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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.
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