Galvanic Isolators, Shore Power and Zinc’s

Jack F. Honey 1996 – Manfred von Borks 2006/2014

The following paper is a combination of four articles and lecture comments by the authors; some information may be repeated…

When two different kinds of metal are immersed in sea water and brought into electrical contact a galvanic cell is formed and an electric current is generated. This process involves galvanic corrosion of one of the two metals. A boat's underwater metal may be protected against galvanic corrosion quite simply by (1), installing one or more zinc anodes on the outside of the hull; (2), wiring (bonding) them to the boat's underwater metals, and (3), monitoring the operation of the zincs and replacing them as they are consumed. In this paper we discuss the operation of the zinc corrosion protection system, its installation and the monitoring of its proper operation…

Zinc Protects a Boat's Underwater Metal!  In a properly protected boat, a ground bonding system is provided which effects good electrical connection between the zinc(s) and all underwater metal parts including the propellers, propeller shafts, shaft struts, metal rudders and their shafts, metal through-hull fittings. etc. Inside the boat the bonding system is connected to the engines, shaft brushes, tanks, engine water intakes, sea water pumps, bilge pumps, etc. The ground bonding system should have only one point in common with the DC negative and AC non­current-carrying ground power systems, usually at a common connection to the engines. No part of the ground bonding system should be permitted to carry any power from the AC or DC power systems.


The zincs in combination with the sea water electrolyte and the bonded underwater metals form a galvanic cell. This is much like an old flashlight cell, in which the zinc case is the anode and the carbon center post is the cathode, both being immersed in the electrolyte inside the cell. This galvanic cell causes a current to flow from the positive carbon center-terminal through the flashlight bulb and back to the negative zinc case, from there the current flows through the electrolyte back to the carbon center post completing the circuit. Electron flow outside the cell is in the opposite direction. When you connect the boat's zinc to the bonding system, you are actually shorting the galvanic cell and you wind up with maximum short-circuit current from the zinc through the sea water to the underwater metals through the ground bonding system and back to the zinc. It's this underwater current flow from the zinc to the underwater metal that results in the desired protection against galvanic corrosion.

The current in the water is carried by H+ hydrogen ions. When these reach the cathode they collect an electron and are plated out on the metal surface just like in an electroplating operation, resulting in an invisibly thin layer of monatomic hydrogen which covers the underwater metal parts. This layer, the voltage across it and the ready availability of more electrons prevent positive ions of the protected metal from forming and leaving the metal surface. This is the corrosion protection desired.

As this hydrogen layer builds up over a period of time, the voltage across the layer increases toward the open-circuit voltage of the galvanic cell and the zinc current is reduced. It is said that the cell is becoming polarized; the hydrogen layer is the polarization layer, and the voltage across the layer, from the metal surface to the sea water beyond the layer, is called the hull potential. The magnitude of this hull potential between the bonded metals and sea water near the boat are excellent indications of the operation of the zinc protection system, and it can easily be measured using a sensitive digital voltmeter and a special probe.

The same voltage that is developed across the infinitesimally thin hydrogen layer on the protected metals also appears across a similarly thin layer on the zinc anode surface. This layer is formed by zinc ions (Zn++) which have dissolved off the zinc surface, one zinc ion for each two electrons conducted from the anode to the cathode via the bonding system. The zinc ions don't get very far because they're heavy and they are snapped up by the hydroxyl (OH-) ions in the water, forming insoluble zinc hydroxide , the whitish mushy material you see accumulating on the zinc surface.

The polarization layer on the protected metal doesn't stay put once it's formed. Water turbulence tends to wash it away, impurities in the water disturb it, dissolved oxygen in the water converts it into more water, and the hydrogen atoms like to pair up and leave as bubbles of hydrogen gas (H2). So a continuous flow of zinc current is required to maintain the layer. This may amount to a zinc consumption rate of one pound for a quarter-amp of zinc current flowing for two months. It must be emphasized that substantial zinc consumption like in this example is normal and necessary and proves the zinc is operating properly. If you find your zincs are lasting for eight months or more without substantial erosion it's proof they're not doing anything.

Zinc Performance: The performance of the zinc corrosion protection installation on a boat can be checked by periodic measurement of hull potential. It usually suffices to check the zincs and the propellers visually during each hull cleaning - say every month or two - but that doesn't show that the zincs aren't working until it may be too late. Many boatmen don't see the need for further precautions, and many hull maintenance people aren't set up to make hull potential measurements.

Caution! Some zinc anodes contains impurities, notably iron, which prevents the normal function of the zinc. A very hard gray surface coating is formed on the zinc in a few weeks which looks like normal zinc but which can completely block zinc current flow. This coating can be removed with a sharp paint scraper, not easy work under water, and the layer starts coming back as soon as you're finished. Years ago, the U.S. Navy established Mil-A-1 8001, a military specification which states the maximum tolerable impurity levels in zinc anodes for Navy use. A number of suppliers to the recreational boating market advertise zincs that meet this Mil Spec. And a number of suppliers do not. It strikes one as humorous that one major manufacturer actually advertised that his non-MilSpec zincs greatly outlasted his Mil-Spec competition.

Hull Potential Measurement (HP): Hull potential is the bottom line of corrosion protection, and it is easily measured. For portable use, one needs only a digital multimeter. These versatile and inexpensive instruments can perform many functions of value to the boatman. The range needed for HP measurement is 0-1.999 DC volts with 1% accuracy and 1 mV resolution.  One also needs a silver/silver -chloride reference cell on a lead about 30 feet long. In addition, pair of meter leads with sharp needle tips is very handy for making good contact with various fittings and elements of the bonding system.

The DVM is set to the 2-volt DC scale. The reference cell is lowered over the side a foot or more away from any underwater metal and its lead is connected to the positive meter terminal. A needle probe is connected to the negative meter terminal and used to contact the common ground point of the bonding system. The hull potential should measure 0.6 to 0.9 volts (600 to 900 mV. An HP of 0.3 indicates little or no protection, and 0 volts shows marginal protection. It should be mentioned that HP and zinc current will both vary with boat use, tidal current and height of tide in a harbor, salinity, temperature, zinc condition and so forth. It's hard to find the ideal HP. If there is reason to believe that the HP is consistently too high, the average may be reduced by inserting an adjustable resistor, say up to 0.5 ohms, in series with the zinc connection to ground. It takes about a day for the HP to settle to its new value when such a change is made. An HP around 700 mV will assure corrosion control yet minimize wood burning in wooden boats.

The HP of all through-hull fittings can be measured individually in this manner. Those that are bonded together should all be at the same voltage. If one of them shows a significantly lower reading, it indicates a poor bonding connection. The connection should be disassembled and cleaned, then rechecked.  The DVM can also be used with two needle probes to measure the voltage difference between different points on the bonding system, starting with the zinc connection. This is an excellent way of spotting bad connections, and the reference cell is not needed.

Zinc Current Measurement: The DVM can be used to measure zinc current directly. A meter shunt resistor of, say, 0.02 ohms resistance, 1% accuracy and maybe 5 watts are inserted in series with the lead from the zinc to the bonding system. The two leads from the meter are connected to the two resistor terminals, and the voltage drop across the resistor is measured using the lowest millivolt scale. The current in milliamps is the voltage across the resistor in millivolts divided by the shunt resistance in ohms. Zinc current in the range or 150 to 500 ma or more may be encountered.

Low zinc current combined with a high HP, say 800 mV or so, indicates a fully charged system. Low zinc current and a low HP indicates that the bonding system is in bad shape or that bad zincs have resigned from the race. A high zinc current with a low HP may be encountered if the zinc is reconnected after a period of time. Thus the combination of HP and zinc current measurements can provide valuable clues to troubles and to a properly operating system. All by itself, though, the HP is the bottom line indication of good corrosion protection. While we're on this subject I'm reminded of the time the twin hull mounted zincs on our 42’ cruiser quit or so it seemed. We heard about cleaning the threads on the mounting bolts when installing new zincs, but we just couldn't believe that all four mounting bolts could be insulated. A needle probe on the zincs told the story - both zincs were completely insulated from their bolts by a very hard layer of white hydroxide. Always clean (wire-brush) the threads.

Shaft, prop and rudder protection will be difficult to monitor if separate zincs are installed directly on the shaft and rudder. One can gain an indication of proper function by measuring the voltage difference between the prop shafts and the central bonding point, same for the rudder shafts. If the difference is small, chances are that protection is effective. Even if shafts and rudders have their own zincs, they may be receiving backup protection through the shaft connections to the engine and through the rudder log connections to the rudder shafts. Either way, the hull potential measurements on these shafts or voltage differences to the central bonding point is a good check. The electrical connection between prop shaft and engine is often subject to intermittent poor or open contact due to oil film insulation in the thrust bearings and reduction gearing. A good quality shaft brush, regularly maintained, is the cure for this condition.

Shore Power Ground: The green shore power grounding lead is brought on board through the shore power connection and tied to the central grounding point. Your ship's zincs are thus presented with the opportunity of protecting the Continental US. to say nothing of other nearby vessels which may not have their own zinc protection? The shore grounding lead - typically several hundred feet of # I 0 wire or so - is much higher resistance than own ship's sea water ground, but the extra drain on the zinc is not helpful and it reduces the HP developed. Installation of a Galvanic Isolator in the green grounding conductor between the hull receptacle and the central ground breaks the connection for low potential yet retains AC fault protection for those on board or dockside.

Galvanic Isolators, Shore Power and Zincs:  The purpose of a Galvanic Isolator is to prevent excessive zinc consumption, and to prevent reduction of corrosion protection when the shore power grounding connection is brought aboard. The purpose of the zinc is to protect the boat's underwater metal from galvanic corrosion. The various underwater metal parts, props, shafts, struts, logs, rudders, through-hull cooling water inlets and so forth are each connected by heavy wire (bonded) to a central point, usually the engine. The boat's zinc is also connected to this point, forming a galvanic cell.

Current from the zinc flows through the water to each of the underwater metal parts and back to the zinc through the bonding connections. This current slowly builds up an insulating layer of hydrogen gas on the underwater metal which prevents corrosion and which gradually reduces the zinc current. This process is called 'polarization'. The voltage across this layer is called the 'hull potential' and is a measure of the adequacy of the protection being provided by the zinc. It may be measured with a digital multi-meter connected between any underwater metal part and a special probe (a silver/silver chloride reference cell) in the seawater near the boat.

This polarization layer is attacked by dissolved oxygen in the water, turbulence and the like. The zinc current is reduced from the initial value needed to form the polarizing layer to just that needed to replenish it. The zinc, like the zinc case of a flashlight cell, is slowly consumed in the process of delivering the polarizing current. That's why it's called a “sacrificial zinc anode”.

The boat's shore-power connection brings all the conveniences of 120-volt ac power on board when you're in your slip. To protect those on board from electrical shock, in the event of a ground fault in some ac circuit or appliance, the shore-power grounding conductor (the green wire) is brought on board along with the two current-carrying conductors, and it is connected to the grounding terminal of all convenience outlets and the cases of all electrical appliances. To back up the protection provided by the shore ground, the green grounding wire is also connected (per ABYC standards) to the boat's underwater metal bonding system discussed above.

This leads to a problem.... The boat's zincs are now obliged to deliver current not only to the boat's underwater metal but to the buried-metal grounding system on shore and to any unprotected boats on shore power in the vicinity. The boat's zincs just aren't up to the task of protecting the entire shore-power ground system and the boat as well. Zinc consumption is very greatly accelerated, and the hull protection voltage is substantially reduced.

Example: Measurements on a typical 42-foot cruiser showed 0.40 amperes dc current in the shore grounding lead, representing the unnecessary loss of 0.77 pounds of zinc a month, plus a substantial reduction in the boat's protection level ...which the Galvanic Isolator cures!

A Galvanic Isolator, installed on board in the green grounding lead from the shore-power receptacle, blocks any significant current due to voltages less than about one volt, which includes the galvanic potential differences which may exist between a protected boat and the shore grounding connection. Yet in the event of a ground fault in the ac system on board, the voltage across the Isolator is enough for it to pass the full ac fault current with less than 2 volts drop, thus preserving full protection for those on board or alongside.

What size Galvanic Isolator? The Galvanic Isolator must be capable of carrying any ground fault current to which it may be subjected. Fault current due to a direct short in the wiring will substantially exceed the trip level of the boat's main and branch circuit breakers and the dockside circuit breaker, and one of these will open after a few moments. So the Isolator must carry very high currents for a short period of time. Other ground faults, such as insulation failure in a heater or refrigerator motor, or water penetration into an appliance or wiring fixture, may involve fault currents less than the smallest breaker rating. The Isolator must therefore be capable of handling current up to the service breaker trip level indefinitely without overheating.

Some boats and more recently some marinas are equipped with Ground Fault Circuit Interrupters (GFCI or GFI) devices in the shore power circuit in place of the conventional breakers. These are similar to those mandated for new-home construction (baths, kitchens, laundry’s, pools...) for a number of years. These devices sense ground fault currents as low as 0.005 amps and open the power circuit within 0.03 seconds, providing outstanding ground-fault protection for those on board, for those alongside and for those in the water near the boat as well.

Even with GFCI, a galvanic isolator is required in the shore grounding lead. If the entire boat is protected by on-board (rather than dockside) GFCI devices, the isolator need not be capable of carrying high current for an extended period of time. With dockside GFCI protection, one would still need high-current isolator capability when visiting an unprotected slip.

A basic (no capaciter and not self-testing) Galvanic Isolator is shown below they are very simple devices and they remain in use on the majority of boats built and modified prior to about 2000. In the past a DIY Kit consisting of instructions and two potted dual 35 amp diodes sold for $5.95, you supplied scrap aluminum for a heat sink; today thousands are out there working just fine.

Typical Mar-Gal Installation: Locate the inboard side of the shore power receptacle and identify the green grounding conductor. It goes to the receptacle grounding prong which is shaped differently that the others. It must NOT be confused with the two or three other current-carrying conductors, which may be color-coded black, white, red, blue, etc. Mount the unit at any convenient spot near the green grounding wire from the receptacle. Arrange it so that the green wire can be cut and its two ends connected to the two Isolator terminals. The green wire from the receptacle should go to the terminal having the insulating washer. Crimp the two ring terminals provided to the two green wire ends and assemble.

When two or more boats are connected to shore power, one side of the necessary circuit required to form a galvanic couple is provided by the AC green grounding wire, which is also connected to the boat ground system, engine, and underwater hardware. The seawater electrolyte provides the other side of the circuit. Galvanic current flowing around the circuit will corrode the least noble metal between the two (or more) boats, in this case aluminum Stern Drive:

Aluminum Sacrificial Anodes - Are They Better Than Zinc? Any discussion concerning marine anode protection must start with the notation “Is your vessels AC shore power system equipped with a galvanic isolator, it should be so check on that immediately. U.S. Marinas and Ports berth many thousands of vessels that use sacrificial anodes to protect underwater metal parts from damaging galvanic corrosion. Most pleasure boats and many fishing/working boats use zinc anodes for this purpose. Our single screw 36’ Motorsailor will dissipate about a pound of zinc anode material a month, larger twin screw trawler hull vessels use much more. Divers who clean hull bottoms typically replace underwater anodes, replacing anodes is an important source of their income. Few divers or boat owner understand the complex electrochemistry involved and as such are unable or just not interested in evaluating the effectiveness of protection thus to “play it safe” there is a tendency to replace functioning anodes that can provide months of useful life. Contrary to popular belief in some small boat applications a quality anode properly installed will continue to protect the more noble underwater metal parts such as thru-holes, propellers, shafts, rudders, etc., until the anode is almost completely consumed. It is relatively easy to determine how effective your anodes are working and if they need replacing requiring only an inexpensive digital multimeter (sometimes available as a freebie at Harbor Freight) and a silver-silver reference cell. Most full service marine dealers offer meters, reference cells and how-to books. For additional information on-line Search “marine galvanic testers”. An on-line supplier is 

In too many instances (like maybe always) anodes are replaced when they are only a third to three-quarters used. An additional major problem is a poor quick & dirty installation and the use of anodes of lesser quality that quickly crumble in salt water. Anodes that are not correctly installed will quickly loosen and/or are not bonded to the host through failure to properly clean the contact points prior to installing the new anode, loose or poorly bonded anodes are essentially “open circuits” they very slowly dissolve away providing no protection whatsoever. If your anode looks like new after a few months you can be sure it is not working.

Are sacrificial anodes damaging the marine environment?  The Canadian Government Ministry of Water, Land and Air published recommended guidelines for zinc for marine life: 10 ppb (µg/L) is acceptable with 55 ppb considered high. After considerable research over a period of 30 years we and many or our colleagues and peers have concluded that zinc and aluminum anode dissipation in a Marina is not toxic to human or marine species, in fact there is a hint that zinc, like trace elements of copper from bottom paint, may be beneficial.

Over the past 20 years boat owners in many European countries have replaced, where appropriate, zinc anodes with aluminum anodes, this aluminum alloy provides the same degree of protection but they are less expensive than zinc, very light and easy to install, and aluminum last about 30 to 50% longer than Zinc. Note: Please do not mix zinc and aluminum external anodes together on the same vessel, use one or the other, better yet go for a bonded system with monitoring capability, in the long term it will save you money and give you peace of mind knowing your very expensive stuff under water is protected.

Boaters in the U.S. have been slow to replace zinc with aluminum, most likely because few boaters know about them and little effort has been devoted by manufacturers, distributors, dealers and divers in promotion of aluminum mainly for economic reasons, like why promote a product that will immediately reduce income potential?

We have a totally bonded system on our 1979 Monk/Roughwater 36’ Sedan using a single Martyr Divers Dream aluminum stern mounted anode. We monitor the system and replace the anode only when necessary. We have been using this system for well over 15 years with excellent results plus considerable savings in anode replacement costs and labor.  For more anode information go to: