My anodes aren't wearing away ?????

[Tranona]

GRP boats do need anodes!

Hull material is (largely) irrelevant. Anodes are needed to protect metals where two different metals are in contact in seawater. As VicS says, not all underwater fittings need protection because they are made of corrosion resistant materials. Those that commonly need protection are stern gear (props, shafts, rudders) where materials, particularly stainless steel and brass type alloys are mixed. Hence the use of anodes such as shaft anodes, buttons or rings on props and anodes attached to the hull close to the stern gear and bonded to it, usually through the gearbox housing.

 

[Robert Wilson]

Hull material is (largely) irrelevant. Anodes are needed to protect metals where two different metals are in contact in seawater. As VicS says, not all underwater fittings need protection because they are made of corrosion resistant materials. Those that commonly need protection are stern gear (props, shafts, rudders) where materials, particularly stainless steel and brass type alloys are mixed. Hence the use of anodes such as shaft anodes, buttons or rings on props and anodes attached to the hull close to the stern gear and bonded to it, usually through the gearbox housing.

Right. THAT sounds more understandable. Thank you.
So if I keep my prop-shaft anodes and wire my "lozenge" hull anode (near the stern) ONLY to the stern gland inside the engine compartment, that should be suitable and adequate?
ANY other bits that should be protected and/or linked? From what you say I think not?

 

[VicS]

Right. THAT sounds more understandable. Thank you.
So if I keep my prop-shaft anodes and wire my "lozenge" hull anode (near the stern) ONLY to the stern gland inside the engine compartment, that should be suitable and adequate?
ANY other bits that should be protected and/or linked? From what you say I think not?

No! Not suitable or adequate
There is probably no electrical connection between the gland and the shaft!

That is why the anode is commonly wired to the engine block, but if there is a flexible shaft coupling it has to be bridged to complete the circuit to the prop
Even then you are relying on contact through bearings etc.

The alternative is to wire to brushes that run on the shaft inboard of the stern gland
Such a system is MGDuff's Electro-Eliminator

 

[Robert Wilson]

No! Not suitable or adequate
There is probably no electrical connection between the gland and the shaft!

That is why the anode is commonly wired to the engine block, but if there is a flexible shaft coupling it has to be bridged to complete the circuit to the prop
Even then you are relying on contact through bearings etc.

The alternative is to wire to brushes that run on the shaft inboard of the stern gland
Such a system is MGDuff's Electro-Eliminator

Gotcha. Thanks. As far as I can tell, that is what I have (i.e. wired to engine block). I'll do a thorough re-check next time aboard. And take wire off any/all seacocks etc.

Great to have knowledgable and experienced advice. Cheers.

 

[xeitosaphil]

Only necessary to connect to one stud on the anode but check the continuity through to the anode itself.

MGDuff recommend 4 mm² cable for bonding.

Just read this, and realised that my bonding cable at the anode connected to the furthest stud from the engine block, actually returns to the engine block in one length making a complete ring circuit.

Will this have a detrimental effect on performance- should I remove it?

 

[Robert Wilson]

Just read this, and realised that my bonding cable at the anode connected to the furthest stud from the engine block, actually returns to the engine block in one length making a complete ring circuit.

Will this have a detrimental effect on performance- should I remove it?

I hope you get a definitive answer!
Having subscribed to this and other threads regarding anodes I find I am getting more confused than ever.
There seems to be so much conflicting advice; circuit/non-circuit, line-of-site-only/wired-up, sea-cocks need connecting/don't need connecting, etc., etc., etc.
I await a response which clearly, definitively and once-and-for-all comes up with the answer(s). As probably do many people.

 

[rob2]

It sounds as if you just have two connections from the engine block to the anode? That'll just spread the current between the two wires. The circuit is actually similar to what happends in a lead acid battery, only the liquid part of this circuit is the seawater between the anode and the metal part to be protected (electrons flow through the water between the anode and prop). In terms of hardware, all you need is to get a good (low resistance) continuity between the anode and the prop or whatever else is being protected. Thius is why on a steel rudder, the anode can simply be bolted onto the blade.

Rob.

 

[VicS]

Just read this, and realised that my bonding cable at the anode connected to the furthest stud from the engine block, actually returns to the engine block in one length making a complete ring circuit.

Will this have a detrimental effect on performance- should I remove it?

No problem with that. In some situations loops are to be avoided because varying or alternating magnetic fields can induce small currents.. can cause hum, for example, in audio circuits. Not relevant in this case.

I hope you get a definitive answer!
Having subscribed to this and other threads regarding anodes I find I am getting more confused than ever.
There seems to be so much conflicting advice; circuit/non-circuit, line-of-site-only/wired-up, sea-cocks need connecting/don't need connecting, etc., etc., etc.
I await a response which clearly, definitively and once-and-for-all comes up with the answer(s). As probably do many people.

There must be a good low resistance electrical connection between an anode and what it is fitted to protect. No circuit = no protection!

Line of sight need not be taken too literally but anodes must be reasonably close to what they are protecting and they must not be shielded eg by a keel or bilge plate. Slight curvature of the hull will be no problem.

It used to be recommended that all underwater fittings should be bonded to the anodes. However skin fittings and seacocks should be made of corrosion resistant materials which do not need cathodic protection so the recommendation is no longer made. In fact one of the recommendations in the MAIB report on the near loss of the F.V. Random Harvest a few years ago is that they should not be connected.

 

[KellysEye]

>- Disc anode on steel rudder - new in February 2012

If you have a steel rudder then I assume it's a steel boat. If so the anodes don't need to be wired to anything. With three anodes I wouldn't expect them to wear at the same rate. Also if it is steel then fit a galvanic isolator as soon as possible, stray current corrosion causes a consiberable number of rust patches all over the hull which quickly become holes. We had a steel boat.

 

[Robert Wilson]

No problem with that. In some situations loops are to be avoided because varying or alternating magnetic fields can induce small currents.. can cause hum, for example, in audio circuits. Not relevant in this case.



There must be a good low resistance electrical connection between an anode and what it is fitted to protect. No circuit = no protection!

Line of sight need not be taken too literally but anodes must be reasonably close to what they are protecting and they must not be shielded eg by a keel or bilge plate. Slight curvature of the hull will be no problem.

It used to be recommended that all underwater fittings should be bonded to the anodes. However skin fittings and seacocks should be made of corrosion resistant materials which do not need cathodic protection so the recommendation is no longer made. In fact one of the recommendations in the MAIB report on the near loss of the F.V. Random Harvest a few years ago is that they should not be connected.

OK. (Thanks). So, shaft anodes; understand that.
Hull anode (lozenge) bolts through the hull (two bolts, one each end) - what do I connect the inside of the bolt(s) to, assuming I'm to protect the engine? A wire from each bolt to different parts of the engine?
Connect them to anything else?

 

[VicS]

OK. (Thanks). So, shaft anodes; understand that.
Hull anode (lozenge) bolts through the hull (two bolts, one each end) - what do I connect the inside of the bolt(s) to, assuming I'm to protect the engine? A wire from each bolt to different parts of the engine?
Connect them to anything else?

A hull anode does not protect the engine!

It maybe connected to the engine as a means of making a connection to the shaft and prop in order to protect the prop. If so any flexible coupling will have to be bridged in order to complete the connection. There are also devices available that make the connection directly with the shaft by means of brushes mounted inboard of the stern gland

The engine may be protected by an anode in the water jacket in the case of a directly cooled engine.
Indirectly cooled engines are protected by the corrosion inhibitor in the coolant ( antifreeze) . There may be an anode in the seawater side of the heat exchanger.
Your engine manual should contain all the details of the engine anodes

MG Duffs website is a good source of info on cathodic protection and fitting anodes

 

[Robert Wilson]

A hull anode does not protect the engine!

It maybe connected to the engine as a means of making a connection to the shaft and prop in order to protect the prop. If so any flexible coupling will have to be bridged in order to complete the connection. There are also devices available that make the connection directly with the shaft by means of brushes mounted inboard of the stern gland

The engine may be protected by an anode in the water jacket in the case of a directly cooled engine.
Indirectly cooled engines are protected by the corrosion inhibitor in the coolant ( antifreeze) . There may be an anode in the seawater side of the heat exchanger.
Your engine manual should contain all the details of the engine anodes

MG Duffs website is a good source of info on cathodic protection and fitting anodes

Thank you. The mist is beginning to clear! And I'll have a look at MG Duff's website. My engine manual is a reasonable "general" guide but there is some doubt as to what type it is. It's a Kubota, marinised by somebody, but Nanni (having looked at photos during last year's SIBS) state it's not one of theirs!
I'll keep nagging away at this.

I'm wondering now why I have a hull anode at all?! Especially when I have a pair of shaft anodes.
Thanks once again.
R

 

[Robert Wilson]

MG Duff a useful and clear site. Thanks.
Still not sure if my shaft and prop are adequately protected by two shaft anodes.
Shall have to investigate material of rudder, stock and P-bracket. The latter seems to be totally encapsulated with GRP (or years of A/F........!)

 

[VicS]

MG Duff a useful and clear site. Thanks.
Still not sure if my shaft and prop are adequately protected by two shaft anodes.
Shall have to investigate material of rudder, stock and P-bracket. The latter seems to be totally encapsulated with GRP (or years of A/F........!)

The shaft anodes should protect the shaft and prop....... probably more effectively than the hull anode ... provided they dont fall off!
Probably the hull anode was fitted before the shaft anodes ... but it is only effective if there is a good electrical pathway to the shaft.

Loadsa boats have anodes protecting the stern gear unnecessarily. The mid-seventies Westerly I used to sail had no anodes ... the prop and shaft are still in good order!
It's folding props, where a mix of materials have been used, that seem to be in most need of anodes.

 

[Bosun Higgs]

If it's brackish water that will reduce electrolysis and fouling. Yes use a multimeter with it's continuity setting - which probably makes a noise when you have continuity. You can check the continuity of the anodes to the bolts from the outside - one prong on the anode, the other on the stud end or nut, and you should get a squeaking noise. Then move inside and check from the bolts to the engine block etc. But all sounds good to me given the brackish water and lack of fouling.

You use the anodes to protect external fitings like the rudder and stern gear. Forget the engine block - just stick the meter probe on the anode surface and the other meter proe on the stern gear and the rudder etc. There must be a low resistance electrical circuit between the two for the anodes to work. If the circuit is there then stop worrying.

 

[Robert Wilson]

The shaft anodes should protect the shaft and prop....... probably more effectively than the hull anode ... provided they dont fall off!
Probably the hull anode was fitted before the shaft anodes ... but it is only effective if there is a good electrical pathway to the shaft.

Loadsa boats have anodes protecting the stern gear unnecessarily. The mid-seventies Westerly I used to sail had no anodes ... the prop and shaft are still in good order!
It's folding props, where a mix of materials have been used, that seem to be in most need of anodes.

The mist has cleared the horizon!! I can now follow the science and the thinking, and breath a sigh of enlightenment.
Thank you.