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Post by MalcolmP on Jun 22, 2019 14:07:40 GMT
On our recent haul out after being afloat for 11 months I found classic signs of dezincification on the P bracket.  I do not leave the shore power connected when we are on board and have a galvanic isolator on the shore feed when we are hooked up. Normally I have just a prop anode and a nearby Ionguard retractable anode and 11 seasons these have only ever shown modest corrosion. I often fully burnish the prop, shaft and P bracket and know there has never previously been any sign of electrolysis. I also have checked the earthing braid that is connected from the P bracket and it looks in good order, but it connects into the wiring loom on our 39i so not quite sure where the factory earthed it to. I am convinced this must have occurred in the marina where we overwintered due to some boat or pontoon nearby having stray mains current on the water, it is salt water. We are now on a new marina and non of the other yachts hauled out there show any signs at all of dezincing The depth of pitting did initially concern me on terms of possible loss of strength of the P bracket, but I then decided to do what Vasko did a few years ago and drill the P bracket to bolt directly on a couple of disk anodes. This confirmed to me at least that the pink copper element is only on the surface as the drilling swafe all was golden bronze throughout. It is just an 8mm hole and I purposely drilled it relatively high up to reduce leverage on the theoretical line of weakness. I am convinced this will not have any significant impact on the brackets real strength and having two big lumps of zinc there hopefully will reduce the risk of future stray current issues. Interestingly neither the prop, an autoprop or the shaft, which is made of duplex stainless now, showed any issue. The photos are linked below, sorry they are not inserted but doing this post from my phone and no WiFi so will have to edit later  www.23hq.com/Jeanneau/photo/56319575/original www.23hq.com/Jeanneau/photo/56319597/original www.23hq.com/Jeanneau/photo/56319621/original www.23hq.com/Jeanneau/photo/56319641/original |
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Post by vasko on Jun 22, 2019 16:34:58 GMT
On our recent haul out after being afloat for 11 months I found classic signs of dezincification on the P bracket. I do not leave the shore power connected when we are on board and have a galvanic isolator on the shore feed when we are hooked up. Normally I have just a prop anode and a nearby Ionguard retractable anode and 11 seasons these have only ever shown modest corrosion. I often fully burnish the prop, shaft and P bracket and know there has never previously been any sign of electrolysis. I also have checked the earthing braid that is connected from the P bracket and it looks in good order, but it connects into the wiring loom on our 39i so not quite sure where the factory earthed it to. I am convinced this must have occurred in the marina where we overwintered due to some boat or pontoon nearby having stray mains current on the water, it is salt water. We are now on a new marina and non of the other yachts hauled out there show any signs at all of dezincing The depth of pitting did initially concern me on terms of possible loss of strength of the P bracket, but I then decided to do what Vasko did a few years ago and drill the P bracket to bolt directly on a couple of disk anodes. This confirmed to me at least that the pink copper element is only on the surface as the drilling swafe all was golden bronze throughout. It is just an 8mm hole and I purposely drilled it relatively high up to reduce leverage on the theoretical line of weakness. I am convinced this will not have any significant impact on the brackets real strength and having two big lumps of zinc there hopefully will reduce the risk of future stray current issues. Interestingly neither the prop, an autoprop or the shaft, which is made of duplex stainless now, showed any issue. The photos are linked below, sorry they are not inserted but doing this post from my phone and no WiFi so will have to edit later www.23hq.com/Jeanneau/photo/56319575/originalwww.23hq.com/Jeanneau/photo/56319597/originalwww.23hq.com/Jeanneau/photo/56319621/originalwww.23hq.com/Jeanneau/photo/56319641/originalThat’s a huge anode  my one a lot smaller |
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Post by Trevor on Jun 23, 2019 12:46:04 GMT
HI Malcolm,
This is an issue we all fear confronting. Two mechanisms are at play in underwater corrosion of expensive metal bits.
1. Galvanic corrosion. This is a spontaneous process (no battery required) caused by two metals being immersed in an electrolyte (salt water) and having an electrical connection between them. The galvanic potential difference between the two metals causes a current to flow between the two metals. The different potential between the two metals can be seen on a galvanic potential scale.
This form off corrosion is limited to voltage potential developed on the Galvanic scale. It is always less than about 1 volt so is not fast acting.
2. Electrolysis. (Stray current corrosion) This is when some external voltage is superimposed on the underwater metal and causes a current to flow.
This form of corrosion can be from the 12 or 24 volt system and can be brutally fast at destroying underwater equipment.
Protection can take two forms.
1. All underwater metal bonded together electrically and connected to an underwater sacrificial anode.
2. Underwater bits remain isolated apart from shaft, prop and P bracket.
Jeanneau do not bond all of their underwater bits and attach those to an anode. Jeanneau only bond prop shaft and P bracket and on shaft drive yachts rely on one anode on the stainless shaft and one on a fixed prop to protect both prop, shaft and P bracket..
Many boats do bond all underwater bits and connect them to an anode.
Advantages of total bonding.
1. In the event of any stray current, the return path is very low resistance and will blow the fuse feeding that path. Leakage current is then brought to the attention of owner prior to damage being caused.
2. All underwater metal is at the same potential and the sacrificial anode is destroyed protecting the underwater metal.
Disadvantage of total bonding.
1. In the event that the yacht is located near an electrically leaky boat, the potential gradient in the water causes current to flow in the bonding system causing electrolysis corrosion on bonded metal.
Advantages of isolated system.
1. Potential gradient in water caused by being located near an electrically leaky boat does not cause any current to flow, hence no impact on underwater metal.
Disadvantage of isolated system.
1. If any voltage accidentally appears on an underwater metal part, it will be destroyed by the electrolysis current.
The braid from the P bracket goes to the yacht ground which is really the engine block. This is bonded to the stainless shaft through the transmission. That connection is important so if a flexible coupling has been used it must be electrically conductive.
The shaft anode changes the galvanic potential of the shaft and the P bracket. ( with respect to a Silver/silver half cell.) It is this test that allows you to know that the anode is doing its job as the anode changes the potential of the entire group of components (shaft, prop, P bracket) Without the anode, the metals will adopt a potential of their alloy group. With the anode, the metals will adopt a potential as provided by the anode.
The anode should lower the potential of the bonded components by about 200mV compared to the lowest potential alloy of the connected metals. In this case that would be the stainless shaft.
If the manganese bronze P bracket has lost Zinc it has been acting as the least noble metal and acting as an anode and not protected.
Perhaps check
1. Electrical resistance between the shaft and the P bracket. That will be isolated at the cutlas bearing but should be VERY low resistance through the engine block and transmission. If a flexible coupling has been used it should be jumpered electrically.
2. Test the potential of the shaft, P bracket and prop by checking voltage potential compared to a silver silver half cell. That will tell you if the anode is doing its job. The engine block, shaft, braid going into the hull at the P bracket should read exactly the same reading.
I doubt this problem is attributed to leakage from another yacht. It doesn't look bad enough for an electrolysis fault which can just be so destructive. It looks like galvanic corrosion.
It is a bad problem because it is hard to know exactly what is happening. You do have a little bit of corrosion on the shaft between the P bracket and the rope cutter but probably not a big problem. Great photos by the way!
I hope this helps.
Regards,
Trevor
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Post by MalcolmP on Jun 23, 2019 13:12:42 GMT
Many thanks indeed Trevor for that comprehensive reply.
I will follow up your suggestions as far as I can, boat is now back in water so will have to measure on the p bracket earth strap.
I did add a CV type joint last year when I put in the new duplex shaft, so will look at that carefully, but it is a bronze and stainless Sigmadrive not a flexible, so expect before testing that it will be fairly conductive.
The cutless bearing was also new, I think quite a few are made of a carbon rich rubber so likely to conduct, bit I can't check that until next haul out.
The zinc's I have added are as Vasko pointed out are on the large side and maybe that might create another issue as aware that you can over zinc.
Might you explain a bit more about the silver cell test, what and how to do
Many thanks Malcolm
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Post by Trevor on Jun 23, 2019 14:43:24 GMT
Hi Malcolm,
In order to know the potential of the metals when under the water, a convenient reference is required. A few different references have been used but a common one these days is called a silver-silver chloride half cell. It is called a half cell because it represents half a battery cell and when combined with another metal in water will produce a complete battery cell and hence produce a voltage that can be conveniently read on a multimeter.
The reference half cell is lowered over the transom into the water about a metre and the end of the lead is connected to the positive input of a multimeter. It comes with quite a long lead so the multimeter can be down in the bilge if required. The negative lead of the multimeter is touched to the underwater metal of interest. If the metal does not have any connection to a sacrificial anode it will have its natural galvanic potential as referenced to the silver-silver chloride half cell. If it is connected to a sacrificial anode the anode changes the potential of the metal, proving it is connected and hence protected.
The potentials are available in a table of the galvanic series. You could check the metal you are concerned about, for instance the braid at the top of the P plate ( provided it is connected) and the prop shaft. They should be exactly the same potential and more negative than the unprotected metal would be. This gives a positive and scientific way of proving the metals are actually protected by the sacrificial anode because the underwater galvanic potential of the metals is greater when attached to the zinc anode.
I forgot to mention that normally the P bracket would be protected by the shaft anode because it is connected to the shaft by the braid. In your case you have separately protected the P bracket with a separate anode. In this case, I figure the the braid could be disconnected without any impact on the protection of either shaft or P bracket, provided both anodes are diligently serviced with each haul out.
I hope this helps,
Regards,
Trevor
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Post by MalcolmP on Jun 23, 2019 19:12:41 GMT
Thanks again Trevor These half cells need a considerable outlay for a solid state electrode eg www.edt.co.uk/ion-selective-electrodes/half-cell-ion-selective-electrodes/1211 maybe there is a DIY option? But I fear I may not understand the results sufficiently in any case... Regarding shaft anodes, I personally have found they risk coming loose then creating mechanical problems so tend to avoid Overall I am hopeful that the new zinc's directly bolted onto the P bracket will be sufficient, but time and the pathways of those erratic loose electrons, will eventually tell I guess 😀 ... Best regards Malcolm |
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Post by Trevor on Jun 23, 2019 22:56:51 GMT
Hi Malcolm, That looks like a pretty fancy half cell with a BNC connector on the lead. I am thinking something that comes in this kit. sterling-power-australasia.com/product/yacht-corrosion-monitoring-equipment-and-work-book/This kit comes with an analogue meter and a scale that tells the user where the voltage should be on the scale. That makes it easier to interpret but it is merely showing a voltage and that voltage is from a table called the galvanic series. The voltage shown is with respect to the half cell used as a reference. Any multimeter can show the same voltage reading as the analogue meter in this kit so you really only need the half cell as a reference. The half cell provides a reference that is fairly universal so people can speak about the problem and their voltages having used a reference. It makes sense that you could simply use a zinc anode as a reference and the voltage difference between your bonded metals and the reference would be very close to zero volts. The problem is some anodes may have a different alloy mix so it cannot really be used as a reference. You and I may use different zincs and our voltages may not match so makes it a little hard to compare. The silver-silver chloride half cell takes away that uncertainty. I noted from your last comment that you do not have an anode on the shaft. In that case you need to ensure that the P bracket braid is very low resistance because the anode on your P bracket will be protecting the shaft. That is perfectly ok but those components do need to be "as one" from an electrical perspective. Also, you mentioned over protecting. That is a real problem for timber boats and aluminium or steel boats but luckily we have fibreglass boats that do not suffer much from any detrimental structural impact of "overprotection". Regards, Trevor |
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Post by MalcolmP on Jun 24, 2019 5:27:07 GMT
Thanks again Trevor for your expertise. I do keep the zinc on the end of the prop as well. Reassuring to know that over zincing should not be an issue on GRP
Maybe you could do an article on electrolysis and cathodic protection for Hints and Tips if you have time at some point? Always seems a black art to many it seems.
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Post by rene460 on Jun 24, 2019 9:13:46 GMT
Hi Trevor, a big thanks from me also, for your clear explanation of the problem and more importantly the practical things we can do. You certainly filled in some gaps for me. Another vote for a hints and tips article when you can find time, it will make it easier to find and refer to.
With two current threads on related issues, it is a timely and valuable contribution.
rene460
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Post by cpetku on Jul 10, 2019 0:19:14 GMT
FYI,Boat Zincs sells a reference electrode at a reasonable price as well. I bought one even though I'm in fresh water with magnesium anodes. Cheap piece of mind to know that there's no stray currents from other boats that could be affecting mine. According to the manual that came with the electrode, over-protection can result in degradation of the bottom paint... www.boatzincs.com/corrosion-reference-electrode.html |
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Post by MickeyB on Jul 10, 2019 6:39:08 GMT
Not hijacking this thread - but a related question I think. My prop and shaft zincs (bar small crustations) look in great health after 2 years. Should I worry? Or does that actually mean my boat is in good shape and not to worry?
Mike
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Post by vasko on Jul 10, 2019 12:43:54 GMT
generally the anodes need to be changed when they are less then 1/2 from the original size.. the one that I have on the shaft and P-bracket are in good condition every year after lift out, the one on the keel is always 80% gone... but I always change all of them every year...
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Post by MickeyB on Jul 10, 2019 12:53:02 GMT
Hmm. I only have 2, one shaft and one end of prop. I don't have a hull one. Any other 34.2 owners out there who can confirm that they only have 2??
If not, I need to work out where it should be, and fairly quickly.
Mike
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Post by shawnkaplan on Dec 21, 2020 5:55:52 GMT
Hmm. I only have 2, one shaft and one end of prop. I don't have a hull one. Any other 34.2 owners out there who can confirm that they only have 2?? If not, I need to work out where it should be, and fairly quickly. Mike Mickey- I’ve wondered the same thing. My zincs are holding up pretty well and I’m wondering if that is a good thing or a bad thing. Reason I’m concerned is my grounding cable to be he P-Bracket corroded off 2 years ago and I’m not so sure I got it reconnected well. Also on a Facebook thread the group there thought I should have a through-hull bolt. I don’t think I have one. (Had the boat for 7 years now). 42DS 2007 |
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Post by sleighride on Dec 21, 2020 16:17:00 GMT
I had a problem several years ago with sight of galvanic corrosion on my MaxProp (pink discoloration.) The boat yard took photos of my prop and shaft and sent them to MaxProp. The prop manufacturer noticed that I had both a new zinc and a corroded zinc still attached to shaft and said that was the most likely problem, namely over zinciing. For years, both the yard and my diver had left on the old zinc when adding a new one between haulouts. I have never had a problem since. I have only the prop zinc and a single shaft zinc.
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Post by sailbleu on Dec 26, 2020 7:53:49 GMT
Hmm. I only have 2, one shaft and one end of prop. I don't have a hull one. Any other 34.2 owners out there who can confirm that they only have 2?? If not, I need to work out where it should be, and fairly quickly. Mike Mickey- I’ve wondered the same thing. My zincs are holding up pretty well and I’m wondering if that is a good thing or a bad thing. Reason I’m concerned is my grounding cable to be he P-Bracket corroded off 2 years ago and I’m not so sure I got it reconnected well. Also on a Facebook thread the group there thought I should have a through-hull bolt. I don’t think I have one. (Had the boat for 7 years now). 42DS 2007 The fact that the grounding cable has corroded off ( they all do after a short period ) , is exactly the reason why boaters should zinc the P-bracket also. I have 3 anodes , one on the shaft ( replacement every two years ) , one on the prop-nut ( replacement every year) and a massive anode on the P-bracket ( the second in 10 years time now ) . However , with winters on the hard they last longer of course. No corrosion on shaft, prop. or bracket in sight here . In the water they all wear down and thereby proof their necessity . Starting from next season our boat will be in a berth the whole year round again and I plan an extra submersed anode on a stainless steel cable attached to the stern pulpit all the time. Not when sailing that is. Regards |
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Post by cpetku on Dec 26, 2020 19:47:03 GMT
Also always make sure you are using the right material for the location of the boat. For you guys in the ocean, making a mistake will cause premature wear of the anode while in fresh water the wrong anode fails to sacrifice itself.
In my case I am in fresh water, the anode was AL and never showed signs of wear. Changing over to the correct material (Magnesium) for my area resulted in anodes needing to be replaced at least every two seasons (6 months each). I'd rather address a cheap maintenance item more often than to risk an undiagnosed (expensive) issue once. I can also recommend the reference electrode from boatzincs.com as easy to use.
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furlin
Junior Member
Posts: 12
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Post by furlin on Aug 17, 2021 18:09:13 GMT
On our recent haul out after being afloat for 11 months I found classic signs of dezincification on the P bracket. I do not leave the shore power connected when we are on board and have a galvanic isolator on the shore feed when we are hooked up. Normally I have just a prop anode and a nearby Ionguard retractable anode and 11 seasons these have only ever shown modest corrosion. I often fully burnish the prop, shaft and P bracket and know there has never previously been any sign of electrolysis. I also have checked the earthing braid that is connected from the P bracket and it looks in good order, but it connects into the wiring loom on our 39i so not quite sure where the factory earthed it to. I am convinced this must have occurred in the marina where we overwintered due to some boat or pontoon nearby having stray mains current on the water, it is salt water. We are now on a new marina and non of the other yachts hauled out there show any signs at all of dezincing The depth of pitting did initially concern me on terms of possible loss of strength of the P bracket, but I then decided to do what Vasko did a few years ago and drill the P bracket to bolt directly on a couple of disk anodes. This confirmed to me at least that the pink copper element is only on the surface as the drilling swafe all was golden bronze throughout. It is just an 8mm hole and I purposely drilled it relatively high up to reduce leverage on the theoretical line of weakness. I am convinced this will not have any significant impact on the brackets real strength and having two big lumps of zinc there hopefully will reduce the risk of future stray current issues. Interestingly neither the prop, an autoprop or the shaft, which is made of duplex stainless now, showed any issue. The photos are linked below, sorry they are not inserted but doing this post from my phone and no WiFi so will have to edit later www.23hq.com/Jeanneau/photo/56319575/originalwww.23hq.com/Jeanneau/photo/56319597/originalwww.23hq.com/Jeanneau/photo/56319621/originalwww.23hq.com/Jeanneau/photo/56319641/originalThat’s a huge anode my one a lot smaller |
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furlin
Junior Member
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Post by furlin on Aug 17, 2021 18:16:16 GMT
What should we be looking for?
Just waiting for a survey on a Sun Odyssey 32, lift keel, just discovered (prior to survey) that the previous owner replaced the prop, shaft and plate (and previous invoice says keel) apparently after electrolysis damage - while the surveyor will no doubt flag any issues that they find is there anything else that we should be checking?
The previous owner is not available to discuss what or why they did what they did, and whether the issue was due to the boat we are planning to buy or an adjacent boat in the marina (which is salt water) looks like the boat has had the same berth for a reasonable number of years.
Any thoughts, including run towards a different vessel, would be useful,
thanks,
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Post by Charlie-Bravo on Aug 17, 2021 20:34:18 GMT
Trevor, a better write up on the subject I have not seen, a Guru you are
CB
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my one a lot smaller 
