MOVED: Galvanic corrosion expert required to identify root cause......

[Dorset Dan]

I have been recommended to post this in the PBO forum rather than the motor boat forum (link here: http://www.ybw.com/forums/showthrea...-to-identify-root-cause#4c3fzGSF5hMAYb6M.99):

I won't say too much of the detail on here, but I have surveyed a boat which has some galvanic corrosion issues on the starboardside in particular (affecting prop, prop nut and more importantly p bracket bolt integrity - 2 x sheared bolts found). Port side generally OK.

I have used a surveyor and engineer who both come very well recommended on this forum and they have duly identified the corrosion issue, and recommended in their reports that the root cause of the issue be found and rectified prior to purchase (noted as a possible issue with the electrical bonding). Galvanic corrosion was also noted on starboardside in a previous survey a few years ago, and starboardside prop was changed only 2.5yrs ago (now severely pitted). The engineer tested with a multimeter each prop to hull anode and highest reading was 5.8ohms on starboardside and portside was also over 5ohms.

Whilst it could be a marina / stray current rather than boat specific issue, the fact that it was reported in a previous survey makes me think it is the boat itself......

Any recommendations as to who to use for this to test both the systems on the boat as well as for any stray currents in the water? I am thinking that a marine electrician who is suitably qualified / experienced in galvanic corrosion issues? Or if not a marine electrician, then who (please bear in mind that I am both time poor and incompetent with all things electrical!)? Preferably within sensible travelling distance of Poole, Dorset.

 

[RichardS]

Is there an anode which is physically closer to the prop than the hull anode?

Richard

 

[lw395]

I would suggest taking care with the multimeter measurements.
5ohms is quite a lot.
The fact that both sides are reading in the 5 ohm ballpark suggests to me that the multimeter might be being misled by the anode potential.
It's quite hard to get a stable 5ohm resistive connection using normal wiring techniques unless the wiring is very long and thin.

 

[lw395]

But to answer the question, I don't know of any surveyors who specialise in this area.
Try their professional bodies.
Or maybe speak to MG Duff, the anode people, they're in Chichester.
http://mgduff.co.uk/contact

 

[bbg]

Do you think any surveyor is going to give a guarantee as to the accuracy of his findings?

"Yes, I have found the problem. I am absolutely certain it is XXX. I am 100% sure that the problem will be fixed if you do YYY. And if I am wrong I am prepared to be liable for the consequences."

Not going to happen, so if you proceed to buy the boat, even if you think you have found the problem, you may be on the hook for a boat that continues to have problems.

Unless you can reach agreement with the current owner that you agree the price is £A taking into account the need to fix the problem, but the value of the boat would be lower (£A - some amount you agree) if the problem can't be found and fixed. And you both agree that an agreed amount be held in escrow for say 2 years to see if the problem is fixed. If it is, he gets the money. If it isn't, it goes back to you.

But you are unlikely to get someone to agree to that.

 

[Dorset Dan]

Thanks all for initial responses above. Some answers as follows:

Is there an anode which is physically closer to the prop than the hull anode?

No, I don't think so although the current owner has now had his engineer install shaft anodes

I would suggest taking care with the multimeter measurements.
5ohms is quite a lot.
The fact that both sides are reading in the 5 ohm ballpark suggests to me that the multimeter might be being misled by the anode potential.
It's quite hard to get a stable 5ohm resistive connection using normal wiring techniques unless the wiring is very long and thin.

Not sure on this one, the engineer is a well respected and experienced chap so I have no reason to suspect that he hasn't tested this properly, but equally this is part of the reason for needing a marine electrician.

Do you think any surveyor is going to give a guarantee as to the accuracy of his findings?

"Yes, I have found the problem. I am absolutely certain it is XXX. I am 100% sure that the problem will be fixed if you do YYY. And if I am wrong I am prepared to be liable for the consequences."

Not going to happen, so if you proceed to buy the boat, even if you think you have found the problem, you may be on the hook for a boat that continues to have problems.

Unless you can reach agreement with the current owner that you agree the price is £A taking into account the need to fix the problem, but the value of the boat would be lower (£A - some amount you agree) if the problem can't be found and fixed. And you both agree that an agreed amount be held in escrow for say 2 years to see if the problem is fixed. If it is, he gets the money. If it isn't, it goes back to you.

But you are unlikely to get someone to agree to that.

I am not expecting a surveyor (or marine electrician) to give a guarantee, rather I want someone to methodically work through the possible causes and if faults are found recommend fix(es) which owner would then undertake. If they find nothing, then I accept that I will still be buying the boat at my risk.

Without doing the above further investigative work, I am in the difficult position of having both a surveyor and engineer say that there is a problem with galvanic corrosion and possible electrical bonding fault that needs further investigation and cannot find an appropriate company / person to undertake this further investigation.

I agree that the amount in escrow is a non-starter, hence I am following my surveyor / engineers advice to carry out further investigative work prior to purchase so at least I am equipped to make my own judgement call on the purchase.

 

[RichardS]

Thanks all for initial responses above. Some answers as follows:


No, I don't think so although the current owner has now had his engineer install shaft anodes

That was certainly not helping the problem as I would suspect that the hull anode was always too far away from the prop to afford sufficient protection, especially with a resistance of 5 ohm which is definitely suspect.

Check the resistance between the prop and the new shaft anode. If that is well below 1 ohm and approaching zero then it should be OK. If the prop continues to erode then there is likely to be something amiss with the boat electrics but we would need more info about shore-power chargers etc.

Richard

 

[Topcat47]

One more thing to consider.....my prop and shaft is connected via a strap across the flexible coupling which ensures electrical continuity from the prop to the engine, and thus is connected to the hull anode. If it breaks, the electrical conductivity between the shaft and prop and the hull anode is lost and the prop loses the protection from anode (I don't have room for a prop or shaft anode). I have a friend with a TSDY and the twin shafts are connected via "brushes" on the shaft. These are wired to the nearest hull anode on either side. Again, brush failure or failure of the cable between the anode and the brushes will cause the protection afforded by the hull anode to be lost.

 

[Keith 66]

None of my boats have ever had bonded fittings or stern gear. None have ever had any problems. I have spent my life in the boat trade & seen more trouble with bonded fittings than not.
Way i have always understood it & what i was taught studying boatbuilding, You put two dissimilar metals in an electrolyte (sea water) you have created a battery & the less noble one gets eaten. This is why we fit anodes. And yet we are encouraged to connect everything together inside the boat with wires, including to the engine with a dirty great alternator on it & batteries, mains power etc. No wonder the bits in the water fizz!
My current yacht still has the same anode on her that she was relaunched with 9 years ago, I keep wirebrushing it every year but its still there.

 

[Tranona]

Perhaps you need to get back to basics. The most common corrosion problems are with props and prop nuts when attached to a Stainless steel shaft. If on P brackets then the easiest way of dealing with that is to use shaft anodes just in front of the P brackets. They are close to the prop and the chances of a good bond are very good. If they are not sufficient for example if the prop is large and made of lower quality alloy such as manganese bronze then a hull anode close to the prop and bonded to the shaft preferably with a brush type fitting will extend the life of anodes because of the greater bulk. You can treat each shaft separately and you may well get different life each side. Not a good idea to connect the two sides together.

P brackets do not normally corrode because they are not in contact with other metals unless fastened with Stainless. In that case insulating the bolts with lots of sealer is usually effective. If the problem still persists it could well be poor material and an anode attached to the P bracket may be necessary.

There is a tendency with some MOBOs to bond far too much. There is no need to use any anodes for skin fittings or seacocks and if each shaft is treated in isolation as suggested there is no need for wires running around the boat.

Should also say rudders may need some attention particularly if they are bronze with stainless stocks but you need to know the exact make up of the system before deciding whether anodes are needed or not.

 

[VicS]

None of my boats have ever had bonded fittings or stern gear. None have ever had any problems. I have spent my life in the boat trade & seen more trouble with bonded fittings than not.
Way i have always understood it & what i was taught studying boatbuilding, You put two dissimilar metals in an electrolyte (sea water) you have created a battery & the less noble one gets eaten. This is why we fit anodes. And yet we are encouraged to connect everything together inside the boat with wires, including to the engine with a dirty great alternator on it & batteries, mains power etc. No wonder the bits in the water fizz!
My current yacht still has the same anode on her that she was relaunched with 9 years ago, I keep wirebrushing it every year but its still there.

We are not now encouraged to do this. In fact we are encouraged not to . (See the report on the near loss of the F.V. Random Harvest, where the bonding of skin fittings to the anodes was considered to be a contributing factor)

The anode(s) must of course be boned to the items they are fitted to protect or they will be completely ineffective.

Bonding of the DC negative ( except when the DC system is completely isolated) and other structures is necessary as part of the safety earthing system when shorepower is installed.

If your anode is showing little or no signs of depletion after 9 years it is doing nothing useful and I would suspect that it is not bonded to anything. You might just as well not have it.

 

[thegasmannick]

i would be carefull as it mite be a issue of boat location material`s/ electrical supplie`s 12volt and 240 in or around boat and position of other boats and marina pontoons and loads of other issue`s as well as other thing`s which are obvious but have seen recently marina with a unsuitable earth which caused problem`s with only 1 boat in marina as it turned out this boat was a perfect earth substitute and the boat had galvonic isolator installed and end result moved boat and 1 year later no problems with boat at all

 

[rogerthebodger]

If your anode is showing little or no signs of depletion after 9 years it is doing nothing useful and I would suspect that it is not bonded to anything. You might just as well not have it.

Vic

My anodes also show little depletion after 9 years and as my anodes are bolted to studs welded to the hull I know there is a good connection.

This year I have seen more depletion than in previous years due to damage to the under water paint work when the marina was destroyed in a storm last year.

Last month the bear metal was painted with epoxy tar which will restore the integrity of the paint reducing the depletion to the previous rate.

The main anode depletion I get is on my shaft anodes not my hull anodes.

This year all hull anodes are the same as when initially launched, its only the shaft anodes I replaced due to depletion overs a 4 year period. one was in fact one fitted at initial launch.

I think the OP problem was that the anodes fitted either too far fron the prop or ineffective connection. The shaft anodes now fitted should have solve the problem IMHO of cause.

 

[vyv_cox]

We are not now encouraged to do this. In fact we are encouraged not to . (See the report on the near loss of the F.V. Random Harvest, where the bonding of skin fittings to the anodes was considered to be a contributing factor)

Boats built in USA are commonly fitted with bonded fittings. Several times per year I receive requests for advice on whether it would be better to remove the bonding. It is difficult to understand why they persist with this practice, particularly since bronze underwater fittings are almost universal over there. The only pointers I have managed to find suggest that the bonding is there to assist with the dissipation of lightning strikes, rather than for corrosion protection.

 

[Dorset Dan]

A quick update - I have someone coming to do some testing on the electrical bonding next week, he is a marine engineer and also a qualified electrician so seems like a man for the job. I will report back with his findings.

 

[bignick]

Another possible cause worth considering, especially since you have stated that the corrosion appears to be affecting one shaft-line worse than the other.
Stray electrical currents can wreak havoc and can cause much more rapid attrition than galvanic action between dissimilar metals alone. Get your engineer/electrician to check the wiring from the alternators; one may be causing a problem and lead to a stray current.

/*edit - just found info on a product called "seabis" to assist in tracing these sorts of problems. Perhaps worth looking up?
Google "stray current electrolysis"*/

 

[jon and michie]

@Dorset Dan - Did you manage to find the root cause ?

 

[Dorset Dan]

Yes and no

Yes in that I’ve now owned boat 5 years with no issues

No in that no specific cause was identified, but starboardside bonding was either cleaned up or replaced so I suspect this was the issue

 

[jon and michie]

Yes and no

Yes in that I’ve now owned boat 5 years with no issues

No in that no specific cause was identified, but starboardside bonding was either cleaned up or replaced so I suspect this was the issue

Spot on.
I had issues on my outdrive a few years back with the ring anode dissolving after 3 months - I added an extra anode to the drive and had a good look around the electrics did a bit of tidying up to some of the crimps
and all turned out good.

 

[Keith 66]

We are not now encouraged to do this. In fact we are encouraged not to . (See the report on the near loss of the F.V. Random Harvest, where the bonding of skin fittings to the anodes was considered to be a contributing factor)

The anode(s) must of course be bonded to the items they are fitted to protect or they will be completely ineffective.

So bonding of skin fittings was considered to be a contributory factor in a near sinking?
Yet anodes must of course be bonded to items they are fitted to protect?

These statements contradict each other.

Look in any old wooden boat where brass or non ferrous screws were used, often they will have a white powdery ring around the fastening, often the wood itself will fuzz up especially below the waterline where more salt water is present.
This is electrolytic rot caused by build up of sodium hydroxide ions around the fastening, Obviously it is impractical to bond every single fastening together with wires so why add more problems bonding skin fittings.
Boats suffered from electrolysis long before they were fitted with electrics.