Sense or Nonsense Hanging Zinc Anode Overboard?

[Cheechako]

A lot of people use them up here. I suspect to avoid stray current in the marina situations. I sometimes see them clipped to the rudder shaft also. If you use one and it's disappearing, it's working!

[Goudurix]

Can't avoid the feeling that a lot of people behave like a herd of sheep...the neighbour does it, so does his neighbour...hmmm...I'd better buy one myself....
Sorry to say so but I haven't heard any really convincing explanations until now, the general feeling being "well it might help last your (normal) zincs longer...so it won't harm".

Well I'll dig up my Boatowner's Mechanical and Electrical Manual , the chapter on galvanic corrosion is quite lengthy but with a good glas of beer.....

Jan

[goboatingnow]

Originally Posted by 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

On a European spec.ed GRP boat, utter waste of time

Dave

[fstbttms]

My opinion is that "guppy" zincs are a bandaid. You should be able to adequately protect your boat with anodes mounted directly on the parts being protected. If it is necessary to add a temporary anode to increase protection, either you do not have enough zinc on the parts in question or there is an electrical issue (onboard or external) that needs rectification.

[Goudurix]

Hi Matt I watched your pro videos nice to see that even as an amateur (I have been a leisure scuba diver > 25 years) I seem to adopt some of your methods!

Off course I do the UW-checks and maintenance on my hull myself, when I do so I always have a look at the "friendly neighbourhood boats" and it is often sad to find things that the owners might have checked easily just taking a dip with a diving mask and a wetsuit.

One of them, a rather new 37ft Beneteau had no zincs anymore neither on the propeller shaft or on the propeller. I warned him in October ast year. "Oh I'll take care of that during haulout" "yeah but you might have a well corroded propeller" "Ooh I'll see". Result on haulout in March: needs to install a new propeller. Sad. He's a young guy he could even have done the relacement while freediving.

Whatever. You do a nice job professionally! Hence I trust the advice you just gave me!

Jan

[Scot McPherson]

Ok well galvanic corrosion is basically a battery effect.

If you have an anode and a cathode and an electrolyte (an acid, or base or the saltwater from combining the two) then you have a battery and electron transmission.

Noble metals? No such thing...two dissimilar metals in an electrolyte will become either a cathode or an anode. Ions will travel from the anode to the cathode.

If there are two dissimilar metals in the salt water, then they already are electrically connected, whether they are wired together on the boat or not. Now connectictinf them with a wire will speed up the corrosion, but disconnecting them will NOT stop it.

We use zincs, because they have lots of free ions to sacrifice, and we therefore use it to STOP something else more
Important from becoming the anode. It's that simple.

If a hanging zinc in the water gets corroded, then it's doing its job, regardless of where the cathode is, whether it's you or your neighbors boat...

If it's corroding, but you think it's unecessary, then at least you are slowing some corrosion on your more expensive zincs...

Remember, every metal in the water is electrically connected, whether it's a strong or weak connection, and whatever becomes the anode will be corroded, as visibly so over
Time.

[Cheechako]

Originally Posted by Goudurix

Can't avoid the feeling that a lot of people behave like a herd of sheep...the neighbour does it, so does his neighbour...hmmm...I'd better buy one myself....
Sorry to say so but I haven't heard any really convincing explanations until now, the general feeling being "well it might help last your (normal) zincs longer...so it won't harm".

Well I'll dig up my Boatowner's Mechanical and Electrical Manual , the chapter on galvanic corrosion is quite lengthy but with a good glas of beer.....

Jan

Actually it's fairly common up here in the PNW for good reason, stray currents in marinas. I went to a corrosion seminar once. He had a Propeller shaft laying on the table. It looked like thin hollow lace. The shaft was 3-4 months old! A Valiant 32 had returned form world cruising and had the shaft etc replaced. They then moored in a Seattle marina and had the bottom scrubbed after 3+ months. The diver told them to have the boat hauled IMMEDIATELY. most of the thru hull fittings were powder and the shaft was as described.
The nice thing about having the "fish" over the side is you can pull it up and see how things are doing without getting in 50 degree water.

[sailorchic34]

I've just started doing this. Though I ran a wire from the engine to a deck stud and then wired up some zinc bits and hung over the side.

I was getting .64V on between the engine and zinc in the water, which just happens to be the nominal galvanic difference between zinc and bronze.

It allows for easy inspection and replacement without paying a diver. My boat is not bonded, so I just connected the wire to the DC ground which is common for the prop too.

[fstbttms]

Pulling up a guppy zinc and checking its state of depletion is not going to tell you if your running gear is corroding or not.

[Scot McPherson]

If there is a zinc on your shaft, your running gear is not corroding because the running gear becomes the cathode.

A guppy zinc isn't a replacement for a zinc on your running gear, it's just an alternate anode that can be placed outside the strong part of the electrical field of the one on the running gear. If it's corroding, it's protecting something else that could have become an anode.

[sailorchic34]

Originally Posted by fstbttms

Pulling up a guppy zinc and checking its state of depletion is not going to tell you if your running gear is corroding or not.

Hey Fstbttms, I knew you would pop up.

Well in engineering speak they are called sacrificial anodes. If the Zinc Anode is depleting itself, that generally means that other more noble metals connected in circuit to the anode in the same electrolyte are not corroding. Not perfect, but for the Cheap/frugal/flat a$$ broke sailor, It does the job.

I could get fancy and create a coupon rack I guess with bronze and SS coupons and inspect them now and then, but its too much hassle. I'll just check the voltage to make sure the electrons are flow the right direction.

Coupons BTW are metal strips used for underground piping and and cooling tower/ boiler piping. They are placed in coupon racks to verify that the various anti-corrosion systems are working as they should

[sailorchic34]

Originally Posted by Scot McPherson

If there is a zinc on your shaft, your running gear is not corroding because the running gear becomes the cathode.

A guppy zinc isn't a replacement for a zinc on your running gear, it's just an alternate anode that can be placed outside the strong part of the electrical field of the one on the running gear. If it's corroding, it's protecting something else that could have become an anode.

A zinc is a zinc or rather an anode is an anode. Placing a zinc in circuit and in close proximity to other metals in the same circuit in an electrolite, that is seawater will protect the shaft and prop.

Yes if there is a prop zinc on the prop and a guppy zinc then there will be some loss from each zinc. But as long as there is a zinc in circuit then the metal is protected. This is done on underground pipelines, with anodes installed every 200-500 feet, depending on soil conditions.

[transmitterdan]

Originally Posted by a64pilot

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.

That does not cause stray DC current. The main fear is that the insulation could be compromised and allow the hot wire to leak current. Nearby swimmers, especially in fresh water marinas, can drown due to paralysis caused by the AC current in the water. This is why all marinas should have residual current devices at the power pedestal. But most in the US do not have them.

[a64pilot]

It's does indeed take DC to have electrolysis, doesn't it? I had forgotten that.

[fstbttms]

Originally Posted by sailorchic34

Hey Fstbttms, I knew you would pop up.

Well in engineering speak they are called sacrificial anodes.

Really? Gosh, thanks.

Originally Posted by sailorchic34

If the Zinc Anode is depleting itself, that generally means that other more noble metals connected in circuit to the anode in the same electrolyte are not corroding.

You are deluding yourself if you think the fact that your anodes are depleting is a guarantee (or even an indication) that your underwater metal parts are not being corroded.