— May 2003
By Ben Ellison
|Lightning and boats: Just how terrified should you be about the mix?|
Every ancient culture had some wonderful explanation for lightning. The Vikings, for one, said it was their god Thor whacking his mega hammer to an anvil while riding his chariot across the clouds. But the Vikings probably weren't standing on the bow of a boat in a thunderstorm when they came up with this explanation. A boat could be a wonderful platform for observing this awesome natural phenomenon if it weren't so terribly exposed. "I play it safe and keep the boat in port when scattered thunderstorms are forecast, even though it is otherwise a nice summer day," a Chesapeake Bay boater wrote us. He went on to ask about the actual probabilities of a boat being struck by lightning, possible consequences, and means of protection.
I'm afraid I cannot give this gentleman definitive answers; in fact, the science of lightning and just how to protect humans and property from it is much more tentative than one might assume. Oh, there are some statistics around that are a bit chilling. An estimated 40 million lightning strikes, which NOAA terms "an underrated killer," account for nearly 100 deaths annually in the United States, making lightning considerably more worrisome than hurricanes and sharks. Florida, with the most thunderstorms, averages more than ten deaths and 30 injuries from lightning per year, and approximately 25 percent of those are water-related. While it's true that most of the marine fatalities occur in open boats, it's also true that National Marine Electronics Association dealers report substantial amounts of lightning damage to big-boat electronics.
The current thinking is that lightning happens when particles within the violent up and down drafts of a thunderstorm cloud become charged through collisions. Different sizes acquire plus and minus charges, then separate into layers, producing enormous electrical potential both within the cloud and between the cloud and ground. There's no question that millions of volts can accumulate, eventually overcome the insulation of the air, and flash in multiple strokes that average five miles long. In the lightning's path the atmosphere is heated to about 50,000°F (three times the surface of the sun), producing a shock wave. Zap! Bang! Since light travels almost a million times the speed of sound, the flash is often seen before the thunder is heard. In fact, if you count the difference in seconds and divide by five, you'll have an approximate distance to the strike in miles. It's also said that just before a close flash, your hair will literally stand on end!
Lightning can strike cloud to cloud harmlessly, cloud to water harmlessly, or cloud to water via your boat. While it's true that sailboats with their tall masts are more susceptible to hits than powerboats, that is not the whole story. Tall masts and tuna towers also provide what's called a cone of protection. If you have a significant conductor overhead, and you can stay out of the conductive chain to ground, you are safer. Thus it is that leading lightning expert Ewen Thomson, an associate professor of electrical engineering at the University of Florida, writes, "An oceangoing power yacht has more risk factors than any other type of boat. Large, open deck spaces with an absence of natural lightning rods raise the risk of a direct attachment to anyone on deck." Moreover, on a power yacht without a prominent mast, "The natural path to ground is via onboard wiring through the main instrumentation cluster, likely destroying most other electronic systems as well."
You may, at this point, be interested in a Lightning Protection System (LPS). But it's a misnomer, as there is no proven way to actually protect a boat from lightning, only a technique for limiting damage when it strikes. Actually, there is a widely held myth, particularly among sailors, that an LPS--which is primarily a straight, highly conductive path from mast to water--causes more harm by attracting lightning than it does good. But there are also numerous documented cases where lightning blew a hole in a boat while trying to find a way out. Thomson conducted an extensive survey on this issue and concluded that grounding a sailboat mast does not increase strike risk and does reduce damage in the event of a strike.
Thomson also cites several studies that question the effectiveness of the one device promoted as a lightning stopper, the so-called "ion dissipater," a sort of stainless steel wire brush you'll sometimes see affixed atop a sailboat mast that is supposed to leak off a boat's attractive ground potential. Yet, despite negative conclusions drawn by multiple agencies including the FAA and NASA, the device is advocated on a large marine insurer's Web site today. As I said, the science of lightning protection is still evolving.
This article originally appeared in the April 2003 issue of Power & Motoryacht magazine.