colhel
Well-Known Member
I know we them and boats fall apart without them, but, why will my bits fall off without them? And how does an anode stop it?
Why do we need anodes?
- Thread starter colhel
- Start date
nimbusgb
Well-Known Member
colhel
Well-Known Member
Quote:
Fe → Fe2+ + 2e-
(1)
O2 + 2H2O + 4e- → 4OH-
(2)
2H2O + 2e- → H2 + 2OH-
End Quote
Thanks. That explains it!
Fe → Fe2+ + 2e-
(1)
O2 + 2H2O + 4e- → 4OH-
(2)
2H2O + 2e- → H2 + 2OH-
End Quote
Thanks. That explains it!
Hamma
Well-Known Member
Quote:
Fe → Fe2+ + 2e-
(1)
O2 + 2H2O + 4e- → 4OH-
(2)
2H2O + 2e- → H2 + 2OH-
End Quote
Thanks. That explains it!
If you want it in more fundamental terms than that:
Metals corrode. The corrosion takes place first on the most metal most susceptible to corrosion. If you have no anode this might be your prop or propshaft. If you have an anode it will be the anode.
You can replace the anode!
Nostrodamus
Well-Known Member
Why don't they use bigger or more anodes to save the boat coming out so often?
Tranona
Well-Known Member
'cos, like everything there is always a trade off. Many boats hardly use anodes at all. My old wooden boat the anode lasts for at least 5 years. In the Med, my saildrive anode was still sound after 8 years. Some GRP boats have no anodes at all and have no problems. Yet others are in situations where there are lots of stray currents and anodes last only a short time. In yet other boats the mix of metals is such that there is a greater possibility of galvanic action. So, you plan your anode strategy to suit your particular boat and location.
VicS
Well-Known Member
For a simple explanation of the chemistry involved see http://www.npl.co.uk/upload/pdf/cathodic_protection.pdf
electrosys
Well-Known Member
There's at least one steel boat which travels the seven seas more-or-less continuously which doesn't have any anodes fitted - it's the Wylo II - but then the designer/builder/owner knows what he's doing. Everything below the waterline is made of mild steel, including the prop and propshaft, rudder fittings etc.
The price to be paid for such a strategy is rust streaking in the obvious areas, and haul-outs every few years to either build-back-up the corroded areas (in the case of the prop shaft) or cut-off and replace the rudder fittings as needed.
Not everyone would be happy with employing such a strategy, but it's worked well for Nick Skeates over the last 20-odd years.
BTW - I'm not advocating this approach, merely mentioning it to counter the usually heard "anodes are de reigeur" mantra.
http://groups.yahoo.com/group/Wylo-II/ for more info ...
The price to be paid for such a strategy is rust streaking in the obvious areas, and haul-outs every few years to either build-back-up the corroded areas (in the case of the prop shaft) or cut-off and replace the rudder fittings as needed.
Not everyone would be happy with employing such a strategy, but it's worked well for Nick Skeates over the last 20-odd years.
BTW - I'm not advocating this approach, merely mentioning it to counter the usually heard "anodes are de reigeur" mantra.
http://groups.yahoo.com/group/Wylo-II/ for more info ...
VicS
Well-Known Member
Wylo II on the beach at Paynes boat yard 3 or 4 years ago
whipper_snapper
Well-Known Member
My boat does not have any anodes.
OK, it is falling apart slowly, but I don't think that anodes would help with that.
OK, it is falling apart slowly, but I don't think that anodes would help with that.
Ardenfour
Well-Known Member
Why does my Volvo MD1 not have any anodes? Orm does it and they corroded away and hence I can find 'em? I also have a mild steel rudder bracket and tube underwater dating from 1966 - should I be worried??
Quandary
Well-Known Member
Anodes
My experience has been that the most vulnerable part is the propeller, I have never had a steel boat and only ever had folders which are made up from bronze and stainless steel in continuous contact. Saildrive legs add even more diverse metals to the mix. I suspect that a relatively cheap fixed prop. on a shaft drive might survive for quite a while without this protection but in the distant past I had one folder whose blades went a distinct pink with a dull sound when tapped after the undetected loss of a shaft anode. The next season the blades were soon rattling about on the pivot pins, I got oversize pins made but the rapid wear continued so the prop went for scrap. After that I always had two anodes on the shaft. So, even though we are rarely on shorepower, it would take a lot to convince me that anodes were unnecessary, they are cheap compared to propellers. It is worth having a walk around a boatyard in winter and having a good look at the condition of the propellers.
I have never (to my knowledge) had a corrosion problem with seacocks, on my current boat these are not even wired together and connected to an anode but they are well separated from each other and from any other metal parts.
Today I have a three blade bronze and stainless steel folder on a saildrive with a stainless rope cutter, there are all sorts of claims about isolation of the metals in the VP blurb but I am sufficiently sceptical to be obsessive about replacing the anodes on the prop. and cleaning the heavier ones on the leg. The speed with which the prop anodes degrade convinces me that something is going on.
My experience has been that the most vulnerable part is the propeller, I have never had a steel boat and only ever had folders which are made up from bronze and stainless steel in continuous contact. Saildrive legs add even more diverse metals to the mix. I suspect that a relatively cheap fixed prop. on a shaft drive might survive for quite a while without this protection but in the distant past I had one folder whose blades went a distinct pink with a dull sound when tapped after the undetected loss of a shaft anode. The next season the blades were soon rattling about on the pivot pins, I got oversize pins made but the rapid wear continued so the prop went for scrap. After that I always had two anodes on the shaft. So, even though we are rarely on shorepower, it would take a lot to convince me that anodes were unnecessary, they are cheap compared to propellers. It is worth having a walk around a boatyard in winter and having a good look at the condition of the propellers.
I have never (to my knowledge) had a corrosion problem with seacocks, on my current boat these are not even wired together and connected to an anode but they are well separated from each other and from any other metal parts.
Today I have a three blade bronze and stainless steel folder on a saildrive with a stainless rope cutter, there are all sorts of claims about isolation of the metals in the VP blurb but I am sufficiently sceptical to be obsessive about replacing the anodes on the prop. and cleaning the heavier ones on the leg. The speed with which the prop anodes degrade convinces me that something is going on.
VicS
Well-Known Member
If underwater bits and pieces are made of a single corrosion resistant material and not connected to each other or to other bibs and bobs then they should not suffer from corrosion.
A bronze , gunmetal or DZR brass fitting in isolation should be fine.
Manganese bronze props on their own are fine.
The boat I used to crew is now 35 years old and has all the original skin fittings and seacocks and the original prop. All still appear to be fine despite the fact that there are no anodes at all.
Problems start once metals are mixed. Folding props for example are likely to have stainless steel in their construction.
Bonding skin fittings to other bits and pieces can be a recipe for disaster. A while back a forumite had his mobo sink on its mooring. The cause seems to have been electrolysis of a skin fitting which had ( for reasons unknown) been connected to a bilge pump which itself probably had a defect in its wiring.
The reasons near loss of the FV Random Harvest make interesting reading too ( see the MAIB report)
You must remember that an anode associated with a copper alloy prop will be consumed... that's the basic electro-chemistry at work. It does not necessarily indicate that it is doing anything useful.
However, if an anode is not being consumed it indicates that it is probably not effectively connected to anything and cannot be be doing anything useful.
nimbusgb
Well-Known Member
On a slightly similar note ...
My perkins 4-108 has a completely isolated block. The instrumentation sensors, starter motor and alternator are all isolated from the battery system. A bit confusing when I tried to use the block as a ground for a meter lead once
The propshaft and prop boss are all 316 stainless and the blades are plastic ( kiwi prop ) and I fitted one of those propshaft bushes with polypropylene spider to reduce vibration.
So in theory there is no reason to have straps across the spider to tie the propshaft to the block ( I have them there at the moment )
I have not see any evidence of corrosion on the prop and shaft but the iron keel appears to have been eaten like crazy previously. Even a couple of weeks a year in the water turns a 1/2 kg anode, mounted just forward and to stbd of the leading edge of the keel, to powder.
The anode is bonded to battery negative.
Anyone got any bright ideas on finding the stray currents?
My perkins 4-108 has a completely isolated block. The instrumentation sensors, starter motor and alternator are all isolated from the battery system. A bit confusing when I tried to use the block as a ground for a meter lead once
The propshaft and prop boss are all 316 stainless and the blades are plastic ( kiwi prop ) and I fitted one of those propshaft bushes with polypropylene spider to reduce vibration.
So in theory there is no reason to have straps across the spider to tie the propshaft to the block ( I have them there at the moment )
I have not see any evidence of corrosion on the prop and shaft but the iron keel appears to have been eaten like crazy previously. Even a couple of weeks a year in the water turns a 1/2 kg anode, mounted just forward and to stbd of the leading edge of the keel, to powder.
The anode is bonded to battery negative.
Anyone got any bright ideas on finding the stray currents?
VicS
Well-Known Member
On a slightly similar note ...
My perkins 4-108 has a completely isolated block. The instrumentation sensors, starter motor and alternator are all isolated from the battery system. A bit confusing when I tried to use the block as a ground for a meter lead once
The propshaft and prop boss are all 316 stainless and the blades are plastic ( kiwi prop ) and I fitted one of those propshaft bushes with polypropylene spider to reduce vibration.
So in theory there is no reason to have straps across the spider to tie the propshaft to the block ( I have them there at the moment )
I have not see any evidence of corrosion on the prop and shaft but the iron keel appears to have been eaten like crazy previously. Even a couple of weeks a year in the water turns a 1/2 kg anode, mounted just forward and to stbd of the leading edge of the keel, to powder.
The anode is bonded to battery negative.
Anyone got any bright ideas on finding the stray currents?
Its a pity you have not started a new topic , However:
Your anode must be bonded with a low resistance connection to what ever it is intended to protect and be close to it.
If you have a fully isolated DC system there is no point in connecting the anode to the battery negative, in fact it is wrong to do so.
I dont think, "The instrumentation sensors, starter motor and alternator are all isolated from the battery system." is quite what you mean. Nothing would work. You mean, I assume, that they are isolated from the engine block!
You appear to have a major problem. I'd start by investigating just what is connected to what.
Do you have shore power..If so is that correctly connected esp with regards to the earth and does it include a galvanic isolator.
No obvious answers to the problem come to mind.
Last edited:
stevensuf
Well-Known Member
What i find funny is the amount of people who paint over anodes to stop them corroding 
nimbusgb
Well-Known Member
Yup right .... isolated from the block.
The keel bolts and the anode are tied together with welding cable. Generally no shore power used while in the water only when the boat is on the hard but it does have a galvanic isolator!
vyv_cox
Well-Known Member
My first move would be to disconnect the bridging connection across the flexible coupling and put a meter across the coupling, with the engine running. If your engine is not separately earthed but is at some positive voltage it would seem it is earthing itself via the propshaft, completing the circuit via the keel..
VicS
Well-Known Member
"put a meter across the coupling, with the engine running"... but not in gear
If the engine "is at some positive voltage" and earthing it self via the prop shaft as you suggest then it will be the stern gear , having now been made anodic, that will suffer from corrosion. The zincs and the keel in the return path will be cathodic and therefore not suffer! Contrary to what is observed.
I've been thinking about nimbusgb's problem.
To get corrosion of the zinc and the keel due to electrolysis they have to be anodic ( ie positive)... How that comes about if they are connected to DC negative is difficult to explain!
I would be looking for a +ve leak to the zincs and the keel ...... but how that would occur I know not! I'd make sure they are not connected to DC positive to start with!
I would check the bonding of the zincs to the keel. I would disconnect the bonding to the DC negative, maybe also disconnect the bridging of the flexible coupling.
I would also disconnect the shorepower earth connection to the DC negative and or engine block if it is currently so bonded but bond it to the zincs and keel.
I will now have the engine block completely isolated but wonder if that is wise.
vyv_cox
Well-Known Member
I take your point about the polarity.
However, the rate of corrosion suggests very strongly that there is an impressed current across the anode and the shaft bridge seems to be the only possible source. What is needed is information about the voltages at various points, so I don't agree that disconnecting a few wires and waiting to see what happens is the correct approach. At the very least the OP needs to replace the anode cable with a meter, checking with and without the engine running. My suggestion of checking the voltage across the flexible coupling also seems valid to me.
However, the rate of corrosion suggests very strongly that there is an impressed current across the anode and the shaft bridge seems to be the only possible source. What is needed is information about the voltages at various points, so I don't agree that disconnecting a few wires and waiting to see what happens is the correct approach. At the very least the OP needs to replace the anode cable with a meter, checking with and without the engine running. My suggestion of checking the voltage across the flexible coupling also seems valid to me.