Category Archives: Boat Technical 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.


Fuel Tank Replacement

This article applies to replacement of  diesel fuel tanks aboard a boat fit with a diesel propulsion engine and a diesel generator.  This article DOES NOT apply to gasoline fuel systems, which carry different risks, and different handling and construction considerations.

There are several choices for dealing with diesel fuel tank leaks.  Most if not all Taiwan built boats have tanks made of “mild steel.”  Also called “black iron,” these tanks are well known to develop leaks at welds and often, on the tops of the tanks.  A common cause of tank top failure is rainwater which leaks through deck fill openings and lies on the top of the tank in the area of the fill tube.

Some tank leaks can be plugged with sealants and/or adhesives, and while that may save up-front costs, it undoubtedly delays the inevitable and impairs the resale value of the boat.  Sanctuary developed a leak that could not be accessed for simple, external remediation.  After careful review of my options, and in consideration of the age of the boat, I chose to physically replace my OEM tanks.  I did this replacement as two completely independent projects, the first being replacement of the STBD tank (2017) that was leaking and could not be used.  The second project was replacement of the PORT tank (2018) as “predictive maintenance.”  This article documents my approach to the tank replacement project.

The major steps of the project plan for replacement of a diesel fuel tank include:

  1. Assess the extent of personal involvement to be invested in this project, based on personal preference, personal skills and boat configuration.
  2. If professional help will be hired, define the scope of the work to be contracted.
  3. Settle on design of the replacement tank solution.
  4. Contract/hire professional assistance.
  5. Empty the tank to be replaced.
  6. Gain physical access to the tank to be replaced.
  7. Perform demolition and removal of OEM/old tank.
  8. Qualify and hire fabricator for new tank.
  9. Wait patiently for the fabricator to complete tank build.
  10. Receive and place new tanks.
  11. Restore disrupted fuel and vent plumbing
  12. Restore vessel infrastructure and any disrupted electrical wiring and plumbing.
  13. Fill and calibrate new tank.
  14. Celebrate completion!

Because I have the necessary skills and tools, I decided to handle many parts of the project work myself.  However, I also decided I would hire a mechanic to cut out the OEM tanks and install the replacement tanks.  Tasks I took on myself included gaining access to the tanks so the mechanic could come in and begin to cut.  The mechanic would manage removal and disposal of the old tank, transport the replacement tanks from the fabricator to the boat, prepare the install location, move the replacement tanks into place, mechanically secure the tanks in place, and re-plumb the tanks.  I would then take over to button-up the work once the new tanks were secured in place, and replace disrupted electrical wiring and fuel system plumbing.  This approach worked well for me, and saved many thousands of dollars of professional hourly-billing labor time.

Aboard Sanctuary, the OEM configuration consisted of two, one-piece tank units of 160 gallon capacity, each, located athwartships in the hull, in a “saddle tank” configuration.  The OEM tanks were placed into the hull before the deck was installed, so physical clearance limitations made it impossible to install a single replacement tank of the OEM dimensions. The OEM tanks were 48” long, with a baffle at the lengthwise midline. It would have been possible to reduce the height of the OEM tank by 3”, but physical placement of a 48”, one-piece tank would have required removal of the engine to gain the needed clearance. Since we live aboard, removal of the engine was a significant impediment. However, two 24” tanks could be fit without engine removal, so two side-by-side 24” tanks became the design point I adopted. This approach also provided equivalence with the midline baffle of the OEM tank.

Using Lotus FreeLance drawing software, I created an engineering drawing for my replacement design, as shown in Figure 1 for my STBD side project.

Fig 1

Figure 1: Design of Replacement Tankage

The complete drawing set for the OEM tank, STBD and PORT replacement units and fabrication notes is here: 20180506_Monk_Fuel_Tank.

Between the mechanic and myself, it was agreed that I would do the site preparation work to gain physical access to the tank. On the STBD side, that involved total removal of the DC electrical system and batteries, relocation of AC distribution wiring to the aft half of the boat, and removal of a non-structural bulkhead covered with soundproofing tiles. Gaining access to the PORT tank involved removal of the main fuel supply rail and primary filter plumbing and removal of the control unit and hydraulic pump for our hydraulic thruster system.  On  the STBD side, the house batteries needed to be removed from the boat, so I used the genset start battery to power the house water pump and the waste macerators for the duration of that project.  Because the OEM STBD tank had leaked fuel, it was already empty.  On the PORT side, I pumped fuel from the OEM PORT tank to the newly replaced STBD tank to empty the PORT tank.

I recommend that frequent photographs be taken at many points as any complex project proceeds. It’s amazing how these photos help at assembly/re-assembly time. Figure 2 is a picture of the wiring of Sanctuary’s main battery box. Figure 3 shows the DC distribution wiring before the start of the project.  This distribution wiring is located on the bulkhead that covers the OEM STBD fuel tank:

Fig 2

Figure 2: Battery Box 1.

Fig 3

Figure 3: DC Distribution Wiring at the Start of the Project

After removal of the DC distribution wiring and temporary relocation of aft-running AC wiring, the soundproofing and bulkhead could be removed. That was a destructive process. The OEM bulkhead was 5/16” plywood – well, since Sanctuary was built in Taiwan, probably 8mm plywood – but non-structural. Figure 4 shows the OEM tank with access gained. At that point, an angle grinder was used to cut out the OEM mild steel (black iron) tank. Careful examination reveals two structural angle iron retainers holding the OEM tank in place. These angle iron retainers were re-installed after the new tanks were placed. Figure 5 shows the hull space, frames exposed, after the OEM tank was cut out:

Fig 4

Figure 4: OEM tank exposed

Fig 5

Figure 5: Tank location showing support frames

The replacement tanks were fabricated of 1/8″ (0.125″ ) Grade 5062 Aluminum.  The work was done by a local SW Florida metals shop. The fabricator pressure tested and certified the tanks. The individual tanks are light enough that they could be handled by one man (a younger, stronger man than I, however). Figure 6 shows the tanks staged on the dock, and Figure 7 shows them in their installed location with the angle iron retainers in place:

Fig 6

Figure 6: New aluminum tanks

Fig 7

Figure 7: New tanks in place

Note the length of fuel hose that interconnects the two tanks at the bottom. That hose is continuously filled with diesel fuel. Use USCG Type A1 fuel hose for that application. USCG Type A2 fuel hose is appropriate for the tank fill hose. Type A2 hose is rated for fuel, but not for applications that are continuously immersed in fuel. Note also that both tanks need to have a vent. Consider the drawing in Figure 1: fuel enters the “A” tank via the fuel fill in the deck, but then fills the “B” tank from the bottom up. The “B” tank must be able to vent captive air or that tank cannot fill. Likewise, for fuel to leave the “B” tank as it is consumed, air must be able to enter the void above the fuel in order for the tank to empty. In our case, the two vents from the “A” and “B” tanks tee into a single vent, which is mounted to an overboard vent fitting in the hullside. Finally, the tanks, the deck fill fitting and the vent thruhull fitting should be electrically bonded to the vessel’s bonding system, if equipped, to dissipate static electricity and prevent galvanic corrosion.

Fuel plumbing also merits special mention. The fuel valves used in diesel fuel systems are commonly made of naval bronze, which is galvanically active in direct contact with aluminum. To minimize galvanic corrosion at the tank fittings, use a 300-series (316L) stainless steel nipple or bushing (adapter) to isolate the anodic and cathodic metals of the bronze valve and the aluminum tank fittings. Bond the tanks to the vessel’s bonding system, if equipped.

With the tanks installed and secured in place, the bulkhead and the vessel’s wiring can be reinstalled. Figure 8 shows the replacement bulkhead in place, with an inspection port that allows access to the interconnecting fuel hose and it’s hose clamps. The temporarily relocated overhead electrical wiring is still evident in this picture. Figure 9 shows the batteries and finished DC electrical distribution system in their restored position.

Fig 8

Figure 8: Bulkhead with inspection port

Fig 9

Figure 9: Electrical Systems re-installed

When filling the new tank for the first time, I put in 10 gallons of diesel fuel at a time, and marked the sight glass meniscus as a fuel level reference. I find this simple calibration of the tank capacity to be extremely helpful in judging my cruising options as I travel.

The loss of 3” in height resulted in a loss of about 25 gallons of total tank capacity. Each boat is different. Each tank replacement project is different. For what I’ve described above, I spent $1750 to have the STBD tanks fabricated, pressure tested and certified. Labor and miscellaneous materials – like the A1 and A2 fuel hose, hose clamps and new fuel valves – was $1800. I invested at least 30 hours of my personal DIY labor doing demo, site prep and re-install work, so for those who choose to contract this total project, consider what that would add in billable cost if performed by a paid professional.  There were efficiencies gained in doing the STBD tank.  The fabrication cost of the PORT replacement tanks was only $1570, and the professional labor component was $1260.

There is no question, this is a major project. With the work done, don’t forget to celebrate.

Polybutylene (PB) Plumbing in Drinking Water Systems

Cruising south in 2017, I became aware that my house water pump was cycling on and off at random intervals.   I proceeded to change our water pump head/valve assembly, but that repair action attempt left the symptom unaffected.  After a period of vigorous self-denial, I had no choice but to accept that I must have had a slow leak somewhere in the house potable water system.

Sanctuary is a 1988 Taiwan-built trawler.  Many boats built in the period were fit with polybutylene (PB) plumbing and PB plumbing fittings.  PB water line “pipes” are gray in color, somewhat flexible, and the fitting are gray plastic.  Our PB system was marketed under the trade name of “Qest.”  Aboard Sanctuary, our potable water plumbing is 3/8” diameter tubing, which means 3/8” ID (inside diameter) and 1/2” OD (outside diameter).  The system fittings are, therefore, either  3/8” by 1/2” MPT (Male Pipe Thread) or 3/8” by 1/2” FPT (Female Pipe Thread).

In the 70s through early 90s, PB systems were used in many building, RV and boat applications.  When it became clear that PB fittings failed as they aged, there was a Class Action lawsuit settlement called COX v. Shell Oil et al.  to compensate PB installation failures in installations between January 1, 1978 through July 31, 1995.  The defective PB fittings were discontinued and the product removed from the market.  Today, replacement Qest fittings of “better” materials are available as replacement parts from a variety of sources, including big box stores, ACE Hdwr and many Internet vendors.

My leak was in the cold water feed to our galley and aft cabin shower, in a predictably inaccessible location.   In my search for the leak, I furthermore identified two non-leaking fittings with visible cracks in the body of the compression nut.  The leakeI had planned to replace two nuts and have some spares.  I wound up using five of those six nuts as I worked on the system.



Anyone with PB plumbing aboard should check it at least once a year for these kinds of failure.

DO NOT OVER-TIGHTEN THESE NUTS; no more than one-quarter turn past hand tight.

Navigation Via PC or Tablet Computer

A long-time cruising friend recently asked: “I’ve been researching a replacement for my circa 2000 RayMarine navigation system.  Clearly, there are any number of commercial systems that integrate chart/radar/depth, etc.  However, I’ve been looking at PC or laptop alternatives.  I’m curious as to what folks may be using out there, i.e., iNavx, etc.”

There are three mix-‘n’match categories of “navigation equipment” that combine into solutions that address this question:

  1. a full suite of made-for-purpose navigation equipment sourced from a major manufacturer of marine products (ComNav, Furuno , Garmin, Lowrance, Raymarine, Simrad, Sitex, etc.), or
  2. a network-connected combination of selected made-for-purpose marine navigation equipment and general purpose PC/tablet computing equipment running navigation software, or
  3. stand-alone PC/tablet computing equipment running navigation software (apps).

In 2017, all three alternatives are possible.  Options are listed above from most expensive to least expensive.  Items 1 and 2 are equally functional for navigation and piloting today.   Item 3 has feature-set limitations because some features are not available in the PC market, (RADAR scanners, AIS transponders, Autopilots, etc) and these features are unlikely to appear in that market in the reasonable future.  There is no “one-size-fits-all” right answer.  This article examines some of the pros and cons.

The value proposition:

Reality: All made-for-purpose marine equipment solutions and PC/tablet solutions have some limitations.

Traditional made-for-purpose marine equipment: is expensive to buy, often requires expensive professional installation, obsoletes quickly (resulting in a short feature-set lifespan), is constrained in its versatility, often requires expensive and/or proprietary charts, is relatively difficult/complex to upgrade and backup, and doesn’t always play well on boats fit with equipment from multiple manufacturers.  On the other hand, made-for-purpose equipment is rugged, weather-resistant,  viewable in bright sunshine, and (because of it’s limited feature-set) has a somewhat simpler learning curve for the end user.  These factors combine to produce a limited value calculation.

General purpose computing devices, including the navigation software applications necessary to run on them: are relatively inexpensive, utilize free NOAA (ENC) and USACE (IENC) navigation charts, are easily replaceable, are light and portable, are easily upgradable (so have a longer feature-set lifespan), and are extremely versatile through the many software applications that are available today.  The user interface for PCs and tablet client devices are based on the operating system they use (Microsoft Windows, Apple Mac OSX, Apple iOS, Google Android), but most operating systems are generally familiar to most people from other life learning and experience.  On the other hand, these devices are generally not made for outdoor use, may be difficult to view in bright daylight, and can be sensitive to over-heating in direct sunlight.  Overall, even with the negatives, this equipment can offer a very attractive value calculation.

There are significant learning curves associated with all marine equipment and general purpose computing products.   The learning curve can be challenging and intimidating for many users.  Some manufacturer’s user interfaces are more intuitive than others.  Personal preference and past experience with technology equipment has a significant effect on both choice and success.

Aboard Sanctuary:

For navigation and piloting aboard Sanctuary, I personally depend on a hybrid solution consisting of a combination of made-for-purpose marine equipment and PC/tablet computing equipment with appropriate software apps (alternative #2, above).  Today, the flow of data in marine data networks is mostly one way, with data traveling from the marine equipment to the PC equipment, via a multiplexor.  (See my article on Marine Data Networks on this site, here:   Aboard Sanctuary, this arrangement allows us to utilize made-for-purpose equipment in a way that lengthens the service life (obsolescence) of it’s aging feature set.  We use it to do the core work of the helm; i.e., run routes via the autopilot and watch for obstructions and marine hazards using RADAR and an AIS receiver.

We pre-plan our routes on our PC before day-of-travel.  When on-the-water, I rely primarily on our Apple iPad for piloting, risk management and risk avoidance operations.  We use our Macbook Pro laptop running Rosepoint’s Coastal Explorer 2011 for route pre-planning.  We rely on our iPad tablet running SEAiq, Navionics, Ayetides, Anchor Watch and various weather apps for general navigation and piloting decisions.  Due to their vintage, our made-for-purpose chart plotters do not support Active Captain.  I rely on iPad apps for ActiveCaptain anchorage and location reviews and marina contact information.  (And yes, my email and Peg’s Facebook are also available via the iPad, even while the nav app “stands watch.”)


Aboard Sanctuary, we have a now-obsolete Raymarine DX500s Fishfinder which serves as our primary depth sounder.  At the time of writing this article, the screen appears to be dying, but the internal electronics and NMEA0183 data network are operational for actual depth measurement.  Because of the capabilities of the iPad app (SEAiq), I don’t need visibility to the depth sounder’s screen.  I’m stuck with the DS500x for now because the Airmar sonar transducer is not compatible with newer versions of depth sounder, so I basically can’t upgrade the device without upgrading the transducer (a “project” to be faced in the months ahead).

I have a now-obsolete two-plotter Raymarine RL70CRC/RN300 GPS/chart plotter system that serves our salon and flybridge.  For cartography, this equipment uses expensive C-MAP chip cartography which I already own, but is prohibitively expensive to update or extend.  The C-MAP cartography works fine, but we very rarely use it anymore, since the SEAiq app on out iPad duplicates it’s capabilities at no cost.  Our Raymarine system has an integrated RADAR scanner.  The RADAR is not up to the capabilities of newer digital HD RADAR, but it is “adequate to the task.”  We occasionally use RADAR for MARPA, but mostly for tracking nearby heavy weather.  All of this is an old technology that continues to work acceptably well for us.

We have full chart redundancy via our made-for-purpose Garmin GPSmap 547xs chart plotter.  The 547xs has a diminutive screen size with tiny text, which limits it’s usefulness.  We use the 547xs almost exclusively for “driving” routes via our Garmin GHP10 autopilot.   The GPSmap 547xs does have modern CHIRP sonar sounder capability, as yet not installed.  The GPSmap 547xs also monitors our ICOM MXA5000 AIS receiver, which the Raymarine chart plotter cannot.  (I recommend AIS transponders be used ONLY for poor visibility, night operations, offshore operations and all operations on the US Inland Rivers.  Otherwise, AIS transponders are not necessary on pleasure craft on the US East Coast, and generally serve to create a false sense of security among users who generally do not understand the limitations of the underlying technology.)

In August, 2013, I installed a DMK 11A “multiplexor.”  The inputs to the multiplexor are our collection of NMEA0183 and NMEA2000 data networks serving our onboard marine equipment (five NMEA0183 networks, one NMEA2000 network and one Raymarine SeaTalk network).  The multiplexor re-formats the data into standard Ethernet data packets, and pumps the data out over wi-fi.  The multiplexor’s wi-fi interface is linked to our onboard Cradlepoint router, to which the multiplexor is just another ordinary client device.  Software apps that can interpret the data and run on any PC or tablet computer allow that computer to become a fully-portable wireless nav station.

I use the multiplexor’s wi-fi feed with Rosepoint’s Coastal Explorer 2011 on the MS-Windows side of my Macbook Pro.  That provides complete navigation redundancy in our salon.  I also use SEAiq Pilot and OpenCPN on the OSX side of my Macbook Pro.  One of the greatest advantages of SEAiq is that the user interface is identical across operating system platforms (iOS, Android, OSX and Windows versions), so regardless of the mix of operating systems, there is only one learning curve for the user of the software.  I use “SEAiq International” on our iPad (iOS).  The iPad version of SEAiq Pilot is professional-quality app that is used by working professional Chesapeake Bay pilots and Harbor pilots worldwide on large ships.   When my brother is aboard, he runs SEAiq on his VerizonWireless Android tablet.   With our multiplexor and suitable software apps that can interpret and display the data, our PC/tablet/smartphone equipment becomes a fully capable, wireless, fully portable chart plotter console.

With the above equipment platform, we have used our iPad since 2013 as our primary navigation device – the device from which our navigation and safety decisions are made.  Our Garmin and Raymarine chart plotters provide redundancy.  An Android tablet with suitable software apps can do what our iPad does, but just as Windows PCs are made by many manufacturers, Android hardware is “versionized” by several different manufacturers.  Depending upon the particular hardware customization, Android software can be finicky to configure and support.   The iPad-based stuff “just works.”

Transit Planning and Cruising:

I have used Coastal Explorer since 2006.  By way of that prior experience, I continue now to create detailed transit routes on my laptop using Coastal Explorer.   I load my routes into our Garmin GPSmap 547xs chart plotter.   Today, we need the Garmin chart plotter to “drive” the autopilot via our NMEA200 data network.  Our multiplexor passes along compass data (HDG), GPS and route data (lat/lon, SOG, COG, DTW, BTW, XTE, etc), sounder data (DPT, DBT, MTW), and all flavors of AIS data (!A).  All of that data is displayed by SEAiq on my iPad.   SEAiq uses the free NOAA and USACE charts, both raster and vector.   I update the charts at my convenience, usually when at a marina that provides reliable and fast wi-fi access to the Internet.   We maintain all of the US ENC charts for the US East Coast from Maine to Texas, the Great Lakes and the IENC charts of the Inland Rivers from Lake Michigan to Mobile and NOLA.   We don’t bother with Puerto Rico, the US West Coast, Hawaii or Alaska because I have no need for them, but they are all available, free.   SEAiq International is about $40, and SEAiq Pilot is about $200.  There are several multiplexor device alternatives; the  DMK11A was $400 from

Below are links to several articles on my website that describe all this in more detail.

  1. describes my Cradlepoint SOHO router configuration and Internet connectivity alternatives that I use on the boat.
  2. is a somewhat dated product description of SEAiq, but it will make the point.
  3.  is a description of NMEA0183, N2K and Ethernet networks, and the role of hardware and software apps that are needed to make up a functional system.

Return on Investment Considerations:

Yes, I use, and rely upon, our iPad for on-water piloting and navigation.

A new Garmin 7212 (now obsolete and no longer in production) would be $3000 or more, without charts.   Current-generation made-for-purpose systems would far exceed that.  Then, absent DIY installation skills, add the cost of professional installation.  A new iPad, app software and a multiplexor together would cost around $1400.   To me, the iPad is a simple, elegant, solution at a price-point that is at least 1/3 the cost of made-for-purpose marine hardware.  The iPad solution is reliable, and easily replaced almost everywhere if something bad were to happen.   Tablets need power to keep batteries charged, but are otherwise fully portable.   Tablets can be hard to see and can be subject to over-heating in direct sunlight, so care in handling is required.   Even considering these limitations, I find my iPad to be a great value proposition!

Specific to the Apple iPad, in the US, FCC regulation requires cell phones to have E911 capability, which means the ability to provide lat/lon location when a caller dials “911” from a cellular telephone.  In the iPad, to meet that requirement, a GPS receiver is built on the cellular telephone chip.   The GPS “comes with” the cellular telephone capability.   Therefore, iPads used for navigation should have cellular telephone capability.   It is not necessary to activate a cellular account in order to use the GPS.   The iPad’s GPS is fast and accurate.  It provides redundancy for position data should the multiplexor ever fail (it never has)…

As described above, I decided on SEAiq for our navigation needs, but other iPad apps are available.  Garmin BlueChart Mobile is a very basic, free navigation app that requires proprietary for-fee charts and bi-annual updates.   BCM includes Active Captain data, which I consider a “must have” in today’s world.  Navionics is similarly basic, also free, also requires proprietary fee charts, but does not provide Active Captain data.  Lack of ActiveCaptain data is offset by two features that people find useful and that I feel give Navionics a slight edge over BCM.   Navionics contains sounding data on the Inland Rivers, useful if cruising the Inland Rivers.   It also has a feature called “Sonar” Charts.   Dozens of cruising boats submit their own actual tracks, and Navionics develops current realtime sounding data in areas of shallow water.   That can be very useful in shallow areas, like SW Florida, the US East Coast ICW, or narrow passes into shallow anchorages on the A-ICW, Chesapeake Bay and elsewhere.   For both BCM and Navionics, chart subscriptions are annual recurring charges, and some features of Navionics, like that sonar feature, turn into a pumpkin at the end of an un-renewed annual subscription period.   The beauty of running these apps on an iPad is that if a user prefers Navionics, but also wants ActiveCaptain data, it’s easy to add an app that shows ActiveCaptain data.  That versatility is simply not possible (today) with made-for-purpose marine devices.  Note: in November, 2017, Farmin bought Navionics.  Garmin also withdrew BCM from the Apple Store.  These events put into question the future of both BCM and Navionics.

Note: In 4Q2017, Garmin discontinued their BlueChart Mobile app, and it is no longer available from the Apple app store.  Garmin replaced BCM with a successor “ActiveCaptain” app.  The ActiveCaptain app consists of the predecessor BCM features and facilities, but adds the capability to communicate with “compatible” Garmin Chart Plotters and share up-to-date cartography back and forth.  The app remains very basic.  It works and, in BCM-mode, will look familiar to previous users of the BCM app.  The advanced features that are new to the ActiveCaptain app are very welcome.  Garmin continues to make these capabilities available only on a proprietary basis with their own branded equipment, but for those with Garmin equipment, the app seems worth having.  Garmin has also purchased Navionics.  The future of that app is unknown at this writing (December, 2017).

If choosing made-for-purpose marine equipment, I recommend that buyers add new equipment made by the same manufacturer as any equipment that is already in place.  This recommendation is largely based on technical design choices manufacturers make having to do with the use of proprietary data.  I consider the “core components” of a navigation system to be the autopilot and the GPS/chart plotter, since more than other devices, these two devices MUST work well together; especially so for Position (lat/lon), course-over-ground (COG), course-made-good (CMG), bearing-to-waypoint (BTW), distance-to-waypoint (DTW) and cross-track-error (XTE).   Other system components should be of the same manufacturer where reasonable, affordable and possible, including depth sounders.  For weather instrumentation, AIS receiver/transponders, VHF radio DSC interface, and some other devices, which are all quite standardized, mixing manufacturer’s may be OK.

Not specific to Raymarine or Garmin, but generally across the marine electronics industry, manufacturers are moving at a very fast pace (Moore’s Law) to implement ever-increasing processor chip speeds, ever larger internal memory capacities, and ever expanding internal software (firmware) capabilities.   The rate at which new function becomes available and old equipment becomes obsolete is very rapid.  That leads to large capital expense outlays for buyers who try to “stay current.”   My personal observation is, the marine equipment manufacturer’s intentionally do not design for “downward compatibility.”  During my career in a fortune’s 10 computer company, one of the critical design issues for new products was “downward compatibility” (“backwards compatibility”) with existing customer equipment.  That was a critical customer requirement necessary 1) to protect the customer’s pre-existing investment base and 2) to allow for a reasonable and minimally-disruptive upgrade path. The same issues are painfully obvious to all of us as boat owners.  As described earlier, I face that issue today with my depth sounder transducer.

The ability of a manufacturer to offer an expanding feature-set is a function of processor chip speed and internal memory capacity.  Chart plotters and depth sounders are really just specialty computers, after all.  But, improvements in the features of marine equipment that are available to users arise from the software (firmware) capability built-in to the equipment, and software requires memory and chip speed.  Upgrading the physical hardware of made-for-purpose marine equipment is not an activity that is supported by the manufacturers, and certainly not a DIY activity.   Upgrades to firmware are limited to what the manufacturer makes available, and are generally not automatic or simple to accomplish.   By contrast, upgrading PC and tablet hardware is usually quite easy and relatively inexpensive.  Upgrading/adding software apps on PCs, tablets and smart phones is both routine and automatic.  This means that new software capability rolls out in tablet apps and PC software at the same rate and pace and at much lesser retail cost than with made-for-purpose hardware.

Finally, none of the marine manufacturer’s do a good job of standing behind their obsolete equipment.  I found a firmware design error in my Raymarine DS500x Fishfinder in the construction of the $SDDPT NMEA0183 sentence.   I reported that to Raymarine via their user’s web support forum.   After some back-and-forth based on the assumption that I had to be wrong (what could an end-user possibly know?) I was finally able to get grudging agreement from Raymarine that I had proved there was an bug in their device firmware.   The conclusion: “Have a nice life, Jim!   The box is out-of-production.”  No matter that the problem was a Raymarine product defect.  There was no way to upgrade the software in the field anyway, so therefore, apparently no need to fix it.  So, I live with it to this day, and every day I reconcile to never again trust Raymarine as my preferred equipment vendor.  That said, who knows if another vendor would actually be any better?

When the end-user posts a problem or a query to the Raymarine support forum, that often draws a lot of potential hints from other Raymarine equipment users.  Sometimes, that is helpful.   But the actual “experts” from Raymarine rarely “jump in” until there as been some largely wasted back-and-forth.  Does the forum work?   For user issues, yes; usually.   For real engineering issues, it depends on how hard you, as a user, push the gorilla to get a satisfactory answer.   If you get tired of the back-and-forth before the gorilla gets tired, you’ll go away empty-handed and frustrated.

Then there is support for the current line of equipment.   Generally, I find Tech Support is not set up to deal with a knowledgeable user.   I am a reasonably knowledgeable (if I say so myself) DIY user.  When I call Garmin, or write to Raymarine, for tech support, I have a problem that I have already researched, both on the Internet and in the manufacturer’s proprietary website support section.  When I call, I can clearly define and clearly explain the issue (or at least I can explain what it is not).  When I call, I have already updated the firmware, and I have done the basic power and wiring checks.   When I call, I am at the point where I know what I need and I know the information is not available elsewhere.   (By the way, the support section of the Garmin website is poor.   I find it largely unusable, with poor search capabilities, many, many hits that are not applicable to the search, and many distractions.)

The initial contact with Garmin tech support is to take callers through the “re-boot,” “re-calibrate” and “update the firmware,” steps before they take you seriously.   That can result in a lot of wasted time and frustration in back-and-forth exchanges, especially of you call from a location that is NOT the boat.   I personally suspect a lot of people just give up.   (But then, I know that many owners can’t operate the advanced functions of their equipment, including such safety items as DSC on VHF radios.)

I experienced an incident with my Garmin then GPSmap 540 chart plotter related to uploading routes.  With two or three routes to upload, the result was a “Route Truncated” error.   That incident lasted across multiple complimentary hardware upgrades and across more than two calendar years.   Very few people are stubborn enough to pursue that.  Indeed, maybe I’m nuts (none other than Jeff Siegel told me I was), but the failure was in a feature I really wanted to work, and the capability was within the published specifications for the device I bought.  But, every time I called Garmin tech support, I got a different technician.   It became impossible to take a technician new to the problem through the long and detailed pre-existing history of that very complex call.   It was a huge usability problem with Garmin tech support.  It took two escalation-to-management calls to get a senior technician assigned to my case and with whom I could just email status, questions, requests for additional data and case progress back-and-forth.   It was not until I got that done that I even began to make progress.

It’s undeniable that general purpose computing devices have their own “usability issues,” of which screen brightness and battery life are two.   But, most functional improvements come from improvements in software capability.   The commercial software applications available today for tablets and PCs are amazingly feature-rich.  In inclement weather, I keep my iPad safe by putting it in a one-gallon Ziploc bag.   Works just fine that way.   I have several different navigation apps loaded there which provide alternatives if needed.


As to the value proposition for all this, I would assume for all of us, boating is a discretionary expense.  Even though I may want the new gee-wiz function a manufacturer has developed, like HD RADAR, I do not want to have to spend thousands of dollars every 2-1/2 to 3 years to upgrade my navigation electronics suite just to be able to take advantage of the emerging features.   When we bought Sanctuary in 2004, there wasn’t a PC/tablet alternative to marine equipment.  I installed the then-current Raymarine chart plotter and RADAR system.   In the ensuing 12 years, that 2004 equipment investment has become several “generations” of Raymarine equipment releases obsolete.  To stay current with Raymarine’s pace of feature development, I would have had to upgrade my equipment three times at a minimum DIY cost approaching $5K each time.   In a word, “horsepucky” to that.   I am reluctant to invest in my system at all any more, because I feel like made-for-purpose equipment is an almost valueless upgrade to the base value of the boat.   Any future buyer of any boat with any navigation system older than a couple of years is buying an obsolete system, and will probably want to upgrade anyway.  There’s no value in making that upgrade for the current owner.

So, there isn’t a clear yes/no to the basic question of PC navigation; just a collection of pros and cons. Both types of solutions have merits, both are completely feasible, and both have limitations.   A very great deal will depend on personal preferences and personal self-confidence.   With the advent of made-for-purpose offerings like the new Furuno 1st Watch wireless RADAR unit (only power required; no signal cable up the mast, app on a tablet to display the RADAR image), PC and tablet solutions become more and more viable for more and more true navigation uses.   Watch this space as it evolves into the [near] future!

Choosing a PC/Tablet App for Cruising: following, I have created a template example of some (but NOT all) navigation application software products and some (but NOT all) factors that cruisers might like to have available.  The matrix, when complete, helps in selecting apps that will work for the personal preferences and navigational needs of different boats and different captains.  There is a great deal of Internet folklore associated with all of these apps.  Some are excellent for beginning cruisers, and some are capable of supporting advanced user requirements.   By way of illustration, I have populated some (but NOT all) specific detail for products I personally own and have personally used.  It’s clear that the matrix can provide a helpful visual means to screen products for personal suitability.  app_matrix

Hardware Considerations: in evaluating one’s interest in PC/tablet navigation solutions, consider the available hardware solutions as well as the navigation apps:

PC/Tablet hardware choice:

  1. cost
  2. network support requirements (NMEA0183, NMEA2000, multiplexor, Ethernet, Bluetooth)
  3. mechanical mounting requirements
  4. contains internal GPS vs requires external GPS
  5. screen visibility in bright sunlight
  6. overheating in direct sunlight
  7. weather resistance
  8. battery life
  9. has data back-up tools
  10. ease of replacement
  11. manufacturer provides good technical support (operating system & applications)

Made-for-Purpose marine hardware products:

  1. cost (product plus installation)
  2. network support (NMEA0183, NMEA2000, Ethernet)
  3. Ethernet interfaces provide for end-user data transfer, not just proprietary manufacturer use
  4. mixed-manufacturer compatibility
  5. time to expected obsolescence (expected feature-set lifespan)
  6. portability limitations
  7. versatility (weather, ActiveCaptain, tides ‘n currents, anchor watch, cruising guides, social media, email)
  8. speed (chip, memory)
  9. ease of data entry (route & waypoint creation/modification) (touch screen vs keypad)
  10. boat motion interferes with touch screen operation when sea states are moderate to lumpy
  11. complexity & frequency of software update(s)
  12. complexity, frequency & cost of chart updates(s)
  13. warranty period
  14. support period
  15. ease of warranty replacement & future upgrade, including backward compatibility
  16. manufacturer provides good technical support (hardware & firmware)
  17. security (insurance deductible, theft)

Marine Data Networks

8/7/2017: Updated “Hardware” section to include Rose Point LLC’s announcement, dated today, of “nemo™” “Signal K” device.

There was recently a question on a forum I follow asking, “are there devices that can allow different network technologies to ‘talk to one another.'”

Just understanding that question requires some knowledge of computer and network technology.  The question is asking if there is the capability to share data created by a software application on one computer with one or more software applications running on another computer.  That capability is actually extremely complex to achieve.

Networks are the “roads” over which digital data travels between computers.  The Internet is just a large and complex “highway system” for digital data travel.  Just as “roads” range in size from dirt trails to interstate highways, computer “networks” range from slow, limited in capacity to very fast and enormous capacity.  Just as Interstate highways can carry more traffic than city streets, some network designs carry more data than others.  Cars and trucks travel on highways.  “Units” of data travel on networks.  Each unit of data on a network is like one car on a highway, in that it can have a different destination than the unit in front of it or the unit that follows it.  Just as there are many sizes/shapes/brands of cars and trucks, there are many different formats for individual units of digital data.  Trains travel on a specific road bed called “tracks,” while cars travel on a specific roadbed called “pavement.”  Networks are designed to handle one – and only one – particular format-type (Syntax) of digital data, so a different unique network is required for each different format-type of data.

Increasingly, the navigation equipment found on pleasure craft are actually computers running operating systems (usually Linux) and software applications (called “firmware”).  Chart Plotters, AIS receiver/transponders, VHF Radios and autopilots are all special purpose computers.  These devices are connected together with network connections consisting of pairs of primary wire, coaxial cable, multiplex cable varieties, or radio waves in the case of Bluetooth and Wi-Fi.  They have imbedded operating systems and run apps of functionally-specific “firmware” which exchanges various kinds of information (depth, heading, course, speed, cross-track error, position, temperature and many more), back and forth among the components of the navigation suite.

So the question, “are there devices that can allow different network technologies to ‘talk to one another,'” is very complicated.  The real answer is neither “yes” nor “no;” the real answer is: “maybe;” “sort of;” “sometimes;” and “it depends.”

Over the past 15 years, the navigation electronics designed for and deployed on pleasure craft has exploded in function and complexity.  There are several excellent and highly competitive marine electronics manufacturers, each with worldwide markets, producing navigation equipment components and systems.  There are also companies specializing in developing sophisticated vessel monitoring and accessory equipment.  That explosion of marine function and technology has been accompanied by a similar explosion by technology companies that manufacture portable, durable, highly functional general-purpose consumer electronics and computing products.  There are many general-purpose computing equipment choices today that boaters did not have as recently as 5 years ago.  It’s highly likely the current rate of development will only accelerate in the near term future.

Whether in the realm of specialty navigation equipment or general-purpose equipment used to support navigation tasks, there are several technical realities that underlie the complexity of the navigation electronics market.  Key technology areas include:

  1.  data formats and data-exchange networks,
  2. capability of hardware devices, including designs for backward compatibility, and
  3. availability and capability of software applications, whether in the form of device-specific  firmware, smart phone apps or PC operating systems with application software, that all need to interoperate.

As youngsters learn to play various sports, they must learn the terms that go with the game.  In baseball, for example, the young ‘un must know what a “ball” is; a “bat;” a “base;” a “diamond;” a “hit;” a “strike;” a “foul;” an “infield fly;” an “umpire.”  In a discussion of digital data and networks, there are terminology and concept basics that need to be understood.   For this article, following are some of “the basics:”

  1. Interoperability – The ability of a buyer to purchase equipment from different manufacturers and be able to install that equipment into an existing suite of equipment with confidence that it will all work together.  When many different manufacturers make products that overlap in capability and are intended to provide the same functional capabilities in the same target market, “interoperability” is an essential requirement of the buyer/end user.  “Interoperability” must be designed into the equipment.  These designs are implemented by adherence to various industry standards and the architectural protocols of the communications network that the equipment utilizes.
  2. Syntax – The specific sequences of control information and user data that make up units of data traveling in a particular network.  In NMEA0183, data units are called “sentences;” in NMEA2000, data units are called “Parameter Group Numbers;” in Ethernet, data units are called “packets.”  The specific format of these units of data are all different from each other, but the construction of each kind of data unit follows very specific architectural rules.
  3. Protocol – Any defined, standardized scheme used to pass data between devices by which the data sent from one device can be received and correctly interpreted by another device.
  4. Simplex – A one-way (uni-directional) communications link between a device that sends data (like a compass sending a heading) and another device that receives data (like a chart plotter displaying a compass heading).  This technology can use a single pair of signal wires.
  5. Duplex – A two-way (bi-directional) communications link, like a telephone conversation.  In digital communications, this technology typically uses three wires, Transmit Data (TD), Receive Data (RD), and signal ground
  6. Serial – Data that is transmitted bit-by-bit, like typewritten words.  Think of a keyboard (typewriter), where the words of this article were created serially, letter-by-letter.
  7. Parallel – Data that is handled in frames of predetermined length.  The two most familiar items here are “32-bit” and “64-bit” operating systems.  What that means is that the internal processor chips and “motherboard” can handle either 32-bits or 64-bits at a time, instead of just one single bit.  Parallel operations add cost but speed up computer and network throughput speeds.
  8. NMEA0183 – A “first generation” marine serial data communications protocol standard of the National Marine Electronics Association (NMEA), used to enable interoperability between other NMEA0183 made-for-purpose navigation devices, including devices made by different manufacturers.  Furuno, Garmin, Raymarine, Sitex, etc, etc. all make GPS receivers, depth sounders, chart plotters, autopilots and weather instruments that can share their data (Interoperability) on a client’s boat because they all follow the same data architecture standard.  This network uses a pair of signal wires (data signal + and electronics ground).
  9. NMEA2000 (N2K) – A “second-generation” marine serial data communications protocol standard used to enable interoperability between N2K devices made by multiple manufacturers.  Faster and more extensive than its NMEA0183 predecessor standard, N2K includes support for data from accessory equipment (engine operating and performance data, battery monitoring data, bilge pump and tank level monitoring data, and more).  This network uses a 5-conductor cable with standardized connectors.
  10. CanBUS – “Controller-Area Network Bus,” is the technology used by the “computerized controllers” found in modern cars and trucks, worldwide.  N2K as used in marine applications is a CanBUS-compatible spin-off of the parent CanBUS technology platform.
  11. Ethernet – The full-duplex networking protocol standards (wired and wi-fi) used by general-purpose computers to exchange data over the public Internet.  The wired form of this technology uses Category 5 or Category 6, 8-conductor cable with RJ-45 terminal ends.  The wireless form of this technology uses two segments of the radio frequency spectrum.
  12. Multiplexor – A simplex (one-way) device that can monitor and forward data passing through NMEA0183 and/or N2K networks, at a minimum.  Some can also include Raymarine Sea Talk network data and Furuno NavNet data.  Multiplexors are designed to bridge data to another network and convert the data format so that it can be used in another kind of network (ex: NMEA0183 to Ethernet).  Conversion of data from one network syntax to another is a function requiring firmware intelligence.
  13. Signal K – An emerging full-duplex (two-way) technology that can convert data between NMEA formats and general-purpose Ethernet formats used by general-purpose computer networks.  This extended function allows the otherwise non-compatible NMEA networks to interoperate with laptop computers, tablets and smart phones using Ethernet (wired or wi-fi) communications networks.

Anyone who has ever read an advertising or marketing brochure for a marine navigation product has been faced with an array of technology terminology (“techno-babble”) like the above.  The “techno-babble” is often confusing, even confounding.  “It sounds wonderful, if I only knew what they were talking about!”  Adding to the confusion, each manufacturer has its own terminology for its features and capabilities.  Furuno has “NavNet.”  Raymarine has “SeaTalk.”  They are the same things by different names.  The manufacturer-specific marketing “techno-babble” adds to the complexity of comparing the capabilities of equipment from different manufacturers.

Data Networks and Data Exchange:

Interoperability is not necessarily a goal of marine navigation equipment manufacturers.   Garmin International has a corporate policy to keep much of their data proprietary.   For other manufacturers, that makes designing for interoperability with Garmin equipment difficult or impossible.  For example, Garmin does not share their autopilot control data syntax with Rosepoint LLC, the developer of Coastal Explorer navigation software.  Thus, Coastal Explorer cannot load route data into Garmin chart plotters.  Garmin’s goal is to “incentivize” buyers of their equipment to stay brand-loyal, since only other Garmin equipment can fully utilize Garmin proprietary data and capabilities.   Conclusion: Garmin doesn’t want true interoperability with other equipment manufacturers.

NMEA0183 (simplex) and NMEA2000 (“N2K”) (full-duplex) are communications network standards for two types of serial networking technologies.   Figure 1 shows the NMEA0183 network model:


Figure 1: NMEA0183 Simplex Network Model

Figure 2 shows the NMEA2000/CanBUS network model:


Figure 2: NMEA0183/CanBUS Full-Duplex Network Model

In an NMEA0183 network, the data units that travel the network are called “sentences.”   In an N2K network, the date units that travel the network are called “Parameter Group Numbers,” or “PGNs.”  The names aren’t important to the average boater.   What is important to know is that these two types of digital data packaging are not compatible with one another.

Within the two NMEA data standards, there are specific sections that provide for manufacturers to use proprietary data syntax.   Several manufacturers, including Garmin, Simrad, Raymarine, Stowe, the Brunswick Corporation, Mastervolt and others, use proprietary data for at least some of their device functions.   If a manufacturer chooses to use proprietary data for any given function, that function may or may not operate correctly in a network involving equipment made by another manufacturer.   More likely, most of the design features will work, but one particular feature – or feature subset – may not.   If that feature isn’t important to the buyer, nothing is lost.   If that feature is important to a buyer, well then, there will be disappointment.   It is not always possible to know in advance if that will happen in any given mix of equipment from multiple manufacturers, so the reality is, there is no absolute guarantee of interoperability.  Adding to the complexity of the technologies is the fact that equipment features and functions change every year as new gear rolls out.  It’s often good advice to stay with a brand if that brand meets your needs.

N2K is an “evolutionary descendant” of another communications protocol called CANBUS (Controller Area Network).   CANBUS is the networking technology used worldwide in automobiles and trucks.   CANBUS is a very fast and very reliable full-duplex serial network.  On boats, it allows modern diesel engine performance monitoring data to be included in an N2K network.  So for example, a marine chart plotter may have the capability to display Cummins or Caterpillar or Volvo engine operating and  performance data.

With the N2K and CANBUS standards, there is no native provision for an interface to an Ethernet network as found on a general-purpose consumer client devices (Server platforms, PCs, tablets, smart phones).   The World-Wide networking standard for general-purpose clients is IEEE 802.11 a,b,g,n wired Ethernet or IEEE 802.3 Wireless Fidelity (wi-fi) Ethernet.   Some marine manufacturers are in the process of adding Ethernet capability to their equipment, as a option for proprietary features/functions if not as a backbone communications network.   Anyone with a requirement to use a general-purpose client device within the Navigation suite will need a way to interface to the NMEA incompatible networks: NMEA0183 and N2K to Ethernet.  Check carefully on any device you purchase that has Ethernet built-in.  It may not be there to support interoperability with general-purpose client computers.


NMEA0183 is a simplex and serial network technology.   The incoming port to a device is known as the “Listener.”  The outgoing port from a device is known as the “Talker.”   Talkers cannot listen, and listeners cannot talk.   By design, an NMEA0183 network is limited to one, single “talker,” and about 4 – 6 listeners.   On most boats, even a “basic” navigation suite of compass, GPS, chart plotter, depth sounder and DSC VHF Radio will need several NMEA0183 networks to function as an integrated system for the user.  Both as new installations and for equipment upgrades, these networks can be a challenge to lay out, can be hard to expand in stages, and will require careful planning and forethought.   Figures 3 is a view of the five NMEA0183 networks I have installed in Sanctuary:



Figure 3: Sanctuary NMEA0183 Networks

Most marine instrument hardware today is made with both N2K and NMEA0183 built-in the unit.   The NMEA0183 interface supports backwards compatibility with older devices that have only an NMEA0183 interface(s).  Today, manufacturers add both NMEA0183 and N2K interfaces to most products in order to support an upgrade path from the old technology to the new.  This allows buyers to add devices with the faster, newer, more functional N2K networking technology in small and affordable increments.  Many – but not all – marine hardware devices support two NMEA0183 listener ports and two NMEA0183 talker ports in addition to an N2K port.   These devices can listen to incoming data on one incoming NMEA0183 listener port and spit it back out again (forward it) on an outgoing talker port.  In that way, data can be bridged to a second NMEA0183 network.   Specific data that can be forwarded is a function of the individual device.  Not all devices can forward all data.

Today, there are devices called “multiplexors” that can translate network data formats into formats needed by other network technologies.   Multiplexors can “listen to” NMEA0183, N2K, Furuno NavNet and Raymarine Seatalk networks and translate that variety of data into Ethernet formats that can be used by a computer or tablet.  Multiplexors can also translate NMEA0183 sentence data into PGN format and forward that data to an N2K network.   Most multiplexor solutions today are simplex (one-way), from the navigation suite to the PC/tablet.   Figure 4 shows the fully integrated suite of equipment aboard Sanctuary, including NMEA0183, N2K, the multiplexor and Wi-Fi.


Figure 4:  Integrated Suite of Equipment, Including NMEA0183, N2K, a Multiplexor and a Wi-Fi Feed For Use By PC and Tablet.

Today in 2017, there is a new development initiative underway.  It is an evolutionary descendent of existing communications network technology, not a new communications protocol standard.   Called “Signal K,” this is being lead not by a manufacturer, or a group of manufacturer’s, but rather a private group (open-source) of software developers.   Signal K is intended to be a full-duplex (bi-directional) solution.  That is, the Signal K hardware (gateway) will assemble NMEA0183 and N2K data and forward it via Ethernet protocols to a PC or tablet, and will receive Ethernet packets from from a PC or tablet and translate that data into NMEA0183 or N2K formats.   The idea is to create an full-duplex network technology platform that truly provides full interoperability.  The developers of Signal K claim that this solution will support Nobeltec, Rosepoint, iNavX, OpenCPN, MacENC, Polar Navy, iSailor, Navionics and other software applications runing on general purpose computing platforms (Servers, PCs, Tablets), all wirelessly via wi-fi feeds.

One such physical gateway is called iKommunicate.  The iKommunicate solution is, in 2017, an emerging technology.  Flash: today – 8/7/2017 – Rose Point Software announced their new “Signal K” gateway, called “nemo.”  Information on “nemo” is available here:  These devices are really highly specialized computers.  They are analogous to the Small-Office Home-Office (SOHO) Ethernet routers that are familiar to most of us.  They don’t do a lot, but what they do, they do very fast and very well.  In the case of iKommunicate, they are data translators, translating between the syntax of data arriving and leaving via different network protocols.
With a multiplexor solution, a computer or tablet application can listen to GPS position data and compare it to a pre-planned route installed on the computer.  But a multiplexor is a simplex device, and cannot talk back to the network, so cannot provide control information to correct the course via the boat’s autopilot.   With the Signal K solution, application software running on the Laptop or tablet would be able to control and correct an off-course condition via the full-duplex Signal K network bridge.


In order to monitor, control and correct for dynamic situations and asynchronous events that occur on the water, a PC or tablet software application solution that has the needed intelligence and decision-making capabilities is also required.  The network alone is not enough.  Today, there are very few software applications that can do that, and NONE that I know of that can do it for all navigation functions.   The two most popular tablet apps – Garmin Blue Chart Mobile and Navionics – can’t do any of this.   MacENC on Mac OSX can do some functions for Mac users.   Coastal Explorer on Windows can perform some functions.   SEAiq is available for iOS and Android Tablets as well as Mac OSX and Windows PCs.  SEAiq can do some driving.   Consider though, if Garmin will not release the syntax of proprietary data to Rosepoint or SEAiq developers, then the apps cannot fully support these manufacturers devices.


So, yes, Virginia, there are devices that can allow different network technologies to “talk to one another.”   But, there’s more to it than just talking.   Just having the network is not enough.  Consider this scenario: put three people in a room, one a speaker of only Mandarin Chinese (syntax), one a speaker of only Arabic (syntax), and one a speaker of only English (syntax); yes, they will be able to talk at one another, but they will not understand one another.   Intelligence is needed to provide translation and understanding.  That is very much what exists in the navigation networking and data realm in 2017.  Any data converter or software application solution will need to understand and translate all three languages (data syntax) in all application areas.

Today, as in the early days of computers, end users of nav equipment must understand more of the technology than they would like to have to understand.  In 1995 or so, my neighbor ran a home-based medical transcription business.  Just to type dictations and send the finished transcriptions to the hospital medical records department, she needed to know a great deal about Windows and network connectivity, for which she had neither background, training nor inclination.  That’s how it is today for navigation electronics on pleasure craft.

Watch this space, though.  In a relatively little time – even today at the high end – we will have equipment that fits into systems, introduces itself as plug ‘n play, and just works.  We will have software apps that allow us to take advantage of all of the features the manufacturers design into their equipment.  And, we will be using PCs instead of made-for-purpose equipment, because it is both less expensive and more functional and flexible.  Were it not for the lack of full-feature software, I would be using only my iPad for navigation today.  As that gap closes, it may well become an all tablet world.

Electrical System Topology

Electrical System Schema:

The schema of Sanctuary’s vessel-wide electrical system contains three major divisions.  This diagram is specific to Sanctuary, showing two 30A shore power connections and a fully-integrated but modestly sized inverter/charger.  That said, the overall model generalizes very well to larger electrical systems based on voltage, inlet, inverter/charger and load capacities and configurations.

  1. AC electrical system division of the vessel includes:
      • 120V Shore Power inlet connections
      • AC Generator (Genset)
      • ABYC-compliant Generator Transfer Switch
      • AC Branch Circuit Distribution Panel(s) – (NewMar – House loads; Weems & Plathe – heat pumps, raw water circulator)
      • Galvanic Isolator
  2. DC electrical system division of the vessel includes:
      • Battery Bank
      • Propulsion engine alternator
      • DC Branch Circuit Distribution Panel(s)
      • Individual component attachments (Thrusters, Windlass, Autopilot, Entertainment, Inverter/charger, etc.)
  3. Interface, or Bridging, or Power Conversion division of the vessel includes:
      • Magnum MS2012 Pure Sine Wave Inverter/Charger

General Topology of the Vessel Electrical System:


An Adobe Portable Document Facility (.pdf) version of this drawing is available by clicking this link: 20161019_electrical_system_topology.

Bonding System Design and Evaluation

My previous post (Corrosion Article) discussed corrosion of underwater metals caused by various stray electric currents in the water.  In that post, I made passing reference to “bonding,” “bonding conductors,” and to underwater metals “being bonded together.”  This article looks specifically at the bonding system of a boat.  The objective is to provide a basic understanding of why bonding is installed, what it does, and consider the maintenance needs of the bonding system.

As boaters, we are constantly involved in discussions of the design, equipment, materials, techniques and components of the AC and DC divisions of a boat’s electrical system. When those systems fail, there are usually symptoms, anxieties and inconveniences that boaters notice. Although Internet boating discussion lists are filled with electrical topics, only rarely does one see discussion related to a boat’s “bonding system.”

As in other electrical technical areas, “bonding” is an area where there is a body of common concepts and terminology that apply across a wide range of AC and DC situations. Just as in the “Corrosion” topic, the concepts of bonding are consistently the same, but an understanding of context is essential to avoiding confusion.  Experienced electrical practitioners often take shortcuts with context. For the layman, the only way to get past that is to invest some time in understanding the concepts. After that, understanding context gets easier very quickly.

The terms “grounding” and “bonding” are often used interchangeably, but in fact, they are different. Following are definitions with which most experts would agree:

Ground” is the single-point of electrical connection between an electrical sub-system (like a boat) and the physical earth. This connection is made for the purposes of:

  1. providing a lightning discharge path,
  2. providing a path to bleed off static charge,
  3. sub-system voltage stabilization, and
  4. reducing RF interference.

Bonding” is an electrical connection (usually a network of electrical connections) which electrically interconnect metallic housings and device enclosure components. Bonding:

  1. provides a low-resistance path for ground-fault currents to ensure circuit protection devices (circuit breakers) trip,
  2. prevents dangerous “touch-voltages” from appearing on exposed metal surfaces, and
  3. provides a path for galvanic currents and AC and DC stray currents.

Figure 1 is a simplified topology overview of the three major divisions (AC division, DC division and Bonding division) of the electrical system of a typical boat. It is representative of the great majority of US-manufactured boats. This topology view is consistent with the “model” electrical system upon which the principal ABYC Electrical Standard, E-11, is based (“AC and DC Electrical Systems on Boats,” July, 2015, Figure 10).

The ABYC E-11 standard treats a boat’s DC System as the “central-most” division of the electrical system of the boat, to which all other divisions are attached in a peer-to-peer relationship. This seems a reasonable assumption, since AC systems and bonding systems are neither required nor essential on a boat, but the DC system is always needed for engine starting and the operation of bilge pumps, navigation lighting and (usually) sound-signaling device requirements.


Figure 1: The Bonding System on a Typical Fiberglass (FRP) Cruiser.

All of the conductors shown in green in Figure 1 are part of the boat’s “bonding system,” or “bonding network.” That entire network of conductors works together. In typical dockside conversation, the “bonding system” is often thought of as limited to the wiring shown on the right-had side of Figure 1. The usual term applied to the AC portion of the bonding system is the “AC safety ground.” Note, however, that the AC safety ground is a part of the overall bonding network of the boat.

In normal operation, all bonding systems are “silent” and “invisible.” When “everything is right,” the bonding system does nothing, and “everything works fine.” Bonding networks are so quiet and invisible that a boat owner might never know if a fault had appeared.

In fact, the primary purpose of the bonding system is to spring into action to protect us when an electrical fault does occur in either the AC or DC system. The only “normally active” purpose of the bonding system is to control corrosion due to DC galvanic currents.

Due to component reliability, the mathematical probability, confirmed by life experience, is that electrical faults are relatively infrequent. Given that the bonding system comes into play only when there is a fault, it probably won’t actually be needed very often. If the bonding system does have a defect, unless there is another fault there will be no failure symptom or danger to people or pets. Yes, there may be an increased rate of corrosion, often interpreted as “electrical issues in the basin and nothing to worry about.” These are “handled” as a routine maintenance item, but the underlying cause is often not corrected. The bonding system adds complexity to the boat, but can save many headaches, much expense and even heartache for the boat owner if it is intact when needed. Some bonding system faults can create dangerous situations leading to fire, electric shock, loss of property and in the ectreme, loss of life.

The heart of the DC division of the boat electrical system is the battery/battery bank, including all B+ and B- wiring and all subordinate DC device attachment wiring. “B+” is the term for the DC positive feed (+12V, +24V) that originates at the positive post of the boat’s battery. “B-” is the term for the DC negative conductor that returns DC power to the negative post of the battery. In the common lexicon of conversation, the DC return circuit is often referenced as its “ground” conductor. However, the B- conductor in the DC system carries DC current back to the battery, so it is more properly analogous to the “neutral” conductor of the AC division.

Bonding circuits are intended to carry only galvanic and fault currents; never currents that power equipment or attachments. To avoid undesirable voltage drops in the bonding system, and problems with accelerated electrolytic corrosion, no B- connections should ever be made to any part of the bonding system. Such connections are analogous to a “code violation.”

ABYC E-11, Figure 10, shows the “DC Main Negative Buss” as the central collection point for all DC B- return circuits, as well as for the “AC Safety Ground” and the bonding network connections. The boat’s AC Safety Ground and the various branches of the DC bonding system are all connected together at one place, and at one place ONLY: the “DC Main Negative Buss.”

Neither ABYC nor NMMA “require” the installation of DC bonding systems. Bonding systems are “optional.” However, ABYC E-11 does specify requirements for the bonding system if one is installed. Among US boat manufacturers, bonding systems are the “normal” manufacturing practice.

The primary purposes of the bonding system are to:

  1. hold exposed metal parts at to “touch potential” that is safe for people and pets;
  2. provide a low resistance path for fault currents to trip “circuit breakers;”
  3. provide a single point-of-access to protect multiple structural metals of the boat from corrosion, via a sacrificial anode (zinc);
  4. provide a path for certain DC stray currents to safely exit the boat via the AC shore power safety ground;
  5. disperse static electricity in high winds and from nearby electrical storms, and
  6. reduce (attenuate) spurious RF electrical “noise” created by on-board equipment (battery chargers, inverters).

Many of the conductors of a “bonding system” are installed in the very hostile environment of the boat’s bilge. The various metal objects tied to the bonding system include:

  1. thruhulls, seachests, sea strainers and packing glands,
  2. rudder “stem iron,” rudders, rudder “shoes” (skegs), tillers and miscellaneous metal support structures of the steering system,
  3. various steering system components (quadrant, cables, hydraulic lines, hydraulic pumps),
  4. trim tabs and thruster systems,
  5. exhaust system fittings and ports,
  6. radio counterpoise and static dissipation “ground plates,”
  7. fuel tanks, fuel filling ports and tank vents,
  8. potable water and black water tank access and vent ports,
  9. generator, battery charger and inverter chassis frames,
  10. solar panel and wind generator frames,
  11. handrail and bridge enclosure frames,
  12. heat pump and circulator pump frames,
  13. stove and water heater frames,
  14. refrigeration (compressor) frames,
  15. etc, etc, etc…

In short, lots ‘o stuff.

Figure 2 shows the hull penetrations on a typical trawler (Sanctuary) built with individual thruhulls (without a seachest).

Figure 2: Typical Hull Penetrations on a Boat with Thruhulls and Without a Seachest

The complete collection of all of these metal components are “bonded” – connected together into a single electrical network – as shown in Figure 3.

Figure 3: Typical Bonding System

Figure 3 is only one example of the construction of a bonding system. Other configurations are acceptable. Take particular note of the large gauge conductor shown in orange. That conductor is the “backbone” of the DC portion of the bonding system. That backbone conductor runs the length of the hull. To the backbone are attached all of the green stranded wire pigtails connecting the metal structures of the boat to the backbone. Also note the transom zinc, which provides primary galvanic protection to all of the metals connected to the bonding system. When the boat is at anchor, away from shore power, it is the transom zinc that is the “ground” connection point. That is, the single point of electrical attachment to the earth, the primary dispersal point for static electricity and lightening and the electrical connection that establishes the “touch potential” for people and pets for the entire electrical system of the boat.

It would not be unusual if a boat’s owner did not know when the bonding network was last tested. It may have been quite some time; perhaps, never. It is possible that weakness(es) are present in the bonding system. I suggest testing of the bonding system should be done every three to five years.

Most if us have measured the terminal voltage of flashlight batteries many times. We have probably all measured our boat’s 12V (or 24V) lead/acid batteries. Figure 4 reminds us of the very simple task of measuring the terminal voltage of a “AA” battery:


Figure 4: Measuring the Terminal Voltage of a Battery

In this “typical” battery, a galvanic cell, there are two “half-cells” (copper and zinc) located in an electrolyte. Since the battery is always seen as a packaged unit, the term “half-cell” is not commonly used except by engineers and battery manufacturers. The terminal voltage is measured with a digital voltmeter. When a load is connected across the battery terminals, current flows.

Key concept: batteries are used to provide the voltage needed for circuits.  With batteries, their intended use means there should be a voltage between the positive and negative terminals.  A direct short circuit across a battery is never desirable, as it will dramatically accelerate the rate at which the battery becomes exhausted.  Inside a short circuited battery, the halfcells will become wasted (corroded) at an extremely fast rate, accompanied by the generation of heat and gasses.  However, in the case of the “accidental” battery created by the electrochemistry of dissimilar metals in seawater, the whole point of the bonding system is to create an electrical short circuit across the various exposed terminals of that “battery.”  Bonding creates a path for electrochemical galvanic currents to circulate.  Bonding holds all of the metal surfaces at the same, safe touch voltage, but in so doing, bonding also ensures the presence of the conditions needed for corrosion to occur.  That is the reason for the presence of the transom zinc in the bonding network.  The transom zinc is the sacrificial anode that protects all of the important and more noble metals attached to the bonding backbone from corrosion.

For measuring and troubleshooting the bonding system of a boat, a reference “half-cell” is used. The reference cell is external to the bonding system.  The reference cell behaves in a known and predictable way when submerged in sea water. The reference cell becomes one of the halves of a “battery.” The metals attached to the bonding network of the boat become the other half-cell. In use, the reference half-cell is immersed in seawater outside the hull of the boat, and that seawater is the electrolyte of the “battery.”

A Silver/Silver Chloride half-cell is the best reference cell with sea water (chemical symbol: Ag/AgCl) because it has known and stabile behavior characteristics. That is, the voltage that other metals will produce against a silver/silver chloride half cell are very consistent across a wide range of temperature and electrolyte salinity.

Conceptually, measuring between the Ag/AgCl half-cell and the bonding network of the boat is the same as measuring the between the terminals of a conventional battery. The bonding system and the half-cell, immersed in sea water, become the battery being tested. The DVM measures the terminal voltage of that battery.

Figure 5 shows the measurement configuration described above:


Figure 5: Measuring the Bonding System with a Ag/AgCl Half-Cell

As a boat owner, there are two ways to proceed with the testing of the bonding system. One is to hire an ABYC-Certified Corrosion Specialist. This analysis is a form of survey, although not all surveyors offer it as a service. Two is for owners to do it themselves.  In the DIY case, one must obtain an Ag/AgCl half-cell, available from and other Internet sources at a cost in the range of $140 – $150.

DIYers will begin their testing by connecting the Ag/AgCl half-cell to the negative terminal of the DVM. Then lower the Ag/AgCl half-cell over the side into the water near the hull, to about the level of the boat’s running gear. The half-cell should not rest on the sea bed. The guiding principle here is, if the bonding system is fully intact and functional, all metals connected to the bonding system are expected to be at the same voltage. Probing any of the bonded metals with the DVM should produce the same voltage reading. If different voltages are noted, something is not right, and corrective action is advised.

The bonding system of a boat – whether connected to shore power or not – should produce a reading on the DVM of between -400mV and -700mV. Knowing that the bonding system has all of its metal structures tied together, we therefore know all of the readings must be found at the same voltage.

To evaluate the integrity of the bonding system, start anywhere that’s convenient and probe each of the various metal objects found all over the boat; that is, all the stuff previously mentioned (thruhulls, packing glands, sea chests, rudder posts and rudders, steering system components, exhaust fittings, main engine/transmission, Generator frame(s), battery charger/inverter chassis frames, solar panel and wind generator frames, handrail and enclosure frames, heat pump unit chassis frames, fuel tanks, fuel filling ports and tank vents, potable water tanks, thruster systems, black water tank, etc, etc, etc). The voltage measured by the DVM should be the same as seen at the shore power connection everywhere. If it is not, something is “wrong!”

The last two steps in this analysis are to discover the cause of any inconsistent voltage reading, and make corrections. Some symptoms one might encounter include:

Symptom Possible Cause
Wide variation of voltages between different metal objects.
  1. Boat is not fit with a DC bonding network;
  2. Damage or corrosion to connections within the bonding system.
Most metal objects have consistent voltages except for one or two isolated objects, “here and there.” Loose, corroded, broken or missing bonding connections to the affected metal object(s).
A collection of several metal objects measure one voltage, but that entire collection is different from the baseline voltage. Broken bonding buss somewhere along the length of the backbone.
The baseline voltage is grossly different than expected (-400mV to -700mV).
  1. Loose, corroded, broken or missing connections to the transom zinc or the shore power ground. Disconnect from shore power, looking for changes and to check the transom zinc by itself;
  2. Overly wasted transom zinc;
  3. Missing shore power ground connection;
  4. B- connection to the bonding system made in error;
  5. Stray DC electrolysis current in the bonding system.
No reading occurs when the metal object is probed. Bonding connections absent.

(Note: this will only happen with metal objects above the waterline and not in contact with the water.)