Bronze shaft log corrosion

[Marrigoround]

Hi all,

I recently pulled off my dripless shaft seal to discover pieces of the bronze tube flake off with it. This was of course disheartening, but I decided to grind away to see if I could discover how far this rot went. After revealing the shiny insides, it looks like the cast material never mixed appropriately or something with various areas of bronze-ish gold-ish color with corresponding density and large pockets of copper color with corresponding softness.

1. Could this be a result of poor manufacturing or evidence of corrosion and some electrical or chemical imbalance? Note that I have not seen this on any other bronze components on the boat.

2. If I were to prep and coat this with resin, could it possibly isolate and stop further degradation? There is still plenty of surface area for the seal to mate with.

Important context:

• See attached photos.
• We are currently in San Carlos, Sonora, Mexico with unlikely access to a new shaft log and shipping parts often takes months if they ever arrive at all.
• The boat is a Hunter Legend 37 from 1987 and maintenance logs are spotty and rare. This may be the original shaft log. If this is the case, the corrosion is very slow.
• The shaft log does not contain a bearing and provides no structure, just a lip for the shaft seal to mate with. The prop shaft does not touch it at all under normal conditions.

[HeywoodJ]

Soft pinkish/copper-colored metal in an underwater casting is pretty good evidence of dezincification (the removal of zinc from the alloy, usually by electrical currents). Dezincification leaves behind a weak, porous, spongy pink shell that breaks easily. Think of it as the zinc in your metal being used as the anode for your boat instead of the “zinc” you usually find on your prop shaft.

FWIW dezincification is also evidence that your stern tube isn’t really a true bronze, as those don’t contain much (if any) zinc. Probably a “manganese bronze” or simply brass.

The risk level (how much further any rot extends) is something you’ll have to decide, but on one past boat we did as you describe. I built a fiberglass and epoxy tube that just fit inside the metal one, glued it in place with thickened epoxy, and then used thickened epoxy and glass to build out the area where the shaft seal attached. Lasted at least a decade.

From your pictures it looks like your shaft is not centered in the stern tube. Maybe that’s just because you have it moved around to remove the shaft seal? If not, it makes repairing by inserting something more difficult. More importantly, it might indicate your shaft is not aligned in your cutless/cutlass bearing, leading to extra wear on that.

[Tetepare]

Well said. I wish there were a reference area for stuff like this.

Originally Posted by HeywoodJ

Soft pinkish/copper-colored metal in an underwater casting is pretty good evidence of dezincification (the removal of zinc from the alloy, usually by electrical currents). Dezincification leaves behind a weak, porous, spongy pink shell that breaks easily. Think of it as the zinc in your metal being used as the anode for your boat instead of the “zinc” you usually find on your prop shaft.

FWIW dezincification is also evidence that your stern tube isn’t really a true bronze, as those don’t contain much (if any) zinc. Probably a “manganese bronze” or simply brass.

The risk level (how much further any rot extends) is something you’ll have to decide, but on one past boat we did as you describe. I built a fiberglass and epoxy tube that just fit inside the metal one, glued it in place with thickened epoxy, and then used thickened epoxy and glass to build out the area where the shaft seal attached. Lasted at least a decade.

From your pictures it looks like your shaft is not centered in the stern tube. Maybe that’s just because you have it moved around to remove the shaft seal? If not, it makes repairing by inserting something more difficult. More importantly, it might indicate your shaft is not aligned in your cutless/cutlass bearing, leading to extra wear on that.

[Orion Jim]

Your shaft log is shot and must be replaced for all the reasons stated previously.
https://catalog.hydrasearchrecreatio...t-shaft-logs-2

[Marrigoround]

Thanks for your reply.

I'd love to hear more about your fiberglass "shoe" that you built. Was this in response to a similar situation to mine? At the very least, I'm hoping that this could hold me over until the next haul out when I can do a proper repair with a new shaft log.

I also initially thought that this was an issue with dezincification, but am thinking of ruling it out considering:
1. the consistency of the copper-y areas. They are definitely soft, but they are consistently dense within themselves; there is no pitting, porosity, spong-iness, etc. It feels and looks like solid copper.
2. the surrounding bronze fittings. I have not seen any other issues like this and there is a bronze raw water intake within inches of this fitting. Of course this does not rule out that this might be the crappiest fitting around...
3. the presumed age. If this is indeed original (ie 34 years old), I would expect the dezincification to have taken more material.

All this to say that I'm thinking that this is a lack of quality control in the casting and it is indeed simply improperly mixed (not sure on the specifics of the manufacturing of these parts or if this is even plausible) and there are large chunks of copper within the bronze. The copper would presumably be slowly washed away by the turbulence and agitation within the unvented dripless shaft seal (the new one has a vent that I will connect to the raw water intake). I think that this would explain the appearance after grinding away the raw edges. If this is indeed the case, I would feel a little better about this short-cut on this crucial area for water ingress. I've got a 150nm passage I'd like to complete next month.

[stormalong]

Heywoodj is on the right path but I would replace the shaft log rather than line it. The builders of my boat, now defunct, recommended replacing the shaft log with fiberglass. They were repowering the 40 ft version of my boat when I visited them. They removed the shaft log entirely, used spacers around the shaft to align a pre-made fiberglass tube and glassed it into place. Seemed like a good idea to me. You can buy fiberglass tube made for exhaust hose connectors in various widths and up to ten feet long here: https://www.fisheriessupply.com/cent...exhaust-tubing or any Centek (Vernalift) dealer.

I have not done this but on the first indication of an issue I would.

[Orion Jim]

Exhaust tubing and shaft log tubing are not the same thing. Shaft log tubing is much stronger as the wall thickness is greater.
https://www.fisheriessupply.com/cent...r-post-housing

[HeywoodJ]

Originally Posted by Marrigoround

....I'd love to hear more about your fiberglass "shoe" that you built. Was this in response to a similar situation to mine? At the very least, I'm hoping that this could hold me over until the next haul out when I can do a proper repair with a new shaft log.....

Ours was in response to a not centered shaft that rubbed a groove in the shaft log. When that groove reached far enough aft there was no place for the shaft seal to seal and we started taking on water (learned a lot of lessons on that one).

Found a dowel/pipe about 1/8” less in diameter than the log. Waxed it and then wrapped it in glass/epoxy until that was just less than the diameter of the shaft log and about 6” long. Gooped it up with thickened epoxy and then slid it down the shaft log. Did another fillet at the aft end to get a good seal. Then used that as a foundation to fill in the groove area, sanded metal plus epoxy plus glass round and smooth and then wrapped that section in a piece of 2” glass tape to hold everything together. The log ended up about 1/16” larger in diameter than original, but was able to fit the shaft seal over it.

[Marrigoround]

Thank you guys, I appreciate your input.

Replacing the stern log / shaft tube is definitely Plan A and I will be implementing this option as soon as it becomes feasible. But as you probably already know that when on a boat, Plan A is the luxury option and we often have to settle somewhere later in the alphabet. It is for this reason that I am reaching out to fellow sailors to help me develop Plan B. And Plan C and D.

Does anybody have any experience in marine metallurgy that might stop or slow the degradation of this questionable bronze for another three years?

[stormalong]

Originally Posted by Orion Jim

Exhaust tubing and shaft log tubing are not the same thing. Shaft log tubing is much stronger as the wall thickness is greater.
https://www.fisheriessupply.com/cent...r-post-housing

Interesting point. The only offering is for 2-3/8 ID do you are talking about a two inch shaft. I doubt the OP's boat has a two inch shaft. I have used this material to replace the exhaust pipe on my boat - 2" OD and the wall thickness is pretty good - about twice as thick as the original builder used.

The only stress (torque) on the tube is from the packing friction. With a PSS or similar packless system the stress would be minimal and there is no need to worry. With packing I can see that over tightening the packing nut would be a cause for concern.

As someone who has tried to make his own fiberglass tube I think that this would be a better starting point than making a tube from scratch. The spiral wrapped fiberglass cord it pretty tough. Adding a few extra layers would be easy.

[misssherry]

I had the same problem when installing a new pss dripless bearing. You can see from the pics that the old log was gone. I was lucky that the cutlass bearing remained attached to the bronze log.

Replacing the old log with a Buck Algonquin new one was not that difficult. I did have to chip away some mastic that encased the two nuts on the inside of the boat.

To prevent dezincification from happening on the new shaft log, I installed a mushroom zinc on the bronze shaft strut. The strut is through bolted to the hull. On the inside of the hull, I connected an eight gauge copper wire from one of the nuts of the strut to one of the nuts of the new shaft log.

[j.g.evans]

Since you are in Mexico where parts are not available, make your own shaft log. I did this over 20 years ago and it is still holding. I assume you can buy epoxy resin and glass in Mexico: This is what I did.

I used a tube wrapped in wax paper for the log ID. Then I used epoxy and cloth to form the log. Make it long. Slide your fabricated tube over the shaft with paper or something around the shaft for centering alignment. Then fiberglass it into the boat using epoxy and glass. Cut it to the desired length inside and outside after it has hardened.

[stormalong]

It is interesting that both Marrigoround and j.g.evans have Hunters. Coincidence? Probably not. I am curious where Hunter got their castings.

[boatpoker]

Originally Posted by Marrigoround

Does anybody have any experience in marine metallurgy that might stop or slow the degradation of this questionable bronze for another three years?

No !. HeywoodJ pretty much nailed it.
You almost certainly have "manganese bronze", a misnomer since MB will have 35 - 40% zinc which places it squarely in the brass category.

Galvanic corrosion requires electrical contact between two metals in an electrolyte. MB comes pre-packaged this way, The copper is eating the zinc within the log. i.e. MB is cannibalistic, corrosion cannot be stopped til' all the zinc is gone. This log must be replaced or it will fall apart.
- Retired Certified Marine Corrosion tech.

PS. add a little stray current and this can happen in hours, not months.

[Searles]

Wot ever it is it’s not bronze ,here in oz we can still buy good G1 or G2 bronze tube,best bet is monel tube heavy wall ,or epoxy glass ,not a big job ,just remember to align the new tube true to shaft c/l ,a simple point but important.⛵️⚓️