Anodes - an idiots guide (please)

[jaminb]

I don't understand how anodes work, although I think I understand the sacrificial nature of them.

I am in the process of replacing my prop and hull anodes, they are obviously working because they are eroded.

A few questions if I may;

I don't have any prop shaft anodes - is that because they have fallen off or I don't need them? The prop and shaft did have some fouling after 2 year in the water but has come up lovely and shiny with no evidence of corrosion.

I have plastic through hulls and a fiberglass hull - what is the hull anode protecting?

When in my marina I leave mains power on for the dehumidifier, I think I have read that this can cause greater 'leakage' is this true? And do I need some kind off additional isolator or will my boat have one as standard (2006 Jeanneau)?

Do I need to meter out the connectivity from the hull anode bolts to somewhere?

Why do lots of boats on the Hamble trot moorings have a wire hanging over the side?

thanks

 

[dankilb]

I don't have any prop shaft anodes - is that because they have fallen off or I don't need them?

Definitely need them. Our Jen has one on the shaft and one on the prop. Is there no evidence of any, even on the prop (nut)?

I have plastic through hulls and a fiberglass hull - what is the hull anode protecting?

Possibly/probably nothing. Is anything connected to it inside (inboard)? Such as a ground or earth? They can be used to earth 240v onboard (without commenting on whether this is ‘correct’ or not)

And do I need some kind off additional isolator or will my boat have one as standard (2006 Jeanneau)?

Couldn’t say whether Jen fitted them in that era - but it’s strongly advisable to have one. They’re not expensive nor hard to install.

Do I need to meter out the connectivity from the hull anode bolts to somewhere?

Is it actually wired to anything? (As above)

I’m no expert in marine electrics but our Jen has always seemed happy with a shaft anode, prop nut anode, and galvanic isolator. (And this seems standard on other production boats of the era)

 

[Tranona]

Just trace the wires from the hull anode. It could be that it is being used as an earth for the DC circuits. The other possibility is that it is connected to the gearbox casing and through that to the shaft, although this is ineffective if you have an isolated flexible coupling.

All you really need is a shaft anode, although sounds like you also have a propnut anode which does the same job as a shaft anode. Nothing else on the boat needs an anode, although some engines do have anodes in their cooling systems.

Yes, wise to fit a galvanic isolator if on shorepower in a marina. check - it will be a box in the wiring from the shorepower inlet to the AC distribution panel, usually close to the inlet.

Wires hanging over the side are "hanging" anodes, which if properly installed will be bonded to the thing they are intended to protect, usually the shaft/prop via the gearbox, or with saildrives to the drive housing. Largely ineffective as if the boat has normally installed anodes they will wear first as they are closer to the things being protected. They do come into play should the main anodes disappear completely.

The basic rule with anodes is that you look at situations where there are 2 dissimilar metals in contact in an electrolyte (seawater ) and electrically bond an anode to one of them. So shaft anodes for prop/shaft (or hull anode through the gearbox with a bridge over any flexible coupling), ring anode for aluminium/stainless mixtures in saildrives and outdrives, bronze hubs for folding and feathering props with stainless pins and gear and so on. Pointless (as previous owners of the boat I have just bought have) to put them on mild steel bilges plates or stainless lower rudder pintles, as they won't stop rust on the former or crevice corrosion on the latter.

 

[VicS]

I don't understand how anodes work, although I think I understand the sacrificial nature of them.

You may find the information on this link helpful :- https://mgduff.co.uk/downloads/Leisure Catalogue.pdf

or a more in depth and scientific document here :- https://www.npl.co.uk/getattachment/research/electrochemistry/corrosion-guides/corrosion-guide-gpg1.pdf.aspx?lang=en-GB#:~:text=The principle of cathodic protection,and therefore do not corrode.

I am in the process of replacing my prop and hull anodes, they are obviously working because they are eroded.

It does not follow that anodes are doing anything useful just because they are eroding but if they are NOT eroding they are certainly NOT working

A few questions if I may;

I don't have any prop shaft anodes - is that because they have fallen off or I don't need them? The prop and shaft did have some fouling after 2 year in the water but has come up lovely and shiny with no evidence of corrosion.

If there is no evidence of corrosion you do not need a shaft anode but one could be fitted to guard against corrosion occurring if your do not / are unable to replace the prop anodes before they are completely eroded away

I have plastic through hulls and a fiberglass hull - what is the hull anode protecting?

It is protecting whatever it is bonded electrically to. Perhaps the prop via the shaft and a bond to the engine or gear box.

When in my marina I leave mains power on for the dehumidifier, I think I have read that this can cause greater 'leakage' is this true? And do I need some kind off additional isolator or will my boat have one as standard (2006 Jeanneau)?

If you leave the shorepower connected you must have a "galvanic isolator" {GI} in the earth connection of the incoming shore power supply ( unless there is no connection between the shorepower earth and any parts, including the anodes , in contact with the water) Without a GI your anodes might erode very quickly due to current conducted by the earth connection. That is why a GI is often called a "zinc saver".

Do I need to meter out the connectivity from the hull anode bolts to somewhere?

There should be a very low electrical resistance between an anode and what it is fitted to protect. Anode Outlet suggest as low as 0,2 ohm. That may be a bit optimistic but should certainly below 1 ohm

Why do lots of boats on the Hamble trot moorings have a wire hanging over the side?

Carrying additional anodes . Probably clipped to sail drives to give emergency back up to the drive anodes in an area which is often troublesome

thanks

 

[jaminb]

Thanks all - very helpful and some useful extra reading VicS.

I do have a prop anode - a bespoke featherStream expensive one!

I still don't understand how the shorepower 'leaks' through the hull and just doesn't return via the earth of the shore power, but I will have a hunt for the GI.

The hull anode is bolted to the hull underneath where the water tank is so access to trace where the bonded wires go but will assume it is connected to something and have a hunt around. Possibly the the rudder shaft?

 

[VicS]

Thanks all - very helpful and some useful extra reading VicS.

I do have a prop anode - a bespoke featherStream expensive one!

I still don't understand how the shorepower 'leaks' through the hull and just doesn't return via the earth of the shore power, but I will have a hunt for the GI.

The hull anode is bolted to the hull underneath where the water tank is so access to trace where the bonded wires go but will assume it is connected to something and have a hunt around. Possibly the the rudder shaft?

Reading not to heavy I hope

The hull anode may have been fitted originally to protect an ordinary fixed blade prop if the Featherstream is a later, aftermarket, fit.

The galvanic isolator is a device which allows the electrical safety aspects of the earth connection to function as normal but blocks current flow from the very low voltage sources that cause galvanic corrosion

Here is a video, from Safeshore Marine, which begins by explaining galvanic corrosion in simple terms then goes on to explaining the function of a galvanic isolator

 

[Tranona]

Thanks all - very helpful and some useful extra reading VicS.

I do have a prop anode - a bespoke featherStream expensive one!

I still don't understand how the shorepower 'leaks' through the hull and just doesn't return via the earth of the shore power, but I will have a hunt for the GI.

The hull anode is bolted to the hull underneath where the water tank is so access to trace where the bonded wires go but will assume it is connected to something and have a hunt around. Possibly the the rudder shaft?

You probably don't need a shaft anode. The Featherstream anode protects the prop from the action between the stainless bits and the hub and the hub and the shaft. Shaft anode would do no harm and probably worth trying one to see if it does wear. Fit it 20mm or so forward of the P bracket

 

[Poey50]

Reading not to heavy I hope

The hull anode may have been fitted originally to protect an ordinary fixed blade prop if the Featherstream is a later, aftermarket, fit.

The galvanic isolator is a device which allows the electrical safety aspects of the earth connection to function as normal but blocks current flow from the very low voltage sources that cause galvanic corrosion

Here is a video, from Safeshore Marine, which begins by explaining galvanic corrosion in simple terms then goes on to explaining the function of a galvanic isolator

In an otherwise clear and helpful video, there seems to be a problem at 3:41. He suggests that all metal parts are to be bonded together and will then be protected by a single anode. I believe that this cross-bonding is no longer considered good practice (except perhaps in more lightning-prone USA to avoid voltage differences between metal parts and therefore the risk of side-flashes) and, even if it was done, an anode several metres away from a particular bonded item would not offer protection.

 

[VicS]

In an otherwise clear and helpful video, there seems to be a problem at 3:41. He suggests that all metal parts are to be bonded together and will then be protected by a single anode. I believe that this cross-bonding is no longer considered good practice (except perhaps in more lightning-prone USA to avoid voltage differences between metal parts and therefore the risk of side-flashes) and, even if it was done, an anode several metres away from a particular bonded item would not offer protection.

I should have said ignore that bit

 

[Major Tom]

Are the various galvanic isolators much of a muchness? - there is quite a variation in price.

 

[Poey50]

Are the various galvanic isolators much of a muchness? - there is quite a variation in price.

I researched this a bit a year or two ago. The cheapest ones have no built in monitoring system, the mid range ones do have some kind of remote monitoring and the most expensive ones are fail-safe - a requirement in the US, I believe.