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Post by davidlaw on Nov 18, 2014 0:16:40 GMT
Help! It looks like my P-bracket is undergoing some very slow electrolysis (Jeanneau 39 DS 2008). This P bracket is obviously insulated from the propshaft and thereby the two anodes on on it (shaft and tip). However, there is a cable on the inside (shown) which according to my engineer connects to the engine (which itself does not have an anode). I have been advised to disconnect this cable but for fear of damage to the engine I am more inclined to fit an anode to the P bracket itself. Any views...please? David Law  |
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Post by Trevor on Nov 18, 2014 3:25:47 GMT
Hello davidlaw,
I have a view but it is unqualified so happy to be corrected by someone more knowledgeable.
The Prop shaft is connected to the gearbox and engine through a metallic fitting so it is fair to say the propshaft is electrically connected to the engine. An electrical connection exists from the P bracket to the engine so it is fair to say that the prop shaft and the P bracket are very solidly electrically connected. As they are bonded together the anode on the prop shaft is actually protecting the prop shaft, the prop and the P bracket.
The anode sacrifices itself to protect the metal bits we don't want corroded by galvanic currents. The theory being that the prop shaft, prop and P bracket are all connected very solidly so the sacrificial anode can do its job and allow galvanic currents through the salt water ( conductor) from the anode to the other metal bits, constantly corroding the anode and perhaps causing a build up of material on the important metal bits. It is required because those metal bits automatically form a small battery cell when immersed in salt water and current flows (galvanic currents) through the water in accordance with the voltage developed by those dissimilar metals in electrolyte ( salt water).
I would not disconnect the P bracket. It is currently being protected by the anode on the shaft.
Electrolysis is the corrosion of parts because of leakage currents. If the yacht is in a marina and leakage currents are flowing because of leakage currents from your yacht or those yachts around you, that is another matter and that is the reason electrolysis blockers are fitted.
I hope this helps on your deliberations on this issue.
Regards,
Trevor
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Post by sailbleu on Nov 18, 2014 5:21:00 GMT
I have put a zinc on my P-bracket aswell.
I advise you ( and others ) to do the same as I notice that anode has been sacrificed too.
Regards
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Post by MalcolmP on Nov 18, 2014 9:34:30 GMT
I would agree with Trevor to leave the wire connected. The P bracket is such a large piece of bronze I would be surprised if you have more than surface issues - otherwise other items like your skin-fittings and prop would have dissolved first. I don't like the idea of drilling the P bracket to attach an anode. Now (as others know) I have a vested interest in this as I developed the Ionguard anode system which allows you to check and replace the anode whilst still afloat. I have this fitted to my 39i which has the same hull as the 39DS since 2008 - I normally avoid advertising here, but they are currently 20% off for Jeanneau owners at £185.00 inc VAT as advertised at: www.jeanneau-owners.com/hintsandtips/hintsandtipsindex.htmlMore info at: www.bruntons-propellers.com/IonGuard/IonGuard.htmlMalcolm |
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Post by davidlaw on Nov 19, 2014 22:50:52 GMT
Thanks very much Trevor, Malcolm and Sailbleu for your helpful replies. I can certainly see the logic now of NOT disconnecting the cable - stoopid of me not to work that out before. (Incidentally, I have also directed the question to Jeanneau so I will circulate their reply for interest's sake when it arrives.) Malcolm - thanks for the suggestion and link (and your candid COI declaration!). One question though.... you point out the P bracket is a solid piece but recommend against drilling into it. Why the reservation? Would it do any harm to fit an anode like Sailbleu? The Ionguard sounds impressive and tempting. However, the info states: "the condition of that (Ionguard) anode will reflect the condition of the other anodes" and I am nervous about that presumption since the tip anode on my prop has always decayed at a rate of knots (sorry!) whilst the anode on the shaft has hardly lost anything at all in years. Can you shed any light on this? Thanks again David Oh..."dreaming of retirement"... just do it....  |
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Post by so36idavid on Nov 20, 2014 1:28:11 GMT
the tip anode on my prop has always decayed at a rate of knots (sorry!) whilst the anode on the shaft has hardly lost anything at all in years. Can you shed any light on this? I had a shaft zinc that didn't lose anything at all for a good, long time. The diver was very suspicious and he changed it out. The new one started losing material at a reasonable rate. So the bottom line is that if a zinc is not disappearing slowly then something else is, and you need to figure out why. At this point it's possible that your prop zinc is the only zinc that's making good electrical contact which might explain why it's disappearing that fast. Maybe swap out the shaft zinc and see if the new one shows the same behavior. If it does then all it's cost you is the price of a spare zinc. |
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Post by Trevor on Nov 20, 2014 5:33:13 GMT
Hello,
I can imagine that the material the prop is made out of is different to the material the shaft is made out of. I am not sure if the prop is folding or fixed but if it were made out of bronze instead of stainless it may require more protection than the shaft. In this case the closest anode would be sacrificed at a rate of knots (also sorry!) faster than the anode on the other material which is the ( requiring less protection ) prop shaft.
That is at least my attempt at a plausable story even it isn't correct!!
Regards,
Trevor.
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Post by so36idavid on Nov 21, 2014 6:07:41 GMT
Hello, I can imagine that the material the prop is made out of is different to the material the shaft is made out of. I am not sure if the prop is folding or fixed but if it were made out of bronze instead of stainless it may require more protection than the shaft. In this case the closest anode would be sacrificed at a rate of knots (also sorry!) faster than the anode on the other material which is the ( requiring less protection ) prop shaft. That is at least my attempt at a plausable story even it isn't correct!! Regards, Trevor. Trevor, I don't buy your story . I don't think that the physical proximity of the zinc to the material makes any difference. If there is a good electrical connection then the electrons will flow to where they are most desired. The shaft, prop and engine usually make good electrical contact so both zincs should protect everything. If the prop and shaft are isolated then the prop zinc should protect the prop and the shaft zinc should protect everything else. As I said before, if a zinc is not disappearing then you aren't "lucky", you have a problem. I had a different problem in my current slip. Zincs requiring monthly replacement. Same with my neighbors. Somewhere in the area was a boat with an electrical problem. I put in a galvanic isolator and that cleared up. At some point the problem boat must have moved out or been repaired because suddenly everyone's zincs started lasting a lot longer. |
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Post by electricmonk on Nov 21, 2014 8:06:51 GMT
This is actually a thorny subject. Years ago - and I don't know if its still the case Jeanneau didn't fit any additional zincs all we had was the prop zinc that dell off in a couple of weeks. Dealers in the UK at least fitted a pear anode near the propeller - mine lasts around 5 years and I have no corrosion issues..
As far as I know the Jeanneau reasoning was that if there is no circuit then there can be no corrosion - fitting a zinc completes a circuit that wasn't there before so corrosion starts - hopefully its the zinc that corrodes.
The problem is that if there no zinc in the circuit as we all know something else will corrode, but if there is no circuit then nothing corrodes.
However the problem is exacerbated if there is a earth path somewhere and hence if there is another available circuit with no zinc then something else (prop, shaft p bracket) will corrode so the proximity of the zinc to item you are trying to protect is important.
The P bracket is a special case as it is isolated from shaft by the cutlass bearing and isolated from the engine (earth) by the hull so in theory it should be almost impossible for it to become part of a circuit. unless its using the water as an electrolyte to the shaft and prop and the wire provided by the maker to bond it to earth, so in theory at least the provision of the wire is encouraging electrolysis of the p bracket. So that wire must be connected to a zinc near the P bracket or it may dissolve or disconnected so there is no circuit.
Galvanic isolator will protect the boat from stray currents from faulty shore power or boats nearby with faulty shore power but will do nothing to protect from faulty on board wiring.
A few decades ago I was called out to boat with no propeller, the thing had rotted away in a week! After an extensive search we found a live 12v wire that had corroded away from its spade connector and fallen into the wet bilge thus completing a 12v circuit with the prop shaft also immersed in bilge water some 12 feet away. The shaft zincs had gone along with the prop and the battery bank was flat, there was no P bracket it wasn't a Jeanneau but the cutlass bearing holder which was not bonded to ground was un damaged.
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Post by Trevor on Nov 21, 2014 8:07:53 GMT
Ahaa..hello so36idavid, Let me try even harder to convince you  . The resistance of a circuit is proportional to the length of the conductors. The electrolyte in this case (salt water) has a certain resistance and that resistance gets higher the more salt water the electrons must travel through to complete the circuit. If you switch the multimeter to measure resistance (ohms) and put the tips of multimeter leads in salt water they will show low resistance (high current) when close to together and high resistance (lower current) when the distance between he tips is further apart. I agree the resistance of the bonding circuit is very low and negligible compared to the resistance of the electrolyte. The more distance through the electrolyte the lower the current. The lower the current the less sacrificial corrosion the anode will exhibit. In other words, all things being equal, if two anodes are protecting a component, the closer anode will disappear faster because the resistance of the salt water is less than the anode further away. Whew! That's my second attempt. How did I go? Regards, |
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Post by electricmonk on Nov 21, 2014 14:11:13 GMT
Trevor, yes you are right - ish. The problem is if your sacrificial anode is a log way away from the item you seek to protect and another circuit is created with another item closer and the least noble metal in that circuit is your propeller - you loose your prop. So to protect your stuff YOUR anode must be close to it.
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Post by MalcolmP on Nov 21, 2014 19:40:16 GMT
Just some quick comments, the variable views coming forward tend to align -or should that be non-align - with the general lack of consensus that surrounds achieving good cathodic protection in both the media and the industry. There are so many variables; salinity, material composition, distance, underwater shape etc etc. No sacrificial system alone will be able to cope with stray current either AC or DC. Anodes do need a good connection, so for example if a shaft anode is not attached to a clean shaft, then it may have too much electrical resistance to work properly. Prop anodes do tend to go first, as they are in direct and immediate contact with the prop and usually are well connected. We found with the Ionguard that it tends to erode later than other anodes such as prop or shaft, hence the comment about being an indicator of the heath of other anodes - if the Ionguard is wearing fast then its time to send a diver to have a look at the condition of other anodes (if fitted). Not all gearboxes provide a reliable low resistance electrical path and equally some carbon rich rubbers used in Cutless bearings will conduct quite effectively, so again generalisations can be misleading. My reference to avoiding fitting direct to P bracket was simply that I don't like the idea of drilling, so potentially weakening the P bracket, but perhaps if the hole is small and well formed then it may not actually weaken it significantly.
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Post by pbunning on Nov 22, 2014 10:45:13 GMT
I concur with Malcolm. Cathodic protection is considered to be somewhat of a black art. The best strategy is to monitor the situation and take appropriate remedial action - despite working for many years in the offshore industry and thinking I had a reasonable grasp, I did experience crevice corrosion on the shaft and some dezincifiction of my prop on my last boat. In the end I had to resort to fitting a hull anode and wiring all skin fittings. I have not had any issue (so far) with my present boat.
Fitting a galvanic isolator (in my mind) is a must have when connecting to shore power (it was the first modification I carried out to my present boat) - I am perhaps a bit paranoid in that I disconnect from shore power whenever I leave the boat.
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Post by sailbleu on Nov 23, 2014 6:29:08 GMT
Maybe the quality of the s/s shaft was not as good allowing pitting to get through.
I' m still here on my boat for 5 days connected to shorepower and will absolutely disconnect it also when I leave. Infact I always do that for the same reasons pbinning has given.
Must look into that galvanic isolator thing too.
Been hearing alot about it , but never got around to it , but than again I have always been on the hard during the autum & winter months. It will be different for the comming years though.
So I guess playing an other game ( around the clock marina time) demands a new set of rules , galvanic isolator can/will be one of them.
But what if your neighbours don't play along ??
Will you boat be protected from their stary currents ?
Regards
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Post by electricmonk on Nov 23, 2014 7:17:26 GMT
But what if your neighbours don't play along ??
Will you boat be protected from their stary currents ?
That's the whole point of the galvanic isolator, you protect your boat from them and dodgy marina electrical installations.
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Post by alex1949 on Nov 23, 2014 7:50:33 GMT
Installing galvanic isolator DIY is really simple and need no change on boat's elect. system. It's low price (around 100$) along with the peace of mind associated ,makes it a must. After I installed it, all other installed anodes kept much longer. No need to consider in-between hull up for change. Again , one of the best cost-effective decisions made. Alex |
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Post by sailbleu on Nov 23, 2014 8:24:47 GMT
But what if your neighbours don't play along ?? Will you boat be protected from their stary currents ? That's the whole point of the galvanic isolator, you protect your boat from them and dodgy marina electrical installations. Well that's the whole point electricmonk . I have never looked in to the galvanic isolator story as such because there was hardly no need for that. As I mentioned above , things have changed somewhat. An another job on the to do list when I return to my boat. @ Alex , thanks for setting my gear in motion  ? Regards |
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Post by sailbleu on Nov 23, 2014 8:46:04 GMT
Just googled a bit and came up with a DIY galvanic isolator. Will not post the article since it is not in english but if you click thelink you' ll see a picture/schematics ( below) Diode bridge and capacitator , that's all it takes no ? www.boothobby.nl/index.php/isolator-voor-schepenRegards. Ps: But where do you connect the isolator ? Could that be on the engine aswell? |
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Post by Trevor on Nov 23, 2014 9:55:17 GMT
The galvanic isolator can only protect you from galvanic currents. The reason is the isolator has two diodes in series for both directions of AC current ( in other words 4 diodes altogether). The two diodes are required so the voltage must be 2 x 0.7V = 1.4 volts before it will conduct. That is insignificant compared to mains voltage so doesn't stop your protection earth from working in case of a mains power leakage problem but will not allow galvanic currents to flow because the potential difference developed as a result of dissimilar metals in salt water is not high enough to cause those diodes to conduct. The voltages developed are too small. That is why two diodes are required. If only one diode were used, the circuit would conduct at 0.7 volts. Dissimilar metals in salt water can develop more than that level of potential so that would not protect in all cases.
If my boat has one type of metal below the water line and the boat next door has a different type of metal below the water line, when we plug in the mains power cable, we join our boats together throughout the earth cable in the mains cord. To protect against galvanic currents we could just disconnect the earth cable in the mains lead but that would render our boat VERY dangerous ( NEVER ever do this). The galvanic isolator breaks that earth cable for very small voltages ( galvanic potential) but does not disconnect the earth for larger voltages (mains voltages) so still allows mains voltage protection.
If a vessel nearby had earth leakage that developed voltages at our boat that was higher than 1.4 volts, our galvanic isolator would conduct and offer no protection at all. If that leakage was from the mains power on the boat, it would surely trip the RCD (residual current device ) that constantly tests for balance between active and neutral conductors ( sorry, probably Australian terms). If they are not balanced, the unbalance current must be going to earth. The RCDs are usually quite sensitive (5-30mA) so they trip with pretty low leakage.
If your neighbour has 12 volts providing the leakage current I guess it could cause problems on your boat but nothing like the fast destruction that could take place on their boat. Most of the current would flow through their metal bits as they are closer and although a small amount may make its way to your boat, it may not have enough potential after a couple of meters of salt water to make the galvanic isolator conduct. However, that is not what the galvanic isolator is designed for despite some of them being wrongly described as electrolysis blockers.
Regards,
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Post by Trevor on Nov 23, 2014 10:05:17 GMT
Hello Sailbleu, I have fitted one and put the details in the H&T section. After I have seen your fantastic and very complex projects over the years this will be the simplest thing you have ever attempted on you boat!!! www.jeanneau-owners.com/hintsandtips/galvanicisolator.htmlI hope this helps. Regards, Trevor |
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Post by MalcolmP on Nov 23, 2014 11:04:45 GMT
Might be a good idea to start a new thread, but I found this interesting article after following a link from another current thread on batteries and Smart Gauges which seems to give an unbiased view on the pros and cons of Galvanic Isolators v Isolation Transformers www.smartgauge.co.uk/galv_tran.html |
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Post by MalcolmP on Nov 23, 2014 11:15:29 GMT
Might be a good idea to start a new thread, but I found this interesting article after following a link from another current thread on batteries and Smart Gauges which seems to give an unbiased view on the pros and cons of Galvanic Isolators v Isolation Transformers www.smartgauge.co.uk/galv_tran.htmlIf post on galvanic isolators could now be added to the new thread I have just started at: jeanneau.proboards.com/thread/3461/galvanic-isolatorsand anything about P bracket electrolysis continued on this thread I think that might be helpful for future readers |
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Post by alex1949 on Nov 24, 2014 6:18:43 GMT
Just googled a bit and came up with a DIY galvanic isolator. Will not post the article since it is not in english but if you click thelink you' ll see a picture/schematics ( below) Diode bridge and capacitator , that's all it takes no ? www.boothobby.nl/index.php/isolator-voor-schepenRegards. Ps: But where do you connect the isolator ? Could that be on the engine aswell? |
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Post by sailbleu on Nov 24, 2014 7:43:33 GMT
Thanks Trevor , Alex , electricmonk and others , switching overto the galvanic isolators topic.
Wintering over in the marinas for the next years makes this and urgent and a ' to be adressed ' new project.
Regards.
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Post by electricmonk on Nov 24, 2014 11:02:01 GMT
Thanks Trevor , Alex , electricmonk and others , switching overto the galvanic isolators topic. Wintering over in the marinas for the next years makes this and urgent and a ' to be adressed ' new project. Regards. Think of the galvanic isolator as a one way valve, it will let current out of the boat but not into it. Its fitted on the earth cable as close as possible to the point where shore power enters the boat. The current from the dock could make a path through the grounded items of the boat, through the water and back to the earth on land, eating away zincs and propellers, shafts etc. as it flows. The idea is that in normal use it just sits there and prevents any current form flowing from the dock to the boat via the earth cable. If there is a problem on the boat it will allow current via the earth cable to flow from the boat to the dock. In a good set up this should activate a trip and switch off the power, the isolator will then continue to protect the boat from stray currents from the dock to the boat via the earth cable. The common ground ie the engine and other items being connected to the earth wire is dependent on how the boat is wired. My boat is wired so the negatives of the 12v system and the earth of the 240v AC system are combined. Hope this helps a bit. |
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. The resistance of a circuit is proportional to the length of the conductors. The electrolyte in this case (salt water) has a certain resistance and that resistance gets higher the more salt water the electrons must travel through to complete the circuit. If you switch the multimeter to measure resistance (ohms) and put the tips of multimeter leads in salt water they will show low resistance (high current) when close to together and high resistance (lower current) when the distance between he tips is further apart.
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