Category Archives: Boat DC Topics

DC Electricity On Boats

About This Article

This Article discusses DC Electricity concepts and terminology at an introductory level.  There are always discussions on boating bulletin boards relating to DC power systems on boats.  This article is intended to help those with little or no background or training in electrical systems to understand those discussions.  I have included the most important sub-topics related to 12V and 24V “low-voltage” DC power distribution systems encountered by typical cruising boat owners.

Electrical Safety

There is one, and only one, absolute when dealing with electricity.  VIRTUALLY ALL ELECTRICITY CAN BE DANGEROUS TO PROPERTY AND LIFE.  Even de-energized electrical circuits can retain enough stored energy to create a life-threatening hazard.   The large batteries and large banks of batteries found on boats can produce explosive gasses and store enough energy to easily start a large, fatal fire.

ALWAYS WEAR SAFETY GLASSES while working around electricity!  WEAR HEARING PROTECTION when working in noisy environments, with running engines or other loud machinery.

If you are not sure of what you’re doing…
If you are not comfortable with electrical safety procedures…
If you are not sure you have the right test equipment and tools for a job…
If you are not sure you know how to use the test equipment and tools you do have…
Well, then, LEAVE IT ALONE until you do!

USE INSULATED TOOLS when working around electricity, and especially around batteries.   Batteries contain enormous amounts of stored energy.  Accidental contact of a metal tool across the terminals of a battery is an emergency situation.  The tool can actually weld to the battery terminals and be both too hot to touch and impossible to remove without external mechanical force. Whenever working around a battery, pre-plan to have a two foot piece of 2”x4” readily available at hand.  If the worst should happen, use the wooden 2”x4” to knock the metal tool away from the battery terminals.  DO NOT TOUCH the tool; assume it will be far too hot to handle with bare hands!  Once this cascade of events has started, the only way to stop it is to break the tool free of the battery terminals.  Otherwise, the battery will get so hot it will melt and may start a fire.

Be very wary of unfamiliar, pungent odors.  Transformers, motors and most electrical and electronic devices that are in the process of failing often heat up and cause insulating or potting materials to give off strong, pungent odors. TURN OFF POWER and use your nose to track down the source.  Treat this as a true emergency.  If you can find the offending device before it bursts into flame, you’re way ahead of the game!  Turning off the power will usually allow the device to cool off.  Do not restart the device!  Excessive over-heating often causes secondary internal damage that you cannot see.

What Is DC Electricity?

DC voltages at their source are characterized by 1) a stable voltage amplitude of 2) unchanging polarity; i.e., the polarity of the voltage between the supply and return terminals never changes.  One battery terminal is considered “positive” and marked with a “+” sign, and one battery terminal is considered “negative” and marked with a “-” sign.  Terminals are either “positive” or “negative” with respect to each other, nit the external world.  The “positive” terminal is positive with respect to the “negative” terminal; the “negative” terminal is negative with respect to the “positive” terminal.  This distinction is important in using a voltmeter to measure voltages.  A DC voltmeter will provide both the amplitude of the voltage that’s present and the polarity of the conductors or components between which the meter is attached.   The amplitude of the voltage can vary somewhat over time, as over the period of time that a battery discharges, but the polarity of that voltage between battery terminals does not change.  This is the fundamental difference between AC and DC electricity, and that difference leads to all of the technical advantages and disadvantages the different electricity technologies offer to users.

Key Electrical Concepts and Terms

The following are some terms regularly used in listserv posts and widely encountered in discussions of electrical systems and circuits.  Boaters will do themselves a great favor by learning these terms and understanding the concepts these terms represent.

  1. Source – Point-of-origin of an electric current.  Typically for DC systems, a battery or bank of batteries.  Electrical sources are “balanced systems,” in that whatever current leaves must return to the source on a one-for-one basis.  If a return path is not available, current cannot flow and useful work cannot be performed.
  2. Load – The components within an electrical system that consume electrical energy to operate; ex: lights, heating elements, motors and electronics.
  3. Circuit – a network of conductors and components carrying electric current from the source to the load, distributing current throughout the load, and returning current to the source from the load.  Circuits are always closed loops that originate AND terminate at the power source.
  4. Supply (or “B+”) – the current-carrying conductor that transports electric current from the source to the load where power is consumed.  In “negative ground” DC systems as required on boats, often called “B+.”
  5. Return (or “B-”) – the current-carrying conductor that returns power from the load back to the source.  In negative-ground DC systems as required by ABYC on boats, often called “B-.”  Analogous the the “neutral” conductor in AC circuits.
  6. Voltage/Volt – the unit of quantification of “Electromotive Force” (“propulsive energy”) that acts on a circuit to force electrons to flow.  Electromotive force is measure across two points in a circuit.  (Volt, millivolts)
  7. Ampere/Amp – the unit of quantification of current flowing through a particular point in an electric circuit.  (Amps, milliamps)
  8. Current – the flow of electrons through a conductor, or the flow of ions through a liquid medium such as salt water; electric current is what performs “work,” i.e., fulfills the purpose of a circuit.  (Ampere; Amp)
  9. Resistance – physical property of all electrically conductive media that acts to retard or impede the flow of electrons through it.  All conductors have resistance. (Ohm)
  10. Conductor (lead, line, cable) – circuit device that transports electric currents.
  11. Ohm’s Law – Mathematical formula that describes the relationships between voltage (V), current (I), resistance (R) and power (P) in a circuit.
  12. Power – the quantification of the amount of “work” that electric current performs in its application.  In purely resistive applications, this will be light or heat.  In turning a motor, this will be the amount of electrical energy consumed in creating torque.  (Watt) (appropriate torque unit)
  13. Ground – a) a universal standard earth reference voltage of “0” volts; b) conversationally, the portion of an electrical circuit to which all other parts are referenced.
  14. Common – any interconnected portions of circuit to which many other parts of an electrical system are also connected.  If reference is specifically to “ground,” this term references a “return” return path shared by many separate portions of an electrical system.  Example: the  positive and return conductors to flybridge nav instruments may be supplied by a “common” B+ power feed conductor (red) and wired with a “common” B- return (yellow) conductor.  Analogies: “ground,” “B+,” “B-” “buss.”  Opposite: “home run.”
  15. Neutral – a non-ground, normally current-carrying return path for electric currents; customarily used in the context of AC circuits.  In DC applications, B- conductors are analogous to the AC neutral.
  16. Fault current – a current flow that follows an abnormal and unexpected path from its source to its return point.
  17. Short circuit – an electrical fault condition resulting from the unintentional connection of a source directly to a return circuit or earth ground.  This unintentional connection often results in the flow of extremely large fault currents. The electrical system should be designed in such a way that fault currents are automatically interrupted by circuit breakers of fuses.  This condition may not cause overload protection devices (circuit breaker) to disconnect the source of power in “ungrounded” systems..
  18. Chase – enclosed spaces in a building or a boat through which wires are run to achieve access to remote locations
  19. Raceway, Conduit, Spiral Wire Wrap, Split Wrap – varieties of supplemental physical enclosure intended to protect electrical conductors from accidental physical damage, excessive ambient temperatures and vibration.
  20. Switchgear – a generic term for all equipment housings in which fuses or circuit breakers and similar disconnecting or switching devices are mounted.  This term is used across the electrical power industry, from generating stations to transformer yards to neighborhood distribution yards to commercial and residential locations.
  21. ABYCAmerican Boat and Yacht Council, Annapolis, MD.  An organization that produces a comprehensive set of safety standards applicable to boats and boat manufacturers, the marine insurance industry, surveyors, attorneys involved in litigation and boat owners.
  22. NECNational Electric Code; United States electrical design standards for Power Generating and Distribution Systems, state, county and community code regulations  and the electrical construction industry.
  23. NFPANational Fire Protection Association; organization that creates and maintains the NEC.

DC Circuits

Fundamental Concept

The essential components of all electrical circuits are:

  1. a source of electrical energy,
  2. a conductor that transports electric current from the energy source to a load,
  3. an electrical load, where useful “work” results when an electric current flows, and
  4. a conductor that transports the electric current back from the load to the energy source.

By definition, an electrical “circuit” must contain all four of the above elements.  All electrical circuits (DC or AC) originate as a pair of electrical terminals that are connected to power-consuming load devices by conductors (wires) of one type or another.  Electric current flows through a circuit.  If a complete electrically-conductive loop is not available from “source” through “return,” an electrical current cannot flow.  Switches, fuses and/or circuit breakers are used to create an incomplete electrical path from the source to the load.

An electric current is the aggregate of millions of migrating electrons (and ions in liquid media).  In DC circuits, it is “convention” to think of the electric current flowing from positive to negative.  This “convention” Is a “working agreement” across all electrical standards bodies, trades and professions.  By mutual agreement, all electrical diagrams of DC circuits and electronics circuits are shown with symbols that assume current flows from positive to negative.  It is a fact of atomic physics that electrons carry a negative electrical charge, so migrate from a more negative place to a more positive place.  As in most “conventional agreements,” as long as the convention is agreed and understood, the pesky facts of atomic physics can be overlooked and left to scientists.

Circuit “Common” Reference(s)

The term “common” applies broadly to circuit elements that are shared among all of the broader network of electrical attachments in a installed electrical system.  The supply buss (“hot, B+”) and the negative return buss (“B-”) are examples of common circuit elements.

Virtually all DC systems encountered by the general public are low-voltage circuits, generally 12-volts, occasionally 24V or 32V.  Examples are 12-volt motor cycle, automobile, light truck, lawn tractor, residential emergency generator, snow thrower or all-terrain vehicle starting batteries, and similar yard and garden devices.  Other low-voltage battery-operated devices include fire/burglar alarms, Uninterrupted Power Supplies (UPS) for computers and data networks, hand-held spot lights, wireless telephone systems and a very wide variety of portable tools.

For applications in the automobile, truck, and outdoor equipment sectors, the return terminal of the battery is typically attached to the metal frame of the vehicle/equipment upon which the  battery is mounted.  The frame is the “common” return path for all sub-circuits.  Electrical components (starter motors, blowers, horns, light sockets, solenoids, sensors, gauges, electronics, etc) have internal electrical return connections that attach to the vehicle’s frame.  The electrical connection is created when the component is bolted to the chassis of the vehicle.  No discrete return conductor is needed because the metal vehicle chassis is the common electrical return path.  This approach simplifies wiring and mechanical design, reduces component design complexity, reduces material and labor cost, and eliminates wiring and connector materials and weight.  The metal frame of a vehicle is perhaps the most obvious place where the term “common” would describe a broadly-shared circuit component.

There are several factors that affect the preceding discussion as it applies to boats:

  1. most small and mid-sized pleasure craft are wired with 12-volt DC systems; 24-volt and 32-volt DC systems are sometimes used;
  2. some medium and large-sized boats have hybrid DC systems of mixed 12V, 24V and 32V systems;
  3. fiberglass (fiberglass reinforced plastic, FRP) does not conduct electricity, so fiberglass boat construction does not provide a functional “chassis,” or “vehicle frame,” return path; and,
  4. electric currents of even the smallest magnitude flowing in metal hulls, metal stringers and/or metal frame members lead to corrosion of the metals, and are always undesirable on boats.

Electrical appliances and utility attachments intended for marine DC applications are designed to have at least two wires; one for the supply of current that originates in the source (B+), and one for the explicit return of that current back into the source (B-).


In all of the preceding discussion, I’ve intentionally referred to the “electrical return path” using that specific term.  In ordinary conversation, the term “ground” is often used to describe the return path of a DC circuit.  This is a technical “liberty” of conversation, since DC return paths are often not actually connected to earth ground.  “Ground” in this context is a term of convenience and convention.  The return path from a low-voltage DC load to its source (B-) is not inherently at zero volts with respect to its surroundings.  A battery held in hand or sitting in a dock cart has two terminals, but neither is referenced to it’s surrounding environment.

Consider a bird sitting on a high-voltage overhead wire in a residential neighborhood.  The wire is at thousands of volts with respect to the earth, and so is the bird’s body and all of it’s little body parts.  But, the bird is safe, the tiny electric currents that make the bird’s heart beat still work, because there is no return path from the bird’s body to enable a disruptive external current to flow.  As soon as the bird flies off, the voltage is gone.  The bird’s body voltage changes, but the bird’s heart still beats normally, and the bird survives, completely unaffected for the contact with that man-made high voltage.

Consider a car, then, that is mounted on rubber tires.  Since rubber is a fairly good insulator, it would be possible for a DC voltage to exist between the earth and the frame of the car.  Normally small, this voltage can be thousands of volts.  Readers who have ever visited or lived in cold climates are undoubtedly familiar with the static shock that can happen when exiting a the car.  That static shock was a blast of high voltage DC caused by the transfer of accumulated charge from the vehicle, through the body, to the earth.  (Well, for purists, electrons flow from the earth, through the body, to the vehicle, but to the shockee, that detail is uninteresting.)  Static electricity and lightening are the same phenomenon, only on a much different scale!  The possibility of static shock is why every gasoline dispensary in the country instructs drivers to remove portable gas cans from cars and place them on the ground before filling them.  Grounding the container disburses any static charge.

It is technically non-trivial to create a reliable earth ground on a car.  Some readers may have seen ground straps dangling from trucks and some cars.  Used mostly on trucks, those straps are intended to protect toll takers and others who might come into contact with the vehicle from static shocks and to provide a safe path to ground static charges.  It is obviously difficult to create a reliable earth ground on a boat, impossible on an airplane.  Historically, earth grounding was not regarded as an important design goal for DC electric circuits.  And of course, experience with low voltage DC equipment generally bears out that assumption.  We get shocks from the build-up of static, but we don’t get shocks when we step off the garden tractor or use the snow blower.  Who among us has never disconnected a car battery when standing on the ground, and that was not a shocking experience.

What this all implies is that, even though the DC return circuit may not actually always be at the electrical potential of earth ground, the DC return circuit in all of our familiar yard equipment, cars and SUVs is referred to in ordinary discussion as the circuit’s  “ground”.  This use of the term “ground” refers to the functional return path ground, not a safety ground.

Safety Ground

As society gained experience with electricity in the early and mid-20th century, it became obvious that there had to be a way to ensure the return path is always at “earth ground” potential in order to  avoid the possibility of personal harm or property damage resulting from accidental contact with electric power.  A safety ground is not required for a circuit to operate correctly, but it does provide other compelling benefits.

Consider a fiberglass boat.  Aboard, there are many parallel DC sub-circuits.  Water pumps, space and nav lighting, nav and entertainment electronics, windlass, thruster, the propulsion engine, etc.  They are all at distances from one another, and the fiberglass frame of the boat is non-conductive.  A safety ground in a DC system (if present) interconnects the external frames and metal cases of equipment, appliances and utility attachments (light switches, outlets, motors, electrical equipment, radios, etc.) to a known point of common potential.  That common point is always the negative terminal of the battery, and under some specific conditions, the water in which the boat floats or the earth itself.  A safety ground is separate from the functional return circuit, and always involves the installation of it’s own individual electrical conductors.

In service, a “safety ground” is never intended to carry current in normal operation.  However, in a circuit containing an electrical fault condition, a safety ground is intended to prevent a personal shock hazard or mitigate property damage risk by ensuring the electrical potential is at earth ground potential.  It is the “safety ground” that provides an emergency path that allows a circuit breaker to function and disconnect power.

Consider, for example, a bow thruster or an anchor windlass.  We would expect to have a battery positive connection to the positive (B+) terminal of the device’s motor solenoid, and a battery negative connection to the negative (B-) terminal of the device’s motor solenoid.  The motor would then be expected to operate correctly with just these two battery connections.   If we also had a separate conductor from the mounting frame of the device to the vessel’s bonding system, that would be considered the “safety ground.”  The thruster would run just fine without the safety ground, but the device could malfunction and place the frame at some non-zero electrical potential.

Vessel Design – “Grounded” vs “Ungrounded”

Designers of DC electrical distribution systems refer to them as either “grounded” or “ungrounded” systems.  The terms “grounded” and “ungrounded” refer to the presence or absence of a safety ground, not the functional return circuit.  A return path of electrons to their source is always required, but that return path is not always referenced to anything else!

There is valid debate among experts as to whether 12-volt, 24-volt and 32-volt boat DC systems can be of the “ungrounded” design or should be of the “grounded” design.  Today, DC grounded systems are not common.  However, new and emerging vessel propulsion systems containing large-horsepower (hP) diesel-driven DC generators and large-horsepower DC motors (systems analogous to diesel-electric train locomotives) are definitely high voltage applications (often between 600VDC and 1000VDC).  Faced with the emerging presence of true medium and high-voltage DC equipment on pleasure craft, this safety ground design choice is now specifically being re-evaluated in the American Boat and Yacht Council’s (ABYC) Electrical Technical Committee.  We await that outcome.

It is to the advantage of boat buyers and all boat owners to understand the low-voltage DC electrical distribution system.  It’s also an obligation of the buyer/owner to understand whether or not a medium or high voltage DC system is also present.  In the majority of fiberglass-hulled boats, it would be unusual to have a separate DC safety grounding circuit installed.  On some boats, nevertheless, one could encounter one of several possibilities.  The electrical system installation on any individual boat depends on:

  1. the prevailing electrical construction standards at the time of OEM fabrication, often related to prevailing standards of the international geography where the boat was built,
  2. how many people may have added to, or otherwise modified, the system over time, and
  3. the electrical skills those individuals who have performed electrical work in the highly specialized marine environment.

The possibilities aboard a vessel include:

  1. no low-voltage DC safety ground at all (most typical today),
  2. partial DC safety grounding on some parts of the system (not recommended; considered technically inadequate), and
  3. full DC safety grounding, vessel-wide.

The ABYC does not require that low-voltage DC distribution systems have a safety ground, but it does make “recommendations” as to how “grounded” and “ungrounded” systems must be interconnected with the vessel’s bonding system.

Polarity  – “Negative Ground” vs “Positive Ground”

Earlier, I pointed out that a battery held in hand or sitting in a dock cart has two terminals, but neither is necessarily referenced to ground.  All that can be said is there is a fixed voltage between the two battery terminals.  Whichever battery terminal is connected to the vehicle frame determines the polarity reference for that DC system.  If the negative battery terminal is connected to the vehicle chassis, the system is considered to be a “negative ground” system.  If the positive terminal is connected to the vehicle frame, the system is considered to be a “positive ground” system.  With the emergence of solid state electronics and economic pressure to reduce manufacturing cost by sharing components across brands, models and manufacturers, the modern automobile industry world-wide (at least since the 1980s) has standardized around negative ground systems.

The ABYC-approved, and by far the most common, DC systems found on pleasure craft in North America are “negative-ground” systems.  On a boat with other than negative-ground DC distribution system, the panels throughout the boat should be clearly marked to identify the manner of connection.  If there is any doubt, always use a voltmeter to confirm the configuration before disconnecting or otherwise making modifications to the system.

Metal Corrosion and Zinc Wasting

2/12/2020: Major update of the section on DC Galvanic Corrosion; minor text edits.
2/15/2020: Correct repeating typo on diagrams; added text to section on what boaters can do about electrolytic fault currents; attempt to simplify descriptions.
5/6/2020: Add approximate concentrations of salts in various types of water.


On the long list of complex technical topics that boat owners face, corrosion of underwater metals is one of the most complicated, potentially most expensive and least well understood.  While it is not possible to ease the complexity or terminology of the topic, I can at least describe several related “stray current” metal corrosion phenomena in this one place.

Some readers may feel this topic is “beyond their pay grade.”  Like it or not, ALL BOATERS have a personal and financial stake in understanding the basics.   At some time in boat ownership, most owners will face one or more corrosion issues.  Even for those for whom the topic is both uninteresting and obscure, all boaters should know how these phenomena are similar and how they are different.  Some familiarity will allow the affected owner to hire the right expert, understand remediation recommendations, and possibly avoid expensive problems in the first place.

AC and DC “stray” electric currents flow in the water.  Not everywhere, but very commonly.  Because these currents flow outside their normal electrical conductors and devices, they are referred to as “stray currents.”  Worst case, all types of fault currents can be present at the same time.  Boaters should consider all electrical currents that flow in the water as a bad thing.


The basic concept in all corrosion is always the same: there is a voltage difference between two or more different metals, or alloys of metal, which are a) connected together electrically and b) immersed in an electrically active liquid.

The three major “stray currents” flowing in water (or in the earth’s crust) are:

  1. AC “ground fault” currents, resulting mostly from wiring errors aboard boats and occasionally from inadequate equipment design, incorrect equipment selection or AC appliance/equipment malfunction,
  2. DC “Galvanic” currents, resulting from the natural behavior of dissimilar metals in mineral-containing ground water, fresh surface water or sea water, and
  3. DC “Electrolytic” currents – a form of DC “ground fault” current – resulting from wiring errors, equipment faults, and improper equipment use.  This is the way electric vats are set up for electroplating, such as in galvanizing chain, but electrolytic currents are very destructive to boats in the water.

While the basic electro-chemical processes and terminology of corrosion are always the same, the cause is always context-specific.  Understanding the context (AC fault current, DC galvanic current, or DC Electrolytic current) is essential to avoiding confusion caused by the shared terminology.

“Electrolytic corrosion,” is  frequently confused with, but very different from, “Galvanic corrosion.”  To repeat, the concepts and terminology are shared and common to both phenomena; it is the “cause and origin” of the driving voltage that is different.  

No matter the terminology, corrosion currents are a silent attack on every boat, and can cost many hundreds or thousands of dollars for those who don’t mount an appropriate and effective defense.


AC fault currents flowing in the water are often dangerous to people, pets and wildlife.

Worst case, AC fault currents can lead to death by “Electric Shock Drowning:”  Children and pets must never swim in a marina’s basin.  Boat owners and professional divers performing in-water maintenance on boats must be alert to the causes and consequences of AC electric currents in the water.

In general, AC fault currents DO NOT deteriorate the underwater metals of boats and do not cause rapid zinc wasting.  

Repeating, In general, AC fault currents DO NOT deteriorate the underwater metals of boats and do not cause rapid zinc wasting.   There is a great deal of technical understanding about environmental AC ground fault currents that comes from the utility and transportation industries (power transmission, buried utilities, pipeline and railroad).   The 60 Hz AC power found across North America changes polarity 120 times per second.   Whatever molecular metal material that might be removed from a metal in one half-cycle is re-deposited in the second-half cycle.  (Cit: “DC Currents in the Bilge – Not AC – Is the Culprit When Metal Fittings Corrode,” Robert Loeser, Seaworthy Magazine, October, 1996).   AC fault currents must be very large before metal corrosion results.   The AC voltages found around pleasure craft docks (less than 600V) DO NOT cause zinc wasting.

Aluminum can be a minor exception.  Aluminum can be damaged by AC stray currents IF the density of the fault current is greater than 40 Amps per square meter of exposed aluminum surface area (40A/M2).  What that means in English is: a relatively high AC fault current in the water will cause erosion to a relatively small chunk of underwater aluminum.  This combination would be unusual, but not impossible, in pleasure craft marinas.  One m2 is equal to about 10.75 ft2, so it would only take 3.7 amps of AC stray current to cause corrosion damage to a 1 ft2 aluminum part.  This is not an extraordinary leakage current, and this amount of leakage current is definitely dangerous to living beings, but 1 ft2 is a small piece of aluminum.  So trim tabs and outdrives may be “relatively” “safe” at levels that would waste aluminum anodes installed for galvanic protection.  On boats without other aluminum parts, aluminum anodes can waste quite rapidly in proportion to the size of a moderate in-water AC fault current.

Readers can find information on testing for AC ground and leakage fault currents in layman’s language on this website.   The reference article helps owners bring their boat into compatibility with National Electric Code (NEC) standards that require ground fault sensors on docks, and it also dovetails well with identifying and eliminating corrosion issues.

Be aware that some in-water AC stray currents can originate from sources on land.  In that case, the fault current will flow on the green AC safety ground wire (a component of the boat’s bonding system), originating in the basin water and flowing back into the shore power infrastructure.  This situation is not caused by a problem on the boat, and in general, is not something a boat owner can fix.  Always report this finding to marina management.


  1. Assess the boat for AC ground fault and leakage fault conditions.
  2. Correct all issues in order to establish a defect-free starting-point baseline.
  3. Consider installing Equipment Leakage Current Interrupter (ELCI) sensors on boat shore power AC service circuits.  (ref: ABYC E-11, 11.11.1 and ELCI Primer)
  4. Where automatic ELCI sensors are not installed, perform frequent manual monitoring of AC shore power cords with a decent-quality clamp-on Ammeter.
  5. Correct any newly discovered issues as soon as they present themselves.


  1. DC galvanic currents are associated with small voltage potentials that are a naturally-occurring characteristic of all metals.  The specific voltage is determined by the atomic structure of the individual metal (or metal alloy).
  2. The Anode is the electrode of an electrochemical cell at which oxidation occurs; the negative terminal of a galvanic cell
  3. The Cathode is the electrode of an electrochemical cell at which reduction occurs; the positive terminal of a galvanic cell
  4. Anodic/Cathodic: terms which relate the polarity of one electrode to another.
  5. A half-cell: either of the polarized components of a battery, either the positive half or the negative half.
  6. A “Galvanic Series” is a list of metals sorted by their naturally-occurring characteristic electro-potentials.  Different “references” can be used for ordering a “galvanic series.”  The best reference for salt water is a silver/silver chloride cell.
  7. A “galvanic couple” is any combination of two or more dissimilar metals or metal alloys connected together electrically and immersed in an electrolyte.
  8. An “electrolyte” is an electrically conductive liquid (generally) medium.
  9. Dry Corrosion” is the direct attack on a metal by dry gasses (air, oxygen) through chemical reactions which result in surface oxidation.
  10. Wet Corrosion” is the direct attack on a metal by an aqueous solution (strong or dilute, acidic or alkaline) through electro-chemical reaction.  Moisture and oxygen can act by themselves.


The underwater metal alloys on a boat together with the minerals in the surface water in which the boat is floating create the elements of a “galvanic cell” (a battery).  The efficiency and strength of galvanic cells depend on the specific materials involved.  Galvanic currents will always be generated when a boat with dis-similar metals occupies water containing dissolved minerals.  A zinc/copper galvanic couple (common “dry cell” flashlight battery) is a “galvanic cell.”  A “lead/acid” automotive or boat battery (wet cells, AGM or Gel) are examples of “galvanic cells.”

Here are some “typical approximations” of salt and mineral content in various geographic types of waters that will affect the electrical efficiency of a galvanic cell:

  1. Potable Water – water fit for human consumption (generally less than 500 ppm).
  2. Fresh Water – water with total dissolved solids (salts & other minerals) generally less than 1,000 ppm.
  3. Brackish Water – water containing more than 1,000 ppm but less than ocean water.
    • Slightly Brackish Water – water that contains between 1,000 to 3,000 ppm salts and other dissolved minerals.
    • Moderately Brackish Water – water that contains between 3,000 to 10,000 ppm salts and other dissolved minerals.
    • Highly Brackish Water – water that contains more than 10,000 ppm salts and other dissolved minerals, but less than sea water (35,000 ppm).
  4. Sea Water – ocean water with Total Dissolved Solids of 35,000 ppm.
  5. Brine – high salt waste water (generally more than 50,000 ppm).

When the metals making up the galvanic cell (battery) are actually the underwater component parts of a boat (bronze, aluminum, stainless steel), naturally-occurring galvanic currents result in corrosion of some of the underwater metal.   The mineral concentration of sodium, calcium and magnesium salts and many others in the surface water affect the speed at which galvanic corrosion proceeds.

The flow of electrons in a DC galvanic current is always from a more active metal (anode) to a less active metal (cathode) on a Galvanic Series.   All environmental surface water, whether fresh or salt, acts as an electrolyte.  Salt water carries more mineral ions than fresh water, so is more “efficient.”

Galvanic corrosion is a slow process that occurs over many months.  Since it’s the anode in a galvanic cell that dissolves, the point of avoidance/remediation is to artificially force the metal(s) to be protected (relatively more cathodic) compared to a sacrificial metal present in the electrolyte (water).  This is done by adding a “sacrificial anode” made of a very active metal (zinc, aluminum, magnesium) to the mix of less active but more valuable underwater metals on a boat.


  1. All metals have a unique and characteristic electro-chemical electrical potential (voltage) that is a result of their atomic structure.  Elemental metals like aluminum, copper, iron, nickel, and tin have unique electro-chemical voltages.  Alloys of metals like steel, bronze and brass also have unique electro-chemical voltages.
  2. In my article on Bonding System Design and Evaluation, I discuss the concept of “half-cells” of a battery.  A “typical” “AA” flashlight battery is a classic galvanic cell consisting of two “half-cells” (copper and zinc) packaged in an electrolyte.  Since the battery is always seen and used as a complete, packaged unit, the term “half-cell” is not commonly used in lay conversations except by engineers, battery manufacturers and technicians specializing in corrosion mitigation.  However, the concept of “half-cells” is important in understanding corrosion.
  3. There are many classification schemes that can be used to quantify and characterize the electro-chemical voltages of metals.  In marine industries, by far the best and easiest is a silver/silver chloride (chemical symbol Ag/AgCL) “half-cell.”  Silver/Silver Chloride “half-cells” produce a stable, definable, repeatable reference voltage across a wide range of temperature and mineral concentrations when immersed in sea water,
  4. When immersed in sea water (sea water becomes the “electrolyte”), the Ag/AgCl reference electrode is one half-cell of a “battery,” and the metal being measured is the other half-cell.  As in all “batteries,” a voltage will be produced between the positive and negative half-cells, so between the test metal and the Ag/AgCl reference cell, small voltages will be apparent.
  5. When compared to the Ag/AgCl half-cell, different metals and metal alloys produce different, unique voltages.  When sorted into a table according to the voltage measured, the result is called a “Galvanic Series,” or “Table of Nobility.”
  6. A piece of ordinary 316 SS immersed in seawater and measured against an Ag/AgCl reference cell will produce around -50mV (0.0V to -100mV).  Similarly, a piece of silicon bronze immersed in sea water and measure against the Ag/AgCl half cell will produce about -260mV (-260mV to -290mV).  In this example, it can be calculated from the voltages measured between the test samples and the Silver Chloride half-cell, there are approximately 210mV BETWEEN THE TWO METALS.  While this is not a very “powerful” battery, it is enough to cause galvanic currents to flow, and those currents can be slowly and continuously destructive to valuable and expensive underwater metals on a boat.
  7. Stainless Steel (Type 316) is an alloy (iron, nickel, chromium, molybdenum) that is “stainless” because the chromium in the alloy forms a strong, clear coating of chromium oxide.  The chromium oxides needs elemental oxygen from the environment in order to maintain and repair itself.  Elemental oxygen is found in the air we breath, and is dissolved in sea water.  Stainless Steel that has a good chromium oxide coating is known as “Passivated SS.”  Stainless Steel without an effective chromium oxide coating is known as “Activated SS.”  The terms “passivated” and “activated” refer to the electro-chemical voltage of the alloy sample being measured.  I will discuss this further later on.

Perhaps some drawings will help:

Sea_Water_Passivated_SSFigure 1 shows a “galvanic couple” of 316 Stainless Steel and Silicon Bronze immersed in seawater.  The silicon bronze is the anodic alloy, so it erodes due to the natural galvanic voltage between it and it’s cathodic 316 SS couple-mate.  Silicon bronze is an alloy of copper, iron, zinc, silicon, molybdenum and tin.  All of these metals have their own unique electro-potential, but when mixed in a “silicon bronze” alloy, the alloy mix produces about -260vM against the Ag/AgCl reference cell.

In this galvanic couple, the 316SS is the cathodic metal (more positive, more noble).  The Bronze is the anodic metal (more negative, less noble).   The anodic metal will always erode, and in this case, the least noble metal in the bronze alloy is the zinc; that zinc is what erodes away first.  This process is called “dezincification.”  Dezincification is a destructive process because it leaves the bronze alloy physically weakened. As it progresses, the bronze turns a pink color, and is easily damaged and subject to mechanical failure.  This is bad and undesirable if it happens to a $2000 propellor, a thruhull, or a packing gland.

There is a way to avoid this damage:

Passivated_ProtectedFigure 2 shows the same SS/Bronze galvanic couple seen in Figure 1, with an added sacrificial anode of zinc (aluminum and magnesium can also be used as sacrificial anodes).  The more negative electro-chemical voltage of the sacrificial anode (zinc ~ -1000mV) forces the bronze component relatively more cathodic in this overall galvanic couple (that is, more positive relative to the zinc anode), so the bronze component is now protected from corrosion.  The zinc becomes the most active (most anodic, least noble) metal in this new galvanic couple.  By corroding, the zinc sacrifices itself as it acts to protect all of the more “valuable” metals from structural damage.

Ongoing maintenance of sacrificial anodes (whether zinc, aluminum or magnesium) is required to provide continuing protection of the more important components of the couple.  Additional valuable corrosion control techniques include the installation of galvanic isolation devices in shore power ground conductors, cable TV coax ground sheathes, and the ground conductors of (now pretty much obsolete) wired telephone and wired Ethernet connections.  Without devices that mitigate against the flow of galvanic currents, the concentration of salt and dissolved minerals in the environmental water will affect the rate at which protective sacrificial anodes are consumed.  Those in areas flooded by tidal ocean waters and in areas of highly brackish water will replace anodes more frequently that those in lightly brackish or fresh water locations.

Stagnant_Sea_Water_Activated_SSFigure 3 shows the same galvanic couple of 316SS and silicon bronze that we have referenced above.  However, this time, we see the galvanic couple floating in highly stagnant water.  As mentioned earlier, Stainless Steel needs a coating of Chromium Oxide to be, and remain, effectively “stainless.”  Oxide coatings – by definition – need oxygen, and some conditions can cause levels of dissolved oxygen to decrease to too low a concentration to maintain an effective chromium oxide coating.  In that case, the SS morphs from a “passive” (or “passivated”) state to an “active (or “activated”) state.  As noted above, 316SS has an electro-chemical voltage of about -50mV in it’s passivated state, but it has an electro-chemical voltage of about -500mV in its activated state.

This situation becomes a serious corrosion concern in at least two cases.

  1. Some boats do not get a lot of use and can sit for years without moving.  If in the water in an area of stagnant sea water, this can cause the SS to deteriorate, affecting all underwater SS components.  In this somewhat rare circumstance, the SS will become anodic and will erode.
  2. More commonly, boats that are not used are at much greater risk of “single metal” corrosion of SS propellor and rudder shafts.  Single metal corrosion is described later in this article.


The propulsion and genset drive engines on most cruising-sized pleasure craft are fit with two-stage engine cooling systems.  In diesel cooling systems, a coolant (“fresh water”) circulates through the engine block, heads, oil-cooler, turbo-charger, and intercooler.  A heat exchanger transfers waste heat from the fresh water coolant to environmental raw water, where it is eliminated via the raw water exhaust.  Commonly, a second heat exchanger transfers waste heat from transmission fluid into exhausted raw water.

Electro-chemically, the raw water passing through the heat exchanger is an electrolyte.   Heat exchangers contain several different alloys of copper and nickel.  The alloys used in heat exchangers are designed to have galvanic voltage potentials that are close to one another on the salt-water galvanic series.  That greatly slows, but does not stop, the galvanic corrosion which occurs within heat exchangers.  The dissimilar metals of the heat exchanger act as the galvanic couple and the raw water is the electrolyte.

If galvanic corrosion in heat exchangers is allowed to continue uninterrupted, pinpoint leaks will develop in the shell or tubes of the exchanger.  Similarly, pinpoint leaks can develop in raw-water cooled oil coolers, transmission coolers and intercoolers.   The result over time is damage to expensive heat exchangers, as well as the possibility of secondary damage to the engine itself.   Boat owners  must be aware that there are zincs located within the raw water channels in engines and heat exchangers.

Boats with wood, steel and aluminum hulls require special anti-corrosion techniques.  Many sacrificial anodes are required to protect the surface area of metal hulls.  Too many anodes can cause paint to peel from a metal hull, and cause damage to the woods of a wooden hull.  Alternatively, systems such as Electro-Guard ( apply a voltage to a metal hull.   These “Impressed Current Cathodic Protection” (ICCP) systems protect the hull plates and welded joints from galvanic attack by making the hull cathodic to its surrounding environment.   This is one of many areas that are “different” for owners of metal-hulled boats vs hulls of fiberglass reinforced plastic (FRP).


SS, bronze, brass and galvanized steel are metallic alloys that contain several elemental  metals within their compounding mix.   Dissimilar metals within the alloy can experience galvanic corrosion.  “Single metal” corrosion results in micro-fractures in the material’s structure, and often results in surface pitting.  The process can proceed to structural failure.

Anodic and cathodic areas form on the surface of alloys due to surface imperfections in the alloy mix, lack of oxygen and/or other environmental factors.  The anodic areas in the matrix give up electron(s).  The ions left behind form into the visible hydroxyl oxidation residue that is shed.  Corrosion currents flow at the expense of the anodic metal of the circuit, which corrodes continuously.

SS shaft logs and propeller shafts, SS rudders and rudder posts, SS fasteners that attach swim platform brackets to an FRP hull, SS keel bolts, SS exhaust port fasteners, etc, etc, are all candidates for a form of single metal galvanic corrosion called “crevice corrosion.”

In brass that contains more than 15% zinc, like the manganese bronze alloy often used in propellors,  unprotected fittings can undergo a single metal galvanic corrosion process called “dezincafication.”  Zinc within the brass alloy erodes away, leaving behind a weak matrix of copper and small percentages of other metals (such as nickel, chromium, manganese) of the original casting.  What’s left is structurally weak and can fail catastrophically.  “Dezincification” leaves a characteristic “pinkish” color to what once had a golden bronze color; particularly so in broken, exposed areas of a part.

In stainless steel, this process is called “CREVICE CORROSION.”  In aluminum, the analogous process is called “POULTICE CORROSION.”  When stale water lies against stainless steel for long periods or time, the water looses it’s content of dissolved oxygen.  Oxygen-depleted water in prolonged contact with stainless steel promotes crevice corrosion, leading to possible structural failure in stainless steel parts and fittings.  Similarly, water that lies in contact with aluminum for long periods of time promotes poultice corrosion.  Poultice corrosion can result in pinpoint leaks in aluminum fuel tanks.

For thruhulls especially, boaters should use fittings of bronze or Marelon; BRASS FITTINGS SHOULD NEVER BE USED UNDERWATER.

For those interested, I have more details on Galvanic Corrosion and the Galvanic Series for salt water on this website, here:

BoatUS has a good article on electrochemical corrosion on their website, here:

David Pascoe has a good article on electro-chemical corrosion on his website, here:


  1. Install a complete bonding system if one is not currently present.
  2. Install zincs to protect bonded underwater metals.
  3. Perform routine maintenance of zincs on underwater metals: propellor shaft, rudder, and other underwater metal structures.
  4. Maintain the “master” zinc that protects the boat’s bonding system.
  5. Maintain zincs protecting engine and transmission cooling system components.
  6. Use deck fill screw-on covers that are galvanically compatible with under-deck fittings to avoid galvanic corrosion and hidden fuel leaks.
  7. Install an appropriately rated Galvanic Isolator in the shore power safety ground if one is not already present.
  8. Install galvanic isolators to telephone, Ethernet and TV Cable feeds that come onto a boat from shore.


The source of the voltage that drives the process is what distinguishes a “Galvanic current” from an “Electrolytic” current.  Recall that galvanic voltages are a function of the natural atomic electro-potential of the metals of a galvanic couple.  Electrolytic voltages are man-made, not naturally-occurring.  The voltages that drive electrolytic corrosion are often significantly larger than galvanic voltages, and the destructive impact of a DC fault causing electrolytic damage is much faster and more aggressive than galvanic currents.

Screen Shot 2020-02-11 at 15.59.41In Figure 4, the metallic actors (SS and bronze alloys) are the same as shown in Figure 1.  In this case, the elemental voltage polarity of the couple has been reversed by the application of an outside source of DC voltage.  This is a DC fault scenario.  The bronze thruhull in this example will disintegrate, freely giving up it’s copper content into the surrounding sea water.

Electrolytic voltages originate with an externally-supplied DC source; i.e., a battery or its equivalent.  Causes can be a wiring error, chafed/frayed DC conductor, defect or age-deteriorated insulation on a bilge pump B+ wire lying in bilge water, a defect in a DC power supply or a DC generator, a failed piece of DC equipment or misapplication of use of DC equipment.   A common wiring error that can lead to electrolytic currents results from incorrectly wiring the neutral return circuit of a DC device to the boat’s bonding system.  NEVER USE THE DC BONDING SYSTEM FOR THE ELECTRICAL RETURN PATH FOR DC CIRCUITS.

DC electrolytic currents are equivalent to the industrial process called “electroplating.”   In a marina, failed DC equipment can deliver a DC voltage into the basin water.  On a boat, wiring error or a failed piece of equipment can apply a DC fault voltage to the boat’s ground buss.  electrolytic current flows IN ONE DIRECTION through the ground path and into the surrounding basin water.  The anode literally dissolves.  The fault can be on the same boat as the failed equipment, on a neighboring boat or in nearby land based equipment.  Or it can be, simply, in between a source point and a return point.  The fault can be located in shore-side infrastructure wiring, or it can be because of misuse of equipment by a contractor, such as a welder or a DC motor on a marine railway or travel lift.

It is a law of physics that electric current always seeks the path of least-resistance back to their source.  Scenario: imagine three adjacent slips on a dock.  In slip #1 is a boat with a fault and dumping a DC electrolytic fault current into the basin water.  Slip #2 is empty.  In slip #3 is a boat providing a path to ground for the fault current via it’s shore power cord.  So far, only boat #1, the boat with the fault, has a serious corrosion problem.

Now a transient boat comes into slip #2.  The fault current previously passed through slip #2 on the way to ground, but when the transient arrived, that boat’s protective electrical system (Bonding System) becomes inserted into the path of the fault current.  The transient’s bonding system has a lower resistance than the surrounding basin water.  The fault current passes into the transient via one or more underwater metals, passes on through the transient’s bonding system, and exits back into the basin “on the other side” of the transient.  The fitting(s) where the current exits the transient will corrode.  That same electrolytic fault current is now causing damage to boat #1 and boat #2.  Boat #2 is a true victim, safe if plugged into shore power, potentially damaged if not.

All fault currents are always opportunistic.  They simply follow the rules of physics to find the “path of least resistance” back home.

The rate at which metal loss occurs is proportional to the voltage involved and to the many Ohms Law factors that determine the magnitude of accompanying current flow.  At its worst case, this process can sink a boat astonishingly quickly (a matter of hours/days), because with large uni-directional electrolytic currents (electroplating), metal mass can erode away from the anodic terminal(s) very quickly.   The part that gives up metal mass will ultimately suffer structural failure if the process is not interrupted.  If it happens to a thruhull on a boat in the water, the boat will sink.   All underwater metals – propellors, rudders, struts, trim tabs and radio ground planes – can be effectively “dissolved” by these stray DC fault currents.

The best articles I know of for an understanding of this topic are by Capt. David Rifkin, who has good reference articles on his website, here:

Nigel Calder, Ed Sherman of ABYC and Steve d’Antonio have also written about these phenomena,  mostly in fee-based subscription publications like Professional Boatbuilder and Passagmaker Magazine, or in their own for-fee publications.


Like all metal corrosion, zinc wasting is a form of electro-chemical corrosion, always due to DC currents.  Electrical measurements of the basin occupied by the boat would be necessary to determine which mix of stray currents are present at any given time, but zinc wasting is a DC phenomena, and by far most commonly, a galvanic phenomena.

Boat owners can do their own basin-water testing, but it is not a process I would recommend for the electrical layman.   An understanding of the theory of these types of faults, understanding the probes and tools that are necessary, and the skills to correctly interpret test results are necessary.  This can be quite confounding, even to experts.

Boat owners that experience corrosion issues would be better served to hire an ABYC CORROSION-CERTIFIED MARINE ELECTRICIAN.  That said, skilled and knowledgeable boat owners can do their own DC testing with a silver/silver chloride half-cell.  High quality Digital Voltmeters (DVM) can detect AC ground fault currents, DC galvanic currents and DC electrolytic fault currents, but detection and evaluation is highly specific and sensitive to the placement of the measurement electrodes, quality of the test equipment, and conductivity of the surrounding basin water.

By the time a layman has bought the tools, learned to use them, and learned to interpret the results, said layman would be better off financially in hiring a professional who could provide the diagnosis and remediation recommendations as a one-time service.


  1. Diligently avoid having DC wiring wetted or submerged in bilge water.
  2. Never use the boat’s bonding system as a B- “neutral” return circuit for DC attachments.
  3. Avoid facilities (marinas, municipal or private docks, boatyards, etc) where the infrastructure appears to be poorly maintained.
  4. Be alert in marinas located in industrial neighborhoods where ground fault currents from shore sources may be more likely; check with the dockmaster for known issues in the basin.
  5. Avoid facilities with numerous boats that are in a poor state of maintenance and repair.
  6. ESPECIALLY FOR BOATS THAT SPEND LONG PERIODS OF TIME IN ONE MARINA BASIN, the final safeguard against serious stray current corrosion is to employ and depend on a diver that has the experience to recognize this type of corrosion.  Develop a good relationship with the diver and demand at least a verbal report stating explicitly the condition of the bottom, the condition of the anodes and, specifically, any signs of stray current corrosion.  Make the diver a “part of the team,” not just someone who scrapes barnacles.



Oxidation occurs with the release of electrons and the simultaneous shedding of positively charged metal atoms which detach from the surface of the metal.  These particles enter the electrolyte solution as positively charged ions.  Chemically:

Fe → Fe++ + 2e (example with Iron);

Zn → Zn++ + 2e (example with Zinc);

Pb → Pb++ + 2e (example with lead).


Free electrons reach the cathode and react with hydrogen ions in the electrolyte.  Hydrogen bubbles will often form on the cathode; clearly visible in lead/acid batteries.  Chemically:

2H+ + 2e → 2H


If acid is not available, water itself will break down (dissociate) to generate hydrogen ions (H+).  The specific chemistry here depends on the composition of the electrolyte.  Assuming water, water dissociates, forming free hydrogen and hydroxyl ions:

H2O ⇌ H+ + OH

Then, metal ions combine to form metallic oxide, which is the corrosion product:

Fe++ + 2(OH) → Fe(OH)2, or

Zn++ + 2(OH) → Zn(OH)2, or

Pb++ + 2(OH) → Pb(OH)2

Galvanic Currents and “Zincs”

Galvanic Corrosion” is a normal, predictable and definitely unwanted electro-chemical phenomenon. However, it does not have to be inevitable!  The science of Galvanic Corrosion spans chemistry, physics and electricity.  There is an entire body of specialty engineering knowledge on the subject arising from the electrical utility, railroad, shipping and pipeline industries.  For boater’s, there are at least four ABYC Standards that relate to it:

  1. A28, “Galvanic Isolators,” July, 2008,
  2. E2, “Cathodic Protection,” July, 2013,
  3. E11, “AC and DC Electrical Systems on Boats,” July, 2012, and
  4. TE4, “Lightening Protection.” July, 2006.

This post is written as an introduction to the key terms and concepts of Galvanic Corrosion.  It is written for boaters and others who have little or no prior electrical background.  In struggling with the technical concepts and unfamiliar terminology used to describe them, one may find that skilled people slip almost casually from one contextual use of terms to another.  For beginners and bystanders, that can be confusing, confounding and frustrating.  My hope is this post will wrap some perspective and understanding around the topic.

There is enormous confusion about corrosion among boaters.  There are two very different causes of corrosion that boaters face: 1) galvanic corrosion and 2) “electrolysis,” or “stray current” corrosion.  Galvanic corrosion is virtually universal – by far the most common – and is the subject of this article.  However, many of the technical concepts, terminology and materials are the same between the two.  The big difference between Galvanic Corrosion and Electrolytic Corrosion are their very different cause.  Symptoms of the two can be the same, but can be very different.  Galvanic corrosion always happens slowly, over the course of months.  “Electrolytic” (DC stray current” corrosion) can happen astoundingly fast, with catastrophic failures in a matter of hours.  The techniques that slow and stop galvanic corrosion may or may not work with stray current corrosion.

For boats, of course, galvanic corrosion is a real and ever-present concern simply because boats float in environmental surface water which contains varying degrees of impurities (dissolved minerals, salts, organic materials and chemical pollutants).  Residential electricians have little to no familiarity with galvanic corrosion issues.  Corrosion is a complex and challenging electrical sub-specialty.

If experiencing corrosion on boat metals, or if corrosion symptoms have noticeably changed in recent days/weeks, boat owner/operators are advised to engage a “certified” marine corrosion expert.  Do not, under any circumstances, rely on a residential electrician to diagnose this phenomenon.  For non-professionals, galvanic corrosion is a somewhat obscure topic.  It is generally not familiar to people who live in – or provide professional services to – residential single family homes or multi-family buildings.  Electrochemical corrosion is generally beyond the experience of the general public.

In any discussion of “Galvanic Corrosion,” there are several inescapable and frequently-encountered terms one must know and understand.

  1. “Galvanic Table,” or “Galvanic Series.”  Because of the atomic structure of metals, every metal has a natural electrochemical ‘static charge,’ or “electrical potential.”  The magnitude of this electrical potential is unique to the particular metal.  Each metal’s electrical potential is different from every other metal.  The electrochemical “galvanic potential” of metals is determined by measurement against a “reference cell.”  Reference cells of different chemical composition can be used for different purposes.  The reference cell most commonly used for marine purposes in surface water containing varying amounts of dissolved minerals is a silver/silver chloride (Ag/AgCl) half-cell.  The “galvanic series” is a table sorted based on the electro-physical charge potentials of metals against the reference half-cell.  Alloys of metals also carry unique electro-potentials.  The proportions of metals that make up an alloy will affect the absolute magnitude of the electrical potential of the resulting alloy.
  2. “Galvanic Couple.”  Any two of the different metals and alloys in the galvanic series are referred to as a “galvanic couple.”  The difference in electro-chemical potentials between any two metals in the galvanic series represents a voltage that can be measured with a high quality Digital Multimeter (DMM).  When several metals are all joined together in an electrical network – as is the case of props, prop shafts, trim tabs, thru hulls all bonded together – they are referred to as a “galvanic collection.”
  3. “Anode,” “cathode,” “anodic” and “cathodic.”  These terms are always used in a particular context.  The context will either be a description of:
    • the position of a specific metal in the galvanic series; that is, reference to the “Galvanic Potential” of a specific metal or alloy, or
    • the relative relationship of one metal to another on a “Galvanic Series of Metals,” or a “Nobility Scale.”  For example, “zinc is anodic referenced to bronze,” and “bronze is cathodic referenced to zinc.”
    • In all electrically connected galvanic couples and galvanic collections, one metal will be “anodic” to the other(s).  Electrically, the anode gives up electrons to the galvanic system.  More importantly, the anode also sheds positive ions of its metallic structure into the surrounding electrolyte solution (surface water containing dissolved minerals).  That shedding is the galvanic corrosion we observe as boaters.

The concepts these terms represent are fundamental to understanding galvanic corrosion.  Relative to any “Galvanic Series,” or “Nobility Scale:”

  • “cathodic” metals are highly “noble,” or considered “passive” metals.  Compared to an Ag/AgCl half-cell, their natural electrochemical potential is more positive with respect to other metals in the galvanic series.  They are naturally more resistant to galvanic corrosion.  Examples include titanium, gold and graphite.
  • “anodic” metals are “less noble,” or considered “active” metals. Compared to an Ag/AgCl half-cell, their natural electrochemical potential is more negative with respect to other metals in the galvanic series. They are moderately to highly subject to galvanic corrosion. Examples with which all boaters are familiar include magnesium, aluminum and zinc, all used as “sacrificial anodes” on boats.

So, context is critical to understanding.

Metals incur galvanic corrosion only when they are in electrical contact with other metals. Therefore, galvanic corrosion should always be thought of as involving two or more dissimilar metals; that is, a “couple” or a “collection.”

For “Galvanic Corrosion” to occur, three conditions are necessary:

  1. Metals must be “well separated” – moderately or greatly – on the “Galvanic Series:”

The larger the galvanic potential difference between the metals involved, the greater the probability of galvanic corrosion, and the faster that corrosion will progress.

  1. The metals must be electrically connected together:

The metals can be wired together, or pressed, riveted, bolted, welded, clamped, or even piled-upon each other.  Normally on boats, galvanic corrosion occurs when metals that are bonded together, so the path connecting them is low resistance.  However, galvanic currents can and do flow when only high resistance paths connect two dissimilar metals.

  1. Both metals must be simultaneously immersed in an “electrolyte:”

An electrolyte is an electrically conductive medium.  The electrolytic medium acts to complete the electrical circuit. If the conductivity of the medium is high, the metal-to-metal corrosion of the less noble metal will be dispersed over a larger area. If the conductivity of the electrolytic medium is low, the corrosion will be localized to the part of the less noble metal nearest to the mechanical connection between the metals. Sea water is an excellent electrolyte, brackish water, less, fresh water, not so much.

When all of the above conditions are met, a difference in “galvanic potential” (a voltage) exists between the different metals in the electrolyte solution. That voltage is the driving force for electrons to flow from one metal to the other metal through the electrolyte. This current results in positive charged ions of the anodic metal of the couple being shed into the electrolyte. Inside a carbon/zinc dry cell battery, when the anode (zinc) is fully depleted, the battery is thought of as “dead.”  Between metals in any mechanical system, this process is thought of as “destructive galvanic corrosion.”  Be aware, “in any mechanical system” in a boat IS NOT limited to prop shafts and propellors.  This deterioration can and does occur inside engines and transmissions, and in metallic structures like potable water and fuel tanks, thru hulls, seachests and strainers, etc.


The following table shows galvanic potentials of many common marine metals in free-flowing sea water as measured with a silver/silver chloride reference cell:

TABLE I – GALVANIC SERIES OF METALS IN SEA WATER WITH REFERENCE TO SILVER/SILVER CHLORIDE REFERENCE CELL [Sea water flowing at 8 to 13 ft./sec. (except as noted), temperature range 50°F (10°C) to 80°F (26.7°C)]
Magnesium and Magnesium Alloys -1600 to –1630
Zinc -980 to –1030
Aluminum Alloys -760 to –1000
Cadmium -700 to –730
Mild Steel -600 to –710
Wrought Iron -600 to –710
Cast Iron -600 to –710
13% Chromium Stainless Steel, Type 410 (active in still water) -460 to –580
18-8 Stainless Steel, Type 304 (active in still water) -460 to –580
Ni-Resist -460 to –580
18-8, 3% Mo Stainless Steel, Type 316 (active in still water) -430 to –540
Inconel (78%Ni, 13.5%Cr, 6%Fe) (active in still water) -350 to -460
Aluminum Bronze (92% Cu, 8% Al) -310 to -420
Nibral (81.2% Cu, 4% Fe, 4.5% Ni, 9% Al, 1.3% Mg) -310 to –420
Naval Brass (60% Cu, 39% Zn) -300 to –400
Yellow Brass (65% Cu, 35% Zn) -300 to –400
Red Brass (85% Cu, 15% Zn)  -300 to –400
Muntz Metal (60% Cu, 40% Zn) -300 to –400
Tin -310 to –330
Copper  -300 to –570
50-50 Lead- Tin Solder -280 to –370
Admiralty Brass (71% Cu, 28% Zn, 1% Sn) -280 to –360
Aluminum Brass (76% Cu, 22% Zn, 2% Al) -280 to –360
Manganese Bronze (58.8% Cu,39%Zn,1%Sn, 1%Fe, 0.3%Mn) -270 to –340
Silicone Bronze (96% Cu Max, 0.80% Fe, 1.50%Zn, 2.00% Si, 0.75% Mn, 1.60% Sn) -260 to –290
Bronze-Composition G (88% Cu, 2% Zn, 10% Sn -240 to –310
Bronze ASTM B62 (thru-hull)(85%Cu, 5%Pb, 5%Sn, 5%Zn) -240 to –310
Bronze Composition M (88% Cu, 3% Zn, 6.5% Sn, 1.5% Pb) -240 to –310
13% Chromium Stainless Steel, Type 410 (passive) -260 to –350
Copper Nickel (90% Cu, 10% Ni) -210 to –280
Copper Nickel (75% Cu, 20% Ni, 5% Zn) -190 to –250
Lead -190 to –250
Copper Nickel (70% Cu, 30% Ni) -180 to –230
Inconell (78% Ni, 13.5% Cr, 6% Fe) (passive) -140 to –170
Nickel 200 -100 to –200
18-8 Stainless Steel, Type 304 (passive) -50 to –100
Monel 400, K-500 (70% Ni, 30% Cu) -40 to –140
Stainless Steel Propeller Shaft (ASTM 630:#17 & ASTM 564: # 19) -30 to +130
18-8 Stainless Steel, Type 316 (passive) 3% Mo 0.0 to –100
Titanium -50 to +60
Hastelloy C -30 to +80
Stainless Steel Shafting (Bar) (UNS 20910) -250 to +60
>Platimium +190 to +250
Graphite +200 to +300
†The range shown does not include sacrificial aluminum anodes. Aluminum alloy sacrificial anodes are available that have a maximum corrosion potential of -1100 mV.
1. Metals and metal alloys are listed in the order of their potential in flowing sea water as determined in tests conducted by a nationally-recognized corrosion research laboratory.
2. The galvanic series may be used to predict whether galvanic actions are likely between two metals. Other factors (e.g., area of the material, flow rate, composition of the electrolyte, crevices, the coupling of copper alloys with aluminum, etc.) affect the relative corrosion rates in seawater.

Source: American Boat and Yacht Council, Standard E-2, July, 2013, Page 10.

A boat with underwater metal fittings in the water is a natural battery (a “galvanic cell”). That natural battery produces a very small DC voltage between the under-water “cathodic” and “anodic” metals of the couple/collection. The water in which the boat is floating is the necessary “electrolyte” in this natural battery, and the dissimilar metals of the propeller, drive shaft, reduction gear/transmission, rudder, thruster components, outdrives, trim tabs, thru-hulls, radio ground plane, speed and sounder sensor bodies, etc., etc., etc., are the relatively anodic (+) and relatively cathodic (-) terminals of the battery.

Electrons carry a negative electrical charge.  Galvanic electric currents consist of a “flow of electrons” out of, and back into, the galvanic cell created by your boat. In a common galvanic corrosion scenario, the flow of electrons that make up a galvanic current originate in the under-water anodic (+) metals of the boat, flow through the electrolyte (water), and return to the  under-water cathodic (-) metals of the boat.  Part of the mechanism of these small DC currents is the shedding of positive ions of the metal, with the ultimate destruction of the anode, the least “noble” metal of the galvanic collection. Hopefully, that anode will be a sacrificial anodes (zinc) and not the more noble metals of bronze props, aluminum outdrives, steel transmissions, SS rudders, steel thrusters, etc.  This flow of galvanic current can be interrupted, and anode wasting (corrosion) stopped, by installing a “Galvanic Isolator” or an “Isolation Transformer.”

The galvanic potential (voltage) that causes the above flow of electrons is determined by the position of the specific cathodic and anodic metals involved in the galvanic series, and a variety of other factors related to metal mass and the physical characteristics of the electrolyte.  The flow of electrons from an anodic metal leaves behind a positive ionic charge.  To balance that charge, anodic metal ions are shed into the electrolyte.  That shedding is the corrosive deterioration we see with sacrificial anodes (zincs).

There are very subtle factors that affect the rate at which anode wasting occurs. These factors vary greatly from place-to-place.  Boaters will fit into all of the affected subcategories, so there is simply no “one size fits all” formula. Examples:

  1. Galvanic currents increase with water circulation over the hull. The protection requirement can be several times that required in still water. Zincs do not have the capability to automatically respond to changing needs as water velocity increases, as active protection devices (“impressed current devices”) do. So a boat in the Beaufort River in Beaufort, SC, or the Ashley or Cooper Rivers in Charleston, SC, may need more protection than the same boat would need in Marblehead, MA, or Miami, FL, or Marsh Harbour, BS.
  2. The ratio of cathode/anode surface area. The larger the relative surface area of the anode, the lower the galvanic current density on the anode, so the lesser the attack.  The amount of galvanic corrosion may be considered as proportional to the ratio of Cathode/Anode surface area.
  3. Boat use.  More frequently-operated boats (cruisers) require more cathodic protection than vessels infrequently used (floating condos, dock mavens). Relates to item 7, following.
  4. The conductivity of the water.  As conductivity increases, the rate of galvanic activity increases. Related to item 5, following.
  5. Water salinity.  Proportionally more protection is required on a given boat in salt water than in fresh water.
  6. pH of the water.  As pH decreases (acid rain fresh water lakes), the cathodic corrosion rate increases.
  7. Condition of bottom paint.  Deteriorating bottom paint increases exposed cathodic surface area, which increases anodic protection requirements.

Furthermore, when connected to shore power, your boat is part of a electrical network of boats – a collection of metals – that are electrically interconnected by the shore power safety ground system. The underwater metals on the collection can dramatically alter the cathodic potential (the amount of protection) of your boat. This is particularly true if your neighbor has aluminum (outdrives, trim tabs) and you do not.

A very common rapid zinc wasting condition occurs when a nearby neighboring boat is poorly maintained.  If you have good, well maintained zincs on your boat, but your dock neighbor does not, you will be glad to know that the noble metals of the neighboring boat are protected.  Your neighbor’s boat is being protected by your zincs, through your generosity, via the shore power network of shared safety ground connections.  Since your zincs are the sacrificial metals in this system, they are likely to deteriorate at a faster-than-normal rate. You may or may not consider this generosity to be a good thing.  This is a particularly common situation at marinas where a high number of absentee owners reside.

Occasionally we hear that we need to be careful not to “over-zinc” a boat. It is possible to “over-zinc,” but the term is frequently misused in context. In the context of that statement, “zinc” does not refer to the metallic substance; it uses the term “zinc” as a synonym for “anode;” i.e., a mechanical object. So more properly, one should say, “it’s possible to ‘over-anode’ a boat.”

Anodes used on boats are available in Magnesium, Aluminum and Zinc metals. Magnesium is best for boats kept long term in fresh water. Zinc is best for boats kept long term in brackish and salt water. Aluminum is often regarded as acceptable for use in all types of water. Using the wrong anode material in the wrong environment can reverse the galvanic potential between some dissimilar metals or non-metallic, electrically conductive materials under some conditions. Magnesium anodes should not be used long term in salt water. Aluminum anodes can cause harmful over-protection which may result in cathodic corrosion of aluminum parts (outdrive, trim tabs) and possible hydrogen blistering of paint, also known as cathodic disbondment. Some oxides of a few metals, including aluminum, tin, lead, and zinc, are “amphoteric,” meaning they are capable of reacting chemically in both acid (low pH) and basic (high pH) environments. These metals are more susceptible to corrosion in alkaline high pH electrolytes (fresh water) than other metals.

Conclusions: my PERSONAL OPINIONS:

  1. In general, most boats are better off having Bonding systems than not having bonding systems. If an owner chooses not to have a bonding system, that should be a deliberate, intentional decision made with great forethought.  It should be backed up by  thorough, professional cathodic measurement testing and with due consideration to dissipate of static electricity and lightening as a related technical matter.
  2. In general, absent an Isolation Transformer, all owners of boats that are fit with shore power service should install a Galvanic Isolator.  Galvanic Isolators block the flow of galvanic currents.  They greatly extend the life of the boat’s anodes, but more importantly, extend the protection of the most noble (and expensive) underwater metals on the boat.
  3. In general, the best protection an owner can afford their boat investment is to diligently maintain their anodes (“zincs”).

If you aren’t familiar with this language and these advanced electrical and materials concepts, you are in the clear majority of the general public and boat owners! Frankly, only true, insanely devoted electrical geeks venture into these “techno-weeds!” Most electrical service professionals – including the great majority of “marine-certified” electrical service professionals, do not really “get into this stuff.”  When faced with an issue, most “professionals” call in “experts” to handle remediation. This is why there can be, and is, a lot of misunderstanding and confusion around this subject.

Diagnosing Engine “No Starts”

The underlying causes of “no start” conditions can be either electrical or mechanical.  In order to start and run, an otherwise operational diesel engine needs fuel and air.  When the ignition key is “turned on,” one thing that happens is an electrical fuel valve called a “fuel solenoid” opens to allow fuel to flow to the injection pump.  No fuel, no start.  If the engine cranks but does not start, think of the humble fuel solenoid as a possible cause.

At it’s most basic, the ignition key or push-button operates the starter solenoid, and the starter solenoid picks, energizing the starter motor. If the engine does not crank (turn over) when the key is operated or the start push-button is pushed, the underlying problem is probably electrical.

In practice, there may be a dozen or more electrical connections in the starter solenoid pick circuit, depending upon how the engine manufacturer designed the circuit.  For example, Sanctuary is fit with a Cummins 4B engine.  On that engine, the starter solenoid circuit originates at the battery, goes to the “start” connection on a key operated ignition switch, goes on to the neutral safety switch (located in the transmission housing; prevents the engine from starting if the transmission is in gear), goes on in turn to the coil of a “secondary relay” and finally returns to ground.  Within this circuit, there are several intermediate connections on a terminal strip in the engine room and at the various switches and the relay.  So, there are many points of possible corrosion or wiring breaks that all must be viable in order to pick the secondary relay.  When that secondary relay picks, it’s the normally open points of that relay that actually put B+ battery power to the starter solenoid.  If that relay is picking when the key switch or “start” button is operated, probably everything in the pick circuit is OK (like that pesky neutral safety switch).  If the electrical connections at the secondary relay are OK, then the secondary relay itself becomes suspect (bad/worn/burned normally open points internal to the relay).

A starter motor pulls many hundreds of amps from the batteries (mine pulls 500 amps locked rotor and it’s a rather small starter motor).  When the starter motor is energized, the very high discharge current causes the battery terminal voltage to instantly fall off.  If battery terminal voltage falls off enough, the solenoid will drop out.  As soon as the solenoid drops, the battery voltage recovers enough to re-pick the solenoid.  This starts the next pick/drop out cycle.  This is called “chatter,” and it almost always means battery circuit problems.  Commonly, the problem is low battery charge.  The cause can be a battery that isn’t fully charged, or a bad battery, or bad connections, or a mechanically locked starter motor rotor.  Locked rotor could be a starter motor problem, or it could be that the engine itself is mechanically bound up.



First, check that the timing pin on the engine is not engaged.  If it is engaged, starting the engine will shear that pin, and that could cause many thousands of dollars in engine damage.  That pin being engaged would only happen if a mechanic had been working on the engine to locate top dead center to do valve adjustments or to install the injection pump, but check it anyway!

If the engine has not been started in several months, suspect that the oil has drained from the cylinder walls and the internal parts (usually piston rings) may be bound to the cylinder walls.  To clear that, bar the engine over (make sure the fuel solenoid is “off” while baring the engine over, or it could start and damage you, the boat hull, the bar, and anything else nearby) and then remove the bar and try normally starting the engine again.


The following procedure requires self-confidence, and is not for the timid of heart.  I learned this test as a kid working on my first car.  It is simple and very quick to complete.  Think carefully through each step, and execute with confidence.  Turn the key on (to open the fuel solenoid) and then take a big (fat, old, sacrificial) screwdriver, and us the metal shaft of the screwdriver to jumper across the battery cable connection at the solenoid to the strap buss that runs into the starter motor from the other large solenoid connection.  These connections on the started solenoid are the two large bolts on the starter solenoid.  Jupering across these two bolts will activate the starter motor by bypassing the solenoid.  B+ battery power will through the metal screwdriver shank to operate the starter motor.  Yes, it’ll spark.  Don’t be timid here.  Stick that baby in there firmly, and expect the engine to immediately turn over and start.  Don’t let the jumper touch the frame of the starter motor or the engine’s block, of course.  That will also spark!  If the engine turns over and starts, then you might assume the solenoid or the solenoid pick circuit is the cause of the no start.

If the engine doesn’t turn over with the solenoid jumpered, that is to say, if absolutely nothing happens, suspect the starter motor itself is bad.  One possible trick is to mechanically jar the starter motor’s internal brush rack.  If the motor has seen many cycles, the brushes will be worn, and eventually will be worn to the point that they do not make contact.  There may still be a few start cycles in that old motor, though.  Hit it with a hammer.  Yes, that’s right, hit it with a hammer.  Not so hard you hurt it, but several times, firmly.  That will jar the brush rack, and may allow the brushes to make contact again.  Perform the screwdriver jumper test again.  Again, expect that motor to immediately start. If this does work, you are definitely on borrowed time.  It wil only work a couple of times before the brushes will be so worn they won’t make contact.  Act immediately to get the starter motor rebuilt.

If the engine does turn over, but only very slowly, then suspect  battery terminal connections or the starter motor terminal connection, and also suspect the return connections at the engine block ground that connects the engine block back to the negative terminal of the battery.   If all connections are all in good condition, then consider low battery state-of-charge.

I once experienced an unusual starter motor failure that could result in a “no start” symptom.  The strap buss that runs from the starter solenoid to the brush rack inside the starter motor body broke inside the starter motor.  To check that, take a piece of wood (***not metal***) like a wooden hammer handle, and push the strap firmly downward into the starter motor body.  AS PREVIOUSLY DISCUSSED, MAKE ABSOLUTELY SURE YOUR BODY AND CLOTHING IS CLEAR OF ANY MOVING PARTS OF THE ENGINE!  While holding that strap down, have an assistant operate the key to start the engine. If it starts, you’ll have to pull the starter and take it to a shop to have the buss bar brazed.