How often do you change your zincs? While this question generally addresses the zincs you install on your prop shaft, we should also discuss zincs used in your engine or cooling system, too. Is the replacement cycle consistent, or does it change from year to year? Have you found any difference in location, brand, size or type you apply? Share your most corrosive opinions here, then vote in this week's Quick Quiz toward the bottom of the home page.
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Water is the difference.
In Lake Erie I can't detect a difference in the zincs from last year to this year, on the shaft or in the motor.
In Lake Erie I can't detect a difference in the zincs from last year to this year, on the shaft or in the motor.
3 Months! Ha! I wish
My boat is in a very hot (electrically, that is) marina. When I first got there my zincs were lasting about 3 weeks. I have since added a galvanic isolator and the lifespan is up to about 2 months. I think I still have a shore power ground leak somewhere. That's one of the projects for the upcoming months.
My boat is in a very hot (electrically, that is) marina. When I first got there my zincs were lasting about 3 weeks. I have since added a galvanic isolator and the lifespan is up to about 2 months. I think I still have a shore power ground leak somewhere. That's one of the projects for the upcoming months.
Once a season...
...which runs from mid-May to mid-November. I carry 3 zincs on the shaft of my L37 and they are usually worn nearly off at that time.
...which runs from mid-May to mid-November. I carry 3 zincs on the shaft of my L37 and they are usually worn nearly off at that time.
toooo cheap
i put two zincs on the shaft...spring and fall...one i left them on for 12 mos....the following sping on was gone and the other was really small..the diver charges 10 bucks plus the 15 for the zinc..its just tooo cheap to not do it...ps the 25 is on top of my cleaning bottum charge...schaefer
i put two zincs on the shaft...spring and fall...one i left them on for 12 mos....the following sping on was gone and the other was really small..the diver charges 10 bucks plus the 15 for the zinc..its just tooo cheap to not do it...ps the 25 is on top of my cleaning bottum charge...schaefer
Depends on the water
When sailing Lake Erie, zincs can last several seasons. But when we sailed our 37.5 from Lake Erie to the Florida Keys, the zincs were gone by the time we reached Ft. Lauderdale, which was three months. It cost us 80 dollars for the diver, but that is cheaper than a new max prop.
When sailing Lake Erie, zincs can last several seasons. But when we sailed our 37.5 from Lake Erie to the Florida Keys, the zincs were gone by the time we reached Ft. Lauderdale, which was three months. It cost us 80 dollars for the diver, but that is cheaper than a new max prop.
Zinc usage
On our 430, when we first got the boat, we were going through the zinc in about six weeks. After installing a UL-approved galvanic isolator, we saw no improvement. This happened whether we were in our own marina in Clearwater, Florida, or in the dealer's yard in St. Petersburg. After four "consultants" who promised improvement, I finally found one who very thoroughly went over the complete DC performance of the boat-problem is DC current leakage, not AC. He found many stray currents (for instance the current for the stereo is constantly on, for the memory circuit, even when everything else is turned off. I now go about seven to eight months before having to install new zincs - I added another one just to be on the safe side, since I ate up a prop when a zinc fell off after only three weeks before the corrections. Pete S/V KAILANI
On our 430, when we first got the boat, we were going through the zinc in about six weeks. After installing a UL-approved galvanic isolator, we saw no improvement. This happened whether we were in our own marina in Clearwater, Florida, or in the dealer's yard in St. Petersburg. After four "consultants" who promised improvement, I finally found one who very thoroughly went over the complete DC performance of the boat-problem is DC current leakage, not AC. He found many stray currents (for instance the current for the stereo is constantly on, for the memory circuit, even when everything else is turned off. I now go about seven to eight months before having to install new zincs - I added another one just to be on the safe side, since I ate up a prop when a zinc fell off after only three weeks before the corrections. Pete S/V KAILANI
too much zinc ???
I was told that you can have too much Zinc on the boat which can attract more corrosion. I asked about adding an extra one to my prop shaft. One zinc gets me thru the season here in New England...
I was told that you can have too much Zinc on the boat which can attract more corrosion. I asked about adding an extra one to my prop shaft. One zinc gets me thru the season here in New England...
which zinc?
My heat exchanger zinc need replacing about every month but my shaft zinc is over a year old and its still 2/3rd's intact and solid. I'll replace it anyway when I return from my next cruise in late May.
My heat exchanger zinc need replacing about every month but my shaft zinc is over a year old and its still 2/3rd's intact and solid. I'll replace it anyway when I return from my next cruise in late May.
Fresh water amkes a difference
I got my boat ('89 H30) last spring. It had been in the water (Lake Ontario) for almost a year before I bought it, and I didn't haul it until the fall. There were no external zincs, and, as far as I can tell, no internal zincs in the engine (2GM20F). When polishing the prop before launch this spring, I noticed a "rash" of very small pits in the prop. I asked a boat maintenance "expert" who told me that zincs are not required in fresh water. I put one on the prop shaft anyway, just in case. I noticed that most of the boats at my yacht club have prop shaft zincs, but most look like they have been on the boats unchanged for many years. I saw none that were eroded, and no one seemed to be putting on new ones this season. I guess fresh water doesn't have enough conductivity to cause serious galvanic corrosion, unless possibly in a marina with serious grounding problems.
I got my boat ('89 H30) last spring. It had been in the water (Lake Ontario) for almost a year before I bought it, and I didn't haul it until the fall. There were no external zincs, and, as far as I can tell, no internal zincs in the engine (2GM20F). When polishing the prop before launch this spring, I noticed a "rash" of very small pits in the prop. I asked a boat maintenance "expert" who told me that zincs are not required in fresh water. I put one on the prop shaft anyway, just in case. I noticed that most of the boats at my yacht club have prop shaft zincs, but most look like they have been on the boats unchanged for many years. I saw none that were eroded, and no one seemed to be putting on new ones this season. I guess fresh water doesn't have enough conductivity to cause serious galvanic corrosion, unless possibly in a marina with serious grounding problems.
Galvanic corrosion
All metals develop a voltage when immersed in seawater, or any conductive solution. The voltage (electrical pressure) developed in each metal is a function of the metal type and its mass (see the Galvanic Table of the Elements). Current (flow of electrons - not pressure), which is what destroys zincs, flows from high voltage to low voltage. The amount of current is determined by the difference in voltage, and the temperature and salinity of the seawater (eletrolyte). Just like a battery, there must be dissimilar metals immersed in a conductive solution in order for current to flow. In the case of the underhull environment, current is measured in microamps, but is still capable of a fair amount of destruction if given the opportunity. Many marinas wind up being branded as "hot", but this is rarely the case. Almost always, the problem is with the yacht. The first line of defence is to ensure that the yacht is properly "bonded". Since all metals below the waterline are developing different voltages, it follows that to connect them together, would cause their voltages to "equalize", ie: become the same. If the voltage is the same in all metals below the waterline, then current flow should cease - no voltage difference = no current = no corrosion. Bonding consists of electrically connecting all through hulls and other underwater metals together inside the hull. This is normally done with 8 ga. heavy strand copper or tinned copper wire. This is necessary since the fiberglass hull is not a conductor. Without bonding, "stray" current will flow randomly and uncontrolled, resulting in the high voltage producing metals giving up electrons to the low voltage metals. Electron loss = metal loss, just like the reverse of an electroplating process. Zinc and/or zinc/magnesium is then connected into the system since zinc tends to develop a much higher voltage than the other metals typically used in marine (bronze, stainless and so on). Often we find that prop zincs deteriorate quickly because the yacht is not bonded. This means that the prop zinc is protecting the prop shaft and propeller, but also dealing with a stray current flow towards the other metals below the waterline. The propeller shaft should be connected to the main zinc and the vessel's bonding system via a shaft brush and wiring inside the yacht. This means that the mass of the prop shaft assembly will be equal to the total hull electrical potential, and that the hull zinc will "assist" the shaft zinc in its work. The shaft and hull zincs' total mass should be sufficient to allow a once a year haulout for replacement. At haulout, there should still be just a little zinc left over, meaning that it has been doing its job well. The reader of this article might be inclined then to think that more zinc is better, but this is not so. More zinc will increase to overall voltage of the yacht (hull potential) by far too much. This will result in greater stray current corrosion, and create even greater problems that before. The zinc must be "balanced". This is done by selecting a zinc that will raise the hul potential about 200mv. above the potential of the hull without any zinc whatever. Example: After bonding, the hull potential is measured and found to be 550mv. A hull zinc and a shaft zinc are installed and bonded into the system - the zinc size being sufficient to raise the hull potential to 750mv. Since the shaft zinc size cannot always be changed as easily as all that, the baalnce of zinc is determined by the selection of the hull zinc size. The zinc will erode away, and as its mass diminishes, so will the effect the zinc has on hull potential. Basically, when the hull potential reduces to the 550mv level, it is time to repalce the zinc. Finally, if you have shore power bonded into the system, or if you have an inverter, the green safety ground should be connected into the bonding system in order to provide for a safety ground in the A/C system at all times. The green ground can be isolated from shorepower with a "zinc saver". This would prevent a nearby yacht with eletrical troubles (and therfore stray current), finding ground to the shore power through your throughulls. A zinc saver is nothing more than a pair of diodes connected in parallel, but at opposite polarities. The diodes will block low voltage stray current, but will still conduct A/C current if a ground fault exists in your system. A zinc saver is installed in your yacht's green wire ground as close to the shore cord as possible, and ahead of any other ground wire ie: nothing between it and the shore power. This will also eliminate the need to hang a zinc over the side of the yacht when in port. There will simply be no need whatever. Hope all this makes sense to you guys.
All metals develop a voltage when immersed in seawater, or any conductive solution. The voltage (electrical pressure) developed in each metal is a function of the metal type and its mass (see the Galvanic Table of the Elements). Current (flow of electrons - not pressure), which is what destroys zincs, flows from high voltage to low voltage. The amount of current is determined by the difference in voltage, and the temperature and salinity of the seawater (eletrolyte). Just like a battery, there must be dissimilar metals immersed in a conductive solution in order for current to flow. In the case of the underhull environment, current is measured in microamps, but is still capable of a fair amount of destruction if given the opportunity. Many marinas wind up being branded as "hot", but this is rarely the case. Almost always, the problem is with the yacht. The first line of defence is to ensure that the yacht is properly "bonded". Since all metals below the waterline are developing different voltages, it follows that to connect them together, would cause their voltages to "equalize", ie: become the same. If the voltage is the same in all metals below the waterline, then current flow should cease - no voltage difference = no current = no corrosion. Bonding consists of electrically connecting all through hulls and other underwater metals together inside the hull. This is normally done with 8 ga. heavy strand copper or tinned copper wire. This is necessary since the fiberglass hull is not a conductor. Without bonding, "stray" current will flow randomly and uncontrolled, resulting in the high voltage producing metals giving up electrons to the low voltage metals. Electron loss = metal loss, just like the reverse of an electroplating process. Zinc and/or zinc/magnesium is then connected into the system since zinc tends to develop a much higher voltage than the other metals typically used in marine (bronze, stainless and so on). Often we find that prop zincs deteriorate quickly because the yacht is not bonded. This means that the prop zinc is protecting the prop shaft and propeller, but also dealing with a stray current flow towards the other metals below the waterline. The propeller shaft should be connected to the main zinc and the vessel's bonding system via a shaft brush and wiring inside the yacht. This means that the mass of the prop shaft assembly will be equal to the total hull electrical potential, and that the hull zinc will "assist" the shaft zinc in its work. The shaft and hull zincs' total mass should be sufficient to allow a once a year haulout for replacement. At haulout, there should still be just a little zinc left over, meaning that it has been doing its job well. The reader of this article might be inclined then to think that more zinc is better, but this is not so. More zinc will increase to overall voltage of the yacht (hull potential) by far too much. This will result in greater stray current corrosion, and create even greater problems that before. The zinc must be "balanced". This is done by selecting a zinc that will raise the hul potential about 200mv. above the potential of the hull without any zinc whatever. Example: After bonding, the hull potential is measured and found to be 550mv. A hull zinc and a shaft zinc are installed and bonded into the system - the zinc size being sufficient to raise the hull potential to 750mv. Since the shaft zinc size cannot always be changed as easily as all that, the baalnce of zinc is determined by the selection of the hull zinc size. The zinc will erode away, and as its mass diminishes, so will the effect the zinc has on hull potential. Basically, when the hull potential reduces to the 550mv level, it is time to repalce the zinc. Finally, if you have shore power bonded into the system, or if you have an inverter, the green safety ground should be connected into the bonding system in order to provide for a safety ground in the A/C system at all times. The green ground can be isolated from shorepower with a "zinc saver". This would prevent a nearby yacht with eletrical troubles (and therfore stray current), finding ground to the shore power through your throughulls. A zinc saver is nothing more than a pair of diodes connected in parallel, but at opposite polarities. The diodes will block low voltage stray current, but will still conduct A/C current if a ground fault exists in your system. A zinc saver is installed in your yacht's green wire ground as close to the shore cord as possible, and ahead of any other ground wire ie: nothing between it and the shore power. This will also eliminate the need to hang a zinc over the side of the yacht when in port. There will simply be no need whatever. Hope all this makes sense to you guys.
Propeller pitting
Bob: Generally, small pitting on a prop is caused by cavitation and has little to do with galvanic corrosion at all. Initially and for some time, galvanic corrosion would cause no apparent change in the appearance of the prop. If there is a discrepancy in prop pitch, or some protuberance etc. that could set up air bubbles in the prop stream, etc., that is far more likely to be the cause of your trouble. You are correct when you surmise that fresh water would not have the effect on corrosion that salt water has.
Bob: Generally, small pitting on a prop is caused by cavitation and has little to do with galvanic corrosion at all. Initially and for some time, galvanic corrosion would cause no apparent change in the appearance of the prop. If there is a discrepancy in prop pitch, or some protuberance etc. that could set up air bubbles in the prop stream, etc., that is far more likely to be the cause of your trouble. You are correct when you surmise that fresh water would not have the effect on corrosion that salt water has.
Who was Galvin anyway?
Great post Mike! A bit of clarification for those still following along. When Mike refers to dissimilar metals, they can actually reside on the same metal. What??? For example, steel is typically manufactured in a variety of compositions. Elements are added to give the steel some unique property. As such, it is impossible to have a perfectly homogeneous mixture from one end of a long I-beam to the other. This creates "pockets" on the beam that have different galvanic properties from other pockets and thus, a potential for current flow. So it's not always different metals that cause the problem, just dissimilar ones. Who cares?
Mark Kissel Kittiwake/98H240
Great post Mike! A bit of clarification for those still following along. When Mike refers to dissimilar metals, they can actually reside on the same metal. What??? For example, steel is typically manufactured in a variety of compositions. Elements are added to give the steel some unique property. As such, it is impossible to have a perfectly homogeneous mixture from one end of a long I-beam to the other. This creates "pockets" on the beam that have different galvanic properties from other pockets and thus, a potential for current flow. So it's not always different metals that cause the problem, just dissimilar ones. Who cares?
Mark Kissel Kittiwake/98H240Cavitation vs. horsepower
Mike, thanks for the explanation. When I saw these tiny pits, the first thing I though of was cavitation. But then, I wondered if a 16 HP diesel has enough power to cause cavitation erosion on a hard bronze prop? Several years ago, I had a power boat that put 260 HP into the water through a single high-pitch aluminum prop. I was careful with the throttle on that boat since it could "boil" the water. Should I be concerned with protecting the prop from cavitation with the little Yanmar? Thanks, Bob.
Mike, thanks for the explanation. When I saw these tiny pits, the first thing I though of was cavitation. But then, I wondered if a 16 HP diesel has enough power to cause cavitation erosion on a hard bronze prop? Several years ago, I had a power boat that put 260 HP into the water through a single high-pitch aluminum prop. I was careful with the throttle on that boat since it could "boil" the water. Should I be concerned with protecting the prop from cavitation with the little Yanmar? Thanks, Bob.
More on cavitation
Bob; Cavitation can happen on any prop, regardless of the size or horsepower involved. Typically, it is caused by turbulence. This can be set up by the blade design itself, by blades of unequal shape (ie: bent), blockage of the water stream and in the case of outboard motors and sail drives, exhaust in the prop stream. Bockage is more commonly seen in full keel yachts where the "deadwood" area of the keel prevents water from feeding into the prop properly. This causes huge troubles when the prop will not "bite" the water - usually when going astern. These yachts are notoriously poor at backing down. I would start by taking the prop off anf having a propeller shop check it for true. The answer may be as simple as that. After that it could be more trouble to sort out, perhaps even demanding the attention of an expert, but I suspect that you will answer you question at the prop shop. Good luck!
Bob; Cavitation can happen on any prop, regardless of the size or horsepower involved. Typically, it is caused by turbulence. This can be set up by the blade design itself, by blades of unequal shape (ie: bent), blockage of the water stream and in the case of outboard motors and sail drives, exhaust in the prop stream. Bockage is more commonly seen in full keel yachts where the "deadwood" area of the keel prevents water from feeding into the prop properly. This causes huge troubles when the prop will not "bite" the water - usually when going astern. These yachts are notoriously poor at backing down. I would start by taking the prop off anf having a propeller shop check it for true. The answer may be as simple as that. After that it could be more trouble to sort out, perhaps even demanding the attention of an expert, but I suspect that you will answer you question at the prop shop. Good luck!
Dissimilar metals
Good point, Mark! A good example of dissimilar metals is your propeller shaft. It is usually made from type 316 stainless steel. This means that it has 3% chromium, 16% nickel with the balance iron. You would expect no troubles with this, right? WRONG! Stainless will attack itself when immersed in deoxygenated seawater. Where do you find that? Around the cutlass bearing on a yacht that spends too much time sitting at the dock! The water becomes stale, and there is no longer sufficient oxygen to maintain the chromium oxide layer on the surface of the shaft. The iron, nickel and chromium attack each other, resulting in extremely deep pits (more like canyons) on the shaft under the bearing and no where else. Solution - simply use the boat more!! If the yacht is seldom used and develops this type of corrosion under the bearing, a shaft can be made from Monel. Monel is much more expensive but is basically inert in seawater, so the problem will go away. Consider a stainless saft with a bronze prop: Iron, Chromium, Nickel, Copper and Tin all in the same electrolyte! Wow! A prescription for trouble second to none! Without the Zinc, the prop will give up first. How can you tell? It will change color and take on the appearance of copper, rather than the more golden appearance normally seen. If you tap a good prop with a small hammer, it will go "ting!" The rotten prop will go "thuk"!! Time to visit the prop shop with your Master Card!
Good point, Mark! A good example of dissimilar metals is your propeller shaft. It is usually made from type 316 stainless steel. This means that it has 3% chromium, 16% nickel with the balance iron. You would expect no troubles with this, right? WRONG! Stainless will attack itself when immersed in deoxygenated seawater. Where do you find that? Around the cutlass bearing on a yacht that spends too much time sitting at the dock! The water becomes stale, and there is no longer sufficient oxygen to maintain the chromium oxide layer on the surface of the shaft. The iron, nickel and chromium attack each other, resulting in extremely deep pits (more like canyons) on the shaft under the bearing and no where else. Solution - simply use the boat more!! If the yacht is seldom used and develops this type of corrosion under the bearing, a shaft can be made from Monel. Monel is much more expensive but is basically inert in seawater, so the problem will go away. Consider a stainless saft with a bronze prop: Iron, Chromium, Nickel, Copper and Tin all in the same electrolyte! Wow! A prescription for trouble second to none! Without the Zinc, the prop will give up first. How can you tell? It will change color and take on the appearance of copper, rather than the more golden appearance normally seen. If you tap a good prop with a small hammer, it will go "ting!" The rotten prop will go "thuk"!! Time to visit the prop shop with your Master Card!
Final results
Final results for thew Quick Quiz ending 5/8/2000: How frequently do you change zincs? 62% 1 year (175) 15% Don't use them (42) 12% 6 months (35) 6% 3 months (19)
Final results for thew Quick Quiz ending 5/8/2000: How frequently do you change zincs? 62% 1 year (175) 15% Don't use them (42) 12% 6 months (35) 6% 3 months (19)
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