The Outdoor Line Forum

Now, I'm just a wife of a recreational fisherman -- but even I know that if you have a zinc that is about gone, that is just pointing to a huge problem. Especially as new as your boat is, zincs don't go away that fast, do they? Didn't hear you really talk about that.

Love your show - we never miss -- and even plan our Saturday around it!

Hey Tyee,
Thanks for listening and you're absolutely right! Actually, according to Chapman's Piloting once a zinc is 50% eroded it's functional life is exhausted.

The situation I found was a one year old zinc that had it's mounting screws come loose and was no longer effective/active. The zinc had was only about 20% eroded but I elected to just replace it. I will use the old zinc in a crab pot!

To further discuss the electricity issues surrounding aluminum boats, here's a question we recieved from a listener and my answer:

"Hey Tom,
On the show today you talked about stray volts on your aluminum boat. I have a 17 footer and was concerned about using the hull for ground. I am upgrading to a 20' aluminum boat. Should I have them isolate the ground and change the way they wire up the boat? Thanks, Jim"

Hey Jim,

Basically, an aluminium boat is a different animal on the water than a fiberglass boat and has the advantage of having a larger electrical "footprint" or grounding surface on the water. The disadvantage is the corrosive potential of the aluminum hull in salt water.

To combat corrosion inside the hull, a dedicated, isolated 12 volt electrical ground system is infinitely preferable to the use of automotive style "frame grounding".

In a salt water application, visible corrosion occurs in a new, "frame grounded" boat in months and accelerates as the boat ages. I know this for a fact as I have seen it with my own eyes. In no circumstance would I ever accept a boat without a negative battery cable system.

External, underwater electrolytic corrosion is a different animal and can be prevented through the use of zinc sacrificial anodes installed on a bare, unpainted, submersed portion of your hull, preferably on the transom. Keep an eye on the zinc as they erode and the general recommendation is to replace them when they are 50% eroded.

However, if you're serious about your fishing, 50% of a zinc is just not acceptable.

Once you see obvious pitting on your zinc, that's a good sign the zinc is functioning properly. Once they are 25% eroded, they're gone on my boat!

To take the guesswork out of the equation, start off this season with fresh zincs and replace them every season! Your boat will not only fish better but will be better protected from that corrosive electrolyte called salt water.

Hope that helps!

And how does a Fiberglass differ from a Aluminum boat?

Hey LM.
It all comes down to the fact that aluminum conducts electricity and fiberglass does not.

Every metal component mounted to an aluminum hull is essentially grounded to the hull and therefore electrically bonded together. Common bonding reduces the potential of corrosion and presents a unified electrical footprint in the water.

To achieve the same common electrical bonding in a fiberglass boat, all of the vessels' underwater metals must connect to a bonding strip or have dedicated wiring providing the common connection. Only by bonding all of the fiberglass boats underwater metals together can they share in the corrosion protection afforded them by your vessels zincs.

"Zinc",... there's that word again

Not to beat a dead horse but the following is a great explanation on the boat-eating process known as electrolysis:

A NON TECHNICAL EDUCATION ON BOAT ELECTROLYSIS
Whenever different metals are placed in a conductive liquid you create a battery. If you connect these pieces of metal together, current will flow. The current will be removing metal from one of the metal pieces is known as "electrolysis". If this piece is the zinc in your flashlight battery that is good, but if one of the pieces is your propeller, lower unit, downrigger wires,... it is bad.

The zincs you use on a boat are called "Sacrificial Anodes". Zinc is used because it has a higher voltage in the water so the current will be more inclined to flow from it than from your propeller. To complete the electrical circuit, the zincs must be connected to the items they are intended to protect. Usually this is no problem because the zinc is bolted right to the shaft or underwater housing. Non metal boats will usually have a copper bonding wire inside that connects all the underwater metal items together so they all share the protection from zinc anodes. Since engines use the metal frame as the negative battery connection and the engine is connected to the prop shaft, the engine and the negative side of your 12 volt system are also part of this bonding connection. This bonding wire is usually connected somewhere to the rigging. This is not for electrolysis protection but for some protection from lightning strikes to conduct it into the water through the items connected together.

If other currents are allowed to get into this bonding circuit they can easily overpower the small voltage available from your zincs and defeat the protection you need. This is usually the most destructive form of electrolysis and you notice it because your zincs get eaten up very quickly trying to keep up. Under normal circumstances, zincs should last at least a year if they are working normally, and much longer if you don't have any problems. If they are being "sacrificed" in a shorter period you need to find where the external current is getting in.

The most common source of this external current is the shore power connection, especially the ground lead. Docks are notorious for bad wiring and often the ground lead is not connected to ground, is connected to the neutral, is being used for carrying current to a mis-wired boat, and all other sorts of problems. So the ground lead should never be directly connected to the ground bonding system we talked about earlier. The purpose of the shore power ground lead is to provide a return path for current if there is a short circuit or power leakage from an appliance or the wiring on the boat. You don't want it to connect all the underwater items on your boat to the underwater items on all the other boats and the dock because now your zinc is trying to protect everyone else too.