Lightning is a powerful, not fully understood phenomenon of nature. While the risks of taking a hit are small, they nonetheless exist. It is important to understand that there is no way to predict the results of the hit, since characteristics vary widely, as does the magnitude of the lightning bolt.

Lightning builds up somewhat as follows: Friction within a cloud creates a stratified charge of positive and negative ions. The positive ions are usually in the upper region of the cloud, while the negatively charged ions generally reside at the bottom of the cloud.

Because like charges repel each other (and unlike ones attract), the area immediately below the cloud typically has only a limited number of negative ions. The cloud does, however, attract a group of positive ions in the area below it (attracted by the negative charge of the cloud base). Now, air being a pretty good insulator, and all other things being equal, there would be no interaction between cloud and ground. Thus, the charge in the cloud just builds and builds. When the area of positive ions that is dragged along the ground by the cloud reaches your boat and if the charge buildup in the cloud is great enough, the cloud may shoot down some "leaders," trying to complete the circuit between negative ions at the cloud base and positive ions around your boat. These leaders take a series of zig-zag steps as they approach the earth, looking for a ground connection.

At the same time this is coming down from the cloud the positive charge on surface of the earth is looking for a jumping-off point where it can accumulate sufficient energy to make the connection to the cloud's leader. The leader itself does not pack much wallop, but it ionizes the air to create a more efficient path for the big charge to follow.At the same time this is coming down from the cloud the positive charge on surface of the earth is looking for a jumping-off point where it can accumulate sufficient energy to make the connection to the cloud's leader. The leader itself does not pack much wallop, but it ionizes the air to create a more efficient path for the big charge to follow.

As the cloud-to-ground connection is made, part, but not all of the energy differential is consumed. The norm is for a series of interchanges to take place along the same path, typically three to as many as 20, all in a split second. The number of these, and the differential between the cloud and earth, determine how big a lightning hit will be.

The issue we now face is how to make the boat less attractive to a lightning discharge. Considering the fact that your mast is a good conductor, and it is by far the highest thing around, it makes a pretty inviting target. The answer lies in reducing the "potential" difference between the mast and the surrounding water. This is accomplished by electrically bonding the mast to the water.

My first exposure to the lightning problem occurred on a lake in the Ozark hills of Missouri, when I was going to college. We were out sailing in a small catamaran when a front came through, bringing with it the usual rain, hail, and lightning. We could see lightning striking the ground on either side of the lake as the rain came toward us. My initial reaction was "My God! Our mast is aluminum and higher than the surrounding scrub." Deciding that discretion was the better part of valor, I capsized my cat and swam for it. That was not a smart move. I didn't think about the conductivity of fresh water if it took a direct hit and we were in it.

At sea, anecdotal evidence indicates that properly protected, a small vessel presents little physical danger to those aboard. There's considerable financial risk, however: a good hit can wipe out all electronic gear aboard, including clocks, alternators, radios, radars, and long-range navigation gear.

Using a Bonding System to Dissipate Static

Your aluminum mast is quite an efficient conductor of electricity. The problem lies in getting the mast connected to the water. This is typically accomplished with a heavy bonding wire (# 4 wire is good for this purpose) connected from the mast heel to the keel bolts. The outside lead ballast is a reasonably efficient ground path to the water. If you also connect the through-hull fittings and engine block (through which the prop shaft and prop are brought into play), you now have a good overall package to dissipate that static charge before it can attract a strike.

Grounding Plate

The only problem with the above grounding system is in its ability to deal with a lightning strike. If a strike does occur, the power will find a way down the boat and into the ground (the sea). It will take the most direct path possible. If your bonding wire at the bottom of the mast makes a right angle to head back for the keel bolts or to some nearby through-hull fittings, the odds are that the lightning is going to take the direct route straight through the bottom, it generally won't take a turn. This could leave you with a large hole.

One alternative is to fit a grounding plate below the mainmast, yielding a more direct path for the lighting hit to exit. This ground plate also help with static dissipation and can be used as an RF ground for electronics.

If you do have a ground plate, be sure to keep it unpainted and clear of marine growth.

Masthead Dissipaters

To further reduce the charge of static you can use a lightning dissipater. This resembles a bottle brush. Essentially, it's a stainless pipe with hundreds of little wires sticking out the top. Those little wires help the ions bleed off so the cloud-to-ground leader looks elsewhere to make its connection. If you have a split rig, it is recommended that you install a dissapater on each spar. This type of device has been used for years on radio towers, power-line pylons, and microwave towers.

The Cone of "Protection"

There is a theory that a cone of protection exists under your bonded mast, and that if you or your gear are inside of this cone, all is safe. However, the theory is controversial, and we've heard lots of anecdotal evidence that "side flashes" can and do occur between the main lightning strike and nearby objects. If an electrical potential exists between the main hit and nearby metal, and if your body becomes the path to equalize this potential, serious damage may occur to you.

Bond All Metal

Side flashes result during a hit when there is a different electrical charge on various parts of the boat. This can be harmful to electronics and bodies if they become the path for the electrical differential to neutralize.

To minimize the risk of side flashes, it is necessary to bond all metal objects in the boat. Bonding keeps everything at the same potential, reducing the risk of side flashes, and it includes all deck hardware, chain plates, lifeline system, pulpits, steering system, and electronics. A #8 wire should be used for this purpose. Be sure to keep the electrical connections clean on these fittings, as well as all the others in the system, especially as time goes on. If the connections corrode, resistance will increase and the efficiency of your system will be reduced.

What To Expect from a Hit

If your dissipating and bonding systems fail and you take a hit, the damage sustained is a function of the quality of your bonding system and the magnitude of the strike. We have heard and seen results ranging from a few areas of burnt paint to the loss of every piece of electronics gear (including unwired handheld electronics).

The more resistance the lightning bolt encounters on its path to ground, the higher the amperage, and the destructive energy, will be. This energy has been known to melt rigging, as well as to blow through-hull fittings away from the hull skin and to explode bonding plates.

Obviously, it is best to avoid the hit entirely. If that isn't possible, get the current to ground with minimum resistance and damage.

Can You Protect Your Electronics?

One of the side-effects of a lightning hit is the creation of huge amounts of magnetic energy. This can be felt sometimes hundreds of feet away. Since most integrated circuits are sensitive to magnetic force, they need to be protected inside of what is called a Faraday cage. This is a simple box made from metal. Wrapping electronics (or spares) in tin foil may help. Also, disconnecting electronics before a hit takes place will reduce the odds of damage, although a big hit will eat even disconnected gear (unless it's in a Faraday cage).

Our friends Al and Beth Liggett took a hit while moored in Guam. They weren't aboard Sunflower at the time, but when they returned they found the VHF antenna gone. The wind instruments and tri-color light at the masthead were still okay, however. Down below, the VHF, pilot, engine alternator, Satnav, stereo, and some of their LED signal lights were fried. Yet other items like normal lights, engine starter, and laptop computer escaped unscathed. They lost a couple of breakers on the main panel; some plastic on the upper lifelines was fried; and the stern light was blown. Sunflower's structural integrity, however, was sound. There were no problems with through-hull fittings.

Metal Boats and Lightning

One of the advantages of a metal boat is that it offers a huge amount of area in contact with the sea to both relieve static charge and dissipate lightning energy. In fact, I have yet to hear of a metal boat being hit.