Is my Guppy Zinc Working?

[Doodles]

I've got one of the guppy zincs having over the side with the lead attached to my backstay. How can I check it to make sure it's working properly? Is there a better place to attach it?

[Paul Elliott]

If you have a clamp-on DC ammeter, you can measure the current running through the guppy's attachment wire. The current will be pretty low, so you can take ten turns through the ammeter's jaws -- the indicated current will be 10X the actual.

For that matter you can use any DC ammeter, such as that on a DVM. You just have to connect the ammeter between the guppy's wire and your boat's ground.

I did this once, and definitely noticed a difference in guppy current when I connected and disconnected my (non-isolated) shore power connection. Now that I have my solar panels the shore power is disconnected and I don't bother with the guppy. Sorry, I don't recall the current I measured, but it was a few milliamps. The current polarity will be important, but I don't remember that either. It is possible to "over-zinc" your boat, but I suppose if the zinc is gradually eroding then it's doing it's job.

[fstbttms]

Originally Posted by Doodles

I've got one of the guppy zincs having over the side with the lead attached to my backstay. Is there a better place to attach it?

Is your backstay electrically connected to the item the zinc is intended to protect? If not, you are wasting your time and money. It's a common misconception that simply hanging a zinc over the side provides protection. It does not.

[Andina Marie]

Agree, to be effective it has to be connected to your battery negative circuit. Places such as the negative battery terminal or engine block or bonding system if you have one. You should not use negative connections on existing wiring because they may not be at negative voltage when current is flowing to existing items through that cable.

[sailvayu]

Just wondering why you have such a thing in the first place? What are you thinking it will do for you?

[Andina Marie]

I often recommend them. Replacing underwater mounted zincs requires diving or hauling and if it is happening too often it is a bloody chore. By hanging a portable zinc in the vicinity of your mounted zincs you can provide an alternate path for electrolysis currents and make your zincs last longer. Since most electrolysis occurs at the dock you don't need protection while you are underway and it is stowed.

In fact cheaper than purchasing the "guppy" zincs, purchase a large "weld on" zinc they use on steel boats weighing many pounds. Drill a hole for a stainless cable and bolt it on.

[fstbttms]

Originally Posted by Andina

By hanging a portable zinc in the vicinity of your mounted zincs you can provide an alternate path for electrolysis currents and make your zincs last longer.

Again, if the anode is not in direct electrical contact with the item being protected, it is doing nothing. It is certainly not helping your mounted zincs last longer.

[conachair]

Originally Posted by Doodles

I've got one of the guppy zincs having over the side with the lead attached to my backstay. How can I check it to make sure it's working properly? Is there a better place to attach it?

I splashed out on a calibrated silver half cell but apparently a piece of silver wire from a jewelers can be used to check the voltage. All revealed here...

Marine corrosion survey

As below the zinc over the side needs a good connection to the metal you want it to protect.

[sailvayu]

I suggest the OP and others read some of the other threads and information out there about this. Would hate to get this topic going anew but will say that these hang over the side things are all but worthless. For them to work at all they need to have a good electrical contact with the metal being protected. that is almost impossible to achieve with the typical spring clamp they employ. And connecting one to the backstay is likely worthless as well as unless the backstay has a good electrical connection to the underwater metal (not likely) they will do nothing. The key phrase being GOOD electrical connection meaning very low resistance. And lastly it is NOT "electrolysis" (google electrolysis) but galvanic corrosion. I know this subject has been covered well so do the reading and you will save time and money. Good luck

[galynd]

Originally Posted by fstbttms

Again, if the anode is not in direct electrical contact with the item being protected, it is doing nothing. It is certainly not helping your mounted zincs last longer.

Do a continuity check from your shaft to backstay (or where ever you will attach your guppy) to your guppy to see that it is protecting your shaft.

[Andina Marie]

I agree that assuming the back stay is grounded to underwater metal can render the guppy useless although on a steel boat it most likely is.

They are not useless, obviously electrolysis currents are flowing however proximity to the objects to be protected helps. Typical radius of protection is about 10 feet in salt water with protection diminishing considerably with distance. Get it as close as possible to the prop or metal to be protected.

So long as the "spring clamp" is connected directly or indirectly to the underwater metal it will be fine. Even if the connection is bad and it has 10 ohms resistance with 5 milliamps (typical) flowing that is only a voltage loss of 0.05 volts which is 10% of typical electrolysis voltages. A good clean contact will typically be less than 0.1 ohms with a voltage loss of only 0.0005 volts.

It is a mistake to connect it to the negative side of an existing electrical cable because when that cable is carrying current the voltage could be considerably different to the underwater metal.

Yes the key phrase is GOOD electrical connection but it doesn't have to be "very low resistance" because the currents flowing are very low.

It is electrolysis. You are transferring (plating) zinc metal through an electrolyte using a low voltage so it is being deposited onto the underwater metal surfaces. So far as the guppy is concerned it appears like corrosion but it is being sacrificed to protect the underwater metal by electrolysis.

[lancelot9898]

I've used a zinc hanging off the shroud for years and one way to see if it's working is just to pull it up and see if material is being lost. I usually go a couple of years before having to buy a new one. The length of the cable is sufficient such that I clip to the shroud and am able to run the zinc aft so that it hangs in the water close (within maybe 8 ft) to the prop.

[cwyckham]

I've only had mine in the water a couple months, but I seem to be losing material.

I have a dedicated electrical cable running from my push-pit to the engine block. I clip the fish to a stud sticking out of the push-pit. I agree that the electrical connection probably isn't spectacular, so I'm glad to hear that the tiny voltage loss (due to low current) shouldn't be an issue.

[conachair]

Has anyone tried the silver wire half cell ?

Marine corrosion survey

Having spent good money on a calibrated unit I'd be happy to hear it doesn't work

But just couple days ago I wired up a lump of zinc to the (steel) boat and measured the voltage from the silver half cell to the hull as the zinc went in the water. Was initially down near -700mV and over a minute or so rose to over -750mV , stabilized now up near -800mV. Been a while since the last lift out and I suspect the zincs are not great.
So try a bit of wire and a good multi meter and see what happens.
Then tell us

[Mark Johnson]

A Guppy, (or any other anode), can protect ONLY the underwater metal that is bonded to it, (inside or out), and then... that metal needs to be close, like within 6' or so. In other words... The clamp end of the Guppy wire must of be clamped to the interior bonding wire.

My shaft zinc protects the nearby strut, even though they appear to be electrically separated by the rubber cutlass bearing. This is because the strut has an interior bonding wire, that connects it to the shaft at the engine, by a "shaft brush".

It is best in many respects, (excepting ABYC compliance), to isolate the mast and its lightning ground, from any bonding wire and anode protection system.

It is also advisable, to have the mast's lightning ground AND the galvanic corrosion prevention system, isolated from each other, AND the boat's DC- wire, which requires some effort, like rubber mounting the VHF antennae's base bracket, putting in a main battery switch for the engine's - wire, etc...

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!

If you have no "issues" with corrosion, or REALLY fast zinc consumption, by all means use the ABYC's suggested "common negative" ground buss, and connect everything to it, I was a member when I built our boat, and I did... but if you later find that you eat zincs like crazy, and don't actually have a substantial leak from the DC or AC system, then your dissimilar metals have created a battery from all of that bonding, and it has combined with the inevitable trickle charges from the boat's systems. In this case, if you ever accidentally go unzinced, you could loose metal FAST!

If you have this problem, ISOLATE the systems! Then protect those that need it, separately. After making the necessary changes, my problems all went away. I went from 5 small zincs, changed every few weeks, to just the ONE on the shaft. I now change it every 6 months or so, still halfway in tact.

M.