I keep my boat at a mooring. I have bronze thru hulls. I have a zinc on my prop shaft. This fall, on hauling the boat, the zinc was reduced in size substantially. This season I was never at a dock. Why the electrolysis?
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There is....
Always electric current in the water. Its just worse at docks usually due to all the metal. There is some metal around the moring I am sure from other boats and whatever it is attached to! So good thing you put a zinc in!
Always electric current in the water. Its just worse at docks usually due to all the metal. There is some metal around the moring I am sure from other boats and whatever it is attached to! So good thing you put a zinc in!
Are your thru-hulls bonded together?
You do not have to be at a dock to get electrolysis. Having a bad ground on the boat next to you and having that pump current into the water makes it more noticeable but all you really need is two different metals connected by a wire and in the water in your boat. Are your thru-hulls bonded to the common ground? It is generally considered to be a bad idea to bond your thru-hulls together unless ALL your thru-hulls and prop, propshaft, keel and rudder stock are the same metal.
You do not have to be at a dock to get electrolysis. Having a bad ground on the boat next to you and having that pump current into the water makes it more noticeable but all you really need is two different metals connected by a wire and in the water in your boat. Are your thru-hulls bonded to the common ground? It is generally considered to be a bad idea to bond your thru-hulls together unless ALL your thru-hulls and prop, propshaft, keel and rudder stock are the same metal.
and of course
Your engine is grounded to the 12v ground via the starter. If you don't have a plastic coupling between your trany and your prop shaft and you also have a short, you will have an electrolysis problem. Your zinc will loose it's mass as electricity leaves it. It is there so that the electricity generated by the different metals (prop bronze and shaft stainless steel) will leave via the zinc instead of the prop. Zincs are a lot cheaper then a prop. It's a good idea to have at least two zincs on the shaft. One will fall off long before the other one, giving you a clear warning sign that something is wrong. Now...when it a good lifespan of a zinc? That is partly determined by the area but in most cases, one zinc should last a year in salt water. Ask other boat owners in your field what the lifespan of their zincs are. Like Trevor said, they are supposed to loose mass.
Your engine is grounded to the 12v ground via the starter. If you don't have a plastic coupling between your trany and your prop shaft and you also have a short, you will have an electrolysis problem. Your zinc will loose it's mass as electricity leaves it. It is there so that the electricity generated by the different metals (prop bronze and shaft stainless steel) will leave via the zinc instead of the prop. Zincs are a lot cheaper then a prop. It's a good idea to have at least two zincs on the shaft. One will fall off long before the other one, giving you a clear warning sign that something is wrong. Now...when it a good lifespan of a zinc? That is partly determined by the area but in most cases, one zinc should last a year in salt water. Ask other boat owners in your field what the lifespan of their zincs are. Like Trevor said, they are supposed to loose mass.
Saltwater
The fact that you are in saltwater and the boat has metal parts (prop, shaft, motor)is what causes the electrolysis. The zinc is sacrafical and is attacked first by the reaction. If you were in fresh water your zinc would last an extremely long time. While it could be stray electric, odds are it is just the salt water reaction. Replace the zinc each year and mabey install larger or more zinc.
The fact that you are in saltwater and the boat has metal parts (prop, shaft, motor)is what causes the electrolysis. The zinc is sacrafical and is attacked first by the reaction. If you were in fresh water your zinc would last an extremely long time. While it could be stray electric, odds are it is just the salt water reaction. Replace the zinc each year and mabey install larger or more zinc.
bonding
Thank you for the responses. My thru hulls are not bonded. The mast is grounded to a keel bolt but no bonding system is present.
Thank you for the responses. My thru hulls are not bonded. The mast is grounded to a keel bolt but no bonding system is present.
These answers are on the right track, but not exactly correct. Anytime you have two different metals immersed in water they create a battery (bronze, through hulls, prop, stainless shaft). Not all bronze components are the same alloy so they will react as well. Salt is a better solution for current than fresh, but the result is the same. If you don't have a zinc in the equation then one of your components will give electrons to the other (the prop will loose electrons to the stainless).
Galvanic versus electrolytic
At the risk of being picky, there are two types of underwater corrosion: galvanic - the interaction between dissimilar metals and electrolytic - the introduction of stray electrical currents. Either one is partly resolved with sacrificial zincs. You don't have to be in a marina to get either, but electrolytic corrosion is more likely there from shore power and/or other boats.
At the risk of being picky, there are two types of underwater corrosion: galvanic - the interaction between dissimilar metals and electrolytic - the introduction of stray electrical currents. Either one is partly resolved with sacrificial zincs. You don't have to be in a marina to get either, but electrolytic corrosion is more likely there from shore power and/or other boats.
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