Is my Guppy Zinc Working?

[Paul Elliott]

Originally Posted by Mark Johnson

While still staying isolated, you can use the boats underwater metal in your SSB's ground plane, by connecting them (by way of diodes), that pass RF energy, but not AC or DC current... (see photo). NO galvanic corrosion induced!

I'm pretty sure you meant to say capacitors there. Diodes are sometimes used to isolate the boat's ground from the shore-power ground. Capacitors are sometimes used in the SSB grounding system, They pass radio-frequency current, but not D.C current.

[Doodles]

Thanks for all the replies. I'm quite sure mine is not doing anything for me since I've seen no wearing away of it over the last year or so, whereas my shaft zincs disappear regularly.

Not to over simplify but I am understanding it that what I need to do is run a cable from my engine ground to the backstay or some similar place where my zinc cable is clamped? I can see no evidence that the backstay is currently grounded.

[CharlieJ]

Andina- I tried to send this to you as a PM but you have not enabled that feature.

From the New Oxford American Dictionary:

1 Chemistry chemical decomposition produced by passing an electric current through a liquid or solution containing ions.

From ABYC Standard E-2 Cathodic Protection:

2.4.35 Electrolysis - The breakdown of an electrolyte resulting from current flowing in an electrochemical cell that includes that electrolyte. Example - the breakdown of water into hydrogen and oxygen gases in a supplied-current electrochemical cell.

NOTE: The term “Electrolysis” is often used loosely to describe corrosion in general, or the operation of supplied-current cells in particular. Its use in this respect is often confusing, and should be discouraged.

From Wikipedia:

In chemistry and manufacturing, electrolysis is a method of using a direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially highly important as a stage in the separation of elements from naturally occurring sources such as ores using an electrolytic cell.

Most of this thread has been referring to galvanic corrosion, a slow process with potentials < 1VDC. Because of these low galvanic potentials, the connections that make up the cathodic protection system must have a low resistance in order to be effective. ABYC E-2 specifies these connections should be < 1 Ω.

Stray current corrosion, on the other hand, is a very fast process and can be incredibly destructive. It comes about when a B+ source comes in contact with underwater metal fittings. This contact can be direct with a "hot" conductor against an underwater metal fitting or indirect with the "hot" conductor in bilge water that then contacts underwater metal fittings. Bonding the underwater metal fittings with electrical connections < 1 Ω will remove the danger of stray current corrosion from being caused by own boat problems as all underwater fittings are at the same electrical potential and current cannot flow.

[noelex 77]

As others have said for the zinc to work it needs to be electrically connected to metal part that is immersed in seawater. As the back stay is not electrically connected to seawater the zinc is not doing anything.
For a zinc to work it needs a very low resistance connection (because the voltages are low) and it also needs to be reasonably close to metal it is protecting (in your case I assume it's it's the prop and shaft).

This is very difficult to achieve with a hanging zinc. Your plan of grounding the back stay introduces a long electrical path with multiple connections. Even if a low resistance path could be achieved the zinc is still a long way from the metal it's supposed to be protecting.

It will not do harm, but it's hard to see it doing much good.

Hanging zincs can be worthwhile on a metal boat, where a low resistance electrical connection is available, or for a non metal boat in situations such as, or example, protecting a rudder stock, where direct conection with the the stock and hanging close to the rudder in seawater can provide significant benefit, but as others have said without a low resistance electrical path and close proximity, a zinc hanging over the side offers no protection.

[cwyckham]

Originally Posted by noelex 77

As others have said for the zinc to work it needs to be electrically connected to metal part that is immersed in seawater. As the back stay is not electrically connected to seawater the zinc is not doing anything.
For a zinc to work it needs a very low resistance connection (because the voltages are low) and it also needs to be reasonably close to metal it is protecting (in your case I assume it's it's the prop and shaft).

...

How close is close? Other posters have suggested between 6 and 10 feet as the limit. Mine hangs about 4' from my prop and is electrically connected to the block by an internal wire. Sounds reasonable??

[CharlieJ]

cwyckham-
Take a resistance reading from your propulsion shaft to the block. If it is < 1 Ω you are ok.

[cwyckham]

Originally Posted by CharlieJ

cwyckham-
Take a resistance reading from your propulsion shaft to the block. If it is < 1 &Omega; you are ok.

From shaft to engine block or to hanging zinc?

[CharlieJ]

Easiest test is shaft to engine block. Guppy is usually cast to the conductor and the spring clamp can be manipulated to get decent contact. The most conclusive test would be to check hull potential and shaft potential against a silver/silver chloride half cell.

[noelex 77]

Originally Posted by cwyckham

How close is close? Other posters have suggested between 6 and 10 feet as the limit. Mine hangs about 4' from my prop and is electrically connected to the block by an internal wire. Sounds reasonable??

4 feet it is near enough to work well. If you have got a low resistance resonably short electrical connection and the engine is not isolated from the propshaft the zinc should be effective.

[foggysail]

Your guppy should be tied to the metal you're trying to protect. Sure, it would not matter in a securely bonded boat, the guppy could be tied anywhere on the bonded structure.

But there is a problem in most boats with the shaft and props caused by how the shaft gets turned by gears sitting in an oil bath. So if its you shaft and propeller you have concerns about, that is where the bond should be made. But now you have another problem because the shaft is not stationary, it rotates.

There are options. First is to mount an anode directly on the shaft as it sits in the water. The second is to consider installing shaft brushes that attach to your bonding system thereby bonding the shaft regardless of what happens to it inside its oil bath.

If you do not electrically attach your guppy to the shaft in some manner, a guppy will only provide protection you seek if there is low resistance between the shaft and the rest of your bonding system....and remember, the shaft turns in an oil bath so what you measure today might be different than tomorrow's measurements.

ZINCS! I long ago moved away from zincs. I use an aluminum anode, one (1) mounted at my boat's stern. This provides all the protection my boat requires. But in order to properly decide if your boat is adequately protected, you must measure the bonding voltage against a known reference half cell. THAT IS THE ONLY WAY TO KNOW! I designed and made my own shaft brushes to bond my shafts. Further, I purchased a silver-silver half cell to measure my wetted metal voltages which are all in allowable range.

Foggy

[kellyp08]

I have been on my boat in the Med (Mallorca) for 5 months. I had the bottom done and new zincs on the Max-Prop placed in mid July. I'd never had a problem with electrolysis/galvanic corrosion in Maine. However, I noticed in Mallorca that the bottom and especially the prop cage was getting fouled real fast. So I just hauled (Dec 4th) and noted that the zincs on the prop cage and Max Prop had virtually disappeared. Once I cleaned all the crap off the prop - (a full days work) and removed and had the prop cage sandblasted, I was happy to find no real metal loss on either. It seems to me that there can only be two sources for the problem: 1. stray currents from other boats in the warm, salty and dirty water of the Med port or the addition of a Victron Isolation transformer (240VAC to 110VAC) and/or 2. a new autopilot (Simrad with hydraullic arm to the rudder post) has set up a new electrical loop. Obviously it's time for a Guppy (never used one before). But I was wondering if anyone had any ideas on the above. Thanks.

[fstbttms]

Originally Posted by kellyp08

It seems to me that there can only be two sources for the problem: 1. stray currents from other boats in the warm, salty and dirty water of the Med port or the addition of a Victron Isolation transformer (240VAC to 110VAC) and/or 2. a new autopilot (Simrad with hydraullic arm to the rudder post) has set up a new electrical loop. Obviously it's time for a Guppy (never used one before). But I was wondering if anyone had any ideas on the above. Thanks.

The cause of rapid anode depletion is usually found aboard the boat experiencing it. If this is indeed the case in your situation, adding a guppy is merely a band-aide and not a true solution. My recommendation is to have a qualified marine electrician track down the source and rectify the problem properly.

[CharlieJ]

+1 to fstbttms comment.

That said, if the the Victron Isolation Transformer is installed correctly, then the normal path for accelerated anode wastage via the safety ground wire is eliminated. Step one would be to ensure that the Victron is installed correctly.

Since the timeline appears to show that the installation of the autopilot coincides with the accelerated anode wastage, then that installation is suspect. Step two is to ensure that the autopilot wiring was performed in accordance with the installation manual and that no short cuts were taken; e.g., are all of the B- conductors lead back to the ship's ground or have they been connected to a convenient bonding bus?

[kellyp08]

Thanks fstbttms.

I agree; I spent most of today in the bilge, engine compartment and sail lockers with a multimeter and could not find any connection not grounded. It's time to call in the Cavalry - the problem is above my pay grade. My boat's in Palma de Mallorca at STP and finding a good marine electrician there should not be too difficult and is my next step.