H25 hull zincs-?

Jim Legere · Mar 23, 2016

I have only ever relied on shaft zincs (and I suspect that my 37C has more metal in the water than your 25) and have seen no galvanic corrosion issues. If you are at a marina, a galvanic isolator on the 120 VAC green wire is a must. Lots of options out there but, since I don't use shore power a lot, I went home-built for about $25. There are pros & cons (lots of info on the inteweb) but I check mine with a multi-tester every season and am pretty confident it does the job safely.

Good to see you still posting here BTW.

Dennis Kitchen · Mar 23, 2016

JCII,
The galvanic isolator prevents small galvanic induced currents from completing a circuit which would allow corrosion to begin. However; if a larger current should occur due to a ground fault or short it will pass the current to shore through the green ground line. This prevents the current from going out into the water and seeking a path to ground to the bottom or adjacent dock structures or boats. This would create corrosion on other boats as well as a fatal death field around your boat for a swimmer or someone falling in.
This occurs on systems which have all the underwater metals bonded together via a ground wire which is attached to the ships ground. The AC and DC grounds are then often joined together. This creates the need for the galvanic isolator.
Since you have a glass boat everything is isolated unless you bond them together. How many pieces of AC equipment do you have permanently connected when on shore power? I only have the battery charger so I can periodically test for a fault easily if I want. I will someday put in an isolator however.
So you probably don't really need zincs. I have myself considered making wiring bracket at the top of the rudder to connect to a zinc on my H25 project. Haven't decided yet.
This brings us to another point, never join the AC neutral to the ground wire on the boat. This union should only be made at the shore power breaker box.
Dennis

fstbttms · Mar 23, 2016

DianaOfBurlington said:
...do I even need hull zincs?

The very great majority of sailboats (regardless of size or whether they have an isolator or are bonded) do not have hull anodes and they all seem to do just fine.

Jim Legere · Mar 24, 2016

JCII, I realize I may be preaching to the choir here, but others may find this useful.

"The purpose [of a galvanic isolator] is to disconnect your wet metal parts from the dock supply in order to prevent electrolysis. The trouble is, however, you need them connected together so that if there is an electrical short on the boat, it doesn't make the boat alive at 120 volts or worse which can give you quite a kick when you step off an aluminum dock!!" - Yandina

According to ABYC best practice, the AC green ground wire (but never the white neutral wire) should be tied to the ship ground buss at one point, thus making it possible for your underwater metal to be in a circuit with every other boat that is plugged in to shore power. The galvanic isolator interrupts this circuit at the very low voltages (< 2V) that are typical of stray DC currents that cause galvanic action. But it also allows AC ground fault currents to pass - a very important safety feature. Note that galvanic corrosion is caused by DC not AC; think of metal plating and you have it. More info here:
http://www.yandina.com/galvanicIsolator.htm
http://www.blackfinforums.com/sites/default/files/10/attachments/abyc-e-11.pdf

Jim Legere · Mar 25, 2016

It sounds like your electrical systems are fine. Since there is no connection between your AC system and the underwater metal, you don't need to worry about stray DC currents from the shore power connection. You don't have an inboard with a shaft in the water and attendant DC ground. As you say, the OB has its own battery so any issues are contained to that system (and OBs have their own zincs). Finally, no inverter or generator, so no need to worry about AC grounds away from the dock. KISS beautifully implemented!

Maine Sail · Mar 25, 2016

DianaOfBurlington said:
I am wondering what, if anything, other H25 owners have done about zinc anodes on the hull. My boat had none when I got her. She now has 115vac but only for dockside, and the green wire carries the stray current ashore (no ground plate). The seacocks are Marelon. The speedometer transducer has a plastic shell. The depth-finder one is mounted inside the hull, not through it. The outboard motor has its own zincs. The only other metals underwater are the keel, sealed in epoxy barrier coating, and the rudder shaft, also painted (mostly) in barrier coating.

I have the opportunity to add these in the next month or so and have considered a 'piercing' through the skeg ahead of the rudder, a 3/8"-ID Garelite tube to allow two 3" discs to be mounted there. I can much more easily do this in the rudder; but it is a small blade and the flow would probably be affected... or the discs would just foul with seaweed.

The larger point is this: do I even need hull zincs?

What is your hull zinc protecting, is the fist question that needs to be answered. What underwater metal does it physically connect to?

Maine Sail · Mar 25, 2016

DianaOfBurlington said:
Jim, is not the galvanic isolator for the wire going to ground/earth in the water? I have no such thing. The 115vac runs only when plugged into the dock, and I rely on the green wire going to the dock for that. I have a crossed-polarity indicator on the panel to indicate when stray current is going to the green (which the isolator would protect against, if it happened). The light went off only once-- when I first hooked up the 115vac on the boat-- and after inspecting every single connection on board I found the fault was in the yard's outlet box (no kidding).

My cousin Rick (ABYC) says the biggest problem is for boats with on-board 115vac (generators or inverters) needing to ground the green wire to 'earth' when there's only a ground plate in the water. So at the dock, these people are running two green-wire grounds (if they don't remember to turn one off). So they have a complete circuit. I don't have anything like that going on... hence my question.

Because our boats AC grounding wire (green) is supposed to be bonded to the DC grounding system, for human safety, this means that when you plug into shore power your underwater metals are connected to your neighbors underwater metals via the green AC grounding wire. If you have no underwater metals, and no ground path to the ocean, then there would be no galvanic corrosion. However if you leave your OB in the water, have a bronze ground plate, your centerboard is bonded, your gudgeons are bonded etc. then you will want a GI at a minimim.

A galvanic isolator is a device that is inserted, in series, into the green grounding wire (safety ground) of your shore power feed to help minimize or reduce the effects of galvanic current from flowing between your vessel & your neighbors. While blocking galvanic level current it also has to allow for the passage of AC fault current. This is why any galvanic isolator should be of the "fail safe" type or have idiot lights that tell you you still have an AC grounding connection.

Bottom line: If you plug into a marina without a GI your anodes will be protecting your neighbors boat who's anodes have already eroded away.

This blockage of low voltage galvanic current is achieved by using two diodes in-series in each direction. Each diode drops approximately .6V or requires more than .6V to open and "Flow". Two of them in series results in approximately a 1.0V - 1.2V threshold for blocking DC galvanic level voltage & current.

GI's normally have two diodes in each direction so the AC green wire is not "check valved" and acts just like a wire normally would. The only difference is it acts as one that won't pass voltages below 1.0V - 1.2V. Simple and pretty effective at blocking galvanic level current. GI's do not however block stray current that exceeds 1.2V...

Think of a diode as an electrical check valve. It allows current to flow in only one direction but not in the other direction. One of the inherent traits of diodes is the voltage drop associated with them, which is usually around 0.6V. In a galvanic isolator application they have used this often assumed bad trait of a diode to an advantage. By wiring two diodes in series you now have a device that can block any galvanic level voltages below 1.0V - 1.2V from flowing into or out of your vessel.

If we understand the voltage potential spreads, between underwater metals, which is usually below a 1.0V differential, it becomes easy to see how a GI works.

Because the dissimilar metals connected together in the electrolyte can't really create more than 1.2V the GI stops your vessels anodes from protecting your neighbors underwater metals.

If plugging in at a marina, with underwater metals that need anode protection, a GI is the absolute bare minimum level of protection that every boater should have.

On personal level you'd never catch me plugged into any marina without an isolation transformer (IT). A simple GI won't cut it for me. Of course the conversation of true isolation versus galvanic isolation only takes on another life and is a whole other discussion & topic. Without knowing what your anode is actually protecting it is impossible to say what to do...

At a bare minimum you should have a fail safe galvanic isolator if you have below water metals.

The standards for AC grounding (green AC wire) are simple and clear:

ABYC E-11
11.5.3.3 The main AC system grounding bus shall be connected to:

11.5.3.3.1 the engine negative terminal or the DC main negative bus on grounded DC systems, or

11.5.3.3.2 the boat’s DC grounding bus in installations using ungrounded DC electrical systems.

Dennis Kitchen · Mar 25, 2016

DianaOfBurlington said:
I was always of the impression that zincs are best for stray current and for the natural bimetallic corrosion of dissimilar metals underwater. Rick tells me that it's the other guy's crossed-polarity and inadvertent arcing-to-chassis problems that you need to be worried about.

JCII,
Your impression is correct regarding dissimilar metals corrosion. "Stray current" is what Rick is describing in your quote. The problem is that stray currents, whether caused by your boat or the guy's next to you is that they could deplete your anodes in a matter of days depending on the amount of current flow. Though anodes can protect your metals from stray currents for a short term, the problems that cause them must be addressed immediately. Purely galvanic currents from your underwater metals can be addressed by your anodes for perhaps several years.
Mainsail's links I'm sure will give you the full scoop and he certainly is more the expert than I. As I understand it ABYC has progressively developed its standard over the years and "one thing leads to another". I agree with MAINSAIL that an isolation transformer is the best solution but at the price they go for it isn't ever going to happen on my boat. So I work backwards through the standard until I have an option that agrees with my system and my budget.
So with your system as described I believe you really have no issue, except perhaps the microwave. Your grounds aren't joined, your metals aren't bonded (what little you have), your shore power is correctly wired. If you have all your outlets on one circuit, be sure the first one in the series is a GFCI type. Inspect the prongs and sockets on your AC cord regularly for tarnishing or burning which could indicate/cause problems. If your microwave is not marineized it could possibly have the neutral and ground joined inside. As for your motor, I've never thought about that one. My guess is that as long as your battery ground circuit is good, clean, and low resistance you'll never have a problem with current flow through the chassis.
Now, after all my generalities, I defer to MAINSAIL who is an expert dealing with this stuff daily. My memory is mush. I better go dust of my "Boat Owners's Mechanical and Electrical Manual" by Nigel Calder.
Dennis

Maine Sail · Mar 25, 2016

Guys,

Stray current is not the same as galvanic current, and I believe it's important to understand the differences..

Galvanic current created by interconnecting dissimilar metals, with varying electrical potentials, in an electrolyte (salt water). By immersing a cathode (most noble metal) and anode (least noble metal) we create a galvanic cell. When you immerse dissimilar metals in an electrolyte you create galvanic current flow between the cathodic and anodic metals. The least noble metal in the electrolyte will give up its metals. This is why a zinc anode is called a sacrificial anode because it is placed there as the weakest/least noble metal to erode, instead of your more expensive metals eroding.

Stray current, also called leakage current, is cause by a failure in the electrical systems & results in direct DC or AC leaks into the electrolyte. AC leaks are far, far, far less damaging & considerably slower, than DC leaks, but AC stray current leaks can be deadly to humans. DC stray current leaks are deadly to your underwater metals.

The most common forms of DC stray current leaks are bilge connections or poorly made terminations. Wires that pass through the electrolyte in the bilge water with cracked or chafed jackets or open un-sealed terminations also rank high on the list..

Improperly wired bilge pumps is another major source of stray current. The simple act of wiring the bilge switch into the negative side of the circuit can create a serious DC leak when the pump seals begin to fail, and I see this major flub violated regularly.When properly wired in the + wide of the circuit the pump only has DC at it when the switch is made. Even if the seals wear it is only for a few seconds that the DC stray current can leak into the electrolyte. When the switch is wired in the negative side the motor always has +12V current at to leak into the electrolyte despite the switch not being made..

A bad DC stray current leak can eat a underwater metals in as little as a day. Bad DC stray current issues can travel many yards through the electrolyte but galvanic current won't. Stray DC current problems that create damage almost always originate on-board your own boat but can originate elsewhere too, and if above 1.2V, or your boat is bonded, the galvanic isolator will do nothing to stop stray current from using your boat as a lower resistance pathway.

fstbttms · Mar 25, 2016

Maine Sail said:
A bad DC stray current leak can eat a underwater metals in as little as a day. Bad DC stray current issues can travel many yards through the electrolyte. Stray current problems that create damage almost always originate on-board your own boat...

This. ^^^
Broken insulation on a bilge pump wire caused this in just a few weeks. The prop was eventually lost.

Jim Legere · Mar 25, 2016

Thanks for the important distinction Maine Sail - obviously a galvanic isolator (GI) only protects you against currents at less than 2 VDC potential, which is to say currents generated by dissimilar metals in an electrolyte (salt water). Hence the descriptor: "galvanic"...

Also of note is that a GFCI cannot protect you properly if the neutral (white) and ground (green) are tied together - I don't know how microwave ovens are wired but it is worth checking into.

Dennis Kitchen · Mar 26, 2016

It may be that in any appliance as modern as a microwave the wiring is going to be modern convention as well and would not have the ground and neutral connected and I am therefore passing on an antiquated warning. Still, worth it to check, huh. Your story of the old house illustrates how conventions have changed over the years as understanding has grown and technology has improved. Putting it on its own breaker is a good idea, it could always be off unless you're on board expecting to use it. Another horror with AC appliances is they could be safe when sitting static but leak current when in use. This is why good galvanic isolators have indicator lights and should be mounted where they can be monitored when AC is being used. A single pole breaker on the black wire should be sufficient when polarity is wired correctly through out the system and AC is not used when a reverse polarity is indicated on the main breaker panel.