Is it possible to "over zinc?" Recently added zinc since old ones were wearing (see photo). Pulled yesterday to work on keel rust and painter in the yard said I was "overzinced."
Overzinced?
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In theory, yes.
Consider what you are doing by using anodes - you are creating a battery with the anode acting as the termination through which the current flows. You can create a battery too large by having excess anode material.
Consider what you are doing by using anodes - you are creating a battery with the anode acting as the termination through which the current flows. You can create a battery too large by having excess anode material.
If you want 'precision' on how big a 'zinc battery' you should be making: http://www.boatzincs.com/corrosion-reference-electrode-specs.html
I have never seen any published argument/experiment that supports the idea there is risk from too much zinc in the case of inboard fiberglass boats.
However, there are risks in the case of metal or wood hulls.
For some good discussion see http://www.martyranodes.com/content/martyr-resources/Aluminum Anodes.pdf and http://www.kastenmarine.com/_pdf/mbqCref.pdf . The booklet published by Boatzincs is useful but does not contain any support for the idea "overzincing" is possible in a fiberglass inboard sailboat either.
These sources are probably more reliable than your painter's opinion but I would be interested in what he thinks will happen if there is "too much" and what information he relies on in concluding "too much" zinc is present on your boat.
Charles
However, there are risks in the case of metal or wood hulls.
For some good discussion see http://www.martyranodes.com/content/martyr-resources/Aluminum Anodes.pdf and http://www.kastenmarine.com/_pdf/mbqCref.pdf . The booklet published by Boatzincs is useful but does not contain any support for the idea "overzincing" is possible in a fiberglass inboard sailboat either.
These sources are probably more reliable than your painter's opinion but I would be interested in what he thinks will happen if there is "too much" and what information he relies on in concluding "too much" zinc is present on your boat.
Charles
Why the hell would you leave those 2 old zincs on the boat? They aren't doing anything anymore.
If those 2 old ones haven't been on long you aren't over-zinc'd, you are under zinc'd!
How many zincs you need and how long they last depend on lots of things. But if they are wasting away more than 50% in a year you need more, if less you may be able to use less. But I would be more worried about under zinc than over.
If those 2 old ones haven't been on long you aren't over-zinc'd, you are under zinc'd!
How many zincs you need and how long they last depend on lots of things. But if they are wasting away more than 50% in a year you need more, if less you may be able to use less. But I would be more worried about under zinc than over.
Yes you can over zinc by using the wrong material such as magnesium in salt water.. It can create halo's or dead spots in the copper paint around underwater metals. Also by placing too many anodes on the vessel they can become ineffective and grow a layer of oxide around themselves causing them to not work as well or work at all.
A fiberglass boat should be somewhere between neg -650 to -1050 mV DC using a Ag/AgCl reference cell..
If you have an aluminum leg sail drive then you want to be between neg 850mV and -1050mV anything less than -850mV and you are under protected for the aluminum and anything over -1050mV and you are over protected. Without a Ag/AgCl reference cell monitoring anodic protection is a crap shoot. I know our boat takes two streamlined shaft zincs but I have many customers boats who are over protected with just one or under protected with two etc.. It all amounts to how much underwater metal weight you are protecting as to how much anode protection you need for the duration intended.
Other things that cause haloing are improper wiring or the use of "bonding" circuits as return paths for 12V DC appliances. Non marine battery chargers can also cause some serious "burning/haloing" as they are poorly "isolated"......
People always assume that with zincs more or bigger is better and it is not always better unless you actually know you need it.
A fiberglass boat should be somewhere between neg -650 to -1050 mV DC using a Ag/AgCl reference cell..
If you have an aluminum leg sail drive then you want to be between neg 850mV and -1050mV anything less than -850mV and you are under protected for the aluminum and anything over -1050mV and you are over protected. Without a Ag/AgCl reference cell monitoring anodic protection is a crap shoot. I know our boat takes two streamlined shaft zincs but I have many customers boats who are over protected with just one or under protected with two etc.. It all amounts to how much underwater metal weight you are protecting as to how much anode protection you need for the duration intended.
Other things that cause haloing are improper wiring or the use of "bonding" circuits as return paths for 12V DC appliances. Non marine battery chargers can also cause some serious "burning/haloing" as they are poorly "isolated"......
People always assume that with zincs more or bigger is better and it is not always better unless you actually know you need it.
Thanks
Thanks you, gentlemen, for the great information. I knew you guys would know. I now have some reference numbers to check when back in my slip. Re old ones, knew I was pulling soon, tight bolts, very bad visibility--murky water. Will pull them off in the yard. Steve
Thanks you, gentlemen, for the great information. I knew you guys would know. I now have some reference numbers to check when back in my slip. Re old ones, knew I was pulling soon, tight bolts, very bad visibility--murky water. Will pull them off in the yard. Steve
The values cited in the ABYC E2 guidelines mentioned above are negative values relative to silver/silver-chloride reference cell. So, for example, when one measures hull potential with a silver probe one is supposed to install zincs which cause a potential shift value of -750 to -1100 mv for a fiberglass boat, inboard, ss shaft, bronze prop, no sail drive, no outdrive.
Acceptable negative shifts for wood boats, metal boats, boats with outdrives, outboard, are set at different levels by ABYC.
A zinc probe (about $5) is plenty accurate and much less costly than silver (over $100.) If you use a zinc probe instead of silver you simply correct the value at the multi meter by -1000 mv.
Too much zinc can cause paint halos and antifoul degradation in a metal boat case but I doubt that would be observable in the case of fibreglass boat shown in the picture above assuming protection is shaft/prop zinc only. In any case - anti-foul paint with copper content over 55% can suffer local halo effect even if the zinc achieves a -750 to -1100 mv potential shift and even then would be extrememly local in its effect.
I still cannot find anything showing that a larger amount of zinc produces a higher negative shift than does a smaller quantity of zinc.
In other words - if mil spec zinc alloy is about -1100 mv on the galvanic scale it is not possible to induce more than -1100 mv by adding more zinc. Is it?
By the way - these negative shift values and the discussion here is useful only if there is no DC stay current involved. If there is DC stray current flowing on your boat then these "norrmal" target values will be impossible to obtain. DC stray current must be eliminated completely before normal hull potential values can be expected.
Charles
Acceptable negative shifts for wood boats, metal boats, boats with outdrives, outboard, are set at different levels by ABYC.
A zinc probe (about $5) is plenty accurate and much less costly than silver (over $100.) If you use a zinc probe instead of silver you simply correct the value at the multi meter by -1000 mv.
Too much zinc can cause paint halos and antifoul degradation in a metal boat case but I doubt that would be observable in the case of fibreglass boat shown in the picture above assuming protection is shaft/prop zinc only. In any case - anti-foul paint with copper content over 55% can suffer local halo effect even if the zinc achieves a -750 to -1100 mv potential shift and even then would be extrememly local in its effect.
I still cannot find anything showing that a larger amount of zinc produces a higher negative shift than does a smaller quantity of zinc.
In other words - if mil spec zinc alloy is about -1100 mv on the galvanic scale it is not possible to induce more than -1100 mv by adding more zinc. Is it?
By the way - these negative shift values and the discussion here is useful only if there is no DC stay current involved. If there is DC stray current flowing on your boat then these "norrmal" target values will be impossible to obtain. DC stray current must be eliminated completely before normal hull potential values can be expected.
Charles
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If I had a dollar for every uninformed and erroneous proclamation I have heard from "expert" boatyard employees or even boatyard owners... well, I'd have a quite a few dollars.
Also it is possible to induce shaft vibration if too much weight is improperly located on the shaft.
2
I have always had 2 Zincs on my prop shaft and never saw any problem,just make sure the Zinc is on good so it does not come off and if and when you have a diver clean your boat let him ccheck the Zincs and if needed have him replace.
I now keep 1 Zinc on the shaft and have a special Zinc on my Flex O Fold prop,had my boat in the water for 4 years before taking it out for new bottom paint here in Burnt Store and only remember changing the Zinc on the shaft once.
I used SR40 bottom paint and never had a diver clean the bottom,I switched to SR60 this past year and I only dive to clean the prop.
Nick
I have always had 2 Zincs on my prop shaft and never saw any problem,just make sure the Zinc is on good so it does not come off and if and when you have a diver clean your boat let him ccheck the Zincs and if needed have him replace.
I now keep 1 Zinc on the shaft and have a special Zinc on my Flex O Fold prop,had my boat in the water for 4 years before taking it out for new bottom paint here in Burnt Store and only remember changing the Zinc on the shaft once.
I used SR40 bottom paint and never had a diver clean the bottom,I switched to SR60 this past year and I only dive to clean the prop.
Nick
I have a related question. I purchased a used 21 Ft trailer-able sailboat two years ago.
A large number of the boats I looked at had very corroded cast iron (?) center boards, but I saw no evidence that any "zincs" where installed on any of the boats. There may have been some on the outboard motors, but the motor is not typically in the water most of the time. Should I be installing these electrodes on the center board?
A large number of the boats I looked at had very corroded cast iron (?) center boards, but I saw no evidence that any "zincs" where installed on any of the boats. There may have been some on the outboard motors, but the motor is not typically in the water most of the time. Should I be installing these electrodes on the center board?
I agree that zinc or aluminum anodes don't prevent rust, but I believe the keel does act as the "ground" for the electrical system and the lightning rod.
This being the case, it seems to me that galvanic corrosion could also accelerate the degeneration of the keel.
From what I read, simply keeping the part clean & painted (good for rust protection) won't prevent galvanic corrosion. The paint could even "get blown off the surface".
This being the case, it seems to me that galvanic corrosion could also accelerate the degeneration of the keel.
From what I read, simply keeping the part clean & painted (good for rust protection) won't prevent galvanic corrosion. The paint could even "get blown off the surface".
Catchment Area
The zinc is the anode, that is where corrosion is supposed to occur, thereby saving vital parts of your boat from corroding. The higher ratio of cathode to anode, the faster the rate of corrosion at the anode (the zinc). Therefore, increasing the size of the zinc anode too much could in theory impede and slow the galvanic sacrificial action of the zinc anode. The ratio of anode to cathode is referred to as the "catchment area principle". The ratio does matter. The voltage can be measured as noted by others in this string.
The zinc is the anode, that is where corrosion is supposed to occur, thereby saving vital parts of your boat from corroding. The higher ratio of cathode to anode, the faster the rate of corrosion at the anode (the zinc). Therefore, increasing the size of the zinc anode too much could in theory impede and slow the galvanic sacrificial action of the zinc anode. The ratio of anode to cathode is referred to as the "catchment area principle". The ratio does matter. The voltage can be measured as noted by others in this string.
If the keel is actually rusting (as opposed to galvanic or electrolytic corrosion) as many iron keels do, a zinc is not going to help it.
Stray DC current
Charles,
I too have a Norsea and the only zinc I have is on the prop nut. Since getting away from the Anacortes marina with it's various unknown currents from other boats and onto a mooring I thought I would use less zinc. As it turns out, anchored in Hunter Bay on Lopez, I'm going through zincs faster. All I can think is the up to 1/2 knot current is creating current. Any thoughts?
Barry
Charles,
I too have a Norsea and the only zinc I have is on the prop nut. Since getting away from the Anacortes marina with it's various unknown currents from other boats and onto a mooring I thought I would use less zinc. As it turns out, anchored in Hunter Bay on Lopez, I'm going through zincs faster. All I can think is the up to 1/2 knot current is creating current. Any thoughts?
Barry
Barry -
Greetings
How fast zinc depletion? I use mil spec zinc. Mine uses about 50% of prop zinc every year. That is the only zinc on the boat. I do not bond (wire together) any underwater metal. No lightning protection wiring.
To your case -- seawater current flow can effect zinc corrosion rate. So will elevated temperature, dissolved oxygen, PH, salinity, etc. - theoretically. If you are using the same brand zinc as at Anacortes and you have changed nothing else then - apart from change in environment -- maybe someting has changed without you noticing.
Maybe a DC leak. The usual suspects are bilge pump (internal high resistence short) alternator positive contacts alternator case via corrosion , positive starter feed contacts starter case case via corrosion. If you have bonding wiring, or any DC grounding wires (usually green), see to it these terminate clean and are in tact. No wiring in blige water if you please -- so check any wiring that goes under the floor and up the compression post, refrigeration compressor/thermostat leads, deck pump, what have you.
After check outs try for measurements.
Hull potential measurement is easy. Zinc probe on end of wire goes over the side, probe wire to multi meter negative, multi meter positive via wire and clipped to propeller shaft. Observe meter readings as follows -- battery fully disconnected, battery connected main switch off, battery connected switched on, motor start key on, motor start key off, radar SSB VHF inverter make sure to turn these on and off respectively to test whether any values change. Record results.
If no changes in values then probably no DC onboard current leak and explanation is elsewhere.
Charles
Edit: We spend many days time in your country every year. Must look you up. Was anchored in Hunter Bay July 11 2012 , very cool setting but did not notice your boat.
Greetings
How fast zinc depletion? I use mil spec zinc. Mine uses about 50% of prop zinc every year. That is the only zinc on the boat. I do not bond (wire together) any underwater metal. No lightning protection wiring.
To your case -- seawater current flow can effect zinc corrosion rate. So will elevated temperature, dissolved oxygen, PH, salinity, etc. - theoretically. If you are using the same brand zinc as at Anacortes and you have changed nothing else then - apart from change in environment -- maybe someting has changed without you noticing.
Maybe a DC leak. The usual suspects are bilge pump (internal high resistence short) alternator positive contacts alternator case via corrosion , positive starter feed contacts starter case case via corrosion. If you have bonding wiring, or any DC grounding wires (usually green), see to it these terminate clean and are in tact. No wiring in blige water if you please -- so check any wiring that goes under the floor and up the compression post, refrigeration compressor/thermostat leads, deck pump, what have you.
After check outs try for measurements.
Hull potential measurement is easy. Zinc probe on end of wire goes over the side, probe wire to multi meter negative, multi meter positive via wire and clipped to propeller shaft. Observe meter readings as follows -- battery fully disconnected, battery connected main switch off, battery connected switched on, motor start key on, motor start key off, radar SSB VHF inverter make sure to turn these on and off respectively to test whether any values change. Record results.
If no changes in values then probably no DC onboard current leak and explanation is elsewhere.
Charles
Edit: We spend many days time in your country every year. Must look you up. Was anchored in Hunter Bay July 11 2012 , very cool setting but did not notice your boat.
My diver is my best friend in this regard, on a previous boat he let me know zincs where depleting much faster than usual suddenly. I spoke with the dockmaster and he discovered the 110 AC was energizing the dock.
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