Boat Electricity

This is an odd post, even for me, but I keep having to answer the same questions on other forums and I will try posting it here and use a link.


Electricity and lightning are actually quite well understood by perhaps a million people, but frequently not by boaters.

As an Engineer, who built my own boat in the mid 1970s and is still sailing her, I hope that I understand, but I can make some doozy mistakes. These are all JUST MY OPINIONS. So, buyer beware…..

1) I totally agree that it is a complicated subject, and often counterintuitive. But actually the right people know a great deal about it. It takes hundreds of hours to teach, so do not expect to become knowledgeable by reading anything I write. My hope is that you will investigate various books and authorities and learn from knowledgeable people. There is a gigantic body of false information, probably coming from well-intentioned people, like myself, they just have the misfortune of writing down things that are wrong, even though they believe them at the time.

I also recommend learning about the electrochemical scale and galvanic corrosion. Basically this is the idea that if you have two different pieces of metal in a conductive fluid, you have usually created a battery. If you allow electricity to flow between the two pieces of metal, by a path other than the ions flowing through the liquid, you have created a battery.

In the same principle that allows electroplating, the ions flowing from one of the pieces of metal, will be pieces of that metal. That metal will get smaller, and weaker.

Commonly, some brasses and bronze will actually lose components of the inside of metal. The metal will become spongy, like tiny nano-termites have been after it.

2) A bad bonding system is much worse than no bonding system.

This is because (usually) ONE piece of metal in sea water does not cause electrolysis. The one exception that I know of is that many kinds of stainless steel are stainless because they contain a lot of chrome, or nickel and the oxides of those components form a clear protective protecting film and protects the iron, which is the main component.

If this protective film, gets scratched, the electrochemical characteristics of the base metal versus the film are different. Immersed in a conductive fluid, the protective film will be noble, or not easily removed, and the area of the scratch that has not been able to form the protective film will be sacrificed. You literally get electrolysis within a single piece of metal.

Because the scratched metal has a very tiny surface area, and the protective film has a huge surface area, it is not rare to have the scratch propagate into the material quickly, and deeply, like a crack, or crevice. A friend saw a demonstration of a new half inch bolt that corroded into two pieces, over a weekend.

Because this was a demonstration, it was kind created in the worst possible scenario. Usually it takes longer. But the entire bolt looked like new, except where the scratch had been, and that looks like the bolt had just broken, like a piece of glass. It was quite sobering.

We used 3/8″ stainless steel U-bolt’s, like you would put at the bow of a trailer boat to pull it onto the trailer, for our safety harness clip-on points. We used them for 10 yrs, until Janet took one out to make it easier to paint that area.

Both legs of the U-bolt had corroded completely into separate pieces, inside the bulkhead, due to the above-mentioned danger.

I immediately fabricated bronze U-bolt’s and replaced the three attachment points on our boat. Bronze is far from perfect, but a metallurgist once generalized that it is 800 times less likely to suffer from crevice corrosion.

Why did I get off on to this sidetrack? Because electricity on boats affects all sorts of things. What I’ve been describing is an electrical phenomena.

When you have electricity flowing through metals and a conductive fluid one of the pieces of metal will be getting smaller. If it’s alternating current, both pieces will take turns getting smaller, 50 or 60 times per second.

My personal opinion, not accepting any liability or responsibility for that opinion, is that all of the DC systems on the boat, should be grounded to one point, if possible.

Most engines assume that the body of the engine is at DC ground. The starter motor, the alternator, the engine instruments, are all designed to use the engine block as DC ground. There are electrically isolated engines and components, but they are quite unusual, and they have their own needs and requirements.

This is already way too long, and to keep it from looking like a set of encyclopedias, I will just say that on my 38ft boat, I have a single, 10 pound zinc, and a slightly more than 12 inch square, thick copper plate at the bottom of the mast. The copper is part of my lightning protection system.

I have number 10 wires connecting that zinc to all of the metal parts of the boat, in the most direct physical path possible. The copperplate is isolated by a homemade capacitor, made of two aluminum plates, about 18 inches square, separated by about a millimeter of air.

In this way the theory, and 41 years of practice, have kept all the metal parts looking like new. One time as we were rowing to shore, we noticed that the Bobstay iron, which is bronze, was looking redish. We immediately returned to the boat and found the break in the bonding system. The Bobstay iron had temporarily been disconnected from the zinc, but was still connected to a different thruhull, so it had a serious problem, that was probably corrected within a few days.

I believe that the ABYC requires that the alternating current ground wire also be connected at the same place as the DC ground wire. It is my personal opinion that this is very dangerous. It is my personal opinion that frequently in marinas and informal settings that there is a voltage present on the ground wire. It is small, but there is no acceptable voltage unless you’re willing to lose metal.

I believe that the ABYC are thinking that in order to prevent electrocution from the AC line that they need to have the ground connected so that tools and machinery that have a grounded case, will protect you. That “protection” could kill you if the “grounded case” became electrically hot, etc.

My personal opinion is that if I plug my tool or machinery into a grounded outlet at the power pedestal in the Marina, that I can get electrocuted but that I’ve done all that I reasonably can in order to protect myself.

If I take that tool, using an extension cord, onto my boat, it is my personal opinion that I still am as protected as I reasonably can be. If I take it down and connect my battery charger, it is my personal opinion that I’m still reasonably safe. And so on, as I connect more and more toys to the AC line.

There are definitely a huge number of, “What if’s.” But, I personally believe that my probability of a long and happy life is acceptable. I personally believe that if I connect the ground wire coming from shore to large metal objects on my boat, particularly the propeller shaft and thruhull fittings, that I have an absolutely insane probability of major electrolysis. Therefore, I make the personal choice, and accept no liability for that choice, of having my alternating current system completely isolated from my DC system. And I tried to take precaution to make sure that some piece of equipment does not, without my knowledge, connect the DC system to the alternating system. I only use battery chargers that do not have any electrical connection between the AC and DC system. I would love to have an isolation transformer for the AC system, but whenever I have had an opportunity to buy one it was more expensive and physically larger than something I wanted to own.

(Update.  There is a silver stud on the left side of my battery charger, that in the manual is called the ABYC Grounding Stud. It is my personal opinion that my boat is safer with that not connected to anything. That is not how ABYC wants it.

You and your loved ones will pay the price if I am wrong. I do not have enough money to be worth suing. All I promise is that I will be really sorry.

As I tried to explain, in many marinas we saw systems that had a voltage on the AC ground. I am pretty sure that the Galvanic Isolators are just a couple of large diodes in opposite directions, so that if there is less than about 1 volt on the ground wire, it will not allow electricity to flow. Usually this is OK, and if there is a GFI on the shore supply, it is SUPPOSED to open as soon as a tiny current flows in the gnd wire. But Murphy’s Law….

With a good Galvanic Isolator, it is probably OK having the AC ground connected to the boat ground through the Galvanic Isolator. I just keep them separate and feel that that is an OK compromise. If I lost the shore ground AND had a short between the AC “Hot wire” and the formerly grounded chassis, I would get hurt. My plan is not without some risk.

End of Update #1)

And, I have heard otherwise intelligent and well-informed people say that there is nothing that anyone can do about lightning protection. They do not realize that there is a whole industry successfully protecting hundreds of millions of dollars of equipment from lightning damage every day. But, like most things, it only works if you know what you’re doing. is just one of many excellent companies making lightning protection equipment. In all parts of the world, cell phone towers, microwave towers, radio transmission towers, etc. are hit by lightning, in some cases hundreds of times a year, with no loss of service. Not through Harry Potter, but through following the rules.

As I said above this would take hundreds of hours to really educate someone, but one of the surprises with lightning is called the electromagnetic pulse, or EMP. I think most of us know that magnetism and electricity are closely interrelated and that electricity can cause a magnetic field, and magnetic fields can cause electricity. A typical lightning bolt is such a massive flow of electricity that it creates a massive magnetic pulse. That magnetic pulse frequently causes devastating amounts of electricity that ruin equipment that can be 100 yards away.

A friend with a correctly protected boat was hit on the masthead. The VHF antenna fell into his cockpit. He replaced the coax, and antenna and was fine.

The reason that he was well protected, was, that a few months before a lightning strike, that he saw hit the beach, near his anchored boat, destroyed nearly everything electrical, including his starter motor. He learned his lesson well.

Another thing that is often not expected with lightning is that the current begins to flow so suddenly that it is like an RF phenomena, even though it is just going in one direction. So, it is direct-current that follows the surprising rules for radiofrequency electricity. One of those being that it does not like to go around corners.

So lightning protection wires should be not only large, but as straight as you can manage.

I will give you the kindness of stopping at this point. This was not intended to educate anyone. The goal is for you to continue your own ongoing education of electricity, magnetism, electrolysis, and all the other stuff you need to enjoy owning a boat. And, installing a SSB, is even more complicated to do right.

All the things I’ve talked about here are very well understood by many thousands of people around the world. If you are not one of those people, I encourage you to please find someone who is and learn from them. There are classes, books, videos, just be suspicious and cautious. As I said in the beginning there are people with wonderful credentials, that are good well-meaning people that don’t realize that they don’t know.

I think it was Mark Twain that said something like, “It’s not the stuff you don’t know that gets you. It’s the stuff that you think you do know, that just ain’t so.”


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