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Thread: Delignification around propeller stern tube in shaft log

  1. #1
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    Default Delignification around propeller stern tube in shaft log

    A year ago during a survey and bottom cleaning an area of soft wood was found in the wood surrounding the propeller stern tube of my 1962 Cheoy Lee Lion. The surveyor (and others) suspected that the boat might have too much zinc, in that the bronze is protected but electrolysis is setup in the wood surrounding the protected metal. I am in the process of confirming this diagnosis with measurements, so that when the boat is hauled this Spring corrections can be made.

    But I need help with how to repair the damage this process has caused so far. I've reviewed many similar threads on this forum, some of which resulted in significant professional repairs, up to and including replacement of the stern post. I'd like to get your collective opinion on the best path forward. For the purpose of this discussion, I will use the term shaft log to describe the wood surrounding the stern tube, since "log" seems to represent wood, not metal. Some threads seem to refer to the shaft log as the tube itself.

    Here is the boat at the time of the survey last Spring:

    2~2.jpg

    A small soft area with flaking paint was found above the stern tube:
    3.jpg
    4.jpg
    5.jpg

    The deadwood inside the boat and surrounding the sterntube is not soft at all, but seems to show evidence of a previous repair. If you look carefully, there is an annular ring around the stern tube, almost as if a large 4" wood dowel had been inserted before the prop shaft was bored. Both the fact that the paint is gone from that area and there is white corrorsion surrounding the bronze, lend credence to the electrolysis theory.
    6.jpg

    So, what are your ideas on how to proceed with a repair? In a month or so, she'll be out for cleaning, painting, and topsides work. We have a more significant repowering project planned for next year, when the boat will be out of water again, mainly because the current propeller is left-handed and the engines/gearboxes we're considering are all right-handed. So, it's possible to do something less complex this year, and leave the major work for next.

    Jim

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    Default Re: Delignification around propeller stern tube in shaft log

    For the sake of completeness, here is a snapshot of Arthur Robb's drawing of how the stern post and deadwood are supposed to have been built. I thing the drawing is fairly accurate in this area.
    7.jpg

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    Default Re: Delignification around propeller stern tube in shaft log

    Well , Right off the bat , if I was walking by your boat, Id speak up and suggest there is too much zinc for a wood boat. We cannot bond a rudder fitting yet leave 3000 screws and bolts un protected. One zinc on the shaft to protect the wheel. One.

    Onward... you wanna do a temp repair now and something else later? I think Id wanna grind the daylights out of the area, pull the cutlass bearing housing and whatever is on the inside for the stuffin box and get a better look and feel for whats doin.
    By n by , I could envision boring the whole lot out , including any previous repair and installing a modern fg/ epoxy tube, one which uses far less metal and does not allow water to reach the end grain of the stern post log.

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    Default Re: Delignification around propeller stern tube in shaft log

    Thankfully, the distance of the log, or bore, or tube, is short . It is not buried in a massive stern post knee that has a bunch of screwy grain.

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    Default Re: Delignification around propeller stern tube in shaft log

    The short fix I recon is to get most of the zinc off , scrape/grind the area, and zap it with CPES .

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    Default Re: Delignification around propeller stern tube in shaft log

    +1 on above

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    Default Re: Delignification around propeller stern tube in shaft log

    I agree with Wizbang. Do it right the first time, and his upgrade recommendations are spot on...

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    I've nothing to offer except to say she looks lovely, and such a clean bilge!

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    Default Re: Delignification around propeller stern tube in shaft log

    Wiz is right. Been there, done that, got the tee shirt on Cheoy Lees of that vintage. Clean up the decayed wood. Dry out well. CPES the heck out of it, tube and all. (Use one of those cotton balls on a wire thingies that come with shoe polish to apply it.) Epoxy in a plastic (whatever flavor - but good stuff) sleeve in the stern tube. Replace cutless bearing hardware. Lose the fooking zincs, except for one on the prop shaft.

    Love those Lions. Beautiful boats and built like a tank.

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    Default Re: Delignification around propeller stern tube in shaft log

    Everyone seems to agree with Wizbang's wize suggestions. Thanks to all for your comments. Removing bad wood, drying the area, treating with CPES, and getting rid of most of the zincs is only a short term mitigation of the problem, but one that should get us through this sailing season. As I mentioned, we're already planning to repower the boat next year which will open that area up (no engine, gearbox, or shaft in the way) and make it possible to bore out an oversize plug taking with it all the previous repair as well as the CPES treated wood. That's the plan, at least until the haulout next month. We'll see how much things have changed since the exterior photos were taken a year ago.

    Jim

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    Default Re: Delignification around propeller stern tube in shaft log

    I've been considering this problem on an iron-fastened boat, and thinking about using cast iron instead of a zinc. It's still slightly more attractive to stray ions than steel fasteners, but a lot less so than zinc. The reaction byproducts should be less destructive to wood ?

    Bad idea ?

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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by Favorite View Post
    I've been considering this problem on an iron-fastened boat, and thinking about using cast iron instead of a zinc. It's still slightly more attractive to stray ions than steel fasteners, but a lot less so than zinc. The reaction byproducts should be less destructive to wood ?

    Bad idea ?
    Pointless I think. You want to protect rusting iron with a piece of iron ?
    First off, is the "iron fastened" iron or steel? We tend to call rusted steel "iron fastening sickness" or whatever.
    Is your boat being damaged by rust or are you trying to head off damage from rust?

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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by Favorite View Post
    I've been considering this problem on an iron-fastened boat, and thinking about using cast iron instead of a zinc. It's still slightly more attractive to stray ions than steel fasteners, but a lot less so than zinc. The reaction byproducts should be less destructive to wood ?

    Bad idea ?
    I have some old INCO literature on stainless steels in seawater that says a mild steel anode will protect stainless steel in seawater. Plain mild steel, not alloy steel. If anything, the lower voltage difference should reduce the alkali formation that is destroying the wood. See the table at the bottom of page 15 of this link Metal Corrosion in Boats: Nigel Warren

    Cast iron has too much carbon, etc. and in a nutshell, gets crusty. Long story short you want an anode that will corrode away without forming any kind of coating that will change its behavior.

    Chris mcm has the best information on galvanic corrosion as it affects wood. Re: Galvanic corrosion Re: Electrolysis In Wooden Boats

    You can't protect the fasteners in a wooden hull with anodes. You can cause problems. The fasteners are buried in wood, which is not conductive enough until it is completely saturated and even then the electrical circuit that is the essential element of galvanic corrosion is just not there. Distance, resistance, very low current flow, low voltage, differential oxidation cells, all add up to a complex situation that occurs over limited distances.

    Metal Corrosion in Boats: Nigel Warren This is a good book that is written for boat owners, not engineers.
    http://www.worldstainless.org/Files/...h_Other_EN.pdf
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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by MN Dave View Post
    If anything, the lower voltage difference should reduce the alkali formation that is destroying the wood ...
    This is what I was thinking of. "Too many zincs" is now understood as being bad; well .... why use zinc at all ? Why not something which is a little bit attractive to electron migration but not nearly as much as zinc ? Same problem eventually but maybe twenty years instead of ten ? And still give some protection to the fasteners ?

    If mine were bronze I'd just drop the zincs altogether but with iron/steel fasteners ...

    edit: looking at a different chart now (the one from Chris McMullen) perhaps aluminum would be more suitable ... and pretty easy to come by.

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    Default Re: Delignification around propeller stern tube in shaft log

    Aluminum anodes are made of an alloy that has the same galvanic voltage as zinc and doesn't passivate or form a protective crust that stops it from working. If you have 2 anodes the same size the Zn will weigh 2.6 times as much and will produce 2/3 as much current in its lifetime. Al is also better in fresh water. Zn can go crusty in fresh water an stop working.

    Most aluminum alloys aren't good for anodes because they will form an oxide film and the galvanic potentials vary. Aluminum is easy to get, but most alloys will frm an oxide film and stop working.

    https://www.boatingmag.com/how-to/ch...l-anode#page-1
    Last edited by MN Dave; 03-09-2019 at 12:56 PM.
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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by MN Dave View Post
    Aluminum anodes are made of an alloy that has the same galvanic voltage as zinc ...
    There's a LOT of aluminum alloys and you don't have to buy from Waste Marine ... look at your chart, some of them are much farther to the left than zinc .... in fact, one nice thing about aluminum is it's so readily available with a chemical composition that you can be certain of.

    All we have to do is find out the electrochemical potential of all the various alloys .... shouldn't be a big problem, right ?

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    Default Re: Delignification around propeller stern tube in shaft log

    The alloying elements added to structural alloys are added for strength and good corrosion resistance. The elements added to anode alloys have been chosen with great care to ensure an even corrosion pattern, reliable electrical capacity and a long working life.

    Aluminum anode alloys used to contain mercury, the absolute worst for corrosion resistance and the environment. Mercury has been replaced with In and Sn. Al-Zn-In, Al-Zn-Mg, Al-Zn-Bi-In and Al-Zn-Sn alloys are commonly used as sacrificial anodes in marine environments. Zn anodes usually contain small amounts of Cd.


    https://www.totalmateria.com/page.as...ite=ktn&NM=375 part 1
    https://www.totalmateria.com/page.as...ite=kts&NM=451 Part 2
    http://forum.woodenboat.com/showthre...ificial-Anodes
    http://forum.woodenboat.com/showthre...n-Wooden-Boats
    http://martyranodes.com/sites/defaul...num_anodes.pdf
    Aluminum MIL-A-24779
    Zinc
    MIL-A-18001
    Magnesium
    MIL-A-21412

    Best anode thread: http://forum.woodenboat.com/showthre...from-corroding
    Last edited by MN Dave; 03-09-2019 at 12:09 AM.
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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by MN Dave View Post
    The elements added to anode alloys have been chosen with great care
    Surely. For fiberglass boats, however. Our problem is different

  19. #19

    Default Re: Delignification around propeller stern tube in shaft log

    Yes, It is the sodium hydroxide by-product of cathodic protection that damages a wood boat. On a fiberglass boat it is not an issue. I am saddened to see the result of bonded anodes on JW Waites beautiful yacht. I have seen it all before. The reason I bother writing is to try and stop people loving their precious wooden boats to death. MN Dave kindly referred to my views on this subject. Re: Galvanic corrosion Re: Electrolysis In Wooden Boats
    Actually, I am only the messenger. You can find the same information in back copies of Wooden Boat , Professional Boat Builder and Classic Yacht Magazines. It is all there, exactly parallel to my 60 years experience as a wooden boat builder.

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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by Favorite View Post
    Surely. For fiberglass boats, however. Our problem is different
    Yes, I have fond memories of my 3 year old daughter asking why. She didn't follow the usual rules. Even at that age she asked for more detail. As an adult, it isn't as cute.
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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by MN Dave View Post
    Yes, I have fond memories of my 3 year old daughter asking why. She didn't follow the usual rules. Even at that age she asked for more detail. As an adult, it isn't as cute.
    Since we're going to get personal, I'll have to say I think you live in a dream world. West Marine "chooses the alloy with great care" ? Yes. Great care for their markup. They could care less what's in the "zinc" they sell as long as they can get it as cheap as possible and sell it for as much as possible. If they still made cookware out of pot metal, they'd be melting it down in back to resell as zincs.

    And the problem truly is, cathodic "protection" of wooden boats screws them up. I am attempting to look for a slightly better answer. Having the fasteners disintegrate is not nice, but having the wood around them disintegrate (as evidenced by the boat which started this thread, apparently twice now) is equally bad if not worse.

    Maybe you should put away your superiority and listen to your daughter more.

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    Default Re: Delignification around propeller stern tube in shaft log

    I made a DC voltage measurement between a silver chloride reference electrode and the prop shaft shown above in post #1. The multimeter read 1.036 Volts, which is significantly overprotected for any boat (see https://www.reliabilitydirectstore.c...ults.asp?ID=71), but especially bad for wood boats: https://waitematawoodys.com/2015/05/...f-leaky-homes/.

    According to the some of the comments above, the zincs aren't doing much for the thousands of copper rivets in the hull. Since we'll be replacing the powertrain (including the propeller) in a year, I'm inclined to remove *all* the zincs for the upcoming year as an experiment. The voltage difference should drop to 0.3 to 0.5 VDC, and we'll see in a year if there is much loss in mass of the bronze propeller.

    Jim

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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by jwwaite View Post
    I'm inclined to remove *all* the zincs for the upcoming year as an experiment.
    I'm trying to get up the nerve to do that also This deligniification thing is even scarier than refastening ...

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    Default Re: Delignification around propeller stern tube in shaft log

    Do you keep the boat in a marina?
    Do you change slips often?
    Do the neighboring boats change often?
    Other boats,stray marina current.
    Consider the copper IN the antifouling paint in the mix! (one of several reasons I put epoxy on my boat bottoms)
    It's ALL scary.

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    Default Re: Delignification around propeller stern tube in shaft log

    wizbang 13 wrote:

    >> Other boats,stray marina current.

    >> Consider the copper IN the antifouling paint in the mix!

    Just to make sure we're not reading between the lines here... Are you saying that some zinc (say, a small one on the prop shaft) can protect bronze or copper fastenings, when there is copper-based bottom paint on a wood boat? I.e., the bottom paint can act as a bond to the fasteners, and the zinc will correctly serve as a sacraficial anode? To avoid having that issue, what are more preferred bottom paint solutions?

    And, yes, my boat is in a marina. Other boats have recorded similar voltage readings using the reference electrode.

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    Default Re: Delignification around propeller stern tube in shaft log

    Oh no. I’m saying the metal in the paint may contribute to the ever changing mess that is corrosion/electrolysis.For better or for worse .

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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by jwwaite View Post
    wizbang 13 wrote:

    >> Other boats,stray marina current.

    >> Consider the copper IN the antifouling paint in the mix!

    Just to make sure we're not reading between the lines here... Are you saying that some zinc (say, a small one on the prop shaft) can protect bronze or copper fastenings, when there is copper-based bottom paint on a wood boat? I.e., the bottom paint can act as a bond to the fasteners, and the zinc will correctly serve as a sacraficial anode? To avoid having that issue, what are more preferred bottom paint solutions?

    And, yes, my boat is in a marina. Other boats have recorded similar voltage readings using the reference electrode.
    Sounds as though either the marina needs to do some electrical work, or else do some checking on what boats there are doing. While wood boats get damage like this, figerglass ones can also be damaged: props, etc. Has anyone done any amperage tests?

    Then there's the whole "what if someone swimming gets electrocuted?" issue. Good piece on it here: http://www.boatus.com/seaworthy/asse...ock-safety.pdf

    BTW - beautiful boat!
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  28. #28

    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by jwwaite View Post
    I made a DC voltage measurement between a silver chloride reference electrode and the prop shaft shown above in post #1. The multimeter read 1.036 Volts, which is significantly overprotected for any boat (see https://www.reliabilitydirectstore.c...ults.asp?ID=71), but especially bad for wood boats: https://waitematawoodys.com/2015/05/...f-leaky-homes/.

    According to the some of the comments above, the zincs aren't doing much for the thousands of copper rivets in the hull. Since we'll be replacing the powertrain (including the propeller) in a year, I'm inclined to remove *all* the zincs for the upcoming year as an experiment. The voltage difference should drop to 0.3 to 0.5 VDC, and we'll see in a year if there is much loss in mass of the bronze propeller.

    Jim
    And you are absolutely right about the fastenings not being cathodically protected. The existing deterioration will not improve until you neutralise the existing caustic soda by-product caused by cathodic protection. It is the caustic Soda that damages the wood. Less zinc just a slower death but death all the same. The message is NO ANODES ON A WOOD BOAT. No brass or Manganese bronze or copper zinc alloy should be used on a wood boat. If it is, just accept it will corrode and in time replace the same with marine bronze. Anodes are a solution looking for a problem. Please believe me.
    Further, and that includes the propeller.
    Last edited by Chris mcm; 03-11-2019 at 03:06 AM. Reason: slight change

  29. #29
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    My boat has mild steel skegs with stainless rudders hanging off the back of them. Bronze prop. Whatever shaft. Copper fastenings. I think the keels might be steel frames encased with glass and filled with lead at the bottom and foam higher up. I have zincs on the mild steel skegs. I can't replace the keel frames, skegs and rudders with bronze. Life's a bitch.

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    Re the OP, whilst not answering the repair question, I wonder why this delignification has apparently only recently reared its ugly head? This is a 57 year old boat after all. I'd be looking more toward shore power and compromised marina electrics before I dismissed the idea of having a couple of zincs on any wooden boat.

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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by Phil Y View Post
    I wonder why this delignification has apparently only recently reared its ugly head?
    Read the post where he describes what appears to be an earlier repair.

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    Quote Originally Posted by Favorite View Post
    Read the post where he describes what appears to be an earlier repair.
    I did, and wondered if indeed it was a similar earlier repair. It's not really clear. Maybe the auger wandered when the stern tube was originally drilled. Who knows.

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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by wizbang 13 View Post
    Oh no. Im saying the metal in the paint may contribute to the ever changing mess that is corrosion/electrolysis.For better or for worse .
    When copper leaches out of the paint it can plate out on a more active metal, especially aluminum, and the microscopic bits of plated out copper cause galvanic corrosion. So yes, the metal in the paint can contribute, but not directly. The paint isn't conductive, probably not even static dissipating, so the paint as a whole doesn't act like an electrode. It is very difficult to make conductive paint, and even more difficult to make conductive paint stay conductive in salt water.
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    Default Re: Delignification around propeller stern tube in shaft log

    Quote Originally Posted by Phil Y View Post
    I did, and wondered if indeed it was a similar earlier repair. It's not really clear. Maybe the auger wandered when the stern tube was originally drilled. Who knows.
    Fifth photo down. "The augur wandered" ... 8" ? Jose he no theenk so. 'specially since I independently came up with that idea for a repair ...

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    Default Re: Delignification around propeller stern tube in shaft log

    To fill in the blanks in some history, I was alerted to the issue just a year ago during a prepurchase survey. The boat has been in the same slip (and remains so) for 16 years constantly “protected” by the zincs shown in the photos, which were periodically replaced by the previous owner. To be honest there were so many other positive aspects of the boat that I decided to accept the position as her new caretaker (for a fee), and deal with grapefruit sized softness in her shaft log. Because of plenty of other items needed attending to, a year has passed. But I wasn’t too concerned since the situation seemed to be fairly static given her longevity on the same dock. There was no mention of a previous repair in that area, and so I assume that may go back before the late 90’s, when the last owner acquired her (and did plenty of other restoration work). I have been enlightened by all your observations and suggestions and at this point, am anxious to see her hauled out next month.

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