Ardenfour
Active member
I want to fit a larger anode on the keel but the existing studs are too short, and due for resealing anyway. Should I replace with mild steel or stainless?
Anode mounting studs
- Thread starter Ardenfour
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neil_s
Well-known member
In theory, mild steel is OK - protected by the anode. In practice, I find they rust inside the boat and get messy. I use stainless, now.
superheat6k
Well-known member
Stainless is prone to crevice corrosion where it passes through the hull. Using mild steel is OK because the first thing the anode protects is its mounting studs. Normally they are supplied galvanised.
Tranona
Well-known member
Is this on a steel or iron keel? If so why are you fitting an anode at all?
Stemar
Well-known member
This should do the job, assuming a GRP or wooden hull
M G Duff M10B Anode Fixing Stud Assembly | Force 4 Chandlery
M G Duff M10B Anode Fixing Stud Assembly | Force 4 Chandlery
Ardenfour
Active member
It's a long keel grp. Anode is wired to Mercedes engine which doesn't have any internal anodes
Tranona
Well-known member
OK - but it won't do anything to "protect" the engine as the anode is in the external seawater, not in the engine cooling system. The engine probably does not need one anyway otherwise the maker would have fitted one. They are usually in the heat exchanger of a freshwater cooled engine or in the block of a seawater cooled type.
However it may well be connected via the engine and gearbox to the prop shaft to protect the propeller from the shaft. If this is the case you should check for continuity between the anode and the propeller. Presumably if you want a bigger anode - what type is it? - it is doing something so it is best to check out exactly what it is protecting. If it is the propeller then it needs to be sited close to it, less than 1m if possible.
Ardenfour
Active member
Ok, thanks. I'll check what's going on.
ducked
Active member
Apparently most transmission/prop shaft connections cant achieve the requisite sub-ohm resistance for effective prop protection via this route, unless you use fancy silver slip ring shaft brushes, so its usually better to mount the anode on the shaft near the prop, or on the prop itself.
Reference Cell Testing; Know Thy Corrosion Protection Level – Editorial: Old vs. New | Steve D'Antonio Marine Consulting
ducked
Active member
Raises the question as to whether operational cathodic protection works against crevice corrosion in stainless.
I would have thought it should, as in I can't offhand see why it wouldn't, but am far from knowing this for a fact.
Maybe it isn't wet enough where crevice corrosion happens for cathodic protection to work?
But then it shouldn't corrode either?
Maybe the hull coverage prevents a circuit forming, so electrons could travel through the fastner, but ions cant travel through the water between the metal surfaces?
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VicS
Well-known member
Crevice corrosion should be prevented by ensuring that the stud is well sealed to the hull by applying a liberal amount of sealant to the back of the welded on plate
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ducked
Active member
It isn't clear to me that that would be the case.
As per the above, I'd think (ignorantly and aloud) that if water ingress is possible, but restricted, so is corrosion.
The restriction might prevent the formation of a cathodic protection circuit and exclude the oxygen which passivates stainless steel, so sealant might promote corrosion.
Sealant would only prevent corrosion if it completely excluded water, a tall order
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Tranona
Well-known member
That is not true - or only partly. If the coupling is solid, as it is on many boats then there is usually a good path which can be checked by measuring the resistance from the propeller to the anode. If there is a flexible coupling it is usually possible to provide a bridge wire across the flexible part. Indeed R&D maker of one of the commonly used couplings include a bridge in the design. Other couplings such as the one on my boat are difficult to get a reliable bridge, so I have used a Duff Electroeliminator on the shaft as in the photo. Clearly you need a fair amount of exposed shaft internally to use that method, and it is commonly used on large powerboats that do have the space.
Not all boats (again like mine) have exposed shafts externally, as yours does so not possible to use a shaft anode. Anodes direct to props are available using a special nut and a cone anode, but these are small in mass and often have a short life but can be complemented by also wiring to hull anode, again as I have done.
It is easy to overthink this subject. For most small simple sailing boats there are tried and tested methods of providing protection, which in most cases with conventional shaft drive is only the propeller. With a saildrive it is normally only the drive housing that needs protection and there is a dedicated anode on the housing for this. Standard propellers are aluminium and isolated from the drive with a rubber bush so need no protection. Folding and feathering propellers are commonly made of a mixture of bronze and stainless and the makers incorporate anodes where necessary. However like cone anodes on fixed propellers their life can be short so a hull anode is often useful with shaft drive installations. Saildrives can be problematic because there is no way of adding additional anodes, but painting to reduce the exposed surface can reduce the amount of galvanic action.
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dansaskip
Well-known member
I would be more worried about rust corrosion than crevice corrosion after all it is not a stressed item. This is my old anode stud before I replaced it with a SS one.
Ardenfour
Active member
That's what mine look like!
