electrolysis

We are having a problem with electrolysis at our thru hulls on our 380. We use a fish when in a marina but we are still are seeing electrolysis when we pull the boat in June in Trinidad for hurricane season. Any suggestions on how to track down the problem and fixing it?

Maybe I can help, but I have some questions....
Can you be more specific about the symptoms or problems you see?
Are your thru hulls being eaten? Since they are made of bronze, I certainly hope not. Or are you seeing problems with bottom paint lifting near the through hulls (perhaps interaction with the copper in the bottom paint). And what about your zincs - are they intact? or all eaten away?
For your fish, how and where do you have it connected?

Best, Drew

Drew,

The bottom paint is lifting around some of the through hulls (not all of them). Our original bottom paint was copper based but in Trinidad we had tin based paint put on. No unusual wear on the zincs. The fish is attached to a shroud. Not the same shroud every time we use the fish.

We have had the boat in the Caribbean since 2007 but only the last three years we have had the problem. We haul the boat in June and get the thru hulls repaired before launch in January. After noticing the problem the first time three years ago we started using the fish for the last two years. It seemed to reduce the problem but not solve the whole problem.

We are in a marina in Trinidad for about 1 1/2 months and then at anchor the rest of the time except for two weeks in a marina in St Lucia.

Genie and John

Hi Genie and John,

We have had similar problems on Shawnee in the last several years, after the yard switched to water based antifouling. I think we only had the problems on through hulls that have bonding wires connected. I have had "remove bonding wires from through hulls" for several years. Just never got to it. (yet) I plan to get the base of the mast connected with a short cable to the nearest through hull, but other than that disconnect them.

I don't have a good explanation for the paint degrading/lifting and neither did the boat yard crew. There would normally be some flow of current from the bronze to the various zincs as normal protection. (the zinc gets eaten) Perhaps some of the current is flowing through the paint as part of the path. Not sure what effect this could have. The solution would be using paint that has copper or tin to keep stuff from growing, but has carrier for the paint that insulates and prevents current flow across/through the paint. Or at least prevents separation if current flows in the paint. But I'm not a materials guy or a chemist. Perhaps others on the list might have some ideas.

We have not had any corrosion on the through hulls. You mentioned repairing them - or was it just repairing the paint around them?

Drew

Drew,

The thru hulls themselves are fine is just repairing the areas where the paint has lifted. Would a bad isolator cause the problem? But if it were the isolator would all the thru hulls be affected?
The same type of question if there was a short in an on board system such as the generator.

Genie and John

A bad isolator would potential allow stray DC currents into your electrical system from the AC shore plug ground. The isolator diode only protects up to 1.4VDC, I have seen one marina where the Shore cord ground had more like 2.25VDC, which overwhelmed the isolator. All the aluminum OB's in the marina, which were on shore power had the lower units damaged.

You can measure the Marina VDC by putting a meter between the AC ground and the water (earth ground). To check the Isolator measure the voltage drop from the shore cord ground side to the boat ground side. The voltage should be the same as measured between the Marina ground and earth ground.

Is "Remove bonding wires from through hulls" a standard recommendation, or a possible solution to a problems. Is this recommended for all IP? Thanks.

Rwirtz wrote: Is "Remove bonding wires from through hulls" a standard recommendation, or a possible solution to a problems. Is this recommended for all IP? Thanks.

It depends. You won't get a straight up or down answer to that question. It is a complex subject and there is no one perfect real world solution. IMO, for our boats, leaving our bonding system connected and installing a galvanic isolator is probably the best defense. But even that is not a universal perfect answer. How and where do you install it to allow for periodic testing of the diodes? And then there is the subject of which GI to use.
To begin to get an overview of the subject consider Nigel Calder's "Boat Owners Mechanical and Electrical Manual"

Thanks for the information. I'm getting Calder's book.

We had this issue with bottom paint separation near through hulls, even though we are on a mooring with no AC line power anywhere around. I think that there is a galvanic reaction going on, perhaps with current flow through the bonding wires to other submerged metal. I think our problem may have started when we switched to enviro friendly (sort of) water based antifouling paint. But not totally sure about that, since my record keeping isn't that complete. Presently using Sea Hawk BioCop - maybe that will do better.

My todo list still includes disconnecting the through hulls (most thru hulls) from the bonding system. I haven't gotten to it yet. The shaft and prop will alway be connected to the bonding, via battery minus, and tie to bonding system. I'll probably leave at least 1 thru hull connected. The bonding system isn't designed to handle direct lightning strikes, so I'm not concerned effecting that.

Interesting discussion. When we re-powered, we added a seacock for the 1" raw water line for the new engine and re-purposed the existing 3/4" seacock for galley raw water, washdown, etc.
I do not think the new seacock was bronze - we could not use original type seacocks due to a height restriction.
Anyway, we connected the new seacock to the bonding system and found that the handle and ball had disassociated so we could not close the seacock. This in 2 years! We replaced it assuming it was defective and then had it fail again 2 years later. The mechanic said it was electrolysis so the third seacock was NOT bonded - it is working fine 6 years later.

I have taken a different approach. I cleaned all the connections of ground and bonding green wires. Then last weekend I installed a fail safe galvanic isolator. I have been going through one prop zinc every five months, so I will be able to compare. Here is the isolator I chose. The install was easy.