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Hi, I have an Oday 30 that has lived all of it's life on the Great Lakes and has not needed any sacrificial metal protection to date. I would sure appreciate any help from you salt water sailors. I am pretty much at a loss on this issue.
Thanks
Phil
 

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Discussion Starter · #3 · (Edited)
I thought that a boat in salt water had to have a sacrifiial metal to protect things like engine, mast, prop and shaft. Am I just borrowing trouble or is this an issue

Hey, I see why you ask about an issue, part of my post didn't make it through. I am thinking of taking my boat to salt water this summer. I should have proof read.
 

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All boats with dissimilar metals in an electrolyte need protection, even in fresh water. I assume you haven't seen any external electrolysis, but it can occur where you can't see it as well. However, salt water is a much better conductor and more important to have a zinc attached to your shaft and/or prop, assuming you have an inboard.
 

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salt water - zinc
fresh water - magnesium
brackish water - aluminum indium

For an anode to work, both mating surfaces must be clean and there must be good contact, something less than 1ohm resistance. Two out of 40 boats I recently checked in a yard had good continuity between anode and the metal to which it was mounted. This can be checked with any cheap multimeter.
 

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I thought that a boat in salt water had to have a sacrifiial metal to protect things like engine, mast, prop and shaft. Am I just borrowing trouble or is this an issue

Hey, I see why you ask about an issue, part of my post didn't make it through. I am thinking of taking my boat to salt water this summer. I should have proof read.
All good.. :) To answer your question, yes, you'll need a zinc and it doesn't need to be an expensive one. There's usually a spot on the prop and/or shaft to fit it to. Talk to the people at your friendly boat-yard.

For an anode to work, both mating surfaces must be clean and there must be good contact, something less than 1ohm resistance. Two out of 40 boats I recently checked in a yard had good continuity between anode and the metal to which it was mounted. This can be checked with any cheap multimeter.
A zinc installed anywhere in contact with the electrolyte (salt water in this case) is capable of 'protecting' metals for a reasonable distance around it - not just the ones it's in electrical contact with - this is why it's possible to be 'overprotected' in certain marina situations. ..but being in electrical contact is certainly preferable.
 

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A zinc installed anywhere in contact with the electrolyte (salt water in this case) is capable of 'protecting' metals for a reasonable distance around it - not just the ones it's in electrical contact with - this is why it's possible to be 'overprotected' in certain marina situations. ..but being in electrical contact is certainly preferable.
That is incorrect. An anode without a connection is just a piece of metal in the water. You have to think back to high school chemistry where a galvanic cell is created by immersing two different metals in an electrolyte and connecting them with a conductive wire. One of the metals become a cathode (more noble) and the less noble metal becomes an anode..... you have just built a battery ! The anode gives up electrons to the cathode. That is why they are called "sacrificial anodes". Without good contact there can be no galvanic current flow and therefore no cathodic protection.

Do not buy off brand anodes. Anodes from China can do more harm than good. Take a look at the Martyr website for more info. They make high quality products
 

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That is incorrect. An anode without a connection is just a piece of metal in the water. You have to think back to high school chemistry where a galvanic cell is created by immersing two different metals in an electrolyte and connecting them with a conductive wire. One of the metals become a cathode (more noble) and the less noble metal becomes an anode..... you have just built a battery ! The anode gives up electrons to the cathode. That is why they are called "sacrificial anodes". Without good contact there can be no galvanic current flow and therefore no cathodic protection.
BoatP, you can build a perfectly good battery without connecting the anode and cathode together "with a conductive wire". An anode (zinc) and a cathode (most other metals) in an electrolyte (salt water) = a battery, and galvanic current will happily flow in the electrolyte. If this wasn't true, lead-acid batteries would never discharge on the shelf.

..at least, that's what we were taught in high-school chemistry. ;)
 

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Do you understand what continuity is or what bond integrity is ? I suggest you visit the previously linked Martyr site and refresh your memory. Glavanic current needs a path it does not just jump around in the water at the millivolt levels involved in galvanism.

Or this report can explain it for you

When reading the report or looking at the Martyr site look for the terms "bond integrity", "electrical contact" and "coupled". All of these terms refer to continuity.
 

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Thanks for the replies. So pretty much if I have a zinc on the prop shaft and insure that everything metal on the boat is connected to the prop shaft as in grounded, I will be good to go.
Zinc on the prop shaft, yes.. but the jury is out on whether or not everything metal should be grounded to the prop shaft also.
 

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The on-line jury made up of experienced sailors.

Fine.. go for your life. After all, it can't do too much harm, right? :)
You go with your "on-line" experts. I'll go with ABYC Standards and what I was taught in courses by Ed Sherman, Paul Fleury and Dave Rifkin

All three of these guys are internationally recognized marine corrosion analysis instructors and consultants and Rifkin is a retired nuclear sub commander and consultant to USCG
 

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I don't have a position to fight for on bonding. Mine are not bonded (nearly 30 of them!! holy moly)

The question I have is over the standard. Logically, it makes sense to bond a thru hull that may be composed of dissimilar metals. However, are there valves that are solely one metal? If so, what would you be protecting, rather aren't you introducing dissimilar metals by bonding? Finally, from a practical standpoint, I've seen electrically corroded thru hulls, but not very often. Is this standard a logical one, with less practical impact than most?
 

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I don't have a position to fight for on bonding. Mine are not bonded (nearly 30 of them!! holy moly)

The question I have is over the standard. Logically, it makes sense to bond a thru hull that may be composed of dissimilar metals. However, are there valves that are solely one metal? If so, what would you be protecting, rather aren't you introducing dissimilar metals by bonding? Finally, from a practical standpoint, I've seen electrically corroded thru hulls, but not very often. Is this standard a logical one, with less practical impact than most?
Since all metals have a natural voltage potential, bonding them equalizes those potentials. if all potentials are equal no current can flow therefore no galvanic reaction.

Bonding does not protect you from stray current corrosion which is another form of electrolytic corrosion that involves leaking current from bad connections usually in a wet bilge/bilge pump or even old style battery chargers or water heaters on vessels with AC/DC bonds but that is an entirely different topic.
 

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Doesn't that also apply, if the thru hull is only one metal? At least, only one metal exposed to the electrolyte (sea water)?
If all below the water line metals are the same, then no bonding would be required but I don't know of any throughull/seacock combinations that don't consist of only one metal.

If you did have one consisting of one metal and another of two metals they would likely form a galvanic couple through a high capacitance bottom paint.

Or you could start another war by proposing all Marelon throughulls :)
 

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No way. I hate marelon. :)

While I get the science on galvanic corrosion, I'm just trying to get my head around why I just don't see much of it on unbonded thru hulls. Some, for sure.

Ours are unbonded and have a stainless steel ball inside a bronze case. Add salt water and they should disappear. But they haven't. Maybe the potential between the two is small? That can't be it. There is some thing preventing disintegration. I'm curious.
 
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