Anodes (zincs) advice

[MrFish]

I have a 11.3m timber hulled keel boat with isolated bronze thru-hulls, a prop shaft with a zinc anode fitted, and a Prop shaft bracket (bronze) which was connected to a large zinc anode by a metal strap. On the inside of the hull, the rudder tube (bronze), the stern gland, and the engine are wired together to the propshaft bracket bolts.
My problem is this - the timber around the prop shaft bracket has gone "dozzy" - is quite alkaline (froths when white vinegar is poured on) and is basically rotten.
This has been fixed and replaced, but how do I stop it from happening again?
How do I connect the P bracket to a zinc anode?
How do I connect the rudder tube and stern gland metal to an anode?
What else should be bonded to an anode?
How big should this anode be - surface area and weight? (I am advised you can have too much zinc on a timber boat!)
All ideas greatfully recieved

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[mickshep]

The problems with timber degredation in relation to anodes is well known and has appeared on the forums several times, IMHO your better off without them. All my old wooden boat's fittings were also bronze. 50 yrs on they are fine, as are the galved iron floors, only rot on her was caused by ex owner fitting anodes. even if you had to replace a stern tube every 10 years it would be a lot easier than replacing a rotten stern post. Cheers, Mike

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[Gordonmc]

I'll go with Mickshep's advice.

I have one anode wired direct to the engine and a small one bolted directly on the rudder pintle which is galvanised mild steel with bronze bushes. All through-hull fittings are bronze and are not wired.

In the winter I had to replace the propshaft because of crevice corrosion, but that's acceptable after 40 odd years. The prop itself is fine. I have seen the effects of electrolysis on deadwood and it isn't pretty.

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[MrFish]

Thanks for your reply.
I have a zinc on the prop shaft - should I fit a small zinc anode onto the prop shaft support bracket?
I could do this by drilling through the vertical and bolting on a disc as used on metal rudders on launches etc.
Or should I not have any anodes on the propshaft bracket or rudder tube at all?
I suspect my problem may have been to much zinc - do you have any ideas on this aspect?
I'm a bit worried about having no anodes at all, but the consequences of that may be less dramatic than the dozzy rot I have just repaired.
All ideas welcome at this stage

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[DMGibson]

Bronze lives very happily underwater by itself, so doesn't need any protection.

If you have only bronze underwater, then you certainly don't need anodes.

If the propellor shaft is made of stainless steel, then you may need to protect it from the propellor, in which case it just needs a shaft anode clamped round the shaft itself.

I have also found out the hard (and nearly dangerous way) about the effect of anodes on wood, when we found that a hull anode (with bonding wires to other fittings) was causing the timber round the through bolts to erode in just the same way as on yours - luckily we found it in time, before the bolts pulled through the timber leaving two holes in the planking if the anode had fallen off.

Anodes might work for the boys in plastic, (although I often wonder about the long-term effect of the electrolytic products on GRP), but use with extreme caution on wooden hulls.

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[VicS]

I confess that cathodic protection is a subject I should know an awful lot more about than I actually do as a result of having had direct access to real experts during my working life. However I think the problems you are experiencing are the result of the large distance between zinc and bronze in the "Galvanic series" and the effect that the products of the electrolysis that results is having on the wood in the vicinity of the bronze.

Even without looking up the voltages concerned I can say that the difference between bronze and zinc is much greater than between zinc and iron or steel so the driving force for the electrochemical reactions is much greater. The positively charged ions in the water (sodium, calcium and magneium) will concentrate towards the cathode, namely the bronze, and the pH will rise. Calcium and magnesium will form insoluble carbonate deposits by reaction with bicarbonate in the water, hence effervescence with dilute acid. We can consider that the sodium forms caustic soda by reaction with the water. I am sure you know the effect of even dilute caustic soda on wood over a long time...well youv'e seen it with your own eyes. Although the same theoretically happens around cathodically protected iron or steel it will be much much less pronounced

So yes you have far to much zinc around and connected to the bronze bits.

I would agree with the suggestion that all you need is a small shaft anode to protect that. Although even that might not be necessary. Theoretically stainless steel is protected by its own oxide coating. It should therefore be kept in an oxidising environment. Making a metal the cathode in an electrochemical cell actually places it in a reducing environment. (I believe stainless steel can be protected by anodic protection when necessary. Something that the bronze propeller will actually achieve.) Thinking about it I wonder whether fitting an anode on a stainless shaft may actually increase the risk of crevice corrosion.

I think I would remove all the anodes and the bonding between the bronze bits and carefully monitor the situation before deciding if a shaft anode is desirable or not. I assume your bronze bits really are truly bronze and not brass masquerading as bronze.

Reserve cathodic protection for iron and steel.

I wish I had kept in contact with some of my former colleagues who were real experts on marine corrosion but alas I have not.

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[MrFish]

Thanks for your reply. It all real;ly makes sence to me (now!) as everything you describe fits my situation pretty well exactly.
What I still don't understand properly is what effect the engine and gearbox have on the need for zinc anodes. Both were/are wired to the main zinc before repairs started. Why?
The engine cooling system has pencil anodes in it, so presumably it is protected that way. Why then bond it to another anode on the hull?

I'm convinced that a shaft anode is all I need for the rudder, prop, prop shaft and prop shaft support bracket as all are bronze except the shaft itself.
I just need to clarify the needs of protecting the engine, and I'll be all sorted.
Thanks for your advice, it's really very helpfull

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[vyv_cox]

Your question is very valid, and one that many users and yards clearly do not understand. In order for an anode to protect an engine and gearbox, electrons need to pass from the engine/gearbox down the wire to the anode, fair enough, but then through the seawater outside the boat, turn a right-angle bend into the seacock, up the hose, past the rubber impeller and so into the engine. They will not do it!

There have been thousands of hours of research and tens of thousands of words in reports to show that it does not happen. Electrons in seawater flow pretty much line-of-sight and, for structural steel and pipework, generally six pipe diameters. This is largely to do with the conductivity and resistance of seawater which, although better than fresh water, is far from being equivalent to a copper wire. It is for these reasons that impressed current protection systems are installed on ships, structures, and even large yachts.

Your final sentence is, IMHO, perfectly correct and is exactly what I have on my GRP boat.

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[VicS]

There's no point in wiring the engine to anodes on the outside of the boat, they are fartoo remote to have any effect. Protection of the engine cannot be improved over ensuring that its own internal anodes are replaced as necessary. (Other than converting to fresh water cooling)

You do refer to the rudder. If this is steel then you shoud fit anodes to it to protect it, one on each side.

You obviously feel that a shaft anode is desirable. While acknowledging that they are commonly fitted I would point to the many boats with bronze props on stainless shafts without one which suffer no problems as a consequence. The boat I sail regularly, a 26 or 27 year old Westerly, has no shaft anode and has suffered no poblems.

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[MrFish]

Thanks for all the advice, experience and opinions. There is quite a consencsus on what I should be doing, so I'll get on with it this weekend.
Thanks to everyone!

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