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Marky · 12-08-2010, 16:19

Hi
Have slight wood deterioration around skin fittings in a wooden boat. What is the best way to stop this. I have herad use vinegar. Please advise

Blue Stocking · 12-08-2010, 17:30

Marky,

Both Professional Boatbuilder and Woodenboat mags have done extensive articles on this subject. It is a very complex situation, with different woods reacting in different ways. Makes for interesting reading.

**Wotname** · 12-08-2010, 17:57

Assuming you have metal skin fittings then the correct procedure is to bond all the fittings together. This was the origin of bonding through hulls etc.

A ROUGH explanation of the circuit is:

Two slightly dissimilar metals (the fittings), sea water as the electrolyte and the (slightly) damp wood as the conductor (or load). Therefore the fittings become the + & - poles of the electrical cell and current flows through the wood and slowly destroying the cellular structure of the timber.

By bonding the fittings, you effectively short out the cell and hold both "poles" at the same potential.

Blue Stocking · 13-08-2010, 06:45

Marky,

Electrical current flows in wood boats, where there are planking fastenings, rigidly mounted bronze shaft logs, etc, can be markedly different from straight forward bonding , as done in 'glass boats.
Different woods, with their particular cell structures, acid levels, and moisture contents, perform in unique ways.
Look into this carefully before proceeding.

S&S · 13-08-2010, 07:14

Quote:

Originally Posted by Blue Stocking

Marky,

Both Professional Boatbuilder and Woodenboat mags have done extensive articles on this subject. It is a very complex situation, with different woods reacting in different ways. Makes for interesting reading.

Quote:

Originally Posted by Wotname

Assuming you have metal skin fittings then the correct procedure is to bond all the fittings together. This was the origin of bonding through hulls etc.

A ROUGH explanation of the circuit is:

Two slightly dissimilar metals (the fittings), sea water as the electrolyte and the (slightly) damp wood as the conductor (or load). Therefore the fittings become the + & - poles of the electrical cell and current flows through the wood and slowly destroying the cellular structure of the timber.

By bonding the fittings, you effectively short out the cell and hold both "poles" at the same potential.

Every wooden boat builder I've talked to has told me:
DO NOT BOND METAL FITTINGS ON WOODEN HULLS!!!
I had a good deal of electrolysis that was caused by bonding. I cut the wires and haven't had a problem since. The bond wire is the ground return of the circuit and makes the problem worse, not better. What you want to try for is electrically isolating the fittings from the wood.

The first question I'd ask is what are the fittings made of? Second, how are they mounted?

**GordMay** · 13-08-2010, 07:43

There seems to be some confusion between Electrolytic (stray current) , and Galvanic (dissimilar metals) Corrosion.

Some excellent reading:

http://www.fpl.fs.fed.us/documnts/fplrp/fplrp229.pdf

http://www.practical-sailor.com/marine/corrosion.pdf

Marine Surveying : Surveying Wood Hulls - Old Boats and Yachts

**GordMay** · 13-08-2010, 07:45

Quote:

Originally Posted by S&S

Every wooden boat builder I've talked to has told me:
DO NOT BOND METAL FITTINGS ON WOODEN HULLS!!!

How does one prevent galvanic corrosion, on a non-conductive hull, without bonding metals?

S&S · 13-08-2010, 07:58

We're talking electrolytic delignification, not electrolysis. Your outside zinc's take care of that.(electrolysis)

From your FPS article:

The formation of alkaline conditions at the
cathode and the resulting wood degradation
describe what occurs in wooden vessels
around metal that is being catholically
protected against corrosion. It is common to
protect metal on ships from corroding by
cathodic protection, by attaching zinc or
magnesium anodes to the vessel and connecting
these either directly or by a conducting

wire to the metal to be protected.

No bonding, please.

Soaking the affected area with vinegar is a good idea to neutralize the alkalai. Then isolate the wood from the metal (epoxy, caulk etc.)

Again from the article:

Cathodic protection
of metal on wooden vessels should not be
attempted for metal embedded or surrounded
by wet wood unless the metal is coated with an
impermeable plastic film, such as epoxy resin.

**GordMay** · 13-08-2010, 08:37

Quote:

Originally Posted by S&S

...

It is common toprotect metal on ships from corroding by cathodic protection, by attaching zinc or magnesium anodes to the vessel and connecting these either directly or by a conducting wire to the metal to be protected...

IOW: Bonding ?

S&S · 13-08-2010, 09:02

Quote:

Originally Posted by GordMay

IOW: Bonding ?

Yep, bad news on wooden hulls.

Worst with acidic woods like white oak (as I can attest)

Marky · 13-08-2010, 17:36

Thanks guys. If rhe metals are bonded (mainly bronze skin fittings and stainless) and the system has a manual controller, is that the best way to go?

**Wotname** · 14-08-2010, 00:20

Quote:

Originally Posted by GordMay

There seems to be some confusion between Electrolytic (stray current) , and Galvanic (dissimilar metals) Corrosion.

Some excellent reading:

http://www.fpl.fs.fed.us/documnts/fplrp/fplrp229.pdf

http://www.practical-sailor.com/marine/corrosion.pdf

Marine Surveying : Surveying Wood Hulls - Old Boats and Yachts

Gord is spot on here; unless one knows the difference between these two quite different concepts and can work out the resulting equivalent circuits, one WILL get tripped up and go off down the wrong track.

Quote:

Originally Posted by GordMay

How does one prevent galvanic corrosion, on a non-conductive hull, without bonding metals?

I am thinking you meant "conductive hull" or at least "semi-conductive hull" aka traditional wooden hull.
On a truly non conductive hull, there is no (or at least very little) galvanic corrosion unless the dissimilar metals are bonded. The circuit is each metal (becoming the poles of the galvanic cell) with the seawater as the electrolyte and the bonding wire completing the circuit. Thus bonding the underwater metals is a bad idea on a non-conductive hull with relation to galvanic problems.

Quote:

Originally Posted by S&S

We're talking electrolytic delignification, not electrolysis. Your outside zinc's take care of that.(electrolysis)

From your FPS article:

The formation of alkaline conditions at the
cathode and the resulting wood degradation
describe what occurs in wooden vessels
around metal that is being catholically
protected against corrosion. It is common to
protect metal on ships from corroding by
cathodic protection, by attaching zinc or
magnesium anodes to the vessel and connecting
these either directly or by a conducting

wire to the metal to be protected.

No bonding, please.

Soaking the affected area with vinegar is a good idea to neutralize the alkalai. Then isolate the wood from the metal (epoxy, caulk etc.)

Again from the article:

Cathodic protection
of metal on wooden vessels should not be
attempted for metal embedded or surrounded
by wet wood unless the metal is coated with an
impermeable plastic film, such as epoxy resin.

Why are zincs fitted to a wooden hull?
Assuming no zincs fitted then if you DON'T bond your below water through hulls etc on a wooden boat, you WILL have to electrically isolate them from the hull. But bonding them together (and keeping them well bonded) will achieve the same electrical effect.
Of course, if you have fitted zincs for some reason and attached them to some metal item below the waterline, the circuit changes and without working it all out, I would probably think that bonding might not be a good idea - however, electrically isolating them from the hull would become paramount IMO.

Quote:

Originally Posted by Marky

Thanks guys. If rhe metals are bonded (mainly bronze skin fittings and stainless) and the system has a manual controller, is that the best way to go?

What is this manual controller that you mention? Surely your wooden hull does not have an impressed cathodic protection system fitted (or something similar).

Blue Stocking is also right, the current flows that occur inside the timber on a wooden hull are complex and one cannot just assume techniques that have become common on fibreglass and steel hulls

Marky · 14-08-2010, 04:13

By manual control I have an 'electro-guard' system fitted where you can manually alter the protection levels.

**Wotname** · 14-08-2010, 05:25

Quote:

Originally Posted by Marky

By manual control I have an 'electro-guard' system fitted where you can manually alter the protection levels.

Wotname gracefully bows out of this discussion as he has no idea how an electro-guard system would be installed on the wooden hull.

However I guess it must just be connected to the underwater metal structures (through-hulls, shaft, prop etc). If this is so, I would expect it would be essential to electrically isolate (insulate) all metal from the wooden hull as the hull WILL be slightly conductive; surely for the system to be effective, it would have to be connected to all the below water metals which is effectively bonding them. I would wonder about the effects of such a system would have on the fastenings in the hull as these cannot be isolated from the wood nor can they be connected to the electro-guard system.

Wotname now takes a back seat and waits for more learned posters to post