Boat Maintaince

Bonding and Corrosion

By Allen Edwards

Definition: Bonding noun 1: fastening firmly together [syn: soldering]


There are few if any topics in yachting that have as much myth and misinformation surrounding them as does the question of bonding various pieces of your boat together. There is a very good reason for this confusion as the requirements surrounding bonding conflict. It is important to understand when bonding is good and when it is bad so that you can know when you should and when you should not bond. I hope this article will help to clear up some of that confusion. I will try and present the topic in an understandable way so that you can make decisions based on knowledge rather than on opinion or myth.

The "big picture" questions that need to be answered are:

  1. What does bonding actually do?
  2. Why do you want to bond pieces of your boat together?
  3. Why do you want to have pieces of your boat isolated (not bonded)?

If we can answer these questions we can make intelligent decisions about what to do.

What does bonding actually do?

Bonding forces the two pieces of metal that are bonded to be at the same voltage potential and it does this by having a low enough resistance that whatever current flows between them can't create any significant voltage. The fundamental equation is that V (voltage) = I (current) * R (resistance). All we need to do is have R be low enough and the voltage generated by the current I will be low. Said simply, bonding makes the resistance between the bonded pieces essentially zero which means there will be zero voltage difference between the two pieces of metal even if current flows from one piece to the other thought the bond wire. Having the two pieces at the same voltage is a good thing. Having current be able to flow between them is often a bad thing. This is fundamentally what creates the confusion about bonding.

Why do you want to bond things together?

The simple answer is as to why you want to bond things is to keep them at the same voltage. But why is it a good thing to have two pieces of your boat at the same voltage? There are two main reasons. Lightning protection is one and I will leave it to the many other articles I have collected HERE. You can read my plan for lightning at the introduction on that page. The other main reason to have two pieces of you boat at the same voltage is if one of those pieces accidentally gets connected to AC hot. In that case, what you really want is for both those pieces to be connected to AC ground so that the circuit breaker trips and you don't die. There are safety regulations that deal with this and most apply to new boats so if your boat had such regulations in place when it was new, you best follow them. You might want to upgrade as well. That is the safe thing to do. The basic idea is to bond all the pieces of your boat to AC Ground that you might touch you if there is any possibility they could also touch AC Hot due to some failure. That makes for a safe boat. Note that any accidental connection between the AC and DC systems potentially puts everything running off DC at risk of killing you as well. But there are two other ways other than bonding to keep your boat safe. One is to make sure you cannot touch the metal. A completely inaccessible piece of your boat, a through hull under the cockpit sole that nobody can get to does not have to be bonded. The second way is to make sure there is no possibility of having AC hot touch a piece of your boat. That means that no AC line can ever touch that piece of your boat even if the insulation melts and the bare wires are exposed. As I said above, it also means that no DC line can touch the metal in question if there is the possibility of the AC and DC lines touching somewhere else on the boat. To take this approach, you are basically not running AC near any metal pieces of your boat or near any DC wire. This especially applies to any pieces of your boat that are connected to DC ground, like your engine, if you let the AC wires touch the DC wires anywhere, you have a big safety problem. You cannot count on the insulation because either the DC or the AC can over current and melt the insulation causing the two circuits to be connected. People have died from this so if you run all your AC and DC close together, you need to electrically connect your DC ground to the AC ground. From any practical standpoint, if you have AC running around your boat, you need to connect the boats AC green ground with the large metal parts of your boat. But never connect your boat to the marina AC green ground. You need to isolate the marina AC ground and the boat AC ground using the appropriate marine isolation devices discussed below or you will shorten the life of you zinc considerable. It all depends on your marina. Some people have reported burning through a zinc in a few weeks when their boat ground was connected to the marina ground.

But before I change subjects, I would like to emphasize this point. If you have some AC equipment that has a failure and connects the AC hot to the DC ground, like melted insulation between your AC and DC circuits, you could end up making the marina water AC hot. This has happened and it has killed people who swim in that water. A GFI breaker should offer protection from such a fault. My marina has GFI breakers at every boat outlet box. Yours should as well but many marina operators don't think GFI breakers will work in a marine environment. They are wrong. I belive new boats are required to have GFI circuits on the boat as well. Sounds like a good idea. But what you really want is for this to trip any breaker and connecting the boat AC ground and the boat DC ground will do this. The bottom line is that having AC on a boat is dangerous and precautions need to be taken seriously.

Why do you not want to bond things together?

Why is it a bad thing to allow current to flow between two pieces of your boat? If those two pieces are immersed in an electrolyte (like salt water) and if those two pieces of metal happen to be at different voltages then current will flow in the wire and metal ions will flow in the water. Absent the salt water electrolyte, the ions could not move and the electrons would build up in the metals until there were at the same voltage and current flow would stop. But because we can move charges in the salt water in the form of metal ions, the current can and will continue until one of the pieces of your boat is gone. Moving metal from one piece of your boat to another may be a good thing for the piece receiving the ions, but it is decidedly a bad thing for the piece losing ions. Absent external sources of voltage, different metals have different natural voltages. There is a table HERE that lists the properties of different metals. More active metals like Zinc or Magnesium tend to lose ions to more Noble metals like Gold or Stainless Steel.. For example, a gold magnesium pair would produce 1.75 volts where a Bronze Stainless pair only 0.15 volts. On a boat we might say that more active metals like Zinc lose ions to more Noble metals like Stainless Steel (0.6 volts). The higher the voltage difference, the bigger the problem and the faster the more active metal will go away. We bond a zinc to the our propeller shaft because the bronze prop and the stainless shaft are bonded together (they have to be or the prop won't turn) and you really would prefer to have the more active metal be zinc and have the plating go from the zinc to both the bronze and stainless steel instead of having the bronze plate the stainless. The zinc will lose ions to both the bronze and stainless steel. The conclusion is if you don't have some other reason to bond pieces of your boat together, don't.

"Your choice is do you want your boat to last as long as a shorted battery or a battery sitting on the shelf in storage"

The process I have been describing is basically that of a battery (two dissimilar metals in an electrolyte). The dissimilar metals are the two terminals of the battery. They are dissimilar in that they have different natural voltages. Because of this voltage difference, electrons move from the negative terminal to the positive terminal through your bonding wire. But now one of the metals has lost electrons so it is positively charged. The metal doesn't like that so it pushes a positive ion (metal) through the salt water and now things are balanced again, except this continues because the natural voltage of the two metals are still different so they will continue to push electrons and ions until one of them completely gone. At the risk of repeating myself, don't bond pieces of your boat together unless you have to for safety or mechanical reasons. I used to tell people that dissimilar metals in salt water form a battery. There is nothing you can do to change that. A bronze battery has a shelf life of about 100 years. A shorted battery has a much shorter life. Your choice is do you want your boat to last as long as a shorted battery or a battery sitting on the shelf in storage. Now, a battery created by pieces of a boat is not a very good battery and cannot generate much current so it will last quite some time even shorted, but not near as long as if it were not. When I first got my boat I measured the voltage difference between different bronze through hulls. There was a measurable difference probably do to them being slightly different alloys.

A Real Life Example

My bilge sump has a sump area about a foot deep, a foot long, and 8 inches wide. It is lined with bronze. It is not bonded. I measured the voltage from DC ground to this metal sump liner. It was 0.7 Volts. Wow. I then removed the fuses from the two pumps that occupy that space. No difference. Next I took a reading of an unbonded through hull. Same voltage. So where did this come from? Bronze is 0.4 Volts below Gold and Zinc is 1.0-1.2 volts below gold. The difference between these would represent the voltage I should expect to see, which is what I see. Next I shorted out the sump liner with the amp meter simulating what would happen if I bonded the sump liner. I registered 25 mA. A shaft zinc of 1 pound can source 324 amp hours so my 25 mA draw on my "battery" would last just over a year, which is what it does. So, adding a bond wire to that liner would double my use of zincs. Bond enough things and I would be replacing zincs every few months or using more than one zinc. I would not like to add more zincs as I will discuss in the section on wood boats.

Make that three reasons to bond for some boats

If you have a gasoline engine like I do with metal tanks, you need to bond your engine and your gas tanks together. The mechanism of the series of bad luck that is required to blow up your boat seems unlikely but as you will probably be on your boat when it blows up, go ahead and bond them to prevent this from happening.

Yet Another Reason to Bond - Internal Stray Currents

Just when I thought I was out of reasons to bond, along comes another one. It goes something like this. If you have a through hull fitting that accidentally gets connected to a stray DC+ wire, it is in big trouble. There will be DC- connected to the prop shaft and now that DC+ fitting is going to plate itself all over the prop. It may do this to the tune of being gone in a few days. So, if you have DC wiring draped across metal that is in contact with sea water, either move those wires or bond the metal so the fuse will blow. This is less of a problem if you don't have a battery charger as shorting the battery to the through hull will probably discharge the battery before the fitting is gone.

There is a variation on this theme where a DC+ wire is exposed to the bilge water. In this case, if there is a through hull fitting near that is also in the bilge water and has contact to the outside water it can get enough stray current from the positive bilge water to have problems. The source of these stray DC+ currents would be the wiring for a submerged bilge pump. My boat has a through hull right next to the bilge pump but it is plastic. It holds the knot meter pickup.


A sidebar on electroplating. When two pieces of metal are in an electrolyte and then connected to a source of DC voltage two things happen. The positive terminal of the source, say a battery, takes electrons from the piece of metal it is connected to. That leaves a positive charge on the metal and it is unhappy. To gain bliss, it sloughs off a positive ion and moves it over to the other piece of metal. That piece, connected to the battery DC- is also happy as it is gaining electrons from the battery and ions through the salt water from the other piece of metal. It is also growing in size while the piece of metal connected to the DC+ is shrinking. The piece of metal connected to the DC+ is going to give the battery electrons and is going to give the other piece of metal ions. It is losing atoms.

How bad is it to bond?

It probably isn't that bad assuming you are talking about just your boat. With a zinc able to produce 25mA for over a year, you probably can have things bonded and if you keep your zincs up, you will be safe. My personal bias is to keep things as they are if you are not having problems. If you are burning through zincs, figure out why and correct the problem. There is an easy test that is fun to do that I will discuss below.

The Marina can be an unfriendly electrical place

There is another topic that needs to be discussed -- the marina. Let us assume that some other boat in your marina has a problem, like a 12 volt hot lead connected to the marina ground by a fault and the negative lead connected to their prop, as is typical. This can cause two problems for your boat. The most obvious is that if you connect your DC negative to your engine and to your propeller and then connect that to the AC ground from your incoming AC cord, you are creating a path for the DC current from the other boat to get to the dock to complete the circuit. You are also creating the conditions for electroplating his boat with your metal. This can eat your prop away in a very short amount of time. If you have a complex AC system on your boat, you no doubt have the AC ground connected to the DC ground. Otherwise , it is probably unsafe because there are just too many places that wires cross or AC wires can touch things that are at DC ground.

AC Marina Ground and Boat Ground Isolation

For the above reason, you can see that you really don't want to literally connect your boat ground to the marina ground directly. But from our discussions earlier, you know you need the AC and DC grounds electrically connected. The solution to this dilemma is to have the AC dock ground isolated from the AC boat ground. This is done in one of two ways. The best way is with an isolation transformer. That is basically what I have in my battery charger. But running an air conditioner or refrigerator requires a very expensive isolation transformer so not everyone goes this route. The other way is a galvanic isolator which is basically a bunch of diodes connected so that they require about 1.2 volts before the dock AC and boat AC are connected. This is enough to trip a circuit breaker in the event of a short, but too much for most electrolysis situations. Galvanic isolators are not perfect isolators, they theoretically cannot be. They can also fail. They are clearly better than nothing and I am sure they will drastically slow any corrosion but I seriously doubt they can eliminate the issue given that diodes have leakage current by their physics. An isolation transformer is better.

Other Marina Currents

Let's assume you have electrically isolated your boat from marina ground. There is still the potential to have a marina based problem. Think of a boat to your starboard with a DC + lead connected to marina ground (a fault) and a boat to your port with a path to ground (no isolator). That sets up a potential between the two boats. If the two boats are 20 feet apart and two parts of your boat are 2 feet apart there is going to be 1.2 volts between the two parts of your boat. If you bond them, you will eliminate that voltage but you will get current between them instead. You are now participating in two electroplating systems just as if your boat was connected to the marina ground. One boat is plating pieces of your boat and part of your boat is leaving and going off to the third boat. This is another reason not to bond pieces of your boat together. If you don't bond them, each piece may still be at a different voltage but it will be like a bird sitting on a 20,000 volt wire, they won't care.

How Bad is it to Connect to Marina Ground?

I am actually surprised to see that many boats do not have isolators. I did a simple test to verify that my battery charger was isolated. I put a zinc on a wire and put it over the side. I connected it to a cheap digital amp meter with the other lead connected to my battery negative terminal. My boat measured about 10mA. When I disconnected my battery charger, I measured about 10mA. No change. I did the same with the boat next to me. His boat measured 6mA and when he connected the battery charger it measured 35mA. He was clearly not isolated. He says our marina is very good as he doesn't burn zincs like he did at the last marina he was in where he got 2 months on a zinc. This can be a useful test if you are burning zincs. Make the test and try changing things to get this number down.

Zinc Protection

I mentioned the shaft zinc that is used to protect the bronze prop from being corroded away because of the stainless steel shaft. It should be noted that if you do bond pieces of your boat together, you need to include a zinc or equivalent in that circuit. That will protect the metal from some kinds of electrical problems but as this article states, not from all of them. But please be aware if you are only bonding for electrolysis protection and not for safety reasons, you are only adding the zinc protection because you are bonding. An isolated piece of metal in the water is not subject to electrolysis. It needs a mechanical connection to a dissimilar metal. Adding zinc is not a free ride. They cost money to buy and install and as discussed below can be a particular problem for wood but the process of protection the metal will also eat the paint away. Bond if you must but protect with zinc and make sure you are bonding for a good reason.

Wood Boats and Zinc

A special mention needs to be made regarding wood boats. I mentioned that we bond zinc to our already bonded bronze-stainless prop and shaft. You need to be careful with a wood boat because in the process of protecting the metal, you are eating away the wood. The zinc-bronze electrochemical reaction creates hydroxyl ions around the zinc. These are corrosive to wood and will eat the wood leaving only the cell walls. I have taken to putting only a single zinc on my prop shaft as we were getting too much wood damage when I had 2 zincs. At one time the yard put a large zinc plate right on the wood with a wire over to some metal. I had significant wood damage around that zinc so we didn't replace it letting the 1 inch bronze rudder shaft protect itself for another 50 years as it had for the first 50. The moral is that on a wood boat, zinc is good and zinc is bad. You need to find a balance. This is, of course, another reason not to bond all your through hulls thinking your zincs will protect them as you will then be eating away the wood right around your through hulls, a particularly bad idea.

"A sailboat was hit here by lightning and sank 100 yards out in the channel, it sank in a few minutes. When they brought it in all the through hulls had been blown out, I looked in it, the mast and all the thrum hulls were bonded."

A Few words on Lightning

I did a rough calculation and find that being hit by lightning is like having some fraction (1/4 to 1/10) of a stick of dynamite go off on your boat. I think on my boat the power came to 8 billion watts. Best not to be on your boat when there is lightning. There a lot of theories about what to do regarding lightning and it is beyond the scope of this article. I offer several of these references on my Lightning page.

Radio Ground

I am not going to talk a lot about RF grounds except so suggest not to let the presence of a SSB radio make you decide to bond anything together in your boat. There is no reason for it. The radio is probably DC ground anyway so no harm in using anything that is already at DC ground as your counterpose. I would suggest the engine block. As for antenna efficiency, you are not going to have a theoretically good antenna on a boat for SSB unless you have a very big boat. Instead you will be using an antenna tuner to "load up" your backstay or a long wire. The ground wire off the antenna tuner is also part of the antenna and can be long or short. It can be connected to another long wire or to your engine block. From an RF standpoint, it really doesn't matter. However, it is nice to have the RF ground on the antenna tuner electrically near your station ground potential. If it is not, you will know it because the microphone will "bite" you. I have had that happen before I got my grounds set up right. You can transmit just fine but it isn't pleasant. I think the issues here are safety in terms of RF energy, which you will have on any wire from the tuner to the antenna out and making sure the station ground (the case of your transceiver) is not RF hot. Here is an example, not on a boat, but useful nonetheless. My HAM radio is 15 feet off the ground in my house. My station ground is an aluminum plate under the radio. There is a 1 inch braid going from this plate down to a ground rod. The antenna tuner is right there at the transceiver. A long wire leaves the tuner and travels over to a tree in my back yard. I have another antenna on my roof that isn't grounded, it just has a bunch of long wires running over my roof as the counterpoise. I have talked to Madagascar from California with this setup, which is half way around the world. There is no reason to bond anything for the sake of your SSB.

One Final Note

You may be wondering why I say not to bond and others say to bond. Why should you believe me? It turns out my boat is very old. The things I am saying have been tested in real conditions on my boat for 50 years. For example, last year I replaced two bronze through hull fittings just because they were over 50 years old and happened to be the last two old fittings, the others having been replaced or removed for other reasons. As I said, bronze has a shelf life in salt water of about 100 years so I was giving myself a 2:1 safety factor. These through hulls had never been bonded in over 50 years. They have been in salt water the entire time and near the shaft and other metals I might add. We cut them in half in the process of getting them out. They were pristine. I could have left them in there another 50 years.


The simple conclusion is to not bond anything together except for safety reasons and try to find other ways to make the boat safe, like not having any AC wiring on my boat but that is just me and my old wood race boat. If you live in Florida where there is lots of lightning, by all means ground your mast but don't connect that to your boats DC or AC grounds if you are concerned about lightning, keep them separate. But do bond your gas tank if you have a gasoline engine. Whatever you do, make sure you have all the systems required to keep your boat safe from AC hazards.

* Allen Edwards is a retired Electrical Engineer with a MS-EE from Stanford University. He has owned Papoose, a Lapworth 36 since 1996 and is an Extra Class HAM radio operator, AA6EX. The opinions expressed in this article are his and are just that, opinions. He takes no responsibility for anything in this article and encourages you to consult an expert in the field before you go and do something that kills someone.


Thanks Allen for your article. I read a magazine article bonding when I first got my Cal 29 that clearly stated that everything metal should be bonded together. So I spent hours doing just that. About 6 months later I realized that two Zincs had disappeared on the prop shaft and you could break pieces off the prop itself. I quickly removed the bonding.

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