Sense or Nonsense Hanging Zinc Anode Overboard?

[Goudurix]

In our marina (Belgium, seawater) more and more GRP sailing boats start to hang extra zinc anodes attached to a S/S wire, overboard, as (extra) cathodic protection.
On european boats we do not have the shore power earth connected to the boat 12V negative DC ground system.
So what is the sense of hanging an extra anode overboard (wire connected to engine blok or DC negative or propeller shaft etc) if one has the adequate zincs alreay installed "according to good practice" = on propeller shat close to the propeller nd if possible and extra one on the propeller retaining nut?

I see them installing commercially available stuff consisting of a stainless steel cable attached to a heavy zinc. Other side of the cable to be hung over a stanchion or similar (providing this stanchion is bonded into the DC ground....)

I thought that to provide good cathodic protection a zinc should:
- be fixed/connected electrically as good as possible (eg. fixed ON the shat or ON the propeller providing a tight electrical contact) => the S/S wire plus"hanging" it over a stanchion doesn't sound like a good low-resistance tight connection to me...
- as close as possible to the metal it should protect

Your comments?
Does it make sense or is it a waste of money?


Jan

[GordMay]

You've got it right, Jan.

[Goudurix]

When I cross a boat in the dock and having that kind of zinc hanging overboard, I try to check the state of such a zinc.

Quite often the zinc is "attacked" e.g. pitted of chunks eaten away (thus "working") but I'm convinced that (especially if the electrical connection of the wire in-board is so-so..) the fact that there is a more noble metal (copper wire or stainless steel wire) connected to the zinc and both hung in seawater, the zinc will protect the copper or S/S wire it is connected to and get eaten away....

Jan

[a64pilot]

I've one, actually had them for years. I've seen them detect stray current from a neighbors boat. You see it every time you go out, so one day you pull it up and a lot of it's eaten away, you start investigating, something changed, new neighbor maybe? Power cord in the water?
I've seen stray current from a boat next door almost completely eat up the props on a larger sport fisherman before, they only started investigating what was wrong when the boat would no longer plane the props were so eaten up, other than props I don't know how extensive the damage was.

[zboss]

If you are then grounding your boat with a zinc, where it once was not, you are in effect creating the issue you are trying to prevent?

This comes down to the bond or not to bond question.

[S/V Illusion]

Originally Posted by zboss

If you are then grounding your boat with a zinc, where it once was not, you are in effect creating the issue you are trying to prevent?

This comes down to the bond or not to bond question.

When you connect a zinc anywhere, you essentially CREATE a battery. There are circumstances where that is beneficial, however, there are also circumstances where it serves only to exacerbate electrolysis.
The specifics are too varied to discuss them all here. Suffice to say that connecting a zinc to Stanton's which are not directly connected to the underwater metal of concern can exacerbate the problem.

Far preferable to fix the cause rather than try to mitigate the effect.

[Goudurix]

In a GRP boat without bonding (typically for european boats) meaning the shore power AC ground is not connected to the ship's DC-ground (battery negative and engine block), with the through hulls unbonded, and AC-powered appliance OK (e.g. battery charger well isolated and no connection whatsoever between AC neutral/ground to boat DC ground) I really don't see how other boats in the marine could cause a problem for galvanic corrosion!???

If a stray current problem exists, it must then come from your own boat (+ leaks to wet bilge, DC-ground loops etc). Correct?

Jan

[djmarchand]

Here is what I have observed- opinion and here is what I think- opinion:

The "guppy" zincs or on my case a think cylinder of zinc hanging over the stern and connected to the bonding system does get wasted away. If the bonding connection is broken for some reason the zinc stops wasting. All fact.

Now for opinion: When such a zinc is connected to the bonding system it creates a "battery cell" just like a zinc bolted on a prop shaft. That cell produces a field of ions that protects adjacent metal. But I think that the field is small, maybe a few feet. So for example a zinc hung over the port side won't protect the starboard prop on a twin engine boat.

I do it to limit wasting of the more expensive shaft zincs. I think it works, but who knows for sure.

David

[MarkSF]

Originally Posted by Goudurix

So what is the sense of hanging an extra anode overboard (wire connected to engine blok or DC negative or propeller shaft etc) if one has the adequate zincs alreay installed "according to good practice" = on propeller shat close to the propeller nd if possible and extra one on the propeller retaining nut?
Jan

People do this in order to extend the life of the underwater zincs, which may require a diver to replace.

[CSY Man]

Been doing it for years, it works.

[Goudurix]

OK thanks for the reactions and opinions so far.

I am a diver so that point is not an issue

Actually I was diving last night to inspect and clean the hull.

The zinc on the prop hub has already wasted quite a lot of material. I guess it will only last 3-4 months more.

The ball-shape prop shaft zinc is quite intact, only minor wasting.

If hanging an overboard zinc (bonded into DC ground) could help prevent wasting the prop zinc, I would be glad to try that out.
To have a better view on that I guess should install a brand new prop zinc, install the overboard zinc, and follow how the prop zinc behaves over several months.

Jan

[fstbttms]

Originally Posted by Goudurix

If hanging an overboard zinc (bonded into DC ground) could help prevent wasting the prop zinc, I would be glad to try that out.
To have a better view on that I guess should install a brand new prop zinc, install the overboard zinc, and follow how the prop zinc behaves over several months.

Consider adding a second shaft anode and save yourself the trouble of dealing with the overboard zinc every time you want to use the boat.

[a64pilot]

Originally Posted by Goudurix

If a stray current problem exists, it must then come from your own boat (+ leaks to wet bilge, DC-ground loops etc). Correct?

Jan

I am not smart enough to explain exactly how, but I am sure stray current can come from you neighbor as I've seen it happen. It can be as simple as them using a car battery charger on their boat for example.
I've not witnessed it, but believe that a shore power cable hanging in the water can cause stray current, but admit that may be a legend.
I believe for the big zinc hanging off the side to be effective it has to be electrically connected to the vessels metals that are in contact with the water, in other words all metal in contact with the water needs to be bonded together, and the big zinc connected to this bond.

Pure opinion only

[Goudurix]

a64pilot,

if all your boat's underwater metals are NOT connected to the shore power earth lead, and no other electrically conductive connections exist between you boat and neighbouring boats it is impossible to get an stray current effect afik!

The 2 dissimilar metals (having a different potential) have to be immersed in a conductive liquid (eg seawater) AND have to be electricall connected (eg by being fixed on one another or having a conductive wire connecting both).

As far as I know in the US you are obliged (by regulations) to connect the shore power ground to the ships bonded DC-ground as a safety measure. If you also have al your underwater metal thorugh hulls bonded into this DC-ground, you are at risk and you will need a more dedicated cathodic protection system (zindcs) installed.

Jan

[a64pilot]

I believe it's pretty common to isolate the ground from shore power with a transformer.

I've seen props eaten up from stray current many years ago, entirely possible that his ground was not isolated. I'd sleep better if I could be sure that stray current could only come from my boat.