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Old 05-01-2017, 05:06 PM   #1
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Ruined prop shaft and corrosion questions?

I already asked this on the BOC and figured I could get some other opinions or experience here as well

Woe to thee whom buys a used boat and starts looking a little to hard :silly:

While working on my list of things I found wrong during my inspections. . .and comparing and checking it against my surveyors list of deficiencies. . .

. . .I'm coming up with a third list of things we both missed inch:

And this first one is a bit of a stomach turner :sick:

As the picture shows, there is a chunk of my stbd. prop shaft missing! It looks like it was broken out, which kind of doesn't make any sense. In all the time I've crawled in and around the bilge, I'd swear I would have noticed something like that. And the reflective tape is right there where surveyor put it on to check the rpms, I assume it would have been in sight. The shiny part of the shaft made me think the engine shifted forward and revealed that spot, but I see no evidence of engine movement and I even ran the engine in gear and goosed the throttles a bit. The shaft didn't move one way or the other. There is newer stuffing box hose on there now, I just assumed the shiny was from when it was changed out. . .any advice, ideas, things to check? Can something like that be fixed? I priced out a new shaft and ouch!

(see attached pic)

This next question relates to underwater corrosion. I dove on the boat to check things out and brush off the zincs, etc. The port prop had 2 spots (one per blade) on it that looked like a thin black ring with pink in the middle, about the size of a fingernail, that I could scratch off. Never seen anything like before, in person or on the net. The shaft is zinc'd and is isolated with a shaft saver. No picture though, sorry

Advice, recommendations, things to check for, etc. are all welcome B)
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Old 05-01-2017, 05:16 PM   #2
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The shiny section of shaft indicates that the shaft probably is moving forward and aft. That has the appearance of crevice corrosion from water trapped between the packing and the shaft. At some point did the boat sit for a long while? If so, the corrosion was accelerated by oxygen depletion on the surface o the shaft. Was the packing recently tightened? If so the gland nut would move move aft a bit and that would expose a bit of the shaft. One work around is to shorten the length of shaft hose by an inch or so, so that the shaft packing will ride on a different part of the shaft.
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Old 05-01-2017, 05:30 PM   #3
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Greetings,
Mr. cb. I second the crevice corrosion observation and the shiny bit could be simply be the old wear surface where the old shaft hose (longer) allowed the stuffing box to sit. You did mention the current hose (shorter) appeared newish so there may be, in fact, no shaft movement at all.
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Old 05-01-2017, 05:33 PM   #4
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Poster child crevice corrosion.

Edit: I downloaded it and zoomed in. Not so sure. Possible element of galvanic corrosion. The wavy part in the foreground bothers me, unless you find a chip of steel in your packing, then definitely crevice corrosion.
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Old 05-01-2017, 05:43 PM   #5
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The shaft is not necessarily ruined. Does the seal leak? Does the propeller turn. If so, then it can be used as is. At the next haul out, shaft could be magnafluxed, if no cracks, welded and turned. If you're really worried, have another one made and waiting for your next haul out. My shafts are 1942. Monel and probably repairable forever.
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Old 05-01-2017, 09:57 PM   #6
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Hmm, maybe that's why the shaft hose was shortened. Maybe the PO could not get the thing to quit leaking and moving the wear point was the solution.
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Old 05-01-2017, 09:59 PM   #7
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Zinc on prop shaft?
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Old 05-01-2017, 10:10 PM   #8
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Hmm, maybe that's why the shaft hose was shortened. Maybe the PO could not get the thing to quit leaking and moving the wear point was the solution.
This was the solution we opted for. We went from packing gland to dripless seals and cut off about an inch of tube to get to a smooth spot on the shafts. Certainly a crap-ton cheaper than two new 2" shafts.

We went to Jim Bircher in Morehead City, on of the best metal guys in three states. I inquired about replacing the metal and he said it doesn't work. The temps of the welding would do more damage to the precision of the shaft, in addition, turning such a long piece of steel is tough without very specialized machinery. In the end, new shafts were a better option. However, as stated above, we took a small step before MAYBE taking a big one later. But it may never come to that.
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Old 05-02-2017, 05:40 AM   #9
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"We went from packing gland to dripless seals and cut off about an inch of tube to get to a smooth spot on the shafts."

I would have tried Duramax or some other modern packing first , as it doesn't expose the boat to the hassles/dangers of drippless seals.

The constant need for water lubrication , and the possibility of flooding beyond multiple bilge pump ability.

A longer term solution to SS shaft problems is a bronze shaft.
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Old 05-02-2017, 04:21 PM   #10
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Yes, zinc on the shaft. I dove the boat that day to make sure it is still on there and wiped it off with a scrubby sponge.

If I, and the surveyor, completely missed that. . .it's been run on all the sea trials and an 8 hour run down the James river (half on plane). If I had never seen it, it would probably be fine. . .but now that I know about it. . .

How might I check for engine movement? I dropped that side in and out of gear at the dock, but no movement was noted. Is there something else I can try? For the amount of movement it would take to show that much shaft, I see no evidence at the engine mounts.

Many thanks for the insight! Any ideas on the weird (what I think) copper spots on the other propeller?
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Old 05-02-2017, 07:46 PM   #11
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The red spots on the propeller are signs of dezincification. The zinc component of the propeller alloy is being consumed by other metals in the propeller alloy and the copper is left behind. This is galvanic corrosion pure and simple. The reason you have a zinc, is to fight galvanic corrosion. You have a zinc, and still have galvanic corrosion, then your zinc is not doing the job. Perhaps the mass of zinc is insufficient and more is needed, or the zinc you have is no longer electrically connected to the system, having become insulated by zinc oxide. A full and fair analysis is not reasonable from the limited information available.

But regardless of a galvanic corrosion problem, the "crevice corrosion" is entirely an different form of corrosion and zinc will not prevent it.. SS needs a free flow of waler to pass oxygen over the surface to maintain the passivation. IF the water is trapped against the SS the oxygen is depleted and crevice corrosion begins and it can act surprisingly quickly. This can happen between the shaft and the shaft packing if the boat sits for a long while.

The other common issue with corrosion is stray current corrosion and none of the symptoms you have reported would lead me to think stray current is an issue.
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Old 05-02-2017, 08:34 PM   #12
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"The shaft is zinc'd and is isolated with a shaft saver."

If the shaft is truly isolated, that could be your problem.
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Old 05-03-2017, 05:49 AM   #13
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From a friend who knows......

"Clad welding and machining before straightening is a very common way to repair shaft damage. It is done all the time.

Look at this site for a shop owned by a guy I have worked with for years whenever this sort of thing pops up." RB

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Old 05-03-2017, 06:05 AM   #14
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From a friend who knows......

"Clad welding and machining before straightening is a very common way to repair shaft damage. It is done all the time.

Look at this site for a shop owned by a guy I have worked with for years whenever this sort of thing pops up." RB

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Exactly.
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Old 05-03-2017, 11:58 PM   #15
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When [how long ago] was the zinc installed on the shaft and what was its condition when you dove and scrubbed it? If the zinc was quite solid and very little material flaked off then it is not working.

Anytime a zinc does not show flaking, assuming it has been on and submerged for a while, it is not working and the usual cause is poor preparation of the mounting area, both on the zinc and the protected item, in this case the shaft.

The mounting areas of both must be CLEAN. I sand the zinc mounting area since zinc can oxidize and the oxide can be a poor conductor.

I then always check continuity with an ohmmeter. Several times over the years I have had to dismount a zinc and do more sanding and cleaning.

Shaft zincs almost always have to be rapped with a hammer to get them on tight after tightening the screws, rapping, screws, rapping untill no more slack develops from the raps.

If a yard installs do not assume they know what they are about as I found out one time. Just lucky I was not short of zincs but one of them was just along for the ride. Ever after I check.

Even more important if there is only one shaft zinc.
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Old 05-04-2017, 12:21 AM   #16
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The pink spots are dezincification and the anodes needs checked/replaced.
If you take the shaft to a hydraulic ram repair shop they will have the gear to hold it. Metal spay welding will bring your shaft like new, if its OK for truck crankshafts it will be fine on your shaft.
I've fitted PSS shaft seals on 2 boats and over thousands of miles during 15 years never, ever had a problem.
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Old 05-09-2017, 09:48 PM   #17
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The pink spots are dezincification and the anodes needs checked/replaced.
If you take the shaft to a hydraulic ram repair shop they will have the gear to hold it. Metal spay welding will bring your shaft like new, if its OK for truck crankshafts it will be fine on your shaft.
I've fitted PSS shaft seals on 2 boats and over thousands of miles during 15 years never, ever had a problem.
I used the Gore dripless packing, has been a great help.
Shafts are from 1970 and do have similar scars, but they just work anyway.

I blame those scars on decades of use of flax packing. I think I used shorter piece of rubber hose years ago to get a smoother area for the packing to seal against, and since changing to GFO packing, no leaking, and still on that packing for 10 years now. For me it truly is pack it and forget it. I do have to tighten it every few years.
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Old 05-10-2017, 08:47 AM   #18
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If you look closely there are other, not as bad, marks about an inch fwd of the gouge.
I wonder if someone tried turning or holding the shaft with a Stilson wrench?
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