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?
It's a long keel grp. Anode is wired to Mercedes engine which doesn't have any internal anodesIs this on a steel or iron keel? If so why are you fitting an anode at all?
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.It's a long keel grp. Anode is wired to Mercedes engine which doesn't have any internal anodes
Ok, thanks. I'll check what's going on.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.
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.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.
Raises the question as to whether operational cathodic protection works against crevice corrosion in stainless.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.
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 plateRaises 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?
It isn't clear to me that that would be the case.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
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.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
That's what mine look like!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.View attachment 188609