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Anodes Question


Peter009

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Hi All, I am getting a bit of conflicting advice so wanted to check here, my understanding is that the hull underneath the anodes should be painted but never the anodes, am I correct in this as I have been advised to not paint the hull under the anodes and also advised to paint the back of the anodes next to the hull and also not to paint the hull at all where the anodes sit,  but am not sure whether that is true or not.

 

Also we have 4 large anodes 2 at the front and 2 at the back of the boat, would it be wise to have one either side in the centre of the hull as well as our survey did pick up that we only had 4 anodes and we have a 60 x 12 boat.  I also am thinking of taking the welded anodes off and putting bolt on ones on for ease of changing them.

 

Any advice would be greatly appreciated as we do not want to have spent all the time on protecting the hull with good paint only to find we have insufficient anodes so want to make sure we have got this right as we are now moving in 5 weeks from a boat yard to the canals thanks

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To explain how anodes work, will help you understand the good advice from Tony B above.  If the boat had no exposed metal (excluding the anodes) then it would not corrode, and you would not need anodes, but with far from perfect paint - bumps, scrapes, old mill scale falling off, exposed prop & prop shaft etc, then there will be bare metal which anodes will help to protect.  So you would not want to deliberately leave areas of your steel boat unpainted as this will accelerate the rate of anode metal loss (protecting the bare steel) requiring more frequent anode replacement - which is good for those that sell and fit anodes.  As to how many anodes, anodes have a protecting effect that decreases with distance, so the longer the boat, the more anodes the better (within reason) but anodes placed mid way along the side are at great risk of being whacked and bashed which is not good, so many people don’t consider the risk of damage to be worth the benefit.  If however it was a static houseboat that never moves, then the risk of impact is nil, and for a big boat mid anodes would be worthwhile.

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But you can also 'over-anode' a boat which can itself causes corrosion.

 

Common Symptoms Of Over Protection

Paint Blistering

The hydrogen blistering of paint occurs because the adhesion between the coating and the base to which it was applied (or substrate) is being destroyed. This can be often be seen near where zinc anodes are attached to fibreglass hulls. The process of blistering is referred to as “cathodic disbondment”.

Paint blistering on wooden or metallic hulls can also be caused by alkali attack. Over protection can lead to the formation of alkaline conditions on metallic hulls when there is insufficient flow of water to return the water to its natural pH, resulting in accelerated corrosion. This problem can be enhanced as the paint blisters whereby creating an even more alkaline condition which leads to more rapid deterioration of the submerged metal.

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Others will disagree but I would put anodes around the stern where dissimilar metals are, prop etc. They are likely to be actually useful. A couple on the front will do no harm, the metal has been bent to shape there and I my experience (certainly not scientific) formed and shaped metal tends to develop more pits ( please don't let this become some sort of urban myth!)  An anode has an effective 'range' so you would probably need quite a few on a long boat. What Alan says is quite correct as well. You will possibly find that the paint a few inches away from the anode has little pin prick blisters, farther away there will be no blisters At the end of the day though its the paint that is important. Good paint, 3 or 4 coats, checked every 2 or 3 years will do the job better than any number of anodes on a freshwater boat.

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1 hour ago, Alan de Enfield said:

But you can also 'over-anode' a boat which can itself causes corrosion.

 

Common Symptoms Of Over Protection

Paint Blistering

The hydrogen blistering of paint occurs because the adhesion between the coating and the base to which it was applied (or substrate) is being destroyed. This can be often be seen near where zinc anodes are attached to fibreglass hulls. The process of blistering is referred to as “cathodic disbondment”.

Paint blistering on wooden or metallic hulls can also be caused by alkali attack. Over protection can lead to the formation of alkaline conditions on metallic hulls when there is insufficient flow of water to return the water to its natural pH, resulting in accelerated corrosion. This problem can be enhanced as the paint blisters whereby creating an even more alkaline condition which leads to more rapid deterioration of the submerged metal.

 

Why would you put anodes on a fibreglass or wooden hull?

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Just now, Mac of Cygnet said:

 

Why would you put anodes on a fibreglass or wooden hull?

Because of all the metal fittings :

 

I have a GRP boat with :

11 bronze sea-cocks / hull openings below the water line.

2x steel shafts

2x Bronze propellers

2x stern glands

2x steel P-Brackets

 

Without anodes the sea-cocks, and propellers would become very brittle and shatter.

 

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5 hours ago, Tony Brooks said:

The anodes must make a good electrical connection to the hull but the welded on straps will do this. Paint the hull, don't paint the anode, locally repaint the strap, hull and weld after fitting.

Thanks Tony that is great.  I have just checked the survey which states the below

 

The vessel has four, believed 5kg anodes, these marked magnesium (i.e. for freshwater), two at the bow and two at the stern, all welded on. The underwater area will be approximately 70m2 which would require around 20kg. There are many variables involved in the amount of erosion and hence anodes a hull might require so given the above is on the lower limit, it is recommended you check the anodes regularly after the vessel is launched. If their erosion is rapid and more are needed, these can be hung from the sides when not underway using wire or chain. Provided they are electrically bonded to the hull there is no need for them to be welded or bolted. Care should be taken to ensure all electrical fittings are well installed and adequately insulated, the engine starting circuit being isolated when not in use, otherwise there is a risk of stray currents causing excessive erosion. Particular care should be exercised with mains/240v current installation which, apart from shock hazards, will rapidly erode underwater metals if there is any current leakage. For complete security a galvanic isolator can be fitted which ensures there is no current leakage to or from neighbouring boats or steel structures.

 

Given the above information putting another 5 kg anodes either side in the centre exactly as you described should help protect the hull more, am I correct that it would be wise to do that ?  also it is mentioned for complete security a galvanic isolator wondering if this is a big job to do as I would like to protect our hull the best we can.

 

Also the hull is painted in Intertuf 2 pack


Thanks again

Edited by Peter009
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3 hours ago, Chewbacka said:

To explain how anodes work, will help you understand the good advice from Tony B above.  If the boat had no exposed metal (excluding the anodes) then it would not corrode, and you would not need anodes, but with far from perfect paint - bumps, scrapes, old mill scale falling off, exposed prop & prop shaft etc, then there will be bare metal which anodes will help to protect.  So you would not want to deliberately leave areas of your steel boat unpainted as this will accelerate the rate of anode metal loss (protecting the bare steel) requiring more frequent anode replacement - which is good for those that sell and fit anodes.  As to how many anodes, anodes have a protecting effect that decreases with distance, so the longer the boat, the more anodes the better (within reason) but anodes placed mid way along the side are at great risk of being whacked and bashed which is not good, so many people don’t consider the risk of damage to be worth the benefit.  If however it was a static houseboat that never moves, then the risk of impact is nil, and for a big boat mid anodes would be worthwhile.

Hi there thanks for this and tony's advice, when you say the more anodes the better within reason do you mean not to put them in the centre sides due to the risk should I perhaps put the anodes next to the existing ones if I get more to add to the hull rather than the centre ? 

 

 

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16 minutes ago, Peter009 said:

If their erosion is rapid and more are needed, these can be hung from the sides when not underway using wire or chain

This is a very common method for lumpy water boats to increase their anode numbers/protection if coming into a marina.

 

canadametal-cmgroupera-02.jpg

 

 

Edited by Alan de Enfield
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1 hour ago, Alan de Enfield said:

This is a very common method for lumpy water boats to increase their anode numbers/protection if coming into a marina.

 

canadametal-cmgroupera-02.jpg

 

Thanks for this where are these available as just had a quick search on ebay and they sell them in the US a lot 

 

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5 hours ago, Chewbacka said:

To explain how anodes work, will help you understand the good advice from Tony B above.  If the boat had no exposed metal (excluding the anodes) then it would not corrode, and you would not need anodes, but with far from perfect paint - bumps, scrapes, old mill scale falling off, exposed prop & prop shaft etc, then there will be bare metal which anodes will help to protect.  So you would not want to deliberately leave areas of your steel boat unpainted as this will accelerate the rate of anode metal loss (protecting the bare steel) requiring more frequent anode replacement - which is good for those that sell and fit anodes.  As to how many anodes, anodes have a protecting effect that decreases with distance, so the longer the boat, the more anodes the better (within reason) but anodes placed mid way along the side are at great risk of being whacked and bashed which is not good, so many people don’t consider the risk of damage to be worth the benefit.  If however it was a static houseboat that never moves, then the risk of impact is nil, and for a big boat mid anodes would be worthwhile.

Hi Chewbacka, when you say risk of being whacked in the centre, is it not the same risk wherever the anodes are on the boat if they are in the same line ?

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Anodes on the swims allow the main hull sides to protect them from impact with rocks etc. On the hull side they are less well protected, especially on hulls vertical sides and little base plate overhang.

 

You can over anode an area and then I understand paint may flake off the hull.

 

Anodes only have a relatively limited area of protection, say 2m all round.

 

You pays your money and makes your choice.

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13 minutes ago, Tony Brooks said:

Anodes on the swims allow the main hull sides to protect them from impact with rocks etc. On the hull side they are less well protected, especially on hulls vertical sides and little base plate overhang.

 

You can over anode an area and then I understand paint may flake off the hull.

 

Anodes only have a relatively limited area of protection, say 2m all round.

 

You pays your money and makes your choice.

Thanks Tony, yes of course that would make sense not to put in the centre, as they are protected where they are, I guess having a couple more anodes above the ones we already have might be an option to increase protection but dont want to overdo it either.  Perhaps I might just put some small anodes that were given to me in the yard towards the stern of the boat and leave the others alone and check the boat in a year after its been launched.

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1 hour ago, Peter009 said:

If they have them on a canal boat, they've wasted 79 quid a pop. We need Magnesium, not Aluminium, anodes.

 

Don't overthink this. Some boatbuilders don't even believe they make a difference; some surveyors recommend everyone has additional anodes along the sides. Most boats have a happy medium of a pair fwd and a pair aft and few are lost without trace.  I'd suggest you're hull us at more risk from being connected to shore supply whilst not having a Galvanic Isolator or Isolation Transformer, so put the effort into getting your earth arrangements right. :)

 

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3 minutes ago, Sea Dog said:

If they have them on a canal boat, they've wasted 79 quid a pop. We need Magnesium, not Aluminium, anodes.

 

Don't overthink this. Some boatbuilders don't even believe they make a difference; some surveyors recommend everyone has additional anodes along the sides. Most boats have a happy medium of a pair fwd and a pair aft and few are lost without trace.  I'd suggest you're hull us at more risk from being connected to shore supply whilst not having a Galvanic Isolator or Isolation Transformer, so put the effort into getting your earth arrangements right. :)

 

Hi seadog, sorry clicked the wrong one meant to copy the magnesium ones.  I agree with you that the effort should be in the Galvonic Isolator as we will be in a busy marina I will ask the electrician if he can fit one as he is in next week I may put the other small magnesium anodes I have on the hull above the existing ones as I have them anyway.  Cheers

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15 minutes ago, Peter009 said:

Hi seadog, sorry clicked the wrong one meant to copy the magnesium ones.  I agree with you that the effort should be in the Galvonic Isolator as we will be in a busy marina I will ask the electrician if he can fit one as he is in next week I may put the other small magnesium anodes I have on the hull above the existing ones as I have them anyway.  Cheers

Just found this on ebay maybe I dont need an electrician if I buy this for the shore power he has good feedback just wondering if this is a good enough solution 

 

https://www.ebay.co.uk/itm/Galvanic-Isolator-Boat-500A-Lifetime-Guarantee-Easy-Install-Top-Quality/292567881656?hash=item441e6797b8:g:kN0AAOSwNNVa~b6J

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14 minutes ago, Peter009 said:

Just found this on ebay maybe I dont need an electrician if I buy this for the shore power he has good feedback just wondering if this is a good enough solution 

 

https://www.ebay.co.uk/itm/Galvanic-Isolator-Boat-500A-Lifetime-Guarantee-Easy-Install-Top-Quality/292567881656?hash=item441e6797b8:g:kN0AAOSwNNVa~b6J

It’ll be fine until it isn’t. How would you know if it’s working or not?

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Safeshore Marine with some form of monitoring.  Even then there are questions over the surge rating. You can get thousands of amps current surge when a short circuit occurs before a fuse or breaker trips. If that blew the diodes in the isolator short circuit you would have no isolation and if they failed open circuit no earth protection. hence the need for a method of monitoring the diodes. If you know how to do it checking with a multimeter is easy enough but as you are dealing with mains probably bets not to do it if unsure.

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14 minutes ago, Alan de Enfield said:

This one does have some idiot lights to show you what is going on :

Unless I’m missing something they still won’t indicate an open circuit. 

 

That’s my biggest worry with a cheap GI - no circuit at all. And you’d never know unless you checked it with a multimeter. 

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10 minutes ago, WotEver said:

Unless I’m missing something they still won’t indicate an open circuit. 

 

That’s my biggest worry with a cheap GI - no circuit at all. And you’d never know unless you checked it with a multimeter. 

What do you use ?

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