Getting the Charge Out of Lightning

No matter how well protected the boat may be, few manage to escape unscathed.


Miraculously, relatively few people are injured in lightning strikes. Frequently, of course, no one is aboard the boat when it is struck, and it is only by after-the-fact detective work that the extent of damage is discovered.

There are, however, two attendant bits of unpleasantness water, and contaminates like dirt, dust, rust, scale, bugs, and bones.

Most boat owners have only the vaguest idea of what is involved in protecting their boats from lightning damage. Many believe that their boats are already protected by the boat’s grounding system. Most are wrong.

Just because your boat may be bonded with heavy cop-per conductors connecting the masses of metal in the boat doesn’t mean that it is protected against lightning. A bon-ding system may be a part of a lightning protection system, but bonding itself offers no protection to the boat unless a good, direct path to ground is part of the system.

The purpose of bonding is to tie underwater metal masses in the boat together to reduce the possibility of galvanic corrosion caused by dissimilar metals immersed in an electrolyte. The purpose of lightning grounding is to get the massive electrical charge of a lightning strikethrough the boat to ground with the least possible amount of resistance.

Most lightning never reaches the earth: it is dispersed between clouds of different electrical potential. The lightning that concerns sailors is the discharge of electricity between a cloud and the surface of the earth, or an object on the surface of the earth, namely, your boat. The amount of electricity involved in lightning can be, well, astronomical. We’re talking about millions of volts.

Granted, the duration of a lightning strike is extremely short. But in the fraction of a second it takes for lightning to pass through your boat to ground, a great deal of damage can be done. And here’s the kicker. No matter how elaborate your lightning protection system, there is no guarantee that a lightning strike will not damage your boat.

Certainly you can reduce the potential damage from a lightning strike. That’s what protection is all about. But to think you can eliminate the possibility of damage is folly. There are too many recorded instances of so-called properly lightning-protected boats suffering damage to believe in the infallibility of lightning protection systems.

The goal of lightning protection is to offer a low resistance path to ground, in this case, the water. On a sailboat equipped with an aluminum mast and stainless steel standing rigging, the basic components of the lightning protection system are in place.

While neither aluminum nor stainless steel is an outstanding electrical conductor, the large cross-sectional area of both the mast and the rigging provide adequate conductivity for lightning protection. The trick, however, is get-ting the electricity from the mast and rigging to the water.

The straighter the path is from conductor (mast and rigging) to ground, the less likely are potentially dangerous side flashes. Put simply, side flashes are miniature lightning bolts which leap from the surface of the conductor to adjacent metal masses due to the difference in electrical potential between the charged conductor and the near by mass of metal. Ideally, therefore, the path from the bottom of the mast and rigging to ground would be absolutely vertical. In practice, this is rarely achieved.

If the boat has an external metal keel, the mast and standing rigging is frequently grounded to a keelbolt. There are pitfalls to this method. First, the connection between the bottom of the mast and rigging to the keelbolt must be highly conductive. ABYC (American Boat and Yacht Council) standards say that each primary conductor for lightning protection systems should have a resistance equal to or less than a #4 AWG copper conductor. (Secondary conductors should have resistance not greater than a #6 AWG copper conductor.)There is no drawback to using an even larger conductor.

Connecting the short conductor to the mast and keelbolt presents some problems. A crimp eye can be used on the end that is to be attached to the mast, but you may have to fabricate a larger eye for attachment to the keelbolt. This can be made from sheet copper. Soldering the connections is not recommended, since the heat generated in a lightning strike could melt the solder.

Then you have to face up to a basic problem. Your mast is aluminum, yet you’re connecting it to ground with a copper cable. Everyone knows that aluminum and copper are not galvanically compatible, so what’s the solution? While it will not eliminate corrosion, a stainless steel washer placed between the copper cable’s end fitting and the aluminum mast will at least retard it. But this connection is going to require yearly examination to make sure that a hole isn’t being eaten through the mast. In addition, of course, the process of corrosion creates wonderful aluminum oxide byproducts, which have very low conductivity. The aluminum oxide may reduce conductivity to the point where your theoretical attachment to ground is in fact non-existent. Once again, disassembling the connection and cleaning it yearly are essential to maintain conductivity. Constant attention to all the conductor connections is essential in any grounding system, whether it’s for lightning protection or grounding of the electrical system.

Even if all the connections in the system are flawless, you’re faced with getting the electrical charge out of the boat and into the water. Keels are always coated with bottom paint. Depending on the type of bottom paint used, the keel itself may be a fairly poor ground. An iron keel properly primed with epoxy mastic before bottom paint is applied is fairly well isolated from the water. If it weren’t, it would rust. The same goes for lead keels prepared in the same way. In practice, the electrical charge will probably be powerful enough to get to ground through the protection system on the keel. The same problem exists, of course, on painted metal boats with their systems of barrier coats. The barrier coats reduce conductivity, but do not eliminate it.

Do not under any circumstances ground the rigging or mast to ballast located inside a fiberglass hull shell. The electrical charge tends to travel on the surface of the conductor. Finding no path to ground from the isolated inside ballast, you may literally blow a line of holes through the hull along the top of the ballast line. Surveyors have reported occurrences of this type of damage to us, as strange as it may sound.

For boats with inside ballasting, or for powerboats, some type of external grounding plate is required. These are usually made from sintered bronze: tiny particles of bronze fused into a porous block. The effect is to give a much larger surface area than defined by the dimensions of the block itself. It is very important to use as large aground plate as necessary, and to position it as close to vertically in line with the primary lightning conductor (the mast) as possible.

Racing boats are not going to be willing to do this, since a ground plate creates a fair amount of drag in light air. Cruisers would be advised to trade off the drag for the protection offered.

A grounding plate installation is not a nail-it-in-place-and-forget-it installation. As with any bare metal underwater, oxides build up in the grounding plate, reducing its efficiency. The manufacturer of the plate can tell you the proper remedies, which may include removing the plate yearly and treating it in an acid bath to restore proper conductivity.

It is probably a poor practice to use the same grounding plate for lightning grounding and grounding of electronics such as Loran. If the lightning charge is too great for the plate to instantaneously transmit to ground, the charge may travel back through the ground wire to your electronics, with disastrous results.

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Getting the Charge Out of Lightning  

Darrell Nicholson
Practical Sailor has been independently testing and reporting on sailboats and sailing gear for more than 50 years. Supported entirely by subscribers, Practical Sailor accepts no advertising. Its independent tests are carried out by experienced sailors and marine industry professionals dedicated to providing objective evaluation and reporting about boats, gear, and the skills required to cross oceans. Practical Sailor is edited by Darrell Nicholson, a long-time liveaboard sailor and trans-Pacific cruiser who has been director of Belvoir Media Group's marine division since 2005. He holds a U.S. Coast Guard 100-ton Master license, has logged tens of thousands of miles in three oceans, and has skippered everything from pilot boats to day charter cats. His weekly blog Inside Practical Sailor offers an inside look at current research and gear tests at Practical Sailor, while his award-winning column,"Rhumb Lines," tracks boating trends and reflects upon the sailing life. He sails a Sparkman & Stephens-designed Yankee 30 out of St. Petersburg, Florida. You can reach him at