Need advice from corrosion experts

[sandy stone]

I recently had to remove the zincs from my prop shaft, in order to slide the shaft back for clearance to replace my transmission. That work is complete, but there have been delays in getting a diver to replace the zincs. In the meantime, I have hung one of those giant fish zincs over the side and connected the lead to a bolt on the shaft coupling. Will that adequately protect my prop until I can get a diver over here?

[fstbttms]

Originally Posted by sandy stone

I recently had to remove the zincs from my prop shaft, in order to slide the shaft back for clearance to replace my transmission. That work is complete, but there have been delays in getting a diver to replace the zincs. In the meantime, I have hung one of those giant fish zincs over the side and connected the lead to a bolt on the shaft coupling. Will that adequately protect my prop until I can get a diver over here?

Fresh or saltwater?
Zinc, aluminum or magnesium?

[MicHughV]

As an aside, my prop zinc seemed to disappear quite quickly at the marina.
In frustration, I hung one of fish zincs over the aft side, which was connected to my engine, with the cable it comes along with.
It was smallish one, maybe 6" long or so....but it seemed to do the trick, as my shaft zinc lasted longer. The fish zinc hung there for quite a while, before I replaced it.

Would interested in fstbttms take on it...

[fstbttms]

Originally Posted by MicHughV

As an aside, my prop zinc seemed to disappear quite quickly at the marina.
In frustration, I hung one of fish zincs over the aft side, which was connected to my engine, with the cable it comes along with.
It was smallish one, maybe 6" long or so....but it seemed to do the trick, as my shaft zinc lasted longer. The fish zinc hung there for quite a while, before I replaced it.

Would interested in fstbttms take on it...

I asked these question of the OP because I see that he is in Louisiana and possibly not in saltwater, which would affect his choice of anode. However, as a rule of thumb I think that installing a fish anode can't hurt, but whether it provides any protection or not is something of a crap shoot and very dependent upon the electrical path/resistance between the anode and the part being protected.

[MicHughV]

It's an interesting topic for me. I am a structural marine engineer by profession (retired now), and have done countless steel bulkhead designs and other marine structures.
In effort to prolong the life of the metal sheet pile, sacrificial zincs are often added to the sheets, both zinc and aluminum.
Upon a yearly inspection, a year later, it is notable that some zincs are completely gone, while a neighboring one is as good as new.
That is but one example and I could list countless others.
It's difficult to pinpoint a cause, because there are invariably long steel tie back rods involved, etc.

I don't want to hijack this thread, but am always on the lookout for these threads, hoping I might learn something new.

[sandy stone]

I'm in brackish water, leaning towards fresh, I think. I'm a good ways up a bayou. AFAIK, the fish is zinc. The shaft zincs, when installed, seem to last a pretty long time. When I sent my Maxprop in to be refurbished, the service guy remarked what good shape it was in for its age.

[fstbttms]

Originally Posted by sandy stone

I'm in brackish water, leaning towards fresh, I think. I'm a good ways up a bayou. AFAIK, the fish is zinc. The shaft zincs, when installed, seem to last a pretty long time. When I sent my Maxprop in to be refurbished, the service guy remarked what good shape it was in for its age.

A zinc anode will not protect your underwater metals in fresh or brackish water. Suggest you go with aluminum.

[MicHughV]

My dr. sez the same thing about me...that I look remarkably well preserved for my age

[Statistical]

Originally Posted by sandy stone

I recently had to remove the zincs from my prop shaft, in order to slide the shaft back for clearance to replace my transmission. That work is complete, but there have been delays in getting a diver to replace the zincs. In the meantime, I have hung one of those giant fish zincs over the side and connected the lead to a bolt on the shaft coupling. Will that adequately protect my prop until I can get a diver over here?

As long as it has a good electrical connection with the prop which from your description sounds like it does then it will work as well as a zinc on the prop.

It is all one electrical circuit with sea water as the electrolyte. While bonding is not popular in the US this is why European boats are sometimes bonded. If everything metal below the surface is bonded together and there is one zinc that zinc can protect everything.

[Statistical]

Originally Posted by MicHughV

As an aside, my prop zinc seemed to disappear quite quickly at the marina.

That would indicate stray current corrosion not just galvanic corrosion. Someone in your marina has a hot boat and leaking AC is making its way back to the pedestal through your propshaft.

Assuming your aren't the source you need a galvanic isolator. Even if your electrical system is perfect you can't avoid stray current corrosion if connected to marina shorepower with other hot boats without being electrically isolated from them (galvanic isolator, isolation transformer, or physical isolation).

[fstbttms]

Originally Posted by Statistical

That would indicate stray current corrosion not just galvanic corrosion. Someone in your marina has a hot boat and leaking AC is making its way back to the pedestal through your propshaft.

I wish I had a dollar for every boat owner that told me his electrolytic corrosion issue was some other boat's fault

[Statistical]

Originally Posted by fstbttms

I wish I had a dollar for every boat owner that told me his electrolytic corrosion issue was some other boat's fault

Indeed. It could also be his boat just pointing out that even if his boat is perfect it is all one circuit when you are on shorepower. So people who are sure their boat is flawless should still get isolation of some kind.

[MicHughV]

Yep, while at a marina, usually have some or other problem with sacrificial zincs. Difficult to point a finger as the problem could come from a myriad of different directions....neighboring boat or boats...shore power pedestal, power lines under floating docks, boats own electrical setup, etc, etc

At anchor, usually have a minimum of problems.

[fstbttms]

Originally Posted by MicHughV

Yep, while at a marina, usually have some or other problem with sacrificial zincs.

I service about 800 boats on a regular basis. Almost all of them have anodes and 100% of them are in marinas. Amongst them, rapid anode depletion issues are not common and almost always due to a problem aboard the boat in question.

[boatpoker]

Originally Posted by fstbttms

I wish I had a dollar for every boat owner that told me his electrolytic corrosion issue was some other boat's fault

Agreed - As a certified Marine corrosion tech I've done hundreds of corrosion surveys and in every case the complainants own boat was master of it's own disaster. I've never seen corrosion caused by another vessel.

PS. Also agree with you about aluminum vs. zinc