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Old 04-21-2014, 09:02 AM   #1
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serious corrosion on prop shaft

When scraping barnacles off the running gear a few months back I noticed there are three or four very deep pits on my stainless steel prop shafts. (I believe they are 3" dia shafts) The pits range from 1/4" to 3/8 dia roughly and up to 1/4" deep. They have the appearance of being impurities in the s/s which has been eaten out by rust/corrosion. I have no idea how long they have been there or if its an old problem that has long since been corrected.

I had a similar problem with the large s/s studs that tighten the packing glands down. These looked like "brand new" except each had one or two pitts so deep you could break the studs in half with your bare hands.

The boat has a Quicksilver galvanic isolator and everything is bonded. Zincs are good but very likely neglected in the past.

Any ideas as to whats happening? I'm thinking of getting in a pro welder to grind out these spots and fill them in with s/s rod. Any thoughts on repairing this way? These shafts are probably 15ft long and I imagine ridiculously expensive to replace.
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Old 04-21-2014, 10:10 AM   #2
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You do have a corrosion problem. Find a good marine electrician who can go through your boat, check your galvanic isolator and find the problem.

And if you do try repairing your prop shaft, find a shop that does repair on heavy industrial (refinery, petrochemical) pump shafts. This is not a trivial welding job and should be done by a real pro. At the very least a prop shop that does this sort of work routinely.

And the metallurgy might be one of the Aquamet alloys, not stainless steel.

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Old 04-21-2014, 10:46 AM   #3
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Sounds like you have crevice corrosion. Only the very highest grades of shafting are immune to this. Do to the size of your shafts, you have an expensive problem going.

You could turn your shafts around, end for end giving you a new surface outside and putting the pitted end at the transmission. You would have machine work and new tapered transmission couplings to buy but maybe it would save these shafts. A pair of these shafts, in 22 alloy, would be about $12,000...
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Old 04-21-2014, 10:58 AM   #4
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Thanks David, not familiar with aquamet but I'll google it & find out. Trying to locate a good marine electrician who is "intimately" knowledgeable on the various marine corrosion issues might be problematic. You wouldn't think it was rocket science but even the experts don't seem to agree. Anyone have a recommendation of a good marine electrical technician around StPete's?

I don't want to pull the shafts so I'm going to try & locate a qualified mobile welder in the StPetes / Tampa area.
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Old 04-21-2014, 11:04 AM   #5
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Creative idea Brooksie but no go, only about 5ft of the shafts are on the inside, the pits would still be in the water.
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Old 04-21-2014, 11:32 AM   #6
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I seriously doubt you have 3" shafts....unless you have 1000hp per shaft.

Anaerobic corrosion of stainless is very common..often called crevice crack corrosion and while true is more rampant than crevices.

Under things like barnacles, in shaft packing or studs embedded in wet areas...corrosion is rampant sometimes without stray currents...but you also may have stray current corrosion which has to be systematically run down.

These are the lag bolts that used to hold my stuffing box to the hull...bronze is a much better choice....
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Old 04-21-2014, 12:02 PM   #7
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Do you know if the damage is new, or could it be old damage you never noticed? Were zincs in place on shafts when you noticed it, and were they partially wasted (working, that is)?

You should pull the shafts if you are going to attempt a welding repair. No way will a firm capable of doing a quality repair do it under the boat.

Also, the shop is going to need to know what exact alloy the shafts are. This can be a real problem as shafts are rarely marked, may have been replaced with a different alloy, and if boatbuilder did keep records, they may not be accurate.

There are hundreds of these type alloys, but only several are popular on boat shafting.

Corrosion on these alloys is not simple stuff. Stainless steel is a real poor term for these. On the submarines, it was called "corrosion resistant steel", or CRES. It does corrode.

Condolences on the mess you are facing. Not fun nor cheap.
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Old 04-21-2014, 12:45 PM   #8
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psneeld, Once again pictures are worth a thousand words. Thanks for posting. The damage on yours seems to be much more widespread, particularly the lag bolts. (At least I assume they started life as lag bolts) The threaded studs are somewhat similar only on mine there is no overall corrosion, they look like they are brand new except for these random deep pits. Perhaps if mine were left long enough they would look the same. Strange why it would attack your stuffing box but not your shafts as it did mine.

Ski, don't know if new or old, could have been like that for years. Zincs are in place and working but again don't know if this was always the case. If I could get the shafts home I'd weld them myself, alas shipping them 5000 kilometers is not a viable option. Condolences appreciated
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Old 04-30-2014, 03:52 PM   #9
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I had the same issue on my starboard 1 3/4 inch shaft although the boat and systems are bonded properly. What had happened over time (before I bought the boat) was the diver who replaced the shaft zincs was not cleaning the shafts. He simply unbolted the watsed zinc and installed the new on the dirty shaft. No protection there at all. Just something to think about later.
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Old 04-30-2014, 09:40 PM   #10
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A couple of thoughts

You said barnacles, but I can't tell if you moor in salt water , fresh or brackish water. If you are in fresh or brackish you should be using aluminum instead of zinc. A good way to check for stray current is with a amp meter that can encircle your shore power cord. If there is measurable current on a single shore power cord you have leakage somewhere to ground. If you have two shore power cords subtract the current reading measured on each cord from each other. If there is a difference again you have stray current. This is a quick way to see if you have trouble. The problem may not be your boat it could be the dock or a neighbor. You can check the dock by extending a wire into the water and measuring the current from that wire to the ground on the dock shore power outlet. Any current is a problem. In fresh water stray current is deadly. You can check your dock neighbor by testing his shore power cord.
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Old 04-30-2014, 10:22 PM   #11
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Stray A.C. current is not a big problem. D.C. current is. If you know enough about welding to do this job yourself you will know that you will need a seriously large shaft bending device to correct the bend after it cools from being welded. Heat applied on the opposite side sometimes works. No way can it be done without pulling the shaft. Bonding was mentioned, which has probably been the demise of more underwater metal than any other single outdated and misunderstood thing ever done to a fiberglass boat. It was devised for wooden boats and anyone that has a basic grasp of electricity will not use it on a fiberglass (insulated) boat.
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Old 05-01-2014, 07:44 AM   #12
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Kulas, your comments on bonding disturbs me greatly, particularly as you seem adamant on the point. Not that I don't believe you but can you steer me to any information on the subject that I can read to form my own conclusions as to this bonding issue. My boat ( a later model) has virtually every piece of metal in the boat bonded by the factory. There is miles of green bonding wires running everywhere all connected to copper strips fastened to the stringers and running the length of the boat. Are you suggesting I rip all this out? I have heard before from others that bonding is not good on fiberglass boats but have never found any professional articles on the subject to make an informed decision.
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Old 05-01-2014, 08:46 AM   #13
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Bonding Systems And Corrosion | PassageMaker
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Old 05-01-2014, 10:01 AM   #14
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Bonding was required on wooden boats because of delignification. Which is caused by damp wood, metal objects in contact with water and the damp wood and dc current. The wood is a semiconductor and the current removes the lignin from the wood (oak mostly)(green/white fuzzy) and leaves it weak, enough so that the fitting can blow out and sink the boat. It took the industry awhile to figure this out. Then, all insurance co.s REQUIRED bonding. And in there ignorance some still do on all boats, wood or FG. It would be a very long post to go thru all of the reasons not to bond FG boats but just do this. Hang a metal thru hull with a plastic string from the dock. Hang another with a copper wire from the dock and hook it to your battery charger. See which one lasts longer. And, if you add a small zink to the insulated one it may well last forever with no dicernable corrosion. Your FG boat is the same.
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Old 05-01-2014, 10:10 AM   #15
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Just read the PassageMaker article. I've not agreed with DeAntonio on more than a few subjects, this being one of them. He is a good writer, thats about it. Dave Gerr on the other hand does know his $hit and does not advocate bonding on FG boats. His personal boats are not bonded. It would behoove anyone interested in boats, especially at our level, to read all of his books.
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Old 05-01-2014, 12:08 PM   #16
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Post bonding/zincs

Kulas, your view is similar to mine on bonding. If there is no possible way for two pieces of underwater metal to be electrically connected they should be left that way. There is no electrical connection to complete the circuit and to cause current to flow thus no corrosion. The question I have for you is do you zinc? If two dissimilar metals are not connected they do not need zincs. Right? The one place I have dissimilar metals connected together are my bronze prop and stainless shafts. I believe the stainless is more noble than the bronze so why do we always see shaft collars installed. The protection is needed for the prop. Also if you look at the ratio of surface area on the bronze prop/stainless shaft combination the shaft should have little effect on the propeller anyway. Correct me on any of this if you think I am wrong. I have tried to discuss this with some pretty sharp guys even electricians that boat but all I ever hear is bond and zinc which I do not think is needed in most cases.
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Old 05-01-2014, 05:39 PM   #17
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I do have zinks on my prop shafts, for the same reason you stated, dissimilar metals. But it probably isnt needed. However it is connected to the engine block (not mine, I use plastic DriveSavers) in most cases and therefore to the batt. charging system. An overcharge situation could cause a serious problem. Solar panels are another source of electrolysis, especially large units with inadequate regulation. what usually gets underwater metal on a bonded boat is corroded connections. After awhile you only have 1 or 2 good connections and those items gets all of the excess DC current. It can be really hard to keep up with the maintenance on all of the bonding connections, most folks dont bother. Grounding and lightning suppression/directing also needs to be addressed.
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