Electrochemical Damage To Wood – the marine version of ‘leaky homes’
Story & photos by Chris McMullen. Edited by Alan H
NOTE: The photo above is the residue left from a heart kauri floor. The keel bolt had been bonded for ten years. Impossible to affect a proper repair as the bolt went through the deadwood.
Today’s post appears on WW for three primary reasons:-
#1 the author Chris McMullen is someone I & most intelligent, thinking boaties respect.
#2 waitematawoodys is all about the study & appreciation of classic kiwi wooden boats – if people do not wake up, there wont be any to appreciate.
#3 this information needs to be stored somewhere like waitematawoodys so when people are searching the topic of electrochemical damage in wooden boats, they will find this & be able to make their own decision based on sound, robust debate like the below.
Most people are aware that Chris is one of NZ’s most respected boat builders and at one time was the Lloyds (Honorary) Wood Boat Surveyor in Auckland. Chris’s (The original Co) “McMullen & Wing” built and repaired wood, steel and marine aluminium vessels. They built the first welded aluminium vessels in NZ.
Public opinion back then was, “They were mad” and the hulls would fiz and corrode in salt water. Not so, and now aluminium is used for not only yachts but all types of commercial vessels.
Chris is also the custodian of the magnificent classic Colin Wild launch ‘Wirihana’.
Chris is constantly asked for advice on kauri vessels with wood degradation problems. His view is somewhat different to many marine industry technicians in NZ but backed up by Yacht Surveyors in the USA and the UK.
If you don’t have time now to read this post today, please bookmark it, as I guarantee it will at the least have you doing a double take.
Update 06-06-2015 – if you are time poor scroll to the bottom, new info & photos added
I’ll let Chris tell the story. Alan H
Chris McMullen – I am absolutely convinced that any wet wooden (caulked) vessel is doomed to a slow death if dissimilar metals or zinc anodes are fitted and bonded by wire to underwater metal. I do not reject the theory of cathodic protection; in fact I use it and zinc anodes on my steel floating dock. The problem is the wet wood component. There is a voltage between any two connected (bonded) dissimilar metals and the wet wood completes the circuit. Any voltage in the wood (from any source) breaks down the lignin in the wood round the cathode (protected metal) and that is the issue.
The white corrosion byproduct formed, Sodium hydroxide, is used for pulping of wood in the paper making industry! Want this on your wooden boat? Yuk!
Included below are some links to technical papers written by wood chemists rather than by metallurgists. These articles are not new but quite convincing and are parallel to my thinking and experience.
It seems however that people don’t like reading technical stuff. And it’s (I guess) easier to read and believe what is written in a local boating publication.
I decided to do an experiment to prove my point.
I used a length of 6”x 1” pine. I bolted to it an old zinc anode and about a metre away bolted two bronze objects. I connected one to the anode with a copper strip and the other is close by but not connected to the anode.
I hung these on a rope in the water of the Tamaki River (salt) off my floating dock.
After six months, I pulled the test rig out and cleaned off the considerable marine growth.
I removed the bronze objects. Under the one connected to the anode was a black stain and the bolt fell out of the hole. The other, the bolt had to be punched out and the timber was clear bar some copper residue.
I photographed the test & also took the voltage readings between the metals and also the metals potential against a silver-silver chloride reference. (click photos to enlarge)
The minor damage done to my test piece is the result of only six months submersion. I will put it back and check in another six months.
It is however, quite obvious to me some thing is wrong and in time the wood will deteriorate further. If this is due to a voltage (less than half a volt) well, you can imagine the damage coursed by electrical stray current, and that is likely on old boats. My test rig is very basic and not influenced by other factors.
Sure, the wood may be less effected if painted and if the bronze was insulated by bedding compound. The damage can take years but our kauri boats should last indefinitely if we don’t do things that destroy them. Someone is bound to say I have over sized the anode on my test and thus the timber damage. I know that is the case but I had to accelerate the process.
On a boat it is almost impossible to size an anode correctly due to wasting and wiring faults in inaccessible areas of the bilge.
The point I am trying to make is:
Eliminate any current flow in the wet wood.
If you bond metals underwater you are inviting problems.
Further, a stray leak from the ships battery will do far more damage in less time. I believe most metal corrosion problems originate from this source. Cathodic protection will do nothing in this case.
Again, bonding will encourage a circuit. Remove the bonding and you have no circuit.
Want to check your electrical system for stray voltage in your wet hull?
Connect a voltage meter between the positive on your battery and any bolt or fastening in your hull. You may get a surprise. It may be 12 volts but high resistance so you can’t light a bulb but enough to cause corrosion.
If you have bonded underwater metal, the surprise maybe an expensive. For a start, try tightening the lag screws fastening the stuffing gland to the shaft log. If you don’t, a surveyor will, if you ever want to sell your ship.
History
In Yachting World Magazine March 1957 Mr. MG Duff wrote a convincing article headed “Stern Gear Corrosion” (cathodic protection for underwater metal fittings on wood yachts”.) In the article he never mentions wood degradation! He did not know until the problem showed up years later, see below.
The subject article date (1957), ties in with my memory of Jack Brook (head of the old D.S.I.R) yes, Robert’s father, and Alan Odell (Professor of Chemistry) fitting an Anode on the Tobin Bronze(H/T Brass) shaft of Alan’s Logan yacht the “Mahaki”.
I was only a kid who happened to be at Devonport Yacht Club with the Odell’s that week end. I remember a crowd of onlookers thinking this was black magic.
I believe this resulted from the Duff article. Later Jack Brook had the DSIR print a pamphlet on the subject. Sadly, (like Mr Duff) these academics never thought about wood damage.
Back to M.G Duff Co. Ironically, now on the company’s current web site (58 years later) they warn about the damage to wood. Please check it out. http://mgduff.co.uk/support/knowledge-base/questions/what-is-electrochemical-decay-in-wooden-vessels
Now using anodes did not catch on here until the late1970’s when a local business man saw it as an easy way to print money. He, like the Duff article was very convincing and I am sure well meaning. Some wood boat owners now refer to him as Dr Death! (I never coined the name)
Since then almost every marine electrician has got in on the bonding scheme. It was a new subject for books and magazines, now some misinformed boat owners are pushing it too.
On a hard stand a boat without an anode stands out and in many eyes shows an uncaring owner and will draw criticism. This is mistaken thinking and sadly is self perpetuating.
Please consider, “Logan, Bailey and Wild never used cathodic protection. They had knife switches and crude electrical systems but no bonding! Their boats lasted 60 years plus. ” Why should we use anodes?
As an apprentice wooden boatbuilder, we were told at night school, I quote “ “Never use dissimilar metals underwater” and further ” never use brass”. These days the advocates of bonding use zinc underwater! What could not be more dissimilar or further apart on the Galvanic scale?.
In the 1960’s and prior to that, boat builders used bronze and copper only. There were no stainless shafts available ex stock in New Zealand.
Despite working on all sorts of boats, I never saw the timber degradation I have sighted in recent times. Sad, as we no longer have the kauri or people with the skills to affect these sometimes major repairs.
The boating public and boating industry technician’s have been mislead by a situation rather like “The Leaky Homes”, just because everyone is doing it, does not mean that it is right.
This problem was aired in Wooden Boat Magazine (30 years ago) Also in Classic Yacht Magazine. I sent some information to New Zealand Boating magazine. They showed no interest. Their advertisers sell anodes! Big business!
Further, the NZ Marine industry is partly to blame. Locally made strut bolts were made from tobin bronze! tobin bronze (these days) is brass and in my opinion unsuitable underwater. Their “Through Hulls” were made of gunmetal that although a bronze, is in my opinion a very poor choice of material. Use aluminium bronze or silicon bronze or reinforced plastic.
Cheap propellers are made of manganese bronze. Again a poor choice (these days) and little better than brass! Use aluminium bronze for propellers.
Bronze gate valves have brass spindles. Use reinforced plastic valves.
A lot of confusion comes from reading old books. (See L Francis Herreshoff “Common Sense of Yacht Design”) He recommended tobin bronze (Trade Name) and manganese bronze.
These are now generic terms. Both materials were (possibly) once good marine metals but over the years the makeup of these alloys or the way they are cast has changed and as a result I believe their resistance to dezincification has been compromised. This is evidenced by. See https://www.flickr.com/photos/109707376@N06/11134934714/
This manganese bronze propeller (Mizen Head Ireland) had been underwater 100 years! I checked it out some years ago. The bronze blades with the “Stones” trademark look perfect. The Stones Co. built ships propellers all from manganese bronze! Yes, the iron hub was an anode and (possibly) protected the bronze. A perfect example of cathodic protection! Note.There was no WET wood involved.
I talked to the new owner of Chatfield Engineering. He tells me they are now using silicon bronze for strut bolts. I say “Not before time”!
Sopac Marine Ltd is importing “Groco” silicon bronze hull hardware from the USA. They also stock aluminium bronze plumbing fittings that will last forever.
Support the local manufacturers? Unless they tidy up their act, they do not deserve your support.
Please remember bonding poor quality underwater metals does not guarantee their security. Read about “Random Harvest” (Link below)
Boats are supposed to be fun but you really have to be an expert to keep your ship afloat.
It all comes down to attention to detail. Use the right marine materials, be sure your wiring is done properly and your boat will not suffer from metal corrosion. You certainly won’t need bonding or anodes.
Sadly, for most it is too late. Cut the bonding & remove the anode but the caustic soda corrosion byproduct previously formed will remain and continue to soften the timber. Remove the effected timber round the cathodically protected metal or live with the problem caused by LOVING YOUR BOAT TO DEATH.
Some hints to eliminate DC Voltage Leaks in Boats.
1. Insulate the negative connection from the frame, on alternators, generators and anchor windless motors.
2. Use two pole senders on the engine alarms.
3. Install a solenoid on the starter motor negative so it is only connected as the engine is started.
4. Battery switch’s can leak. Have a second switch on the negative.
5. Be sure your bilge pump is wired correctly and in good order.
6. Use an insulated gearbox to shaft coupling.
7. A Furuno depth sounder is two wire but the bronze housing (like an alternator) connected to the negative! Sleeve it with plastic so there is no connection to the wood. Leave it, and it is unintentionally bonded and you have a circuit.
8. If you have shore power, use an isolating transformer.
9. If you still must bond for safety purposes? Well, accept the fact you have made a circuit and face the consequences, maybe you should not have a caulked wet wooden boat!
Why go to all this trouble?
The negative wiring in the ship can act as bonding wires if the appliance on the end has been designed for automotive use (chassis metal is usually the negative ground on a vehicle.) and touches damp salty wood. Say, for example, you have a negative leak in the autopilot drive that is coupled to the rudder that carries an anode. You once again have made that dreaded circuit and the wood round the rudder gland will suffer.
On a metal boat all the above is mandatory.
Put up with some minor corrosion.
It is easier, to replace a metal fitting than the wooden hull structure. Further it is pointless putting an anode on a aluminium bronze rudder (as is often seen) Of course the anode will erode. The bronze is a marine metal and does not require cathodic protection. If it has a stainless Shaft well that is dissimilar metal underwater. If it corrodes, change the shaft to bronze is the best advice.
If you are worried about your stainless propeller shaft, have a cast iron (nut type) sacrificial anode made. It should only have contact with the shaft not the bronze.
I write this (as a boatbuilder and certainly not as an expert) It is the result of my experience and research into a problem, I first noticed thirty years ago but seems more prevalent in recent times.
I share my observations, for the benefit of classic boat owners to help assure the long life of their vessels.
There is nothing in this for me.
For those interested I would encourage you to click on these links & read carefully.
If nothing else page 4. Written by a Wood Chemist in the USA.
http://www.fpl.fs.fed.us/documnts/fplrp/fplrp229.pdf
Page 6 at least http://www.michel-christen.com/2T-H.pdf
Read at least page 15 Section 2.5 This is ten years old!
https://assets.digital.cabinet-office.gov.uk/media/547c7179ed915d4c0d000131/random_harvest.pdf
Also Wooden Boat Magazine (issues)
Number 65 1985
Number 93 1990
Number 115 1993
Number 167 2002
“Prevention of Decay of Wood in Boats” (refer below)
Forest products research bulletin # 31 (Ministry of Technology (UK)
I can supply copies of the above if anyone is interested
Also Classic Boat Magazine.
And see. http://www.kastenmarine.com/_pdf/mbqCref.pdf
See beware of brass. https://www.proboat.com/beware-the-brass.html
See http://coxengineering.sharepoint.com/pages/brassandbronze.aspx
See WW Dec 8 th 2011. Electrolyses.
Nothing has changed, except I gave up, trying to convince people. It is no fun swimming against the tide.
See below on how to make paper.
Soda Pulp
Soda pulp is the original chemical pulp and is produced by cooking chips of (usually) deciduous woods in a solution of caustic soda under pressure. This leaves a relatively pure cellulose pulp which is then washed and bleached. Soda pulp produces relatively soft, bulky papers (as a filler with other pulps) used in books, magazines and envelopes. Caustic soda dissolves most of the lignin in wood while having little effect on the cellulose. Cooking liquor is recovered during the washing process.
It Is Not Just A Wooden Boat Thing
The photos below show a carbon fibre boat and a fibreglass yacht bonded and with electrical issues. The point here is that all boats can have electrical issues. On these boats it showed up. On a wooden boat the damage is invisible until it is too late.
06-06-2015 A Short & Hard Hitting Update From Chris – Read It, Its Pretty Simple Really!! Alan H
Less zinc, more zinc, over zincing are terms used by those who have recommended bonding to their clients and when things go wrong and they will. They have to have an excuse.
Please consider the following.
Sodium hydroxide or caustic soda is the chemical that damages the wood. This chemical is used for pulping wood and used in the paper industry
To make sodium hydroxide in a laboratory see youtube https://www.youtube.com/watch?v=m0LADyIfHRs
You will find the requirements are a positive and a negative electrode of say carbon, a container of brine or salt water and a power source i.e. a battery charger or a battery.
Now on a bonded boat you have a zinc anode that is positive and the protected metal (the cathode) is negative. You have the sea water electrolyte and if you have any stray current (and that is likely) you have the power source.
You don’t actually require the power source as there is a natural battery with a continual current flow from the electro-positive zinc anode towards the protected metal or electro-negative cathode.
So in effect you have a sodium hydroxide manufacturing plant (factory) incorporated in your bonded boat. Now this plant runs 365 days per year or until you sacrifice the anode.
If there is an electrical leak on the boat, well the plant ups it production.
Now the sodium hydroxide coats the protected metal and the old wet wood assures an all over electrical connection.
So it is the sodium hydroxide that causes the degradation of the wood surrounding metal on all bonded wooden boats.
Unfortunately, there has to be a current flow between any connected dissimilar metals and zinc is way apart from copper and bronze on the galvanic scale.
So it is obvious you should not use zinc anodes and bonding on a wooden boat.
Sorry, this is not what some of you wanted to hear and it is contrary to popular belief in NZ. Remember, popular belief does not mean it is right.
If you don’t take my free advice, it will not be long before a boat repairer welcomes you to his yard. He may not be as charitable.
Chris McMullen
Auckland. New Zealand.
1/6/2015
Remember – click on the photos to enlarge & read captions 😉
Movie showing gas coming off the bronze cathode The Anode Zinc accelerated with 12 volts – click link below
04-07-2015 Additional reading below on the electrochemical degradation of wood in boats from Chris McMullen.
I have heard that some boat builders/ repairers are of the opinion that bronze and copper stern gear is the cause of damage to the wood in shaft logs.
Their fix is to remove this hardware and replace them with a carbon fibre tube and reinforcing.
Sounds like an answer to a problem that does not exist.
There is nothing wrong with the bronze and copper stern gear, it should last the life of the boat or longer. The problem is the fact, that the hull was bonded and catholically protected with a zinc anode. The unnecessarily protected cathode produced sodium hydroxide (caustic soda) and this chemical destroyed the wood.
Possibly, and likely there has also been an electrical leak and this has accelerated the process. The bonding encourages this scenario by creating a circuit.
Sodium hydroxide is a used to pulp wood chips for making paper!
I ask, why not just renew the wood, replace the existing bronze stern gear and remove all the bonding and anodes?
But no, that is too easy, the unfortunate bronze and copper get the blame and is sold for scrap and the new carbon fiber miracle material comes on the scene.
No one it seems has bothered to look at the ‘Galvanic Series of Metals’.
Carbon is right at the top. It is highly conductive and electro negative. All metals are anodes and sacrificial to carbon.
Zinc is at the bottom electro positive and anodic (sacrificial) to all metals.
So if there is any bonding (intended or otherwise) there will be a current flow from all the metals on the boat through the salt water to the carbon fibre and all the copper and bronze in the boat are anodic (sacrificial) to the carbon.
You now have an even bigger battery sitting in the marina than you had with the bonded bronze, copper and zinc. A floating sodium hydroxide factory! This is working 365 days per year making a chemical that will destroy the wood in your boat.
Don’t believe me? Check out the movie showing my experiment.
You can try it yourself. A piece of carbon tube, an old zinc anode and some salt water electrolyte in a glass or plastic container. Couple the positive of a battery to the zinc and the negative to the carbon. Wait two hours and you will have a thick layer of sodium hydroxide or caustic soda. Exactly what happens on your boat but accelerated.
If you have a wood degradation problem please insist the boat be repaired exactly as it was built and replace the hardware.
Remove all the bonding and the zinc anodes and I am sure you should have no further problems.
However, be aware that a bonded boat will have the dreaded caustic soda round all the bonded metals and that will remain even when the bonding is removed.
Sorry, there is not a lot you can do other than remove the hardware and wash it out.
Sodium hydroxide Na OH is an alkali. It is neutralized by acids (vinegar)
This can be a big job if done properly but less expensive than replacing timber. Stop the producing the chemical is easy and the most practical solution.
If you have already used carbon fibre underwater on a wooden boat (dread the thought) make sure it is not bonded. Do everything possible to isolate the carbon and on no account use zinc anodes.
Bonding and anodes are the biggest risk to our classic wooden boat fleet.
We are only custodians of our heritage boats. Eventually, someone else will take over our roll.
If you want your grand children to enjoy owning a classic wooden yacht, I urge you to remove all anodes and bonding from your boat.
I am an experienced boat builder, not an expert, this is just common sense. I write because I care about your classic wooden boat, unlike your bank balance it can’t be replaced.
waitematawoodys.com #1 for classic wooden boat stories, info, advice & news - updated daily 
In fact if there is anyone who has the knowledge and time to advise me on how to save my beautiful boat it would be hugely appreciated!!!!!
Text or call 022 174 5275. Emerson. Thanks kindly.
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Hi Chris. I am a new boat owner. I purchased M. V. WhiteStar last December. She is 1.5 inch Kauri BridgeDecker. (has been in WW post before) I am such a rookie and see this annode issue onboard. I’m afraid of it getting out of hand and was wondering if you a little spare time you could take a look and advise please? 022 174 5275 Thank Kindly. Emerson
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In defence of electricians, we own a classic A class in Waikawa and have these problems which we asked our electrician for advice and he immediately suggested removing the zinc node, this was some 15 years ago.
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Hello thank you for advice. I own Jack brooke ‘Glennis’ 1954. Presently I am working to gringo her back to glory but iv found soft rib wood next to keel.
How do I tell the difference between rotten wood and degraded wood by electrolysis?
Cheers Raewyn
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As a former trawler built under survey the rudder and propeller will likely be aluminium bronze and the shafting 316 SS. These materials are close on the galvanic scale and live happily together. Bonding them to an anode is unnecessary and the result will certainly be softening of the timber round the Rudder and propeller shaft tube and glands. The protected metal or cathode will produce Sodium Hydroxide (Caustic Soda) and that destroys the lignin in the wood.
If you have a metal erosion problem it could be you have brass or manganese bronze underwater. These metals are an alloy of copper and zinc and can not last in salt water without an anode. You may get away with this on a fibre glass boat but not on a wood boat. You should check out your electrical system. Check out the polarity and look for a positive leak from the ships battery. A positive leak turns any connected underwater metal into an anode. There are other reasons your metal may be eroding. Cavitation,shore power wiring and the wrong battery charger can cause problems.
I hope that helps.
For anyone interested.
I inherited the late Max Carters collection of “Professional Boatbuilder” magazines. In volume 65 June /July 2000 there is an excellent article “Wood Behaving Badly” I had never seen it before. Interesting, the author used the same words that I have used. I quote, “Critics often cite bonding as a Solution looking for a Problem”
Use Anodes and Bonding on a wood boat and you will make Sodium Hydroxide round the protected (bonded) metals. This has to happen. It is a scientific fact.
Less zinc with bonding just means the wood will take longer to soften.
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HI, I have stainless steel shaft and brass rudder and propeller.
I have not tested yet but was told there is an erosion problem and was tasked to bond all under water metals (mainly brass according to owner).
15 meters Wooden boat Keel – former trawler.
I understand I need to get rid of the anodes but I am not sure what to with stainless steel shaft and prop (brass) + rudder (brass).
Any advice (urgent) welcome.
Best regards and Many thanks for information.
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As a caretaker of a boat that has been in the family for 70 years, I’ve been astounded with the amount of misinformation and ignorance in the marine industry.
Over the years, trusting these people while trying to love our boat, almost loved it to death.
With anodes on the boat we got extensive wood damage around the keel bolts. After removing the anodes 5 years ago, the wood around the keel and floors has changed from a greasy mess with lots of white / green growth around the keel bolts, to now dry powdery wood.
The horrid smell inside the boat has almost gone. The days of going boating and getting home to find your clothes and things had that particular boat smell, and your skin would smell and feel greasy are gone.
We had been adding more and more zinc anodes on the advice that this would help the keel fastenings and wood damage around them. The zincs would be gone in a year and we had trouble with paint, both antifouling and waterline enamel sticking to the boat. We also got lots of marine growth in one year.
Five years ago we noticed with horror that the Kauri planks just above and below the waterline were soaking wet, soft, stringy, paint wasn’t sticking to it, and tapping them replied with a dull thud. In desperation the boat was returned to the water with no anodes. So called experts that knew we were doing this advised us we were making a big mistake. People have continued to offer us advice that the boat needs anodes. Last week the boat was up on the hardstand. It has now been 5 years since losing the anodes. We slipped the boat 18 months after, and noticed that the antifouling was unusually clean, and for the first time was still in good condition. Normally it was flaking everywhere, and we always use the same antifouling. There was no signs on the propellor that was telling us anything was wrong.
We slipped again 18 months later, and again the exactly the same result. We used to struggle to get 12 months between antifouling.
The last slip I stretched it to 2 years because of circumstances that I could not control. I was expecting it to be bad underwater, however the big surprise was that the hull was clean and the antifouling was in the best condition I’ve ever seen it. There is no tell tail signs that anything was attacking the propellor or any other metals. However the biggest surprise / relief has been those kauri planks that have been causing us so much concern have now dried and give a reassuring sound back when you tap them.
During this time we have kept the boat in the same marina berth, used the same paints, and have done all the work ourselves in the same way that we always have. Nothing seems to have changed.
There has been no electrical work done on the boat at all.
I cannot think of any thing that is the reason for all the improved results. Maybe it is coincident, but I do not think so. I’m really pointing at removing anode.
One thing that perhaps makes the big difference in our boat is that the problems started 35 years ago when a steel shoe and lots of anodes added to protect this steel. This started the wood deterioration, and we removed the steel shoe approximately 25 years ago. (again against the advice of many in the marine industry) Possibly if the wood damage was not there at this point the anodes by themselves after steel shoe removal may not have been so aggressive.
I’ve been struggling in my mind whether we were doing the right thing. Everyone that I asked advice from told me that it was wrong. This week I came across this website and have read Chris McMullen’s advice. For the first time I know I am doing the right thing.
It may not suit everyones boat, but I recommend everyone take this on board and think about it, and really question any advice that you receive from the marine industry.
While it is a highly complicated subject that is beyond my comprehension, Chris manages to catch the obvious simple basics of the subject.
Don’t just dismiss what Chris is trying to inform us!
Thanks Chris, Cheers Peter
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Hello Mr. Rolf Everson
Thank you for your response to my article on the Electrochemical Damage to wood.
You are right; there are so many experts with different views on this obscure subject.
There are many books and Magazine articles on Marine Corrosion but very few consider damage to the wood.
I have a loan of a most beautiful Classic Wooden Launch and made it my priority to find the cause of this problem.
A search of the Internet was not much help. The same old 1970’s theories (proved to be wrong) have been copied by well meaning people.
Here is some of what I have learned from my research and experience.
Cathodic Protection (anodes) are essential on steel structures in salt water.
Copper, Aluminium Bronze and 316 Stainless Steel require no anodic protection. They are expensive materials and their corrosion resistance is the reason they were used when the boat was built.
Brass, Manganese bronze and 304 Stainless Steel should not be used on boats. Anodes may help prevent corrosion of these non marine metals but with side effects. (See below)
Now back to your ship.
You have removed the bonding and the Anode. That is all good.
You have retained an Anode on the propeller shaft. Why?
Being a commercial vessel the shaft will be 316 Stainless Steel and you say the propeller is Aluminium bronze. These metals are so close on the Galvanic scale they will live happily together. You will not harm the metals by using an anode but (in time) you will destroy the shaft log. Just like the timber round your Rudder Gland. If you like to recall what I wrote earlier. I quote from my 1960’s Boatbuilding Night School Teacher. “Never use connected dissimilar metals underwater and never use brass!” What could be more dissimilar than bronze and Zinc. Please look up the Galvanic Scale.
Please consider this.
You have a Bonze propeller and a Zinc Anode on a Stainless shaft. The same most likely runs in a Cutlass rubber bearing. At the forward end there is the stuffing box with a White Metal Bearing.
Now see the similarity to your rudder gland that was bonded to the Zinc Anode. The Stainless steel shaft (although not as good a conductor) is the bonding wire in this case.
The (unnecessarily) protected Bronze Stuffing Gland will produce Hydrogen Gas in the salt water and the Chemical By-product is Sodium Hydroxide ( Caustic Soda) and that is the white residue that destroys the surrounding wood.
I have assumed your shaft is isolated (electrically) from the Engine. If it is connected and you have corrosion and timber degradation it will be due to an electrical problem. Read my article on WW’s on this subject.
Now, let’s consider your steel rudder.
The same thing applies as with the stern gear. In a perfect world you should not use steel underwater on a wooden boat. Steel requires an Anode as it is an alloy that contains a small amount of Carbon and other impurities. It, like brass eats it’s self in salt water. Note where Carbon is on the Galvanic scale. Almost every metal is anodic to Carbon.
By using an anode you will protect the steel but the timber round the rudder gland will degrade in time. Somehow, isolate electrically the rudder from the bronze gland if possible and this will allow you to use anodes. Whatever, use only one and make it small.
If you would like to try and understand this complex subject, without getting too technical. I recommend you do some experiments like I have (see WW’S) and more to come. They are easy to do and seeing is certainly believing.
For a graphic example of Electrochemical Damage to fastenings on a wooden boat check out “ Tips from a Shipwright”. https://www.youtube.com/watch?v=ox93snKVuaM
You can even see the offending anode about one minute into the short program.
If you own a Wooden Boat, I recommend you watch this excellent series.
Further, please read the links I provided on WW’s
I hope that helps. I am not a consultant but happy to share the results of my research, providing I continue to get some interesting feedback. Thanks for writing.
Chris
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Post from Rolfe Everson
I own a 46ft tuna boat. It is constructed of double cross planking and
is used as a commercial boat. Although I am not sure what type of wood
has been used it is but it remains in very good order. The boat is 31
years old and I have owned it for 10 years. I recently had MNZ go
through the boat as an audit, they noticed something that I had known
for sometime, the timber construction around the rudder housing was
disintergrating it was spongy with a dry white crystal type substance
on top of the wood. The timber itself could easily be penetrated with
a screw driver. All of the grounding wires were directly connected to
the rudder housing which is a cast bronze unit with the customary
grease cap and gland. A further solid grounding wire was then put
across to a bolt that went through the hull to a rather large zinc
anode. There are also tear drop anodes on the steel rudder. The prop
shat is 3″ stainless steel with the 35″ propeller made out of
aluminium bronze.
The MNZ auditors claimed that someone must have used the wrong wood in
the construction, (I offered no additional information to them, other
than the nod of the head in agreement, their limited knowledge is a
major concern, but what can you do, they hold all the cards).
While the boat was in no danger of leaking or sinking and no
suggestion on the part of MNZ that something had to be done. I knew I
had to act.
I had asked a number of people concerning galvanic protection and got
not one similar answer. Most people have no idea and will always leave
the status quo as it is in fear of exacerbating the situation.
I found and read your article concerning the bonding and galvanic
action on wooden boats.
I took the boat out of the water and cut out and dried the affected
area around the housing. Cut a new piece of jarrah to act as the top
support for the gland housing, sealed the area of the existing boat
that had been dried and put it all back together. I placed new tear
drop anodes on the steel rudder, coated the stripped rudder face with
lanolin. Placed 3″ collar anode on the prop shaft. And disconnected
all of the bonding wires. The large zinc anode below the hull is now
obsolete, which is good because that was $230 a piece. I don’t have
the equipment to test for leakage but at least I have isolated the
rear components from the rest of the boats electrical system.
The boat comes back out of the water in 6 months for its 4 year survey
so I will reassess it then. What I didn’t want to happen was to be
directed to and how to fix the problem by another party.
I will report back when the audit has been completed and the
components below the waterline been inspected.
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Excellent thanks Chris, I thought this too, as an Engineer (although I’m not electrical). I will go naked and keep an eye on her gear.
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Nathan
Sorry, Your rudder and stock are all Stainless but the rudder gland will be bronze. If you fit an anode you set up a current between the zinc and the bronze in an electrolyte (salt water) The stainless steel will act as the bonding. The bronze is the cathode and the surrounding wood will soften in time. That’s the issue that should concern all owners of our classic boats. The reason I write is because I care and hope to make owners aware. There are a lot of myth,s and wrong information out there. I can’t believe the rubbish I hear from some people in the Marine Industry. BEWARE.
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Hello Nathan
In a perfect world you should stick to copper alloys, an aluminium bronze rudder and shaft will last indefinitely unless subject to stray current corrosion.
The 316 SS rudder should in theory be electrically insulated from the bronze shaft. However , Stainless Steel and bronze are very close on the galvanic scale so you should be fine. Many boats have Aluminium Bronze propellers and 316 shafts and they seem fine without anodes. That is once again, providing there are no electrical faults.
Stainless steel is certainly not the best material underwater and certainly must not be used as fastenings through wood.
To answer your question. NO ANODES EVER ON A WOODEN BOAT!!!
I have been discussing the issue of anodes on wood boats with Glen Bishop in Australia.
Glen designed the Seabis instrument that monitors even very low current DC leaks.
I purchased one for Wirihana. It picked up a problem on the 230 volt generator.
The notes below are part of what I sent to Glen.
I quote
With respect, I beg to differ in the view that anodes don’t cause timber degradation.
I agree Electrical leaks are the main culprit. I have noticed only wooden boats with Anodes have timber (chemical) degradation problems. I am sure the boats I know that don’t have anodes and wood pulping problems will have less than perfect electrical systems.
Here are some of my thoughts about anodes for you to consider. I have written this for you to check my logic.
If you say Zinc Anode protected copper alloys in salt water do not make Sodium Hydroxide (Caustic Soda) you are surely flying in the face of a basic chemistry fact.
A positive and a negative electrode in saltwater produces hydrogen round the (negative) cathode; the by-product is Sodium Hydroxide or Caustic Soda.
The copper and Zinc are a battery in salt water so you don’t require DC current. Add DC current with the right polarity and it accelerates the rate the Sodium Hydroxide is formed. Sodium Hydroxide is used for pulping wood in the paper industry.!!!
In my opinion the anode protected underwater hardware (on a wooden boat) produces a very small amount of Hydrogen that is trapped round the (Cathode) in the salt laden wood for maybe ten years or more, forever increasing, as the zinc anodes are replaced by the boat loving owner each year. Any Sodium Hydroxide on the outside is washed off so the chemical is invisible until too late.
I fail to see the point in spending money on Aluminium Bronze and pure copper that lasts for years in salt water if you have to use anodes. Feeds a whole new industry, I guess, Advertising pay’s.
A British company called MG Duffs first promoted anodes to protect stern gear on wooden yachts. He wrote a convincing article in Yachting word Magazine March 1957. Now 58 years later the same company write a very good article warning people about the use of anodes on wooden vessels. As you likely know MG Duffs are a major company dealing with Cathodic protection of ships and oil rigs. It is in my opinion and theirs that anodes should only be used on “Steel vessels” and underwater structures. please read the link. see http://www.mgduff.co.uk/pdfs/Electro-chemical_decay_in_wood_vessels.pdf
These opinions are based on my experience and research. I hope it helps.
E&OE
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Hi Chris
I have had to replace the rudder with a new one of SS 316 as the bronze one had alignment issues that I was not happy with.. So now I have everything Bronze Except the rudder and its shaft. Should I use an anode, and if so, where?
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Hi Rick
Yes ,and there are many very good boat builders in New Zealand who promote the use of anodes and bonding. Unless they have been involved in repair of boats they may never know or care. If this is not a problem in Denmark, why in NZ does the Sodium Hydroxide forms round the metal on bonded Wooden Boats and destroys the wood. I don’t believe salinity of the water would make much difference.
I have investigated the cause by experiment and provided links from Wood Chemists. I am sure I am on the right track.
Without a doubt leaking DC current plays a major role but without a bonding circuit it would be harmless.
Proven by the fact Kauri Boats lasted almost indefinitely until modern people introduced Cathodic protection of Copper and Bronze that are known to be salt water resistant.
Again, any metal will corrode if subject to a positive Electrical leak. Anodes will not help in this case.
A negative leak causes the wood round the protected metal to degrade.
I believe I have proven this beyond doubt.
I have the Seabis. I will report on it shortly.
Chris
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I meant to say Danish boatbuilder and yes on Bornholm Island in the entrance to the Baltic the rocks had bright green slime – same as in Rio but not so bad.
On 10/09/2015 8:45 p.m., waitematawoodys.com – the #1 classic wooden bo
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Semi tropical waters exact their toll on the boat’s fabric I guess. Remember how ships with iron fastenings go well until they come south (mind you it was different iron back then maybe). The iron fastened Brit yachts last well at home but down here….. BTW the Baltic looked quite toxic last time
i saw it,
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Hi Chris Thanks for the reply-I have been away also and in my travels I met a very experienced and traditional wooden boat builder. I showed him your article on electromechanical damage to wood and he had no idea that zincs can damage wood and has never seen any damage in the Baltic. So… I guess the Baltic is less saline and therefore less conductive and that the timbers used are not damaged?? Anyway the Seabis device sounds great. I look forward to your thoughts on it. Thanks Rick
On 31/08/2015 12:37 p.m., waitematawoodys.com – the #1 classic wooden bo
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Hello again Rick
You wrote. As suggested in your article above I have ..”Connected a voltage meter between the positive on your battery and any bolt or fastening in your hull. You may get a surprise. It may be 12 volts but high resistance so you can’t light a bulb but enough to cause corrosion.” You then asked the question . There is indeed 12 volts. Why is this and what do I do if anything to eliminate it?
I answered to the best of my knowledge but it troubled me that these leaks are hard to find and you really have no idea how much damage the same is doing until you see the damage to metal or wood. I searched the net to try and find an answer. By chance I came on this site. http://www.seabis.com/index.html. This is an South Australian company run by Glen Bishop. It appears that Glen has come to similar conclusions as mine but further he has developed an instrument that gauges the current rather than the voltage. Now, I knew no way of doing that so this is a breakthrough. I understand his instrument will tell whether the (voltage) current is damaging or not. I have written and talked to Glen briefly and he seems to know what he is about. Below is what he wrote to me answering my questions.
My concern is not so much the metal protection but the Caustic Soda byproduct that will continue to form while there is a negative leak from any source.
I am not sure whether Glen is fully aware of the electrochemical wood boat damage. Maybe there are not as many wooden boats in Australia. I know that is the case in Queensland.
Glen wrote.
Hi Chris,
The simple part first: Earth leakage lights measure voltage, not current
and are far too insensitive by 1000:1.
The equally simple part: electrolysis is at least 10,000:1 more destructive
than any other form of corrosion so must be eliminated first.
When electrolysis is eliminated then other test have validity.
“.Proved to me any metal can become an anode if connected to the positive
side of a battery.” I agree, never connect an anode to any battery
potential.
The tests you are carrying out are largely valid. The bottom line is that
electrolysis must be proven to be eliminated before all else. Electrolysis
is caused by current, not voltage. SeaBis measures current. Every other
devices, World-wide measures voltage. That is why we provide a 5 year money
back guarantee when no one else does.
I am going to order a Seabis to see if it helps
I will keep you posted.
Chris McMullen
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Thanks Chris. You may think you are not an expert but the advice you give and the knowledge you have is greatly appreciated. Rick
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Hello Rick
As you know my concern is saving the wood from Electrochemical damage.
If the shaft and rudder are electrically insulated from their glands and drive gear, in theory you can use a Zinc anode but it will waste away very quickly. I would just leave your rudder as it is but give it a good coat of paint. Unfortunately, your shaft may be Tobin Bronze (a High Tensile brass.) However, it sounds like it is old and could be something better. I suggest you try nothing for a while and watch.
A metallurgist will always recommend an anode and without a doubt it will protect the metal unless there is a positive electrical leak. A negative Electrical Leak will protect the brass like an Anode but can seriously degrade the surrounding wet timber. That’s how I see it. Hope that helps.
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Hello Rick. I have been away and not checked WW’s for about a month.
To try and answer your question. Very few old boats will have an electrical system that does not have stray current. Please try and eliminate it. Voltage in the wet wood is not good. If you don’t have a problem just live with it. In affect you have bonded the subject fastening through the meter. Remove it (the voltage meter) and it is no longer bonded.
It is the bonding and connecting dissimilar metals that causes the trouble. An electrical leakage into a bonded wooden boat can work two ways. A Positive leak can turn ALL the bonded hardware into an Anode and corrode even marine metals. A negative leak protects all the bonded hardware. It then becomes the Cathode and produces the Sodium Hydroxide that is so damaging to the wood. The same effect as using an anode but accelerated. You will complicate your life using the corrosion meter. Just isolate the hardware from the electrics and don’t connect dissimilar metals including anodes on a wooden boat.
I am not an expert, I am embarrassed to answer your question as if I am. This is just common sense. Prior to about 1970 boats never had timber electrochemical degradation problems. The boat builder’s never had Corrosion meters, used anodes or bonding. The boats sure did have electrical leaks. Keep it simple and enjoy your boat.
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I’m in the process of putting the running gear back in my resto job. The prop is AB2 and the shaft is the original bronze with no corrosion. Rudder ‘socket/shoe’ is bronze, as are all bolts etc. The rudder has much pinking etc in localised areas suggesting Mang Bronze. I am going to do no anodes on prop and shaft but have an anode on the rudder. Insulated prop shaft and insulated tiller connection. Is anyone going to advise me against no anode on the shaft? No anode on shaft beforehand and still had the perfectly functional 1930 bronze prop.
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Thanks Chris for making this major threat to the NZ wooden boat fleet public. I endorse your comments 100% as evident by ‘Mistral’ a 30 year old “modern” wooden classic. Fastenings are silicon bronze and copper, both positive and negative battery poles are isolated when we are not on board. She has no zincs, no bonding and no corrosion or deterioration of wood or metal.
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Hi Chris
As suggested in your article above I have ..”Connected a voltage meter between the positive on your battery and any bolt or fastening in your hull. You may get a surprise. It may be 12 volts but high resistance so you can’t light a bulb but enough to cause corrosion.”
There is indeed 12 volts. Why is this and what do I do if anything to eliminate it?
The boat now has no anode and no bonding, s/s prop shaft, bronze and s/s feathering prop. negative earth engine, insulated prop shaft to gearbox coupling (and it has a different voltage on each side measured with silver half cell so I know the coupling is non-conductive)
Using the silver half cell the engine has .600-.700v, (presumably due to the heat exchanger anode?) the prop shaft reads 150 -.200 and the bronze stuffing box .300. The keel bolts are monel with a lead keel and reading on them is .300-.400. All the voltages are the same with batteries connected or not.
Thanks
Rick
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Wrong choice of words, thus my email.
Thank you kindly.
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Alan does mind when people ‘copy’ material from ww 😦
Pam thanks for your email asking – I have sent you a word file of the above, which I’m sure Chris is more than happy for me to share. 🙂
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Thank you Chris for putting this in a language I and I’m sure others can now understand.
I’m sure Alan shan’t mind- we shall copy this and pass it on to the woodies that come by and use the slip.
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Updated reading added, , on the topic ex Chris McMullen
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More compelling data & photos added. AH
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I now own my third wooden classic and sadly all three have suffered/are suffering to some extent from obvious signs of electrochemical attack to the backbone timbers. Luckily, Mapuna is in the very early stages with some local softening around a couple of keel bolts. The affected bolts are the closest ones to bronze stern bearing which is bonded by wire to the one and only zinc block mounted on the bronze rudder shoe (this is not surprising in light of reading all of the above).
Thanks Chris for this informative piece and others who have shared their similar thoughts. Needless to say, the zinc anode and all bonding will be coming off Mapuna at the next haulout!
She still has a bronze propeller shaft and all the underwater metal is marine bronze so it would seem electrolysis is unlikely without zinc protection, but further wood degradation is assured if I keep the status quo.
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Yep, when I bought the Woollacot ketch Gloaming in ’91, the survey revealed an area of fizzy looking fastenings on one area stb in way of the main mast step. All the signs -green verdigris and unhappy looking timber/paint around. I sought an opinion from an expert (old and experienced) who came well recommended. He had a look on board and then looked underneath. He zeroed in on a copper plate that ostensibly was an earth for the VHF. “Disconnect it now!” was his recommendation and he also wanted the zincs taken off. The latter went against the grain of all experience- who didn’t replace the zincs every year? Oh he also said to put vinegar on the fizzy fastenings -there weren’t many. Wasn’t a problem after that. Mind you, I hardly ever use the radio unlike some people who seem to be on the air all day.
In passing, my brother in law was in the RNZN and they used to read hull potentials every watch on the frigate HMNZS Waikato. There was an impressive control panel with meters that read them and a potentiometer that adjusted something or other. I guess there are lots of stray currents on those things. Like there are in marinas with all the boats plugged into shore power systems.
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John
You made some very good points in your posting and were generous with your comments on my article. Thank you. I agree with most of what you said and I admit to getting a bit carried away with issues that were not related to what you wrote. I have strong opinions on this subject due to the amount of carnage I get to see. I have met some really nice people who have been badly hurt by owning a wooden boat. Interesting, I knew the late John Hager who had the St Clair rebuilt.
I spent a lot of time trying to encourage my friend not to bond and put anodes on the St Clair. Whether it was done by John or a later owner I don’t know but it so disappointing to hear. It happens all the time. People ask my opinion and the talk to a friend who knows more. Wonder why I bother.
Thanks John
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Hello John
With respect, what you are suggesting is what a Metallurgist would recommend and this theory has been aired in many books and magazines for some years. It was even written in the specifications of Wood Yachts designed by The renowned Sparkman &Stephens. However, it has proven to be very wrong and many yacht surveyors are now recommending no anodes or bonding on wooden boats.( see my references.) Wood chemists have a different view to a Metallurgist.
I quote ” .
“It is often overlooked, however, that the alkaline reaction
product at the cathode, in time, can result
in strength loss to the adjacent wood. The end
result is that, although the metal does not corrode,
the wood surrounding the fastener may
fail. The vessel can literally “stew in its own
juices” until the wood disintegrates near the
protected metal. It probably requires more
than 10 years to produce conditions that can
cause much loss in strength to the wood”,
You can read the rest on the link http://www.fpl.fs.fed.us/documnts/fplrp/fplrp229.pdf
In my view any voltage in the wood is bad and dissimilar metals including zinc should be avoided. Your suggestion of not over zincing simply means a slower death to the boat, but death all the same. In my article I mentioned the use of Tobin Bronze, Manganese bronze and brass underwater. If you use these metals there will be corrosion but surely that is not a good enough reason to use Cathodic protection.
Let these metals corrode at their natural rate and replace them as required with proper marine metal. Why, get technical and use a Silver/Silver Chloride ref on your multimeter. I too have a dedicated hull potential meter purchased in 1971 and I use it extensively. But it is hardly necessary on a simple Wooden boat.
We in NZ are way behind the times and my reason for writing was to try and stop the rot (so to say) Unfortunately, Boating People follow what is written in out dated books and Magazines. Further, the out of date theory is encouraged by some marine electricians and the shop assistants in ship Chandlers, Both have motivation other than the good of your boat to encourage this practise.
Last but not least. Poor wiring is the main course of metal corrosion on boats attend to that and you will eliminate many problems. Cathodic protection will not save metal subject to a flawed Electrical system. Please remember, “just because every one does it does not mean it is right.” In my view Anodes have their place but not on a wood boat. It is a free country follow the sheep and part with your hard earned cash to buy anodes if you choose. Huge business! for the Chandlers and more for the boat repair yards so it can’t be all bad.
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Great article. We also had the same problem with St Clair. When we brought her she failed survey because of this alkaline wood delignification issue caused by a big block of zinc and there were literally birds nests of damp wood pulp around each of the keel bolts hidden under the rear sealed cockpit floor. Colin Brown and his apprentice Josh fixed this for us. It’s interesting that it seems to occur below the water line starting from the inside of the boat. Its like the caustic salts migrate upwards towards the surface. The deeper caustic salts seem to be diluted by the sea water slowly diffusing through the wood. In our case the damage was confined to the top 3 inches or so, with no damage lower down .So we fixed the problem and before launching put a much smaller zinc on. However after 2 or 3 months the paint around an on the keel bolts started to blister and pouring vinegar on would cause them to fizz as the alkali is neutralised. Eventually I figured that once the bronze fittings are painted and the prop and shaft sealed with prop speed there is minimal exposed metal that needs protecting so the zinc anode can be either be dispensed with or has to be really really small. If you are really concerned about the metal fittings then you need to balance the anode anode size and measure the voltage of your rudder stock and prop shaft from inside the boat with reference to a silver/ silver chloride reference electrode once the boat is in the water. See
http://www.reliabilitydirectstore.com/RDI-CRE-Corrosion-Reference-Electrode-p/rdi-cre.htm
The difference should be between 0.5volts and 0.75volts. Any more than 0.75 volts you will risk burning the wood. Over a span of months as more paint protection flakes off and the anode shrinks the voltage difference will drop. Eventually the voltage will drop below 0.5 volts but at least the wood will be ok.
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Very happy that we have Chris’s well researched article out there now, it backs up what I have observed in practice over the last forty years. Two of our boats have no anodes and no sign of issues. I would add that even on multi skin glassed timber boats I preach this line of thinking after observing well meaning engineers bonding a Townson yacht in the 70’s and all the problems that quickly caused. Whilst promoting a new type of anode some years ago my ideas that the kiwi fleet was largely over compensating and causing issues were confirmed when testing a large variety of craft with a cell for potential . The close proximity of craft on marinas and connected to shore power often without proper galvanic isolators also has a bearing on this growing problem.. At least the word is getting out there slowly, I still prefer bronze skin fittings and valves for their full flow benefits – however some of the locally made glass reinforced nylon ones are getting better and will pass survey now. Just thank your lucky stars your hulls aren’t carbon, we are seeing some wild action in craft built this way already!
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Harold Saunders trawler built Paremata 1962.
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Thankyou Chris.
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I had the unfortunate experience of exactly this problem. Solved $20k later! Great article and a topic that all wooden boat owners should be aware of.
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good to hear that i have been preaching from the correct book 🙂 now if i could only convince certain classic owners that the cracking noise they can hear on their boats is shrimp and not electrolysis… 🙂
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Like Baden, I have always been fascinated with the process in which boatbuilding knowledge, skills and lore are transmitted from master boat builder to apprentice, teacher to student, in an unbroken line going back generations, millennia, in fact Here’s a great example of that process.
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A great piece of information
Well done Chris
It takes a lot to get me to reply to an article but this information is a MUST for all wooden boat Owners.
I am also impressed that Chris remembered information that he learnt at Night School many years ago
Having been involved with teaching Apprentices for many many years, yes I did tutor Chris one year, it is great to hear that it was worthwhile.
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Not the Stewart Orion, by chance?
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This is a radical article. Chris, I can totally believe you are right. I am the empirical type, so I’ll test your thesis with my own observation and experiment! I have just bought a wooden boat again, it must be about 18 years since we sold the Amazon (1885). Now we have the Orion (1962) and high on my to-do list is to identify, repair, and prevent further damage from electrolysis and stray currents. It’s a mucky old job wriggling about under the boards with a torch and a multimeter, but it has to be done. Thanks too for the names of suppliers.
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I’m very interested in this, am keen to do the best I can for Lucinda when she goes back in…. I Dug out two craters of it around two keel bolts, luckily it was not tooo deep. Keen as a kipper to do it right.
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Brilliant material.
Chris’ advice halted the rampant decay on ROMANCE II.
I was able to prise great lumps of wood fibre off the shaft log with my fingers when we bought her.
The huge zinc was thrown away.
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Being involved on a daily basis in the marine electrical trade I can only agree with everything Chris says. My observations are that at least 90% of our classic wooden boats are now affected by electrochemical wood degradation caused by indiscriminate use of zinc anodes.
The trade itself, including boatbuilders, electricians and suppliers are all to blame. Perhaps mostly culpable are the electricians who in many cases seem incapable of understanding the basic principles of what is going on. Its not rocket science! Its well scientifically documented. I have even observed some electricians adding even more zinc to damaged vessels in the belief more zinc would cure the problem.
With the advent of non metallic through hull fittings and highly insulating products such as Propspeed on shafts and propellors, the amount of metal left exposed underwater is negligible anyway.
Be aware that inadvertent bonding can happen via such paths as the copper ‘worm’ strip on the bottom of a keel. On my own boat this had bonded several of the keel bolts due to the shallow countersinking of the bolts into the keel. This copper strip was attached to the bronze shoe supporting the rudder. And guess where a large chunk of zinc was mounted? Correct. On the shoe.
Net result, a very expensive replacement of four keel floors. The solution was to of course to remove the zinc altogether but also to replace the copper protective channel with a fibreglass one. (no one will know you have fibreglass on your classic since it will be well hidden underwater!)
Well done WW for bringing this very serious issue to the forefront.
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What a great article. Just an observation but when I have crawled around boats with this earth bonding system they have always had this problem…Worryingly I have also noted quite significant degradation of the copper fastenings as well….
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Great article Chris, I hope every one reads it
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Fantastic! I think I know that keel….
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