sgball
New member
Friends are having problems with their prop being eaten away. within a year the anodes were gone. More anodes have now been put in place but are decaying very quickly. Could the mooring be a problem?
Electrolosis
- Thread starter sgball
- Start date
davidwf
Active member
If they are in a Marina and using Shore Power it could be another boat with a fault. Check with the marina and get them to check out the other boats near them.
Is the prop being eaten away or is it just the Anodes? if its just the Anodes then its possible that they are just doing their job, my anode attached to the prop itself is almost gone after a year, I used an anode over the side last year and that helped me get a full year out of it, before it was gone in 8 months.
Is the prop being eaten away or is it just the Anodes? if its just the Anodes then its possible that they are just doing their job, my anode attached to the prop itself is almost gone after a year, I used an anode over the side last year and that helped me get a full year out of it, before it was gone in 8 months.
sgball
New member
Yes they are in a Marina and quite a few boats around them are linked to the shore power.
5 Anodes are in place, 1 prop, 2 rudder, 1 keel and 1 over the side, they are all being eaten. We know there doing a job but surely this is a bit excessive.
Is there anyway to check the amount of electrical current in the water?
5 Anodes are in place, 1 prop, 2 rudder, 1 keel and 1 over the side, they are all being eaten. We know there doing a job but surely this is a bit excessive.
Is there anyway to check the amount of electrical current in the water?
lezgar
New member
www.corrosion-doctors.org and look in contens
Lynette
Member
Have a look at http://marinesurvey.com/yacht/corrosion_in_marinas.htm, which I've found very helpful.
Unless the mains supply is earthed out through the shaft of your friend's boat, marina mains power is not the most likely cause of the damage to their propellor.
Are they sure the damage is due to electrolysis rather than cavitation?
Unless the mains supply is earthed out through the shaft of your friend's boat, marina mains power is not the most likely cause of the damage to their propellor.
Are they sure the damage is due to electrolysis rather than cavitation?
G
Guest
Guest
One of the first things that needs to be done is to have a "galvanic isolator" installed. If the AC ground (earth) buss on the boat is properly connected to the DC ground (earth) buss then the boat needs to be galvanically isolated. Nigel Calder and others have written extensively on this issue. Also, it would behoove the boat owner to possible have a discussion with the marina owners as it may be a leak of electricity from the shore power mains. Our marina had a similar problem....and in our case the ground (earth) wire for the dock had corroded through completely, and the neutral wire had been corroded halfway through. Needless to say, this situation caused a number of problems.
Trevor_swfyc
New member
The boat is on a mooring so whats all this talk about mains power about!!
I look after the moorings for our yacht club and have not heard of anybody having this kind of trouble. It could only occur if the chain riser was getting attached in some way to the prop and by this I mean physically and electrically so check this is not happening.
A tip here, I have a buoy where the chain passes through the buoy. The riser chain is passed through old/discarded toilet outlet pipe the chain is greased with water proof grease. The only part of the chain directly exposed is where it attaches to the swivel. Initially this caused all the galvanic corrosion to take place on the exposed chain to the point of the last link only lasting two months. This was overcome by fitting an old 50% worn yacht anode bolted onto the chain and onto the shackle between chain and the swivel. I now get what looks like zero loss on the riser, my riser is in its fourth year and is untouched.
So this leads me back to your problem the prop is attached to the shaft which is attached to the engine so thank god the shaft anode worked. You should have a larger anode on the hull electrically connected to the engine and shaft this would result in less loss on the shaft anode. Finally it is not uncommon for an anode to only last one season ie be reduced by 50%, also only purchase anodes from a known supplier to ensure it is of good quality zinc, poor quality zinc will self destruct in salt water as the inpurity metal will set up a cell.
I hope this is useful and at least gives you something to "zinc" about.
All the best Trevor.
I look after the moorings for our yacht club and have not heard of anybody having this kind of trouble. It could only occur if the chain riser was getting attached in some way to the prop and by this I mean physically and electrically so check this is not happening.
A tip here, I have a buoy where the chain passes through the buoy. The riser chain is passed through old/discarded toilet outlet pipe the chain is greased with water proof grease. The only part of the chain directly exposed is where it attaches to the swivel. Initially this caused all the galvanic corrosion to take place on the exposed chain to the point of the last link only lasting two months. This was overcome by fitting an old 50% worn yacht anode bolted onto the chain and onto the shackle between chain and the swivel. I now get what looks like zero loss on the riser, my riser is in its fourth year and is untouched.
So this leads me back to your problem the prop is attached to the shaft which is attached to the engine so thank god the shaft anode worked. You should have a larger anode on the hull electrically connected to the engine and shaft this would result in less loss on the shaft anode. Finally it is not uncommon for an anode to only last one season ie be reduced by 50%, also only purchase anodes from a known supplier to ensure it is of good quality zinc, poor quality zinc will self destruct in salt water as the inpurity metal will set up a cell.
I hope this is useful and at least gives you something to "zinc" about.
All the best Trevor.
AndrewB
Well-known member
Is Calder right about a common AC/DC ground?
Yes, I know he's the guru but ...
The argument as I understand it is that the AC ground, via the prop-shaft etc into the sea, provides a safety measure in case of a short on a piece of kit causes the casing of a piece of kit to become live. This safety is worth the aggrevation of electrolysis caused by electrically linking all yachts in a marina (which can, as you say, be reduced by a low-voltage tripping isolator).
However:
(i) Why is the marina shore-side ground to earth not adequate protection? On my marina berth the resistance between the incoming ground and my prop-shaft is quite small, suggesting I have little to gain (other than my neighbour's anode zinc) from my own separate AC ground.
(ii) Many yachts are now fitted with an RCD in circuit to protect against power surges caused by shorts.
(iii) Virtually all AC kit used aboard is plastic cased and would not go live in the case of an internal short. In fact the ground wire is rarely connected internally.
Yes, I know he's the guru but ...
The argument as I understand it is that the AC ground, via the prop-shaft etc into the sea, provides a safety measure in case of a short on a piece of kit causes the casing of a piece of kit to become live. This safety is worth the aggrevation of electrolysis caused by electrically linking all yachts in a marina (which can, as you say, be reduced by a low-voltage tripping isolator).
However:
(i) Why is the marina shore-side ground to earth not adequate protection? On my marina berth the resistance between the incoming ground and my prop-shaft is quite small, suggesting I have little to gain (other than my neighbour's anode zinc) from my own separate AC ground.
(ii) Many yachts are now fitted with an RCD in circuit to protect against power surges caused by shorts.
(iii) Virtually all AC kit used aboard is plastic cased and would not go live in the case of an internal short. In fact the ground wire is rarely connected internally.
HaraldS
New member
There can be several reasons for your zincs being eaten away fast. Figuring out what is going on isn’t always easy and if you have no other corrosion or de-zincing of brass fittings you might be ok.
Normally zinc anodes help prevent galvanic corrosion, but they are quite limited in helping with electrolytic corrosion. Dissimilar metals in the water that are electrically interconnected cause galvanic corrosion, they generate their own voltage like a little battery. A zinc anode basically creates another offsetting battery with opposite polarity offsetting, and usually overcompensating the others. That process naturally eats the zinc and it is largely depending on the metal surface size. If there are large metal objects or bodies to protect, the zincs will naturally melt away relatively quickly. It does depend on the protected metal surface and it’s galvanic characteristic how fast this goes. The zinc itself needs enough surface to protect the other metal(s), but from a certain surface size up, the zinc will not loose mass any faster. Below that surface level you will not have enough protection. Also the zincs need to be relatively close to the objects they protect.
Electrolytic corrosion you get when you introduce electric currents through the water, which are not caused by dissimilar metals, but may come from various electrical sources. Such a process may also eat zincs faster, but at the same time might as well eat other things too.
One simple to understand case is, current flowing through the water that is not originating or terminating in or through your boat. That would for example be the case in the vicinity of a high power DC-cable terminal. These modern submarine power cables have only one conductor and use the seawater as the return. Large grounding plates pass the current to the water near the terminals, and in the vicinity, say around a mile near the terminal, current flow and voltage drop in the water would be measurable and would be able to give you a corrosion problem: Say you have a bonded sea-cock at the bows, and a grounded prop and prop-shaft. Part of the current through the water would choose your better conducting grounding system, that way starting the fatal current flow.
Similar currents occur in smaller areas, like marinas, when some boats have a faulty AC-system and generate current return through ground. Often small currents happen through good devices that have capacitors as RF filters. More aggressive are things like cathodic protection systems for harbor structures. If you hang two plates of similar metal of the bows and stern and measure the voltage between them with a sensitive multimeter, you could confirm the presence of such stray currents.
The third variant is current that you introduce with your boat, with a defective electrical system. First, make sure no DC return ever runs through the grounding system, instead of its private wire. These may cause enough of voltage drop across the ground system to be a problem. But boats with a sensible installation rarely have that problem, except maybe while cranking as most engines to not have isolated minus. Another possibility is AC currents going through the ground system. This happens when people mix up ground and neutral or connect the two in some places.
Not connecting the AC ground with the ships earth, (and typically also DC minus), could prevent that, but is no good advice from a potential shock perspective. Most appliances that we use are built with the assumption that chassis would be safely tied to ground and a loose hot wire would be shorted to the ground, rather than setting the whole outside a washing machine to 230 V. Best is always an isolation transformer for the boat. In this case no current will flow through the water, even if you have a ground fault internally. If all boats had an isolation transformer, than most of these stray current problem wouldn’t exist.
Going back to these currents that are not your fault, there are only a few defenses you can take. One more and more popular way of doing boat installations is to keep things isolated and have ground to seawater contact only in one place. This is completely opposite than bonding everything together. For example in the US it was long considered best practice to bond every sea cock, rather than leaving it isolated on its rubber hose. An increasing amount of de-zincing in marinas was the result. If you now go to West-Marin’s web site, you find an excellent paper on grounding and how to do to isolate everything properly.
A second and also increasingly popular approach is active cathodic protection. Since you have the boat mainly in the marina, this might be a quite good investment, which will for sure stop your zincs going away. Since this needs a little bit of power, it’s mainly a marina thing to run. It is a little electronic sensor that measures voltage levels below your boat and introduces and offsetting current, through a little non-eroding titanium plate. For galvanic corrosion it does the same as the zincs, it is just using some power instead of material, and it is also a bit better capable to offset foreign stray currents under your boat.
Normally zinc anodes help prevent galvanic corrosion, but they are quite limited in helping with electrolytic corrosion. Dissimilar metals in the water that are electrically interconnected cause galvanic corrosion, they generate their own voltage like a little battery. A zinc anode basically creates another offsetting battery with opposite polarity offsetting, and usually overcompensating the others. That process naturally eats the zinc and it is largely depending on the metal surface size. If there are large metal objects or bodies to protect, the zincs will naturally melt away relatively quickly. It does depend on the protected metal surface and it’s galvanic characteristic how fast this goes. The zinc itself needs enough surface to protect the other metal(s), but from a certain surface size up, the zinc will not loose mass any faster. Below that surface level you will not have enough protection. Also the zincs need to be relatively close to the objects they protect.
Electrolytic corrosion you get when you introduce electric currents through the water, which are not caused by dissimilar metals, but may come from various electrical sources. Such a process may also eat zincs faster, but at the same time might as well eat other things too.
One simple to understand case is, current flowing through the water that is not originating or terminating in or through your boat. That would for example be the case in the vicinity of a high power DC-cable terminal. These modern submarine power cables have only one conductor and use the seawater as the return. Large grounding plates pass the current to the water near the terminals, and in the vicinity, say around a mile near the terminal, current flow and voltage drop in the water would be measurable and would be able to give you a corrosion problem: Say you have a bonded sea-cock at the bows, and a grounded prop and prop-shaft. Part of the current through the water would choose your better conducting grounding system, that way starting the fatal current flow.
Similar currents occur in smaller areas, like marinas, when some boats have a faulty AC-system and generate current return through ground. Often small currents happen through good devices that have capacitors as RF filters. More aggressive are things like cathodic protection systems for harbor structures. If you hang two plates of similar metal of the bows and stern and measure the voltage between them with a sensitive multimeter, you could confirm the presence of such stray currents.
The third variant is current that you introduce with your boat, with a defective electrical system. First, make sure no DC return ever runs through the grounding system, instead of its private wire. These may cause enough of voltage drop across the ground system to be a problem. But boats with a sensible installation rarely have that problem, except maybe while cranking as most engines to not have isolated minus. Another possibility is AC currents going through the ground system. This happens when people mix up ground and neutral or connect the two in some places.
Not connecting the AC ground with the ships earth, (and typically also DC minus), could prevent that, but is no good advice from a potential shock perspective. Most appliances that we use are built with the assumption that chassis would be safely tied to ground and a loose hot wire would be shorted to the ground, rather than setting the whole outside a washing machine to 230 V. Best is always an isolation transformer for the boat. In this case no current will flow through the water, even if you have a ground fault internally. If all boats had an isolation transformer, than most of these stray current problem wouldn’t exist.
Going back to these currents that are not your fault, there are only a few defenses you can take. One more and more popular way of doing boat installations is to keep things isolated and have ground to seawater contact only in one place. This is completely opposite than bonding everything together. For example in the US it was long considered best practice to bond every sea cock, rather than leaving it isolated on its rubber hose. An increasing amount of de-zincing in marinas was the result. If you now go to West-Marin’s web site, you find an excellent paper on grounding and how to do to isolate everything properly.
A second and also increasingly popular approach is active cathodic protection. Since you have the boat mainly in the marina, this might be a quite good investment, which will for sure stop your zincs going away. Since this needs a little bit of power, it’s mainly a marina thing to run. It is a little electronic sensor that measures voltage levels below your boat and introduces and offsetting current, through a little non-eroding titanium plate. For galvanic corrosion it does the same as the zincs, it is just using some power instead of material, and it is also a bit better capable to offset foreign stray currents under your boat.
lezgar
New member
What is the west-marin web site?
thanks
thanks
HaraldS
New member
jfkal
Active member
At this rate I assume that the boat is connected to shore power. Thanks an galvanic process involving thr ground wire the anodes are been eaten like cake.
This can be solved by using a grounds isolator consisting of 2 pairs of diodes which will isolate low DC voltage <1.4 volts but would fully conduct on ground fault.
This can be solved by using a grounds isolator consisting of 2 pairs of diodes which will isolate low DC voltage <1.4 volts but would fully conduct on ground fault.
chriscallender
Active member
All of this talk makes me wonder why boats need to be connected to shore power when the owners are not present.
Reasons I can think of are to run dehumidifiers, heaters or battery chargers. However I'd prefer to take the cushions/curtains/batteries home in winter than take the risks with a permanent hookup - as well as galvanic corrosion the risk of fire from faults in unattended heaters etc. I do make sure I heat and ventilate the boat every couple of weeks or so.
Where I keep my boat, the owners remove all the plugs that are left in every night, and although it can be a bit of a pain at times, it seems like it will eliminate a lot of problems. What electricity you do use while there is free.... and its saved me from running out to buy a dehumidifer ;-)
Chris
Reasons I can think of are to run dehumidifiers, heaters or battery chargers. However I'd prefer to take the cushions/curtains/batteries home in winter than take the risks with a permanent hookup - as well as galvanic corrosion the risk of fire from faults in unattended heaters etc. I do make sure I heat and ventilate the boat every couple of weeks or so.
Where I keep my boat, the owners remove all the plugs that are left in every night, and although it can be a bit of a pain at times, it seems like it will eliminate a lot of problems. What electricity you do use while there is free.... and its saved me from running out to buy a dehumidifer ;-)
Chris