Bronze Propeller anode depletion to bonding bronze skin fittings

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haydude

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Last November end I relaunched with a new Volvo Penta bronze propeller. This weekend I checked the propeller anodes and they were 80% depleted. That is in less then 3 months!

My boat has a saildrive, the propeller is electrically isolated from the shaft and saildrive. The saildrive anode (also new last November) was showing that it was working, but with extremely little pitting yet.

Before fitting this VP folding bronze propeller I have never been concerned about bonding bronze fittings and anodes. My boat does not have an hull anode, and was delivered by the factory without skin fittings bonding. The shore power earth is not bonded to negative.

My previous boat had exactly the same configuration, no skin fittings bonding and saildrive with original aluminium fixed propeller, and I have never had problems with premature anode depletion.

I searched the web and the forum looking for ever more clues and I read everything I could read about the issue. I haven't found an answer to the rapid propeller anode depletion. The most encouraging post mentioned that his anode depletion rate reduced making it last one season when the propeller aged beyond two years, although he could not tell why.

I have never been concerned about my bronze skin fittings that are not bonded, nor I have ever noticed any unusual corrosion. My previous boat, a Dufour Classic, was surveyed when I sold her, aged 8, I was there when the surveyor scraped the antifouling from the bronze skin fittings and checked from dezincfication and he found none.

What can I do to improve the anode depletion rate on the new propeller, bearing in mind that with a saildrive the prop is isolated so an extra anode is not an option?

Should I fit bonding for my skin fittings?

If bronze skin fittings are fine after 8 years without bonding and anode, why does a bronze propeller need an anode?

What would happen to the propeller if the anodes were gone?
 
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You can console yourself with the thought that it would be the propeller that's corroding away if the anodes weren't on it. I've had a couple of saildrive propellers corrode away, admittedly they didn't have anodes but were electrically isolated. Propeller suppliers will have you believe that with electrical isolation anodes are unnecessary, obviously incorrect. I came to the conclusion without actual proof that stray currents are the problem.
 
A relatively small anode attached directly to a bronze prop with a relatively large surface area is simply going to corrode quite rapidly. That's the chemistry of the situation ... dissimilar metals.. galvanic corrosion and all that stuff.

A plain bronze prop almost certainly, in the majority of cases, does not require the protection of an anode.
Folders however are made of a mixture of alloys. A anode becomes necessary for that reason. Inspect it regularly and replace as appropriate to maintain the protection of the prop

Don't worry about your bronze skin fittings and dont be tempted to start bonding them together or to anything else.

Provided your shorepower installation includes an RCD which protects the whole installation dont worry about the fact that the earth is not bonded to the Dc negative. Not ideal perhaps but acceptable.
 
The corroding of the "bronze" prop is because of the mixed metals in the prop, nothing to do with the through hulls or the saildrive. Not all bronze props have this problem - my Flexofold (on a saildrive) has no anode, but some are well known for behaving like yours. check that you have a galvanic isolator if you are on shorepower. Unfortunately there is no way of adding extra protection by way of larger anodes.
 
check that you have a galvanic isolator if you are on shorepower.
Click to expand...
A galvanic isolator is not necessary if the shorepower earth is not bonded to the DC negative ( or hull anodes when fitted etc)
 
VicS said:
A galvanic isolator is not necessary if the shorepower earth is not bonded to the DC negative ( or hull anodes when fitted etc)
Click to expand...
A friend contacted me about the potential for corrosion of skin fittings on his Dufour. Following a protracted exchange of emails, Dufour have advised that the fittings supplied by them are not bronze, nor DZR, but are leaded brass supplied by Guidi, Italy.
 
vyv_cox said:
A friend contacted me about the potential for corrosion of skin fittings on his Dufour. Following a protracted exchange of emails, Dufour have advised that the fittings supplied by them are not bronze, nor DZR, but are leaded brass supplied by Guidi, Italy.
Click to expand...

but presumably complying with ISO 9093 which apparently states that,

“The materials used shall be corrosion-resistant ........”​
and defines corrosion-resistant as:

“a material used for a fitting which, within a service time
of five years, does not display any defect that will impair tightness, strength or
function.”​
 
VicS said:
A relatively small anode attached directly to a bronze prop with a relatively large surface area is simply going to corrode quite rapidly. That's the chemistry of the situation ... dissimilar metals.. galvanic corrosion and all that stuff.
Click to expand...

My Volvo Penta folding has stainless steel pins (large) holding each of the three blades, the pins are held in place by stainless steel screws with hex head.

What if those were bronze instead of stainless steel?
Would that small mass cause the premature anode depletion?
Could stray current in the marine be causing anode depletion even if the propeller is isolated from the shaft and saildrive?
 
Proper bronze through hulls are made with 85-5-5-5 bronze and rarely suffer from dezincification. Propellers on the other hand are most often manganese bronze which contains up to 39% zinc.
Not sure about the Italian brass through hulls though.
 
VicS said:
but presumably complying with ISO 9093 which apparently states that,

“The materials used shall be corrosion-resistant ........”​
and defines corrosion-resistant as:

“a material used for a fitting which, within a service time
of five years, does not display any defect that will impair tightness, strength or
function.”​
Click to expand...

Indeed, and the basis of YM's seacock campaign. I have a statement from Guidi, who supply many upmarket boatbuilders with underwater fittings in the same materials, to the effect that this is excellent material for the duty and they see no problem with it. Presumably the words 'Random Harvest' mean nothing to them.
 
haydude said:
My Volvo Penta folding has stainless steel pins (large) holding each of the three blades, the pins are held in place by stainless steel screws with hex head.

What if those were bronze instead of stainless steel?
Would that small mass cause the premature anode depletion?
Could stray current in the marine be causing anode depletion even if the propeller is isolated from the shaft and saildrive?
Click to expand...

You might want to ask Volvo why it has happened. Expect they have been asked many times so should be able to give you a good answer!
 
mitiempo said:
Proper bronze through hulls are made with 85-5-5-5 bronze and rarely suffer from dezincification.
Click to expand...
Interesting. I asked Aquafax what their bronze through hulls were made of and they replied " UNI 7013/8 G-CuSn5Zn5Pb5 frequently referred to as LG2 Gunmetal". Is that the same stuff ?

Boo2
 
Boo2 said:
Interesting. I asked Aquafax what their bronze through hulls were made of and they replied " UNI 7013/8 G-CuSn5Zn5Pb5 frequently referred to as LG2 Gunmetal". Is that the same stuff ?

Boo2
Click to expand...

You appear to be quoting the same analysis as mitiempo!

Admiralty gunmetal contains 10% Sn and only 2% Zn.

LG2 is often used as a substitute for admiralty gunmetal. I imagine the small % of lead improves its machinability. It is sometimes known as red brass
 
When I bought my boat new I was told that there was a known problem with the propellor eating anodes and that Volvo was aware of it. The prop was a 3 bladed Volvo folder on a 120S Saildrive. Volvo's solution was to supply me with another propellor made with a different metallic compostion. So maybe the problem is with the Volvo prop. Sorry to disappoint but the new problem actually made the problem worse. It's now in my den acting as a hi-fi headphone stand.
 
I have this problem too. I would like to know for sure if the anode is really necessary.
Also, I wonder if the Ambassador rope stripper affects the circuit.
The saildrive anode doesn't erode much. The prop anodes went very quickly during the first year.
 
We have a similar set-up on a new boat (launched April 2011). After 6 months, the prop anode was about 50% gone, and we replaced it. After 10 months, we've just replaced it again (again about 50% gone). We are keeping the remnants in case we get caught short!

The saildrive anode, after 10 months, was about 30-40% gone. But here lies a problem - it is a standard Volvo anode machined to accommodate a stripper, and we are having to mill away some perfectly good zinc from the Volvo replacement to get it to fit! Again, we'll keep the eroded one just in case.
 
Chi34 said:
The prop anodes went very quickly during the first year.
Click to expand...

Do you mean that following years were better? If that is the case I read a similar report. Could it be that the new metal is more active and it stabilizes after the initial oxidation?

In the meantime I found out that Volvo Penta decalares that their folding propellers are a nichel+aluminium+copper alloy.
 
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