Prop Anode Protection

[Tex]

I have a galvanic isolator but despite this my prop anode does not appear to last much longer than before this was fitted.
Each season I polish up prop before relaunch and do not treat with anything or paint. The results are a pretty clean prop at the end of the season with minimal fouling but I wonder if having the prop like this increases the deterioration of the anode.
Would painting the prop with antifoul help?

 

[vyv_cox]

Prop anodes are quite small in most cases, so adding a shaft anode, if you have a shaft, will improve their life by a significant amount. My Autoprop anode alone lasts one season, but with a shaft anode it does two seasons and the shaft anode does three. Painting the prop reduces the area affected by galvanic corrosion and will improve anode life.

 

[Quandary]

Prop anodes are quite small in most cases, so adding a shaft anode, if you have a shaft, will improve their life by a significant amount. My Autoprop anode alone lasts one season, but with a shaft anode it does two seasons and the shaft anode does three. Painting the prop reduces the area affected by galvanic corrosion and will improve anode life.

Surely if you paint a prop, particularly a folder you still have a smaller area of surface unpainted and exposed which coincides with the location most vulnerable to corrosion, extending anode life at the expense of deterioration at folding gears or bearing surfaces is not really an economy? And a galvanic isolator gives protection against stray currents but does not necessarily mean that normal galvanic deterioration will be reduced. A hanging anode may be another option if it is a saildrive rather than a shaft, though my experience is that it extends the life of leg anodes but does not benefit the shaft, not surprising given the mix of metals in close proximity down there?
Or is my understanding of galvanic corrosion too simplistic?

 

[VicS]

Surely .......
Or is my understanding of galvanic corrosion too simplistic?

Painting a prop, even the blades of a folder. will reduce the area "interacting" with the anode(s) so should reduce the rate of loss of zinc from the anodes. It should not reduce the effectiveness of the anode to protect vulnerable parts.

The galvanic isolator is fitted to block the path of galvanic currents in the earth conductor of the shorepower supply. It is necessary if the shorepower earth is bonded ( as the latest standards require it to be) to the DC negative/ anodes etc and if the shorepower is connected for prolonged periods. It has no effect on local galvanic corrosion occurring as a result of mixed metals on board.

A hanging anode must be connected to what it is to protect. Easy to clip its lead to a prop shaft inboard of a stern gland to enhance the protection of the stern gear. Likewise can be clipped to a sail drive to improve the protection of the saildrive leg but not really possible to clip it to the prop on a sail drive to protect that.

 

[AliM]

That is very helpful, Vic

We have a Volvo sail drive and Volvo folding prop. The sail drive leg anode lasts several years, but the prop anode only lasts about 5 months. Is there anything we can do to make the latter last longer?

 

[Tex]

Thinking of using TRILUX 33, can anyone advise on this and will the prop need any special preparation?

 

[Kerenza]

Volvo folding props are a good case to illustrate galvanic corrosion. With the leg isolated and the prop isolated from the leg, it's effectively just hanging around in the water. The fact that the anodes corrode is simply due to the components in the prop - the body and blades of one material and the stainless fittings, including the screws holding the anodes on.
By painting or otherwise protecting some of the surface area of the blades and body there is a reduction in the area available to react, as stated previously. However good results are had by further isolating any of the stainless parts possibl - the ends of the pin bearings and especially the anode screws. I use nail varnish, although no difference is detectable between ruby red and apple green yet.
I do wonder if a specialist screw, possibly of non metallic material, would help - I always make sure the anode makes good clean contact with the prop, without relying on the screw for electrical continuity. Also perhaps a non metallic bearing pin.
?

 

[VicS]

Volvo folding props are a good case to illustrate galvanic corrosion. With the leg isolated and the prop isolated from the leg, it's effectively just hanging around in the water. The fact that the anodes corrode is simply due to the components in the prop - the body and blades of one material and the stainless fittings, including the screws holding the anodes on.
By painting or otherwise protecting some of the surface area of the blades and body there is a reduction in the area available to react, as stated previously. However good results are had by further isolating any of the stainless parts possibl - the ends of the pin bearings and especially the anode screws. I use nail varnish, although no difference is detectable between ruby red and apple green yet.
I do wonder if a specialist screw, possibly of non metallic material, would help - I always make sure the anode makes good clean contact with the prop, without relying on the screw for electrical continuity. Also perhaps a non metallic bearing pin.
?

It is not the mix of materials in the prop that is the cause of the anode corroding rapidly ..... it would probably corrode just as fast if the prop was was all bronze or whatever the major material is.

The anode is fitted because the prop is made of a mix of materials. Without it the most anodic components would corrode, the bronze parts if other bits and pieces are stainless steel.

 

[Kerenza]

Hmm.... Are you indicating an isolated alloy prop suspended in electrolyte will galvanically corrode at a significant rate?

Without doubt the addition of either a stainless shaft or other coupled components will accelerate the local reaction, so anything which can inhibit this is a good thing - I don't think polarity reversal through corrosion is a problem for us.

 

[vyv_cox]

Hmm.... Are you indicating an isolated alloy prop suspended in electrolyte will galvanically corrode at a significant rate?

Without doubt the addition of either a stainless shaft or other coupled components will accelerate the local reaction, so anything which can inhibit this is a good thing - I don't think polarity reversal through corrosion is a problem for us.

If your aluminium prop was entirely made of that metal and you attached a zinc anode to it, immersed in seawater, the zinc would corrode by galvanic action. If the aluminium prop had pins and other parts of a different metal, or was mounted directly onto a stainless steel shaft, the anode would still corrode, protecting both the prop and the other parts.

 

[GrahamM376]

The anode on my Featherstream connects to the prop all around its base area but was eroding very badly around two the bolt holes, its thinnest part, so having to be binned with plenty of meat left on the rest. I found that antifouling the bolts and bolt holes stopped this and it now erodes more evenly, giving two years insted of one.

 

[vyv_cox]

The anode on my Featherstream connects to the prop all around its base area but was eroding very badly around two the bolt holes, its thinnest part, so having to be binned with plenty of meat left on the rest. I found that antifouling the bolts and bolt holes stopped this and it now erodes more evenly, giving two years insted of one.

When I first had my Bruntons Autoprop the anode bolts were a stainless steel of some sort. The same thing happened and sometimes the anode would fall off after quite a short time. Now they are supplied with nylon bolts instead. I always abrade the prop and anode mating surfaces to ensure a good electrical contact. They last two years at least.

 

[Kerenza]

If your aluminium prop was entirely made of that metal and you attached a zinc anode to it, immersed in seawater, the zinc would corrode by galvanic action. If the aluminium prop had pins and other parts of a different metal, or was mounted directly onto a stainless steel shaft, the anode would still corrode, protecting both the prop and the other parts.

Sorry to labour the point, but you didn't answer the question.

 

[VicS]

Hmm.... Are you indicating an isolated alloy prop suspended in electrolyte will galvanically corrode at a significant rate?

Not necessarily Galvanic corrosion occurs when two dissimilar metals are connected together electrically and immersed in an electrolyte.

 

[Kerenza]

Not necessarily Galvanic corrosion occurs when two dissimilar metals are connected together electrically and immersed in an electrolyte.

Something strange going on here- the email telling me there was a reply to my post read:

No. Galvanic corrosion occurs when two dissimilar metals are connected together electrically and immersed in an electrolyte.

Which is a much clearer reply.

Now if the said item is unlikely to corrode, why would you attach an anode to it?
As you said :
it would probably corrode just as fast if the prop was was all bronze or whatever the major material is.

I don't think there is any difference in our Appreciation and understanding of underwater corrosion here.

One or more Dissimilar metals electrically connected and suspended in an electrolyte will lead to one of the metals corroding more than the others. By introducing a sacrificial element we can influence which corrodes the most.

Other methods available include reducing the area of metal we wish to protect by coating and preventing the electrical connection of component parts, by insulation or use of non conducting fastners.

BW
Kerenza

 

[VicS]

Something strange going on here- the email telling me there was a reply to my post read:

No. Galvanic corrosion occurs when two dissimilar metals are connected together electrically and immersed in an electrolyte.

Which is a much clearer reply.

E mail jumped the gun a bit there. Edited to read "Not necessarily" just to be not quite so emphatic about it because some pedant would be bound to find an a exception to a plain "No" (Thinking about a 60:40 brass for example which will dezincify all on its own in complete isolation I understand by a similar mechanism between the α and β phases.)

Now if the said item is unlikely to corrode, why would you attach an anode to it?
As you said :
it would probably corrode just as fast if the prop was was all bronze or whatever the major material is.

No need to fit an anode to something that wont corrode .
I was referring to the anode corroding just as fast if the prop is all bronze as it would if the prop was mix of metals or alloys.

One or more Dissimilar metals electrically connected and suspended in an electrolyte will lead to one of the metals corroding more than the others. By introducing a sacrificial element we can influence which corrodes the most.

The idea of a sacrificial anode is that it is the component which corrodes and in so doing protects the other components

 

[PabloPicasso]

That is very helpful, Vic

We have a Volvo sail drive and Volvo folding prop. The sail drive leg anode lasts several years, but the prop anode only lasts about 5 months. Is there anything we can do to make the latter last longer?

The MOST IMPORTANT THING is DO NOT USE COPPER BASED anti foul on your aluminium sail drive leg!

I have always had problems with Volvo folding prop anodes on a sail drive. Often changing the hub anodes after just 3 months afloat. This is a known Volvo prop problem (why don't they offer supersize prop anodes as an option?).

But this year I had to move the boat from where it's always been based in a big commercial port on a river, to a quiet salt water harbour away from the marina. And just one set of annodes, which have clearly been doing their job, but for the first time ever I'll have to consider leaving them on for second season! Wow!

 

[fastjedi]

"why don't they offer supersize prop anodes as an option" ... damn right!

We moved from Portsmouth Harbour (Wicormarine) where our Volvo folding propeller anode would comfortably do a season afloat to the River Hamble. Last year (March > October) we went through 2.5 sets of prop anodes. This year (April > October) we will just about do it on 2 sets. I have free dived under the boat 4 times this year. Three times to check and once to change them. I can do it in 6 dives ..... 3x remove + 3x fit. Oh how I wish someone would make oversize anodes!

BTW: Non genuine Volvo anodes are less effective / last longer!

 

[VicS]

"why don't they offer supersize prop anodes as an option" ... damn right!

We moved from Portsmouth Harbour (Wicormarine) where our Volvo folding propeller anode would comfortably do a season afloat to the River Hamble. Last year (March > October) we went through 2.5 sets of prop anodes. This year (April > October) we will just about do it on 2 sets. I have free dived under the boat 4 times this year. Three times to check and once to change them. I can do it in 6 dives ..... 3x remove + 3x fit. Oh how I wish someone would make oversize anodes!

BTW: Non genuine Volvo anodes are less effective / last longer!

Maybe some reason why the anodes are not lasting . Is this in a marina with shorepower connected

 

[SimbaDog]

"why don't they offer supersize prop anodes as an option" ... damn right!

We moved from Portsmouth Harbour (Wicormarine) where our Volvo folding propeller anode would comfortably do a season afloat to the River Hamble. Last year (March > October) we went through 2.5 sets of prop anodes. This year (April > October) we will just about do it on 2 sets. I have free dived under the boat 4 times this year. Three times to check and once to change them. I can do it in 6 dives ..... 3x remove + 3x fit. Oh how I wish someone would make oversize anodes!

BTW: Non genuine Volvo anodes are less effective / last longer!

http://www.asap-supplies.com/marine/hanging-mooring-anodes/mgduff-zinc-hanging-anode-zd57-812029