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Old 02-24-2011, 02:26 PM   #21
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Help me out here....

The shaft is isolated from the strut by a bering of some sort. Right?

The shaft zinc is the anode for the prop. Right?

The stainless bolts holding the strut to the skeg are not isolated?

Two dissimilar metals in a salt water solution = a battery.

The stainless bolts become the cathode and the strut (Bronze Less noble) becomes the anode

The stainless bolts are not corroding because they are the cathode.

This would equate to the transfer of electrons from the strut to the sal****er.

The stainless bolts look to be to long as if they are recent additions i.e. not original.

This is how I am understanding the issue .

Tell me if I am wrong.

SD

PS. *Calder says that there are two ways to defeat this problem encapsulate the stainless bolts in bottom paint or some other way or replace them with bronze bolts.



-- Edited by skipperdude on Thursday 24th of February 2011 03:37:14 PM

-- Edited by skipperdude on Thursday 24th of February 2011 03:47:11 PM
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Old 02-24-2011, 03:04 PM   #22
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RE: Help me out here....

1)..Yes...the cutlass bearing.
2)I would assume....although the PO had a theory on why not to have a shaft zinc on this particular boat. I listened to him. That shaft zinc is brand new and has not been there during the past 3 years!!!
3)Not sure I understand. But, yes, they are not connected to the bonding system other than touching other metal that is.
4)Understand
5)I thought Bronze would be more noble...although a completely ignorant statement!
6)Maybe.....

So to summarize, you think the stainless bolts are the issue??? Now I know electrolysis causes the pink stuff....does galvanic corrosion cause the pink stuff also??? You are saying I have a galvanic problem when I am thinking it is more of an electrolysis issue??? I am not disagreeing with you. I am just discussing and thinking out loud.
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Old 02-24-2011, 03:32 PM   #23
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RE: Help me out here....

Galvanic corrosion, often erroneouslycalled electrolysis, is generated by galvanic
action, or the flow of electrical current between
dissimilar metals immersed in an

electrolyte such as seawater.

This is a quote from Nigel Calder the guru on marine corrosion.

As i* Understand it you have a problem with galvanic issues.
the pink is just the indication that the tin in the bronze is migrating thru galvanic action out of the alloy. It makes it week and almost like a sponge i.e .full of holes.

You just can't hav two dissimilar metals in a sal****er solution without forming a galvanic cell and transfering electrons out of the least noble metal to the seaGraphite
Monel
Stainless Steel
Bronze
Brass
Copper
Tin
Mild Steel
Aluminum
Zinc

Thes is the scale of metals most often found on boats.

a fitting made of metal at the bottom of the scale will waste away in seawater
if it is in contact with metal higher on the scale

So to answer to your question it is my belief that you have a galvanic corrosion issue due to dissimilar metals.

SD
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Old 02-24-2011, 03:44 PM   #24
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RE: Help me out here....

Hmmmmmmmmm
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Old 02-24-2011, 03:47 PM   #25
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RE: Help me out here....

Okay...let's assume it is galvanic corrosion. Why would the aft part of the shoe be unaffected??? And would not the standard bonding system cause the circuit to be connected to a zinc and thus be protected? Again, not disagreeing in anyway. Just thinking out loud. I think you might be on to something for sure.
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Old 02-24-2011, 03:57 PM   #26
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Help me out here....

You said the rudder zinc is new. It looks to be well pitted.

**Meaning it is doing it's job and sense it is connected to the shoe it is affording some protected by the rudder zinc. the furthe away from the anode (zinc )the less protection.

I am no expert at this but I have done a lot of reading on this stuff.

You said the bolts were loose. the corrosion could have caused the disassociation of the contact thereby negating the effectiveness of the bonding loop.

If you take out one of the stainless bolts is there any evidence of crevas corosion stainless will corrode in the absence of oxygen. white powder in the stainless bolt


SD




-- Edited by skipperdude on Thursday 24th of February 2011 05:13:49 PM
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Old 02-24-2011, 05:44 PM   #27
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Help me out here....

Quote:
skipperdude wrote:
So to answer to your question it is my belief that you have a galvanic corrosion issue due to dissimilar metals.
Yes there is, but that's why there's a bonding system connected to a sacrificial anode.* If the anode is at the bottom of the galvanic chart, or at least significantly lower on the chart than the other metals you have in the electrolyte (the salt water) or that are physically connected together and connected to the bonding circuit, the anode is what goes away first.* Once the anode is gone then the next lowest metal is "attacked."

So if the bonding system is intact, the connections are good, and the anode (zinc in this case) is good, that is what makes connecting dissimilar metals on*a boat possible without deterioration of the metal in the important components.* The "unimportant" zinc deteriorates instead.

Which is the long way of saying that having dissimilar metals in contact with each other either physically or via an electrolyte is fine as long as there is an anode protecting them.

-- Edited by Marin on Thursday 24th of February 2011 06:46:02 PM
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Old 02-24-2011, 08:59 PM   #28
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Help me out here....

Quote:
Mike wrote:

Is that a standard Mainship installation of the prop shaft strut, and rudder skeg?

I can't help you with your underwater metal problem, but I did notice that your propellor shaft nuts are incorrectly installed, which would cause me to question the competency of the yard that works on your boat.

If they didn't know how to install those 2 simple nuts, maybe they didn't know how to install a zinc, i.e clean surfaces, good contacts, and quality parts.

Not all zincs are the same.

Good luck with your problem. I hope it doesn't ruin the upcoming boating season.

Mike
Merritt Island, FL.
That is a stock installation. *It has nothing to do with the yard that works on my boat.....it is not their work. *The prop has never been *removed under my ownership. * *There is plenty of good metal left in the strut...again the reason for this haul. *I just don't want it to get worse. If you read the post, zincs were never an issue or in question here....nor was their installation. *It is the bonding system that is potentially in question.


I do appreciate your input(sincerely). *It just didn't help any other than to point out something MIGHT be wrong and then for whatever reason not share it with me(or the rest of us). *

Sorry, but your post came off as elitist and condescending. *You made some incorrect assumptions and then judgements based on those assumptions. *I probably took it wrong and the context was lost on this here internets.

*

ANd if you are interested in the design of the strut and rudder shoe, it appears that you could cut off the shoe and the lower part of the strut leaving a relatively conventional set up. *It appears to me that Mainship added the shoe for protection of the running gear. *THe strut and rudder are very properly mounted in their own rite regardless of the existence of the shoe...IOW, the shoe and the lower part of the strut are not structural.....again I think Mainship did it for protection.

Edit: *I did do some research reference the prop and I do see what you are talking about if you were referring to the sequence of the nuts. *I also understand why and it is also stated in the ABYC to have the smaller nut forward. *It was interesting that I did come across a few articles stating that it didn't matter. *I guess my question would be...how much does it matter? *In theory it matters I guess. *But it reality, I don't think it would cause any sort of failure in the prop or any sort of damge to it or any surrounding equipment. *But thanks for pointing that out... IOW, I won't lose sleep over it and correct on the next haul out.





-- Edited by Baker on Friday 25th of February 2011 12:19:29 AM
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Old 02-24-2011, 09:14 PM   #29
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RE: Help me out here....

Quote:
Marin wrote:

*
skipperdude wrote:
So to answer to your question it is my belief that you have a galvanic corrosion issue due to dissimilar metals.
Yes there is, but that's why there's a bonding system connected to a sacrificial anode.* If the anode is at the bottom of the galvanic chart, or at least significantly lower on the chart than the other metals you have in the electrolyte (the salt water) or that are physically connected together and connected to the bonding circuit, the anode is what goes away first.* Once the anode is gone then the next lowest metal is "attacked."

So if the bonding system is intact, the connections are good, and the anode (zinc in this case) is good, that is what makes connecting dissimilar metals on*a boat possible without deterioration of the metal in the important components.* The "unimportant" zinc deteriorates instead.

Which is the long way of saying that having dissimilar metals in contact with each other either physically or via an electrolyte is fine as long as there is an anode protecting them.

-- Edited by Marin on Thursday 24th of February 2011 06:46:02 PM
*



Yep....that is kinda what I was thinking. *Will put a wire in the water tomorrow.
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Old 02-24-2011, 09:23 PM   #30
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RE: Help me out here....

Baker

Not intending to come across as elitist, but these are simple things to do right with your yard guys on the ground and nearly impossible to do from long distance via the internet. Your yard guys should have the answer, if they don't, oh-oh.*

Yes use several additional zincs, but please ask your yard guys to get a VOM and check the entire bonding wire integrity and also the conductivity for each and every bonding system to metal to hull connection. I believe this has been*touched on*earlier in the thread.
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Old 02-24-2011, 09:24 PM   #31
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RE: Help me out here....

Quote:
Baker wrote:


There is plenty of good metal left in the strut...again the reason for this haul.
I certainly don't understand all the potential issues at work here, but I believe the reason that bronze turns pink in these situations is because the copper is being leached out of the bronze.* This weakens the bronze.* What I don't know is how deep the leaching goes or how fast.

So unless you know or have been assured of this already, it might be worth trying to find out not how much metal is left but how strong*that metal really is.*Is there a risk that the strut has been weakened--- even though it looks as substantial as it ever has--- to the point where the stress of the loads and vibration it's subjected to could cause it to fail?

I have no idea if there is any sort of test for this, like magnaflux (which I assume doesn't work on bronze), acoustic frequency*testing or whatever.
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Old 02-24-2011, 09:45 PM   #32
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RE: Help me out here....

Marin, we installed a zinc on the strut just for added protection and "in the mean time". In order to do this, we had to drill through the strut. My fear obviously is that the drill would go through the strut like butter. That wasn't the case. It took us 20 plus minutes while taking turns on the drill with a good cobalt bit. Not only that, the 5/16s hole in the strut allowed a good sample view of the condition of the interior metal....it was shiny pretty bronze all the way thru except on the surface.

Sunchaser, will certainly do although the people that run the yard generally don't do the work on my boat....just the haul and paint if needed. My reasons for posting this was the inconsistencies of what could be going on...that is all. I was confused as to what could be going on because it didn't add up. I was just throwing it out to y'all to see if I was missing something....that is all. I am well aware that it can't be solved over the internet. I just had an issue and figured I would put it out there....instead of talking about anchors and dinghies I figured I would get some help with some problem solving....and obviously learn about problems I didn't even know I had....
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Old 02-24-2011, 09:59 PM   #33
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RE: Help me out here....

Quote:
Baker wrote:

Not only that, the 5/16s hole in the strut allowed a good sample view of the condition of the interior metal....it was shiny pretty bronze all the way thru except on the surface.
Well, you can't get better evidence of the metal's condition than that.* So you can cross that problem you didn't know you had off the list

*
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Old 02-24-2011, 10:56 PM   #34
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Help me out here....

The zinc is leached out of the copper. It is the remaining copper that gives the pink colour, not the other way around. I'm also going to guess that your bronze is less bronze than brass like. Good bronzes have a very small % of zinc, maybe 5%, to none. Good bronzes replace the zinc with tin and other metal which are much closer on the galvanic scale to copper.
Manganese 'bronze' is about 20-40% zinc, is really a brass, and it is likely what you have for the strut. Some brasses can be good if used properly and protected. Witness many manganese bronze propellors which last for years IF they are protected.

Nonetheless the metals should be zinced. There is lots of metal in the strut so if it looks like mostly surface then you have years to go, zinced.
I see you have installed a prop shaft zinc - good. You might even go fo a second one. Just keep it at least two to three shaft diameters from the cutlass bearing.

I'll wager that you lost electrical continuity between the various parts. Even if the bolts were tight eventually seawater can creep into the joints and create enough oxidation to cut the continuity through the joint. The joints should be disassembled, cleaned and redone. I may be off base here but I think you could use a sealer or caulk to keep out the water for a long time. The bolting pressure will squeeze the bulk out leaving a metal to metal contact but close off the remaining gaps, and they willl be there, to water entry. Confirm it with an ohmmeter.
All the connections internal should be cleaned and checked. If any are the least bit dirty or oxidized they may lose continuity and thus protection

The voltages here are quite low so it doesn't take much resistance to upset things. An ohmmeter, at any connection, should show less than 1 ohm.

You might also do some quick checks to ensure you don't have some DC electrical leakage from something like a faulty bilge pump or switch. They can leak into standing bilge water if the insulation or seals are compromised in any way. From your description this may not be the case but be sure.

If any bolts from the forward parts of skeg and strut come through the hull and you have access to them you may be able to check the zinc protection levels over the year. Silver-silver chloride half cells are available over the net and with a dmm can be used to monitor. I think I posted a link which included how to make one by going to a jewellers store for a few bucks.

This subject can be huge and the variables widely variable so if you have any further questions and it appears that you still have a problem get someone to give you a hand and repost.

Your transom zinc should be checked also for CLEAN mounting and continuity to the internal bonding. I had a fellow do mine for a couple haulouts and one of the zincs didn't waste away. Next H.O. ohmed it - isolated - so I pulled it and cleaned the mount with the grinder and a disc. I've also found that sometime I have to grind the zincs, the contact area, as any zinc oxide will interfere with a good connection.

Hopefully a full cleanup of all bolted connections and wire connections will take care of it. But you can do a lot to check in the meantime, before next H.O., to see if you have the problem under control.

Good luck any good boating.
Clark


EDIT:

*** There are microohmeters but the last time I checked they were pricey enough that even though I drooled , I decided I would have to do without.* Too close to retirement even though I could have used it a bit at work, I couldn't justify it.** Take a look if any are interested as a few years have now passed and some amazing tools and meters are 'affordable' compared to just a few years ago.

-- Edited by C lectric on Friday 25th of February 2011 11:30:16 PM
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Old 02-25-2011, 10:25 AM   #35
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RE: Help me out here....

John,
The reason I asked the question earlier about the prop shaft being grounded is it clearly has a zinc in the photos. If this is a new zinc and the shaft was not zinced prior, I would say your problem is solved. I would also check the prop shaft to make sure the ss shaft is connected to the grounding system and a bonding wire has not corroded off on the tranny for example, as is pretty common. The electric current will take the shortest path to ground and you want to eliminate the 'battery effect' of the SS reacting with the bronze as there is no question that these are dissimilar metals with a known propensity to interact with each other. I believe you want the shaft tied into the bonding system as close to the coupling as practical to provide the shortest ground path; on my boat for example, there is 10ga bonding wire coming off the back of the transmission close to the shaft coupling even though the engine is of course bonded directly to the earth side of the 12v system as well as the bonding straps.
Marc
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Old 02-25-2011, 11:09 AM   #36
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RE: Help me out here....

Marc

Your*response raises the eternal debate, does the shaft need a grounding brush or contact strap following the coupling? This strap or brush*would be in addition to the transmission and engine grounding wires and shaft zincs you mentioned.
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Old 02-25-2011, 01:02 PM   #37
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Help me out here....

Quote:
sunchaser wrote:

...does the shaft need a grounding brush or contact strap following the coupling?
On the advice of our electrical shop we added shaft wipers to our boat.** They did not say they were essential, only that they could provide another layer of protection*if we wanted it.* We also run a pair of shaft zincs on each shaft.* I don't know if the wipers contribute anything significant to the boat's protection but they can't hurt and they were very inexpensive to install.

-- Edited by Marin on Friday 25th of February 2011 02:03:00 PM
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Old 02-25-2011, 06:53 PM   #38
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RE: Help me out here....

John,* Donald Cave in Bermuda has your same boat.* SOMERS is his member name.

Maybe you can PM him and compare notes on your boats.

Just a thought.


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Old 02-25-2011, 10:24 PM   #39
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RE: Help me out here....

I use a shaft brush that rides the shaft.

I keep three 2" zincs on the shaft (haul every 2 years) but since these sometime can be thrown or loosen, I finally added the brush. My understanding is to NOT depend on any connection through the coupling which depends on the gear. You do not want any electrical current, though small, running through the bearings to the gear case. Even if you don't damage the bearing there is always a chance that when not running, the gears and bearings could float in a bath of oil effectively isolating the shaft and prop. from the zincs.

Just my understanding.
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