Galvanic activity sluething...

[banjoman]

When hauling Starduster we noticed some crazy galvanic activity (see my "New Pintles" post for evidence. Our Indigo prop also suffered mightily.

When finding the cause, where should we look first.

Particulars:
-Modified H28 built by Far East in 1965
-No shore power
-Atomic 4
-3 batteries
-One automatic bilge pump
-Been docked at Glen Cove Marina in Vallejo for the past year
-Last haulout was a little over a year ago and did not notice anything wrong
-Power was always turned off upon leaving the boat short of the bilge pump (first clue).

Some fasteners were removed to check and they were fine. So far the only victims have been the prop and pintles.

[PAlien]

Well, more info would be helpful. How is the boat/prop zinc'd? Shaft zincs? What condition are any zincs in? Is this a relatively new location for the boat, i.e. new slip, new marina, etc.?

As for the pintles, they could most likely be bonded to a transom zinc or any nearby existing bonded point and that should help a lot.

[Larks]

Hi B'man, what other vessels have been parked around you? It may be another vessel if what you are seeing is out of the ordinary. The other thing to look at is if any work has been done on the marina or fixtures near your vessel.

[S B]

There was a problem here, years ago, when the longliners moored for the winter, in the area of the runoff from a chicken farm, had the props eaten completely, not enough left to move them. Your problem may not be of your own making.

[banjoman]

No idea if it is our own making. The shaft and bolts are stainless. No zinc on the shaft per the manufacturer of the prop's instructions. The bearing is zinced and it is noticeably deteriorated.

[Andrew Craig-Bennett]

I have a healthy distrust of automatic bilge pumps, but you will know if you have persistent battery losses.

What sort of neighbours do you have - if they are hooked into shore power that might have something to do with it.

[Hwyl]

Are your gudgeons grounded?

[Andrew Craig-Bennett]

Having looked across to your other thread, which I should have done in the first place, i don't think its galvanic or electrolytic corrosion. I think it's regular crevice corrosion of stainless steel, triggered by a low oxygen environment.

[banjoman]

Are your gudgeons grounded?

They are not.

[banjoman]

I have a healthy distrust of automatic bilge pumps, but you will know if you have persistent battery losses.

What sort of neighbours do you have - if they are hooked into shore power that might have something to do with it.


Glenn Cove is a hodge podge. To one side we had an M26 and a Bayliner on the other. The BL was on shore power.

The prop shows a lot of wear but I do not have pictures of that at the moment.

[Andrew Craig-Bennett]

I'm pretty sure that it is crevice corrosion and your bronze replacements will do the trick.

[Canoeyawl]

I’ll bet that it is your neighbors causing this. You may need to hang some sacrificial zincs in the water to just diffuse your neighbors activity. A sensitive voltmeter may tell the tale.
I have seen serious damage caused by “hot” neighboring vessels. That bayliner next door may have a charger hooked up full time. Perhaps you could unplug him mometarily and measure before and after for stray current.

If the pintles were a brass alloy (manganese bronze, etc), the zinc is going away – it looks like that in the photo. What type of metal were the cotters in those pintles?
Propellers are manganese bronze, which contains zinc.

[BrianM]

Some marinas own instrumentation to measure stray currents. Ask your harbormaster to have the water near your boat tested. Stray currents in the water can kill people which is the more important motivator to get it fixed if this is the issue.

[banjoman]

The good news is we are not going back to Glen Cove so won't be investigating that marina.
The bad news is...
The cotters were stainless which is a major issue.
The prop is pretty messed up as well.

[Ray Frechette Jr]

Anodes on the prop shaft?

[Canoeyawl]

I don't like Glen cove, it was problematic getting in and out of there without an engine...
You might want to take a good look at the standing rigging at and around the chainplates, they are "connected" via the concentrayed salt above the waterline.

[Windsong]

Hang an extra big zinc over the side when docked or hang one over the side of the dock near your problem hot area. Careful not to over zinc the area.
Cheers
Lars

[banjoman]

Anodes on the prop shaft?

None per the instructions of the prop manufacturer.

[banjoman]

Prop
http://i193.photobucket.com/albums/z98/brewmastermsu/boat001.jpg

http://i193.photobucket.com/albums/z98/brewmastermsu/boat003.jpg

There used to be fins that projected aft. Seems at least a 1/2 inch is gone. New prop will arrive on Thursday. We have been very happy with the performance of this prop and Indigo has been nice to deal with during this. They did discount the replacement.

[Hwyl]

I think moving the boat is a good solution. I'm surprised there's no zinc on the shaft, do you have a rubber coupling that isolates the shaft. Is the prop electrically connected to the zincs. If it's not too difficult I'd try bonding the gudgeons or putting small zinc washers on the pins to kep an eye on. The pins seem much worse than the prop.

I'm a little perplexed.

[banjoman]

For all I know the pins are 60 years old but the prop has only been in the water for a little over a year and a half. The lack of a zinc on the shaft was per the prop manufacturer's instructions. The goal was to prevent barnacles which it did...shaft and nuts had them but prop was clean.

New prop (that should arrive on Thursday) is the same design but stainless instead of bronze.

[nedL]

Yikes! that looks like a lot more than galvanic corrosion (which is all that zincs will help reduce). Corrosion that extensive in a year & a half looks like stray current corrosion (not controlled by zincs), - I 'd say its a good think you are moving to another location.

[Ray Frechette Jr]

I can't quite figure on the no anode on shaft idea.

If you have a stainless steel shaft and it is electrically connected to a bronze prop... And it will be if the two don't have some sort of electrical isolation...

And you place it in an electroyte whcih you do in salt water... Than it is no great shakes that you r prop would disolve.

I think you have a case against the prop manufacturer giving you bad advice.

Stainless is more nobel than bronze, bronze disolves...

No rocket science here.

Of course the Bronze was clean. It was corroding so fast barnacles couldn't attach.

Barnacles can be chipped away. Bettah than corroding bronze in my book.

[Plover]

Thers a lot said in this one! Zinc's are going to help in all regards as far as stray currents unless it is very excessive and then it will just eat them right off! I wouldn't trust what the prop manufacturer say's about no zinc's, its that simple. (Not to mention, they are in the bussiness to sell props!) Zinc for salt Magneisium for brackish and fresh. Also don't over look a simple corroded bonding wire if there are any?
One story worth mentioning is a neighbor of my brothers with an automotive battery charger on his starboard side, the guy left it on for the weekend and literaly ate the starboard trim tab damn near right off! It cost the guy plenty! And to this day I have never seen such fierce corrosion.
Good luck with it.

[redbopeep]

If your marina doesn't own a half cell to do the testing for stray currents you may want to buy one--however--I have no idea of where you could get one here in the USA. They used to be easily obtained at boating supply stores (I've heard) but I've never seen one there. Perhaps another forumite knows where you can get one?