I recently submitted a question to BoatUS expert Don Casey: Do-It-Yourself Guru. My original question: Last year the zinc anode on my bronze shaft of my sailboat was about 75 per cent dissipated at the end of the season. I had connected to shore power only for about 10 hrs the whole season. Thinking I would gain protection this season, I put on two zinc anodes. When hauled a few weeks ago, both anodes were equally dissipated also to about 75 per cent gone. Only this season I never connected to shore power at all. Does it make sense to double the number of anodes? Would agree that the problem would seem to be ambient electricity in the water at my marina? What else could cause this problem? Thanks. Our expert's answer: Current flowing to your boat from an outside source would indeed eat away your anodes, but this requires a complete circuit, meaning your boat would need to be plugged in to the shore circuit. A more likely scenario is that your shaft is connected to the ground side of your electrical system through the ground lug on your engine, and somewhere in your electrical system is a small ground-fault that is energizing the shaft. This assumes that you feel confident that the anodes are not sacrificing themselves simply due to dissimilar metals in the water, given that your shaft is bronze rather than a steel alloy. There is also the issue of what alloy these anodes are compared to the one you had installed last year. Not all anodes are the same on the galvanic scale, which will affect how long they last. This seems like a case where you should check all of your onboard circuits for leaking current. You can find instructions for doing that in Sailboat Electrics Simplified. If this effort does not reveal a current leak, then the recommended treatment is what doctors now call watchful waiting. Put on fresh zincs and keep an eye on them.
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On the subject of electrolsis, I need help.....
Conditions: Boat is located in salty waters of Gulf of Mexico; shore power is disconnected majority of time; neighboring boats are not having electrolsis problems. Because my two shaft "torpedo" anodes were nearly disappearing in three months, I attached a copper wire directly to the engine and to a large anode which hangs off the stern when the boat is in port. There is significant electrolsis on this anode in the five months its been in use. Two days ago I measured 1.877 ohms of current flowing between the engine and the overboard anode with all electrical devices turned off. I don't seem to be loosing battery power. The boat has four six volt batteries in series for house power and one 12 volt starting battery, all separated by two control switches. Previous owner had no known electrolsis problems at a different location. All batteries were replaced in May '05. My knowledge of electricity and wiring is very limited. Any ideas? I'm practicing the 'wait & see'method with new anodes but I'm not getting anywhere. Do you know of someone with expertise in the north gulf coast area who may be able to help with onsite inspection? D. Dixon s/v CD EXPRESS
Conditions: Boat is located in salty waters of Gulf of Mexico; shore power is disconnected majority of time; neighboring boats are not having electrolsis problems. Because my two shaft "torpedo" anodes were nearly disappearing in three months, I attached a copper wire directly to the engine and to a large anode which hangs off the stern when the boat is in port. There is significant electrolsis on this anode in the five months its been in use. Two days ago I measured 1.877 ohms of current flowing between the engine and the overboard anode with all electrical devices turned off. I don't seem to be loosing battery power. The boat has four six volt batteries in series for house power and one 12 volt starting battery, all separated by two control switches. Previous owner had no known electrolsis problems at a different location. All batteries were replaced in May '05. My knowledge of electricity and wiring is very limited. Any ideas? I'm practicing the 'wait & see'method with new anodes but I'm not getting anywhere. Do you know of someone with expertise in the north gulf coast area who may be able to help with onsite inspection? D. Dixon s/v CD EXPRESS
Dick
just to clarify, what were you measuring - 1.877 ohms resistance between zinc and engine block or 1.877V volts?
just to clarify, what were you measuring - 1.877 ohms resistance between zinc and engine block or 1.877V volts?
Don....
Yes, I was measuring 1.877 ohms between the zinc and the engine block. Prior to measuring, I disconnected the copper wire leading to the overboard zinc from the engine. Using a meter set to the lowest ohms settings, I touched one probe of the meter to the engine and the other to the wire leading to the zinc. Dick....
Yes, I was measuring 1.877 ohms between the zinc and the engine block. Prior to measuring, I disconnected the copper wire leading to the overboard zinc from the engine. Using a meter set to the lowest ohms settings, I touched one probe of the meter to the engine and the other to the wire leading to the zinc. Dick....
sounds about right
the resistance you measured is that through salt water indicative of a good electrical bond between the shaft and block. The path you measured, if I understand correctly, is the wire to overboard zinc through the water to the prop, shaft, coupling and back to the block and the resistance is a combination of the connections and salt water. No rpoblem there; the overboard zinc is making a good electrical connection and doing it's job and the reading you obtained is similar to mine since I do the same thing while wintering in the water. The rate of zinc loss is a bit excessive so you might want to do a different test.If you have an ammeter, you can do a crude test to determine how much current is actually traveling that path - the current is what is eating the zinc(s). This isn't the best method but unless you have a silver chloride anode, it will suffice: Measure the current in milliamps to get a baseline reading at which time you can start checking various onboard electrical devices, beginning with the bilge pump wiring and keel bolt connections to see what, if any, change occurs as you disconnect each. Obviously, any significant change will point to a problem. Some folks try to do the same thing by measuring voltage but the change is so small as to render any typical voltmeter indiscriminant enough to be useless. Adding more zincs, contrary to conventional wisdom, will only exacerbate the problem. If it's any consolation, as long as you don't have corrosion of the prop and shaft, it's not that big a problem.
the resistance you measured is that through salt water indicative of a good electrical bond between the shaft and block. The path you measured, if I understand correctly, is the wire to overboard zinc through the water to the prop, shaft, coupling and back to the block and the resistance is a combination of the connections and salt water. No rpoblem there; the overboard zinc is making a good electrical connection and doing it's job and the reading you obtained is similar to mine since I do the same thing while wintering in the water. The rate of zinc loss is a bit excessive so you might want to do a different test.If you have an ammeter, you can do a crude test to determine how much current is actually traveling that path - the current is what is eating the zinc(s). This isn't the best method but unless you have a silver chloride anode, it will suffice: Measure the current in milliamps to get a baseline reading at which time you can start checking various onboard electrical devices, beginning with the bilge pump wiring and keel bolt connections to see what, if any, change occurs as you disconnect each. Obviously, any significant change will point to a problem. Some folks try to do the same thing by measuring voltage but the change is so small as to render any typical voltmeter indiscriminant enough to be useless. Adding more zincs, contrary to conventional wisdom, will only exacerbate the problem. If it's any consolation, as long as you don't have corrosion of the prop and shaft, it's not that big a problem.
Don....so you're saying....
a flow of 1.877 ohms flow is a bit high but somewhat normal in the conditions presented? Since there is no shore power hooked to the boat during the readings, does that mean the flow is coming from the 12 volt battery system? A sailing buddy a few slips down from me has no reading whatsoever when placing the same ohm meter between his engine and overboard zinc. Does that mean that since he has no current flow at all he isn't getting protection? His overboard zinc had no corrosion at all after more than six months of service. Thanks for the help.... Dick
a flow of 1.877 ohms flow is a bit high but somewhat normal in the conditions presented? Since there is no shore power hooked to the boat during the readings, does that mean the flow is coming from the 12 volt battery system? A sailing buddy a few slips down from me has no reading whatsoever when placing the same ohm meter between his engine and overboard zinc. Does that mean that since he has no current flow at all he isn't getting protection? His overboard zinc had no corrosion at all after more than six months of service. Thanks for the help.... Dick
Ohms
As I understand it Ohms are not a measure of a flow of electricity but a measure of electrical resistance. Zero ohms means no resistance. 1.877 ohms means your zinc is connected correctly to the engine block. It is a fairly low number though I think someone else thought it could be lower. I too am on a mooring 99.9% of the time and only a few days a year do I tie into shore power. When I do I add a spare zinc overboard that I grounded to the engine block. My shaft zincs are usually at least 50% gone at the end of the season. I have some blistering on my keel which exposes some lead due to a poor paint job when the boat was delivered. I always attributed the loss of zinc to the dissimilar metals between my S-steel shaft and the keel. I'm sure that if I have this all wrong someone will correct me.
As I understand it Ohms are not a measure of a flow of electricity but a measure of electrical resistance. Zero ohms means no resistance. 1.877 ohms means your zinc is connected correctly to the engine block. It is a fairly low number though I think someone else thought it could be lower. I too am on a mooring 99.9% of the time and only a few days a year do I tie into shore power. When I do I add a spare zinc overboard that I grounded to the engine block. My shaft zincs are usually at least 50% gone at the end of the season. I have some blistering on my keel which exposes some lead due to a poor paint job when the boat was delivered. I always attributed the loss of zinc to the dissimilar metals between my S-steel shaft and the keel. I'm sure that if I have this all wrong someone will correct me.
Zincs erroding
If the zincs are going there is current flowing to cause it. Couple of questions; Is the engine/transmission/shaft & prop electrically isolated from the keel/thru hulls/other "bonded" items? Does your bonding ground connect to the 12 volt electrical power ground? Does your VHF antenna ground connect to the 12 volt electrical power ground? West marine has a great page that talks to all this. See the link below
If the zincs are going there is current flowing to cause it. Couple of questions; Is the engine/transmission/shaft & prop electrically isolated from the keel/thru hulls/other "bonded" items? Does your bonding ground connect to the 12 volt electrical power ground? Does your VHF antenna ground connect to the 12 volt electrical power ground? West marine has a great page that talks to all this. See the link below
Dick
I think I have succeeded in confusing you - not my intent. For purposes of this discussion, forget about the ohm reading. It has nothing to do with stray current. The measurement of ohms, which is resistance, only shows that the circuit (in your case, the connection of the zinc to the engine) is good; not perfect which zero ohms would mean but still good. Knowing the resistance only shows the zinc overboard is making good conenction to the engine; it has nothing to do with the source or measure of stray current which may be causing your zincs to dissipate rapidly. Neither does it indicate that your friend's boat is free of stary current corrosion. Only an ammeter reading would show that. Presuming neither boat is connected to shore power, the cause of the zinc loss is DC current and the only places current can come from are 1. your 12VDC system or 2. a dock neighbors'. If it's your boat's systems, my prior post explains how to find the source of the 12V current eating your zincs. Some ideas for you regarding possible sources of stray 12VDC current are: Most common problem - multiple DC grounds on the boat (there should only be one) If you leave battery switches on normally, it could be a corroded battery cable connection (either + or - cables anywhere in the boat) TV antenna on mast ( most have 12V powered amp and are grounded to mast) same with gps or radar antennas, and grounded brass thruhulls with corroded connections.
I think I have succeeded in confusing you - not my intent. For purposes of this discussion, forget about the ohm reading. It has nothing to do with stray current. The measurement of ohms, which is resistance, only shows that the circuit (in your case, the connection of the zinc to the engine) is good; not perfect which zero ohms would mean but still good. Knowing the resistance only shows the zinc overboard is making good conenction to the engine; it has nothing to do with the source or measure of stray current which may be causing your zincs to dissipate rapidly. Neither does it indicate that your friend's boat is free of stary current corrosion. Only an ammeter reading would show that. Presuming neither boat is connected to shore power, the cause of the zinc loss is DC current and the only places current can come from are 1. your 12VDC system or 2. a dock neighbors'. If it's your boat's systems, my prior post explains how to find the source of the 12V current eating your zincs. Some ideas for you regarding possible sources of stray 12VDC current are: Most common problem - multiple DC grounds on the boat (there should only be one) If you leave battery switches on normally, it could be a corroded battery cable connection (either + or - cables anywhere in the boat) TV antenna on mast ( most have 12V powered amp and are grounded to mast) same with gps or radar antennas, and grounded brass thruhulls with corroded connections.
Thanks Don.....
A local friend who once made his living as a marine surveyor has offered to help locate the problem. Thank you for your advice and information. Admittingly, I am a total idiot when it comes to electricity...my friend knows that and has taken pity. I'll stick to what I know best....sailing and marine woodworking. Again, thanks for the information....as this thing plays out I am learning a lot of valuable lessons. Dick
A local friend who once made his living as a marine surveyor has offered to help locate the problem. Thank you for your advice and information. Admittingly, I am a total idiot when it comes to electricity...my friend knows that and has taken pity. I'll stick to what I know best....sailing and marine woodworking. Again, thanks for the information....as this thing plays out I am learning a lot of valuable lessons. Dick
Galvanic Isolator
You can put a Galvanic Isolator in the ground circuit between your boat and the shore power. It is basically two diodes wired in series going one way and two more diodes wired in series again going the other way. The diodes must be big enough to handle short circuits to trip the breaker on shore (e.g. 30A). The theory is that galvanic actions starts lower than 1 volt. Putting two diodes in series will give 1.5 volts thus blocking the DC current. Galvanic action is just like electro plating (e.g. Chrome plating). AC doesn't do galvanic action. The reason for the reverse diodes is for the DC current going that other way. That is your boat is the bad guy. I got the diodes for about $4 a piece, fixed them on a heat sink just in case, and pour some epoxy to seal them off. Or you can buy them for a lot more money. http://www.boatus.com/seaworthy/galvanic/default.asp
You can put a Galvanic Isolator in the ground circuit between your boat and the shore power. It is basically two diodes wired in series going one way and two more diodes wired in series again going the other way. The diodes must be big enough to handle short circuits to trip the breaker on shore (e.g. 30A). The theory is that galvanic actions starts lower than 1 volt. Putting two diodes in series will give 1.5 volts thus blocking the DC current. Galvanic action is just like electro plating (e.g. Chrome plating). AC doesn't do galvanic action. The reason for the reverse diodes is for the DC current going that other way. That is your boat is the bad guy. I got the diodes for about $4 a piece, fixed them on a heat sink just in case, and pour some epoxy to seal them off. Or you can buy them for a lot more money. http://www.boatus.com/seaworthy/galvanic/default.asp
Zinc corrosion for dummies...
Of which I am one
OK, you've been forewarned... Three things can corrode your zincs. 1) dissimilar metals (galvanic corrosion), part of life, the slowest of zinc eating phenomenons. 2) 12V system leaks - this would usually be from your own boats system leaking voltage into the water. Quick fix - You can stop this by disconnecting your batteries. 3)120V dock power - this can be caused by your own power leak, or a nearby neighbor. Quick fix - you can unhook the dock power. Eventually course you should atempt to find the origin of the leak (root cause) & have it fixed, as covered by the folks who know alot more about electricity than I...
Of which I am one
OK, you've been forewarned... Three things can corrode your zincs. 1) dissimilar metals (galvanic corrosion), part of life, the slowest of zinc eating phenomenons. 2) 12V system leaks - this would usually be from your own boats system leaking voltage into the water. Quick fix - You can stop this by disconnecting your batteries. 3)120V dock power - this can be caused by your own power leak, or a nearby neighbor. Quick fix - you can unhook the dock power. Eventually course you should atempt to find the origin of the leak (root cause) & have it fixed, as covered by the folks who know alot more about electricity than I...- Status