For the last two months I have written about bonding and grounding systems and how these can solve serious and expensive problems for boat owners. Besides reducing the damaging effects or galvanic corrosion (Understanding Galvanic Corrosion) and eliminating electrolysis (Understanding Electrolysis), bonding and grounding systems also protect people and equipment from the damage caused by lightning.
Lightning is certainly the most compelling of all bonding and grounding issues. Though lightning strikes are infrequently reported, they occur millions of times during the tens of thousands of thunderstorms each year in the US. However, even though facts and figures regarding lightning are collected every day by satellites and research laboratories (I frequented one in Tucson, AZ), thorough knowledge of this phenomenon has not yet been attained. Lightning assumes several forms. Most everyone has seen and heard bolt lightning and sheet lightning; but other effects of lightning such as ball lightning, St. Elmo's Fire, and other atypical static discharges have been described for centuries and still are not fully understood.
Basically, lightning is caused by a difference of potential created during atmospheric disturbances; most commonly in and around thunderstorms. The difference of potential causing lighting is measured in millions of volts. Lightning occurs when a current can be conducted between bodies of potential difference. In the case of cloud-to-ground flashes of lighting, which are the most devastating to boaters and our central concern here, the difference of potential is so great that it creates a condition called ionization right in the air, which allows the normally insulating air to conduct electricity. Ionized air is created in steps by the great difference of potential.
The notion of a lightning strike being a bolt of electricity coming out of the clouds and zapping the earth or object on the earth is incomplete. In most cases, the creation of ionized air begins at both
ends of the path that the lightning will travel. A lightning bolt does not usually consist of a single zap to equalize the difference of potential; sometimes several exchanges in both directions occur to dissipate the vast electrical energy involved.
It is very difficult to study lightning for many reasons. It occurs randomly even within ideal conditions and varies greatly in its attributes; a bit akin to snowflakes, no two lightning bolts are alike. It is also an extremely brief event of an immense magnitude. For our purposes, we only need to study the results of lightning, what effects it has when occurring in the vicinity of our boats. Because lightning is the most allusive problem related to bonding and grounding, with the most controversial solutions, I will discuss it in three separate articles.
Nature's Fireworks If galvanic corrosion or electrolysis affect your unbonded or ungrounded vessel, they will eat away at it. At worst these problems will cause severe damage over the course of weeks or months that could render your boat unseaworthy. A lighting event, which typically occurs in a couple of tenths of a second, could cause catastrophic damage to your boat's hull, rigging, equipment, and systems. At worst, it might severely injure or kill crewmembers. Statistics tell us that half of all persons affected adversely by lightning are killed. A rusty prop shaft is one thing; even a melted turnbuckle isn't so bad, but the prospect of getting killed is scary for any sailor.
|"A rusty prop shaft is one thing; even a melted turnbuckle isn't so bad, but the prospect of getting killed is scary."|
To make matters worse, nothing you do short of burying your boat in a deep, deep hole will assure avoidance of a lightning event. Brand new boats have been hit while sitting on stands in a boatyard. Boats docked under roofed facilities have been severely damaged (a metal roof over the dock actually could make a lightning event more likely). Powerboats have been struck and even small fishing boats, so you don't have to have a mast to be a target of lightning.
The mechanics of a lightning event on a sailboat begin with a great difference of potential between the earth and clouds created by a thunderstorm. The exact theory of this electrical generation is debated; I believe it is created by the change of states (mostly condensation) of water in the atmosphere. However this difference of potential is made, it develops astonishingly fast and has amazing power. A voltage of 100 million volts or more is common in lightning strikes! And it's really amazing when you compare that to the few volts involved in electrolysis or tiny fractions of a volt for a dissimilar metal galvanic condition.
The small voltages of corrosion tend to equalize by the transfer of ions from one of the metals through seawater. For most of its rapid journey, lightning doesn't have a conductor to flow through, but has such vast power that it literally makes its own path by ionizing the air. When a sufficient charge exists in the clouds, it draws the opposite charge on a large mass as close to it as possible—say, for instance, the top of your boat's mast. I mean at the very top of your mast, a huge concentration of ions (positive ions, 90 percent of the time) will accumulate. A corresponding accumulation of negative ions will form in the edge of the thunderhead nearest your boat. This charge is so concentrated and robust that it defies the insulating qualities of air and leaps off toward the opposite charge with positive ions streaming up from your mast and negative ions streaming down from the cloud. They attempt to connect, but usually do not on the first try. But each successive attempt builds on the prior jumps and breaks down the insulating qualities of the air on that path.
When they do connect, a transfer of power occurs that sometimes surpasses even the amount of initial voltage present and the lightning overcharges the sailboat. Milliseconds after the initial lightning strike another transfer of energy usually takes place, returning the excess electricity back along the same path. Sometimes the first exchange does not transfer enough energy, and successive transfers attempt to equalize the difference of potential along the well-ionized path in the air. Several exchanges are common, and some investigators speculate that a single lightning strike could involve dozens of these transfers, prolonging the visual and audible results of the event.
So, what can sailors do to prepare themselves for a lightning strike? I suggest there are three important goals to achieve in lightning protection:
- Not allow this dangerous voltage to come aboard in the first place.
- Minimize the amount of time the vessel is charged with millions of volts.
- Reduce the damage caused by the effects of a lightning strike.
There really are no guarantees when it comes to lightning strikes, and the lightning protection devices and methods that are available today to the private boat owners are by no means the ultimate solution. There is no such thing as lightning-proof. We can simply do our best to avoid being a prime target of Thor by employing certain methods believed to be effective in practice and theory. Common sense is perhaps the most underused weapon against lightning threats. Simply put, don't go sailing when there's the threat of a thunderstorm! Of course we can't always exercise that kind of control, so in my next column I will present some ideas you can use to keep your boat from suffering the excessive damage that can result from an encounter with lightning.
Striking the SourceIf you'd like to become better informed regarding lightning protection systems, get a copy of any one of the following sources:
NFPA 780 Standard for the Installation of Lightning Protection Systems
NFPA 302 Fire Protection Standard for Pleasure and Commercial Motor Craft
NFPA 70 National Electrical Code
ABYC standards E-2,4,8,9&10 www.abyc.com/Publications/Standards/descript.htm
There are also various lightning-protection products on the market, some of the available right here at SailNet. (Lightning Protection). Others can be viewed at the following websites:
The author also recommends the following websites as sources for information on lightning protection:
www.yachtdoctor.com (marine electrical checklist)www.bayacht.com/aaa/nl-artic/lightng/lightart.htm
You can also research real-time lightning events in the US at www.lightningstorm.com/ls2/gpg/lex1/mapdisplay_free.jsp or check out the exhaustive bibliography on lightning at www.nssl.noaa.gov/~holle/LtgAlpha.html.