a Galvanic Isolator but being a cheapskate happy to keep the shore power off unless for short periods.

Brian

 

[jeremyshaw]

Your experience on Ronhilda is interesting. Have you tried using aluminium anodes as I understand they are better for brackish water though I have no personal experience. See http://www.defender.com/html/zincs_info.html.
I should note that the marina in question for these damaged props is salt only - in the Med at that.

Interesting mix of opinions on this thread.

Can only add what I have found on Ronhilda.

Ronhilda is kept in Chichester Marina at the far end of the marina so sits in more fresh water in winter and more brackesh in summer. Couple of years ago I started leaving the shore power on 24/7 and found the anodes disappeared pronto. Last year trialled keeping the shore power off unless I was going out when I would put it on couple of days before hand.
Have just toddled around to Bosham for a scrub ( normal slime and small tuffs of green weed, the mullet always seem to do a good job of nibbling the weed down).
I was expecting to have change all hull, shaft and Gori prop anodes as per previous year but no, anodes were less than 50% gone - changed the prop shaft one just in case. Rate of anode corrosion now seems to be back to what it was when Yacht was kept on mooring in Itchenor Reach.
Concluded it must be due to better paths for the electricity through my hull than return cables on Premiers electrical system. If I wanted to keep the shore power on all the time then I would be looking at fitting a Galvanic Isolator but being a cheapskate happy to keep the shore power off unless for short periods.

Brian

 

[jeremyshaw]

looks like the blade broke off possibly due to hitting an underwater object and not corrosion.

I agree it does look like damage. But
a) both props have lost blades (one all of them)
b) props about 12' apart.
c) deep water marina
d) boat parked stern to
e) no other sign of damage

So it kind of looks like a duck and walks like a duck, but no ducks in those parts - if you follow me!

It is of course possible that the marina staff moved the boat, managed to break the props then put it back and kept mum. But it seems highly unlikely esp as only 0.6 engine hours since the previous visit!


Thanks

 

[rogerthebodger]

I hear you but the prop with the lost blade does not look like corrosion. It looks like a fractured surface.

Corrosion would show much more pitting.

 

[vyv_cox]

It occurred to me overnight that maybe these props are fabricated, i.e. Welded? Never having owned one I don't know. The fracture photo of the failure seems to be where a weld would be. In that case maybe preferential corrosion of the weld filler metal?

 

[rogerthebodger]

It occurred to me overnight that maybe these props are fabricated, i.e. Welded? Never having owned one I don't know. The fracture photo of the failure seems to be where a weld would be. In that case maybe preferential corrosion of the weld filler metal?

IF they are fabricated and the failure is a weld line it seems to me to be a strange fabrication method.

My own prop is fabricated from stainless steel and consists of a boss made from thick wall tube with the 3 blades from laser profiled twisted flat plate TIG welded to the boss at the root of each blade.

 

[Tranona]

It occurred to me overnight that maybe these props are fabricated, i.e. Welded? Never having owned one I don't know. The fracture photo of the failure seems to be where a weld would be. In that case maybe preferential corrosion of the weld filler metal?

Pretty certain they are castings like the equivalent Volvo.

 

[KellysEye]

>You are confusing two effects. The mixture of salt and fresh water makes the anodes ineffective so the props which they were protecting corrode.

'The mixture of salt and fresh water makes the anodes ineffective', no it doesn't the anodes are effective until they get Calcium on them, we got calcium on our anodes from diluted sea water and dived on them and cleaned them. What I said was ' Both boats had been left in the water so long that the anodes were badly eaten away and then covered in calcium, clearly you misread that and missed the words 'Both boat had been left in the water so long that the anodes were badly eaten away. Obviously Calcium then makes make the situation worse.

Since you still don't seem to believe that a mix of salt water and fresh creates stray current corrosion that corrodes props and metal boats I suggest you go and park you boat in Lagos for a year and then let us know what happened to your prop.

 

[VicS]

>You are confusing two effects. The mixture of salt and fresh water makes the anodes ineffective so the props which they were protecting corrode.

'The mixture of salt and fresh water makes the anodes ineffective', no it doesn't the anodes are effective until they get Calcium on them, we got calcium on our anodes from diluted sea water and dived on them and cleaned them. What I said was ' Both boats had been left in the water so long that the anodes were badly eaten away and then covered in calcium, clearly you misread that and missed the words 'Both boat had been left in the water so long that the anodes were badly eaten away. Obviously Calcium then makes make the situation worse.

Since you still don't seem to believe that a mix of salt water and fresh creates stray current corrosion that corrodes props and metal boats I suggest you go and park you boat in Lagos for a year and then let us know what happened to your prop.

What is all this about calcium. Calcium is a reactive metal, so reactive that it reacts with water. It is not going to cover anything underwater.

As far as mixing salt and fresh water creating stray currents that cause corrosion is concerned you were challenged to produce a detailed scientific explanation. We are still waiting for that.

 

[Tranona]

>You are confusing two effects. The mixture of salt and fresh water makes the anodes ineffective so the props which they were protecting corrode.

'The mixture of salt and fresh water makes the anodes ineffective', no it doesn't the anodes are effective until they get Calcium on them, we got calcium on our anodes from diluted sea water and dived on them and cleaned them. What I said was ' Both boats had been left in the water so long that the anodes were badly eaten away and then covered in calcium, clearly you misread that and missed the words 'Both boat had been left in the water so long that the anodes were badly eaten away. Obviously Calcium then makes make the situation worse.

Since you still don't seem to believe that a mix of salt water and fresh creates stray current corrosion that corrodes props and metal boats I suggest you go and park you boat in Lagos for a year and then let us know what happened to your prop.

You are still confusing the process. The corrosion of the props is because the anodes are ineffective. It would be exactly the same if the boats were moored in just salt water.

Fresh water can reduce the effectiveness of zinc anodes and magnesium anodes are suggested for fresh water. Brackish water invariably presents problems for anodic protection - frequently discussed here by folks who keep their boats such situations.

The props corroded away because they were unprotected by anodes - nothing to do with stray currents from a mixture of salt and fresh water.

Happy to consider your "theory" if you can link to a proper scientific paper that supports it.

 

[Piers]

Sold the boat last year after ten years. Original props still good if a little pitted - passed by the surveyor.

The new buyer got on the boat for the first time this year and found no drive on one engine (Yanmar SD saildrives). Then discovered the prop had "fallen off".
Hauled and found the blades had fallen off one prop and the other prop looks pretty sick.
But the anodes look good.

They have kept it on our berth where it has been for a few years.

I assumed this was caused by stray electrolysis. We never kept the boat plugged in but I am not sure if they did -or another boat nearby could have caused an issue?

But how come the anodes look barely worn? Or are some kinds of electrolysis not mitigated by anodes?

Might the yard have put the wrong paint on the props? Hard to see it would cause so much damage though...

As you'll have realised my electrolysis/galvanic knowledge is sketchy. I a puzzled like this and would like to give the new owner helpful advice.

Many thanks

If the nodes looked good but the props had been eaten away, the likelihood is that the props/drive were not (properly) connected to the anodes so the anodes could not protect the props.

If the anodes had been eaten away but not the props, that would indicate the anodes were doing their job.

If the anodes had been eaten away in a lesser time than 'normal' and the boat was connected to shore power, fit galvanic isolators to isolate the boat from other boats connected to shore power but which don't have anodes so are relying on your boat to provide the protection.

 

[Daydream believer]

Saildrives and propellers are isolated from everything else on the boat (or should be). Presumably your stray current explanation applies to those components that are connected to the mains/earth.

Surely the drive from the engine down through the clutch & gear train are all connected
Once one gets to the prop shaft the props often have a rubber bush. However on some ( i cannot speak for all) props the part where the nut actually clamps the prop completes the circuit to the prop. However, that being said on my prop the nut tightens the central bush. & the rubber suporting the blades is outside this . Unfortunately the rope stripper overrides this isolation.
So i am not convinced that any saildrive is isolated from the engine. As the engine is connected to the electrics then the whole lot is connected
Where is the isolation?

 

[Piers]

Surely the drive from the engine down through the clutch & gear train are all connected
Once one gets to the prop shaft the props often have a rubber bush. However on some ( i cannot speak for all) props the part where the nut actually clamps the prop completes the circuit to the prop. However, that being said on my prop the nut tightens the central bush. & the rubber suporting the blades is outside this . Unfortunately the rope stripper overrides this isolation.
So i am not convinced that any saildrive is isolated from the engine. As the engine is connected to the electrics then the whole lot is connected
Where is the isolation?

Jeremy, Daydream believer asks a bottom line qn. I suggest you ask the manufacturer for the definitive answer on isolated or not from which your question of why are the props eaten, will come.

 

[VicS]

Surely the drive from the engine down through the clutch & gear train are all connected
Once one gets to the prop shaft the props often have a rubber bush. However on some ( i cannot speak for all) props the part where the nut actually clamps the prop completes the circuit to the prop. However, that being said on my prop the nut tightens the central bush. & the rubber suporting the blades is outside this . Unfortunately the rope stripper overrides this isolation.
So i am not convinced that any saildrive is isolated from the engine. As the engine is connected to the electrics then the whole lot is connected
Where is the isolation?

Some sail drives are completely isolated from the engine . Some VP units for example. In some cases where they are not isolated the engine electrical system is fully insulated.

Afaik this Yanmar SD is not insulated from the engine.

If a prop is intended to be insulated it could be a mistake to fit rope stripper that bridges this insulation. A bronze prop will be insulated or it will cause corrosion of an aluminium SD leg. Bridging this insulation will cause rapid rate of consumption of the sail drive anode and put the leg itself at risk.

 

[vyv_cox]

Surely the drive from the engine down through the clutch & gear train are all connected
Once one gets to the prop shaft the props often have a rubber bush. However on some ( i cannot speak for all) props the part where the nut actually clamps the prop completes the circuit to the prop. However, that being said on my prop the nut tightens the central bush. & the rubber suporting the blades is outside this . Unfortunately the rope stripper overrides this isolation.
So i am not convinced that any saildrive is isolated from the engine. As the engine is connected to the electrics then the whole lot is connected
Where is the isolation?

VP saildrives are isolated from the engine and everything else. There are dire warnings in the operating manuals about not bridging the saildrive by attaching cables, or even wire-wound hoses. Savageseadog advises that Yanmar units are not isolated, which I did not know.

 

[Tranona]

If the nodes looked good but the props had been eaten away, the likelihood is that the props/drive were not (properly) connected to the anodes so the anodes could not protect the props.

If the anodes had been eaten away but not the props, that would indicate the anodes were doing their job.

If the anodes had been eaten away in a lesser time than 'normal' and the boat was connected to shore power, fit galvanic isolators to isolate the boat from other boats connected to shore power but which don't have anodes so are relying on your boat to provide the protection.

The anode on a saildrive does not protect the prop, but the housing. It is not intended to be in contact with the prop which has a rubber bush and an isolating washer to keep it from contact with the shaft. An aluminium prop (such as the OP has) does not normally suffer from galvanic corrosion as it is not connected to anything else. If a bronze prop is fitted, particularly folding and feathering props which often have stainless pins then an anode is usually fitted to the prop. On some folding props these disappear very quickly.

 

[Chris_Robb]

VP saildrives are isolated from the engine and everything else. There are dire warnings in the operating manuals about not bridging the saildrive by attaching cables, or even wire-wound hoses. Savageseadog advises that Yanmar units are not isolated, which I did not know.

VP used to supply double insulated starter motors to complete the isolation, but you can no longer buy them - even the pattern types. I had to re do all my wiring to match the single pole starters

 

[VicS]

VP saildrives are isolated from the engine

Not always, as Nigel Mercier found out