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).  Followup: in February, 2019, I was able to buy a Raymarine DX600s Fishfinder from for $195.  The DX600s was plug-compatible with the failing DS500X, and saved replacement of the existing transducer a6 a cost roughly equivalent to repairing to failing unit.

I have a now-obsolete two-plotter Raymarine RL80CRC/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 (since migrated to Aqua Maps Master, because SEAiq dropped support for ActiveCaptain) on out iPad duplicates (improves upon) 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 highly-desirable 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, Garmin bought Navionics.  Garmin also withdrew BCM from the Apple Store (BCM was never available for an Android platform).  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.)

Metal Corrosion and Zinc Wasting


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

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

AC and DC “stray” electric currents flow in the water.  Because these currents flow outside their normal electrical conductors and devices, they are referred to as “stray currents.”  Worst case, all types of fault currents can be present at the same time.  Boaters should consider all electrical currents that flow in the water as a bad thing.


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

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

  1. AC “ground fault” currents, resulting mostly from wiring errors aboard boats and occasionally from inadequate equipment design, incorrect equipment selection or AC appliance/equipment malfunction,
  2. DC “Galvanic” currents, resulting from the natural behavior of dissimilar metals in mineral-containing ground water, fresh surface water or sea water, and
  3. DC “Electrolysis” currents, a DC “ground fault” current, resulting from wiring errors, equipment faults, and improper equipment use.

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

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

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


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

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

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

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

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

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


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


  1. DC galvanic currents are associated with small voltage potentials that are a naturally-occurring characteristic of all metals.  The specific voltage is determined by the atomic structure of the individual metal (or metal alloy).
  2. Different metals have different naturally-occurring electro-potentials.
  3. A “Galvanic Series” is a list of metals sorted by their naturally-occurring characteristic electro-potentials.  Different “references” can be used for ordering a “galvanic series.”  The best reference for salt water is a silver/silver chloride cell.
  4. A “galvanic couple” is any combination of two or more dissimilar metals or metal alloys connected together electrically and immersed in an electrolyte.
  5. An “electrolyte” is an electrically conductive liquid (generally) medium.
  6. Dry Corrosion” is the direct attack on a metal by dry gasses (air, oxygen) through chemical reactions which result in surface oxidation.
  7. Wet Corrosion” is the direct attack on a metal by an aqueous solution (strong or dilute, acidic or alkaline) through electro-chemical reaction.  Moisture and oxygen can act by themselves.


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

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

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

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

Perhaps a ”before” and ”after” view:


Figure 1 shows a ”before view” of a galvanic couple lying in seawater.  The stainless steel (SS) is the anodic alloy, so it erodes due to the natural galvanic voltage between it and it’s cathodic bronze couple-mate.


Figure 2 shows an ”after view” of the same galvanic couple with the addition of a sacrificial zinc anode.  The zinc forces the SS relatively  more cathodic (relative to the zinc), so the SS part is now protected from corrosion.  The zinc is the most active (anodic) metal in this new couple.  By corroding, the zinc acts to protect all of the more “valuable” metals.

Ongoing zinc maintenance is required to provide continuing protection of the more important components of the couple.  Additional valuable corrosion control techniques include the installation of galvanic isolation devices in shore power ground conductors, cable TV coax ground sheathes, and the ground conductors of (now pretty much obsolete) wired telephone and wired Ethernet connections.


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

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

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

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


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

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

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

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

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

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

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

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

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


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


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

In Figure 3, the metallic actors (SS and bronze alloys) are the same as shown in Figure 1.  In this case, the elemental voltage polarity of the couple has been reversed by the application of an outside source of DC voltage.  This is a DC fault scenario.  The bronze thruhull in this example will disintegrate, freely giving up it’s copper content into the surrounding sea water.


Figure 3: A DC Fault Injecting Power to the Bonding Buss Resulting in Corrosion of Important Underwater Metal Alloys

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

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

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

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

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

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

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

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


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

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

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

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


  1. Diligently avoid having DC wiring wetted or submerged in bilge water.
  2. Never use the boat’s bonding system as a B- “neutral” return circuit for DC attachments.
  3. Avoid facilities (marinas, municipal or private docks, boatyards, etc) where the infrastructure appears to be poorly maintained.
  4. Be alert in marinas located in industrial neighborhoods where ground fault currents from shore sources may be more likely; check with the dockmaster for known issues in the basin.
  5. Avoid facilities with numerous boats that are in a poor state of maintenance and repair.



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

Fe → Fe++ + 2e (example with Iron);

Zn → Zn++ + 2e (example with Zinc);

Pb → Pb++ + 2e (example with lead).


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

2H+ + 2e → 2H


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

H2O ⇌ H+ + OH

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

Fe++ + 2(OH) → Fe(OH)2, or

Zn++ + 2(OH) → Zn(OH)2, or

Pb++ + 2(OH) → Pb(OH)2

AC Electrical System

AC System Overview:

Note: An electrical diagram of Sanctuary’s AC Power Distribution  System as described in this article is located here (Adobe Portable Document Facility (.pdf) file): 20161022_ac_electrical_distribution_system.

The ship is wired to operate from two single-phase, grounded-neutral, 120VAC, 30A shore circuits originating in a dockside shore power system.  Neither the shore grounded (white) neutral conductor (white) nor the ungrounded energized conductor (black) is connected to the safety ground (green) aboard the ship (ABYC E11,

The ship’s AC safety ground (green) and DC negative buss (black) are connected together in the engine room (ABYC E11, and subs).

The ship is fit with a ProMariner® Prosafe-1™ galvanic isolator.  AT the time of it’s installation, the device complied with the then requirements of ABYC A28, 28.13 (now obsolete). The diode pack of the device is installed in series with the ship’s incoming green safety ground wire. The isolator and its control module are located in the ship’s electrical closet. The enumerator/monitor is mounted at the ship’s electrical control center located stbd, in the companionway to the vee berth.

Note: the ProMariner® galvanic isolator control module was disconnected in May, 2016.  The design of the enumerator/control module places a ground fault on the incoming shore power circuit.  That ground fault is used by the device to test the ship’s safety ground wire for continuity.  That ground fault can trip shore power ground fault sensors.  Disconnecting the enumerator does not affect the purpose or operation of the diode pack itself, but it does defeat the self-checking feature of the OEM design of the enumerator/monitor.

The ship is fit with two 120VAC, 30 Amp SmartPlug® marine shore power inlet connections (ABYC E11, and subs). “Shore 1” corresponds to a Newmar® ACDC-1™ “house” AC Load Center (identified with the numeral “1”). AC branch circuit breakers (ABYC E11, and subs) serving the genset battery charger, refrigerator/freezer, 120VAC water heater, inverter/charger and several house utility outlet circuits are installed. “Shore 2” corresponds to a Weems and Plath® “heat pump” Load Center (identified with the numeral “2”). AC branch circuit breakers (ABYC E11, and subs) serving two reverse cycle heat pumps and a raw water circulator pump (“air conditioner” units) are installed.  It is not necessary to connect both shore power circuits in order to use either one; each is completely independent of the other.

The ”AC Master Breaker” on the NewMar® “house” AC Load Center and the ”Master Breaker” on the Weems and Plath® “heat pump” Load Center function as shore power disconnect switches (ABYC E11, and subs and 1). These double-pole breakers isolate their respective AC shore power circuits from the ship’s on-board AC distribution system. The 120V energized current-carrying (“hot”) buss on the AC side of the NewMar® Load Center has been modified from its OEM configuration. Breakers 1-4 on the NewMar® panel are fed by shore line-in or generator power. Breakers 5-8 on the NewMar® panel are fed by the onboard inverter/charger as described later in this document.


  1. NoteThis section of E11 was upgraded to include ELCI in the July, 2012 release of the standard. Sanctuary complies with the July, 2009, release, E11, Sanctuary is not fit with ELCI at this time.

The ship is fit with an ONAN® MDJE™ onboard generator. The generator is powered by aa Onan 2-cylinder, 4-cycle diesel engine. Diesel fuel for the generator engine is drawn from the ship’s onboard fuel tanks. The generator has a 2-stage (fresh water with heat exchanger) cooling system. The generator is rated for continuous operation at 7.5kW, 60Hz. Generator operation (glow plugs, start/stop) is controlled by two rocker switches mounted at the ship’s electrical control center.  The generator and its diesel engine are mechanical devices that require periodic preventive maintenance. Refer to the Onan manual for maintenance schedules.  The generator’s AC output is wired in a 240VAC configuration. The generator starter motor is fed through a BlueSea Systems ML-RBS remotely operated DC disconnect switch (ABYC E11,,

The ship is fit with a Blue Systems® p/n 9093 manual Generator Transfer Switch. The GTS transfers the ship’s AC distribution load centers to either the shore power inlets or the onboard generator. When shore power is available, and the load center disconnect switches are set “on,” the corresponding green “Shore Power” LED on the GTS operator’s panel becomes illuminated. The GTS is of the break-before-make design. This prevents simultaneous cross-connection of incoming shore power source(s) and the ship’s generator (ABYC E11, This allows the generator to be started and run for servicing while the ship is simultaneously connected to energized (live) shore power connections.

Shown following is the wiring layout of the GTS as installed. Shore power service cords feed “Source 1” and “Source 2.” The 240V onboard Generator feeds “Source 3.” The NewMar® AC Load Center for “house” loads is connected as “Load 1;” the Weems & Plath® Load Center for  heat pumps is connected as “Load 2.” The GTS is shown in the “Shore” position. If the generator is “running,” the green “Generator” LED on the GTS operator’s panel becomes illuminated. If both shore and generator power are available at the same time, both sets of LEDs will be illuminated.


The ship is fit with a Magnum® MS2012™ Pure Sine Wave Inverter/Charger.

  1. When either shore or generator power is available, the device operates in “Passthru” mode to forward AC power to utility outlets via circuit breakers 5–8 on the NewMar® AC Load Center and to simultaneously charge the ship’s battery bank. When shore or generator power is not available, the device operates in “Invert” mode as the AC power source for circuits 5-8. The device automatically switches between its “passthru” and “invert” modes as availability of shore or generator power changes.
  2. The neutral buss for branch circuits powered by the inverter/charger is isolated from the neutral buss for branch circuits powered solely by shore or generator power. Magnum requires this separation, which they base on ABYC A31, However, AC neutrals are defined to be “grounded conductors.” Therefore, this ABYC reference seems obscure, since it refers to separation of “ungrounded conductors.”
  3. Configuration of the operational status of the inverter/charger is manually selectable via the ME-RCtm Remote Control mounted at the ship’s electrical control center area.
  4. The “fault” lamp on the ME-RCtm Remote Control indicates a problem that prevents normal operation of the inverter/charger. The two most common faults are spike voltages and out-of-tolerance frequency deviations. These faults sometimes occur when the ship is operating on the generator as its AC power source and heavy loads cycle “on” and “off.” These “faults” can be manually cleared by recycling DC power via the inverter/charger battery disconnect switch in the engine room space.
  5. DC electrical energy to power the inverter/charger originates from either 1) the propulsion engine’s alternator (supplemented by the battery bank), if the ship’s propulsion engine is running, or 2) solely by the ship’s battery bank, if the ship’s propulsion engine is not running. In the absence of shore or generator power, the DC ampere-hour (aHr) capacity of the ship’s battery bank can be conserved by discontinuing use of the Inverter/Charger and its attached AC loads.

Finally, the ship is fit with a 700-watt Xantrex® Modified Sine Wave (MSW) utility Inverter. This inverter is an alternative AC power source that can feed a utility outlet power strip on the ship’s salon nav station. This inverter is a stand-alone device that is not integrated into the ship’s AC distribution system. It is available to power the satellite TV receiver/DVR and the TV. DC energy supply for this inverter is as described above. The device mounted on the aft bulkhead of the ship’s standing closet.

DEFINITION: “Secured State” of the Ship’s AC Electrical System

Aboard Sanctuary, a “secured state” for the AC Electrical System is defined to exist when all of the following conditions exist:

  1. All individual AC house circuit breakers on the NewMar® AC Load Center are in the “off” position,
  2. All individual heat pump circuit breakers on the Weems & Plath® Load Center are in the “off” position,
  3. The “AC Master Breaker” on the Newmar® AC Load Center and the “Master Breaker” on the Weems and Plath® Load Center are both in the “off” position,
  4. The generator transfer switch is in the “off” position,
  5. Shore power service cords are disconnected and stored aboard,
  6. The generator is not running,
  7. DC power to the Xantrex® MSW Inverter is discontinued via it’s disconnect breaker, located in the electrical closet,
  8. DC power to the Magnum® MS2012tm system-integrated Inverter/Charger is discontinue via the DC rotary disconnect switch located in the engine room, stbd bulkhead.

References to ABYC E11 contained in this document:

All references to ABYC E11, AC AND DC ELECTRICAL SYSTEMS ON BOATS, are to the July, 2012, release of the standard unless otherwise noted.

DC Electrical System

DC System Overview:

Note: An electrical diagram of Sanctuary’s DC System as described in this article is located here (Adobe Portable Document Facility (.pdf) file): 20161022_dc_electrical_distribution_system.

The ship’s DC electrical system is a “negative ground” system. (ABYC E11, 11.4.24 and

The DC system is of the “ungrounded” system design (ABYC E11, 11.4 and E11,

The ship’s AC safety ground (green) is bonded to the Ship’s DC negative buss (black) in the engine room. (ABYC E11,

DC energy for the “house” and “engine start” services originates in a single battery bank comprised of six, 6 volt, flooded deep cycle “Golf Cart” batteries. Over-current protection (OCP) for the battery bank is provided by BlueSea Systems®, Type MRBF™, 200A fuses. (ABYC E11, and and subs).  Details of that design decision are discussed in my article, “Battery Bank: Separate vs Combined,” on this site.

A single Group 27 start-service battery is used to start the ship’s generator. A BlueSea Systems® model ML-RBS™ remotely operated disconnect switch, with p/n 2145 remote switch, is fit in the generator’s battery feed circuit.

The ship’s Main DC Disconnect Switch (ABYC E11, and subs) for the DC feed to the NewMar® Load Center and other attached DC loads is located in the engine room, stbd bulkhead, above the ship’s battery bank.

The ship’s OEM NewMar® Model ACDC-1™ Branch Circuit Load Center/Distribution Panel is located to stbd in the forward companionway to the vee berth. Because there is a combined battery bank for house and start functions, the OEM Battery Selector Switch is discontinued and removed. The left half of the Newmar® Load Center serves as the DC Load Attachment Center for many DC attachments, including:

  • navigation, anchor lights and deck lights,
  • LPG safety shutoff valve,
  • refrigerator/freezer DC feed,
  • house drinking water pump,
  • raw water wash-down pump,
  • DC lighting,
  • salon VHF radio,
  • windshield wipers and horn,
  • toilet macerator, and
  • stereo.

This panel provides over-current protection for attached loads via circuit breakers. (ABYC E11,

The ship is fit with a Blue Sea® System WeatherDeck™ DC Circuit Distribution Panel, located stbd on the flybridge. It is the primary load attachment center for the ship’s navigation instruments, including:

  • chart plotters (including Radar),
  • auto pilot controller,
  • VHF radio,
  • depth sounder,
  • under-console AIS receiver and wi-fi multiplexor, and
  • flybridge DC utility outlets.

This panel provides over-current protection for attached loads via circuit breakers. (ABYC E11,

The ship is fit with a Magnum® MS2012™, Pure Sine Wave (PSW) Inverter/Charger. DC over-current protection is provided by a 300A Class “T” fuse (ABYC A31, and subs; The DC Disconnect Switch for the inverter/charger is located in the engine room, stbd bulkhead, above the ship’s battery bank. (ABYC E11, and subs)

DC Power for some ship equipment attaches to the vessel’s DC Distribution System independently of the NewMar® load center. These attachments have individual over-current protection. These attachments include:


Location of Attachment’s
Disconnect Switch
or OCP

(ABYC E11, and subs, 11.10 and subs)

1 propulsion engine
a) starter motor feed and
b) control, instrumentation and instrument lighting circuits
fwd surface of galley drawer cabinet, floor level, aft of stbd salon door; switch is accessible without opening the engine room. This circuit is without OCP. (ABYC E11,, Exception 1)
2 Lewmar® H900 windlass tagged; on vessel’s electrical control center
3 Dickson® stern thruster Inline fuses, respective operator’s panel
4 Garmin® GHP10tm autopilot system tagged; engine room, stbd, mid-ships, above battery boxes
5 Magnum® MS2012tm, 2kW, PSW inverter/charger tagged; engine room, stbd, mid-ships, above battery boxes
6 ICOM® 706MKIIGtm Amateur Radio Transceiver Inline fuses, electrical closet, NewMar® B+ buss
7 bilge pump tagged; electrical closet aft bulkhead, eye level (tubular glass fuse)
8 high bilge alarm tagged; electrical closet aft bulkhead, eye level (tubular glass fuse)
9 shower sump pump tagged; electrical closet aft bulkhead, eye level (tubular glass fuse)
10 SirenMarine® Spritetm Boat Monitor inline fuse, electrical closet
11 salon +12VDC utility outlet. inline fuse, electrical closet

Battery State-of-Charge Status & Monitoring:

The ship is fit with six, Duracelltm, EGC-2, Golf Cart batteries.  These batteries have a 20-hour Ampere Hour (aHr) rating of 230 aHr and a Reserve Capacity rating of 448 minutes. These batteries are manufactured by East Penn Manufacturing® and are sourced from Sam’s Club. The batteries are combined in a series/parallel configuration to provide 12VDC.

The overall total rated ampere hour capacity of the ship’s battery bank is 690 aHr.

A Magnum® ME-RCtm Remote Control, and a Magnum® ME-BMKtm Battery Monitor Kit, are installed to monitor the ship’s DC electrical operating parameters and the state-of-charge status of the ship’s battery bank.

“Standard operating practice” aboard ship is to adhere to the “mid-capacity rule,” which states that lead-acid batteries (flooded wet cells, AGM or Gel) should not be discharged beyond 50% of their rated capacity and that discharging a lead-acid battery in excess of 50% of its capacity shortens its service life. Aboard ship, the ME-RCtm/ME-BMKtm Battery Monitor displays the “State-of-Charge” of the battery bank. Less overall depth-of-discharge is better.

The approximate eight-hour overnight (summer hours-of-daylight) DC system electrical consumption (refrigeration, anchor lighting, evening TV watching, computer use, etc.) aboard ship is 125 – 150 aHr. The approximate eleven-hour overnight (winter hours-of-daylight) DC electrical consumption is 150 – 200 aHr.

Battery Charging:

Shore Power/Onboard Generator:
When the ship is receiving AC power from either 1) shore power or 2) from the ship’s onboard generator, the ship’s battery bank is charged by the Magnum® MS2012tm 2kW Inverter/Charger 1.

Under way:
The ship’s engine alternator charging system consists of a single, 12-volt, 110 amp, Balmar® high-output alternator, model 712110, with a Balmar® MC-614tm external regulator. The external regulator receives its DC input power supply from the propulsion engine’s starter solenoid 2.

DC Distribution Wiring: 3

  1. Three attachments originate at the positive battery post of the ship’s battery bank:
    1. Attachment 1 is BlueSea Systems®, Type MRBFtm Fuse Block, p/n 2151, fit to the designated positive output terminal. (ABYC E-11, This fuse block is fit with two, 200A, Type MRBF fuses.
      • Fuse 1 feeds the ship’s Main DC Power Disconnect Switch 4, a Blue Sea® Systems, p/n 6006, rotary switch, via a 2-0 AWG red wire. A 2-0 red wire continues from the disconnect switch to a BlueSea® Systems, 150A, Type ANL fuse block. The house DC feed is a 2-0 AWG red wire from the ANL fuse block to a 600 amp, 4-post terminal block. The terminal block is the ship’s Main DC Power Distribution Buss.
      • Fuse 2 connects the ship’s Balmar 110A alternator to the battery bank, via a #6 AWG red wire.
    2. Attachment 2 is a 2-0 AWG red wire that feeds the ship’s propulsion engine starter solenoid through a BlueSea System®, p/n 6006, disconnect switch located in the salon, stbd, beneath the fold-down table. This circuit is not overload protected (ref: ABYC E-11,
      • The propulsion engine starter solenoid is the DC system attachment point of the engine’s DC sensors, controls, instrumentation and malfunction alarms, and the external Balmar® MC-614tm voltage regulator which energizes the on-engine Balmar® alternator unit.
    3. Attachment 3 is a 2-0 AWG red wire (ABYC, E-11, that feeds the ship’s inverter/charger through a Class “T” fuse block fit with a 300 Amp, Class “T” fuse. A 2-0 AWG red wire continues from the Class “T” fuse block to a BlueSea Systems® p/n 6003e disconnect switch. From the disconnect switch, a 2-0 AWG red wire feeds the Inverter/Charger.
  2. House loads aboard ship are fed from the Main DC Power Distribution Buss.
    1. A 1-0 AWG red wire runs from the terminal block to a terminal block in the electrical closet. This is the DC attachment point for the ship’s bilge pump, high bilge alarm, NewMar® DC Load Center, BlueSea Systems® WeatherDecktm Load Center, Xantrex® Inverter and shower sump pump.
    2. A 1-0 AWG red wire from the terminal block feeds the vessel’s 70 amp windlass circuit breaker. From the circuit breaker, a 1-0 AWG red wire joins in the electrical locker with an OEM 38mm2 red wire to feed the vessel’s windlass contactor, located forward, in the chain locker overhead.
    3. A #8 AWG red wire feeds a 40 amp circuit breaker which feeds the Garmin® GHP10tm autopilot system.
    4. A fused attachment feeds the Magnum® ME-BMKtm battery monitor module.


  1. Note: Setup options for the Magnum Inverter/Charger and ME-RC Remote Control are found in the MS-Word document entitled “Magnum_Remote_Control_Setup_Customization.doc.”
  2. Note: Setup options for the Balmar ARS-5 external regulator are found in the MS-Word document entitled: “Balmar_ARS-5_Setup_Customization.doc.”
  3. Note: the written description contained in this document is diagrammed in the document entitled ”DC_Electrical_Distribution_System.PDF.” Details of electrical system attachment points are documented in a table entitled “DC_Branch_Circuit_Functions.XLS.”
  4. WARNING: This  disconnect switch removes power from the bilge pump, sump pump and high-bilge alarm circuits. Therefore, it is to be used only for attended servicing of the electrical system. It is not intended for long-term disconnection of the battery bank while the boat is in the water!

Balmar® 110A High Output Alternator:

An alternator is a “self-limiting” device (ABYC E11, 11.4.26). That is, “a device whose maximum output is restricted to a specified value by its magnetic or electrical characteristics.” What that means is that an alternator can only produce just so much output current – in this case, 110A – regardless of how much drive is applied to its field winding. If the alternator fails, output generally stops. For self-limiting devices, a fuse is not “required” by ABYC E11,, but it is a reasonable precaution.

Aboard Sanctuary, the alternator connection to the ship’s DC system is protected by a 200A, Type MRBF, fuse. Two Hundred amps may seem too large a rating. The maximum rated output of the alternator is 110A. The ampacity of the #6 AWG conductor, derated because it’s in an engine space, operating in a 12V system with a 3% voltage drop and 105ºC insulation is 80A, and substantially higher for the 10% voltage drop case. So it may appear there is no scenario where the 200A fuse would protect the alternator connection wiring.

It’s very important to understand that we DO NOT want that fuse to open in the absence of a true, sustained over-current situation. If the fuse were to open while the alternator was operating in its designed power output range, the effect would be to instantly disconnect the electrical load (the battery bank) from the alternator. The magnetic field inside the machine would instantly collapse, creating a very high voltage “spike” inside the windings of the machine. That spike would almost certainly destroy the alternator’s internal diodes and render the device inoperable.

Alternators contain solid state full-wave rectifier diode pacs. Internal diodes prevent DC power from flowing backwards through the alternator when the engine is not in operation. If diodes fail in a welded-closed state – “shorted,” or “short circuited” – the result would create a direct path from the battery to ground via the alternator’s stator windings. The purpose of this fuse is to protect against that true over-current condition. In the case of a shorted diode, the 200A fuse protects the charging wire from becoming overloaded and, thus, a fire risk.

Magnum® MS2012tm Pure Sine Wave Inverter/Charger:

The positive and negative DC wires for the ship’s PSW Inverter/Charger are 2-0 AWG BC5W2 boat cable. Round-trip distance from battery-to-device and return is approximately 10′. The rated ampacity for 2-0 AWG primary wire inside an engine space is 280 Amps DC (ref: Magnum’s MS2012tm specs state that maximum battery charging current is 100 Amps DC, and maximum full load inverter draw at rated AC output load is 225 Amps DC. Optional configuration options are available via the ME-RCtm Remote Control to limit Inverter/Charger operating currents.

DC Return (DC “Ground”):

There are two terminal blocks in the ship’s engine room that comprise the ship’s DC return circuit to the main battery bank. The DC branch circuits and AC Safety Ground are all collected on a terminal block located fwd in the engine compartment, at deck level, starboard. This terminal block is connected to the main DC negative buss terminal block via a 1-0 AWG black wire.

The main DC negative buss is a Blue Sea® Systems 600A terminal block located amidships in the engine room, starboard, above the battery boxes. Joined at the main DC negative buss terminal block are:

  1. the consolidated branch circuit return terminal block, via a 1-0 AWG black wire,
  2. the Magnum® MS2012tm PSW Inverter/Charger, via a 2-0 AWG black wire,
  3. the propulsion engine block, via a 2-0 AWG black wire, (returns the Starter Motor circuit and the Propulsion Engine DC controls, instrumentation and instrument lighting), and
  4. attachment to the battery monitor’s 50mV, 500A measurement shunt, via a 2-0 AWG black wire.

The return connection to the negative terminal of the main battery bank is made from the battery monitor’s 50mV, 500A shunt, via a 2-0 AWG black wire.