William J. Becker
University of Florida
"One minute the fisherman was sitting atop his elevated seat
aboard his boat. The next minute he was dead--the victim of a
lightning bolt."
This was the lead paragraph in a recent Florida newspaper
article. These accidents can and do happen--and yet they need not.
Florida has more thunderstorms--and thus, more lightning
strikes--than any other state (see Figure 1). Only three states have
a higher death rate from lightning than Florida, and no state has
more deaths or injuries.
Florida averages more than ten deaths and thirty injuries from
lightning per year. Approximately fifty percent of the deaths and
injuries occur to individuals involved in recreational activities,
and nearly forty percent of those are water-related: boating,
swimming, surfing, and others.
Those who enjoy Florida's waters certainly should understand the
phenomena of thunderstorms--lightning and the precautions to take in
order to keep these activities pleasurable--and how to prevent
tragedy.
Most lightning strikes occur in the afternoon--70 percent between
noon and 6:00 p.m. As the air temperatures warm, evaporation
increases. This warm, moisture-laden air rises and evaporates,
forming fluffy cumulus clouds. As more moisture accumulates, the
clouds darken and change into cumulus nimbus clouds--thunderstorm
clouds--frequently, with a flattened top or anvil shape, reaching to
40,000 feet or more (see Figure 2).
The upper portion of the cloud develops a positive electrical
charge, the lower level a negative electrical charge. The air,
because it is a poor conductor of electricity, restricts the regular
flow of electricity between these, attracting electrical charges.
While this phenomenon is occurring in the clouds, a similar
phenomenon is occurring on the surface.
Negative charges repel negative charges and attract positive
charges. So, as a thunder cloud passes overhead, a concentration of
positive charges accumulates in and on all objects below the cloud.
Since these positive charges are attempting to reach the negative
charge of the cloud, they tend to accumulate at the top of the
highest object around. On a boat that may be the radio antenna, the
mast, a fishing rod, or even you! The better the contact an object
has with the water, the more easily these positive charges can enter
the object and race upward toward the negative charge in the bottom
of the cloud.
Lightning occurs when the difference between the positive and
negative charges, the electrical potential, becomes great enough to
overcome the resistance of the insulating air and to overcome the
resistance of the insulating air and to force a conductive path
between the positive and negative charges. This potential may be as
much as 100 million volts. To help you understand the magnitude of
this voltage, the voltage needed in an automobile to cause a spark
plug to fire is only 15 to 200 volts! And the spark plug gap is but
a fraction of an inch!
Lightning strikes represent a flow of current from negative to
positive, in most cases, and may move from the bottom to the top of
a cloud, from cloud to cloud, or most-feared, from cloud to ground
(see Figure 3). And when the lightning does strike, it will most
often strike the highest object in the immediate area. On a body of
water, that highest object is a boat. Once it strikes the boat, the
electrical charge is going to take the most direct route to the
water where the electrical charge will dissipate in all directions.
Let's consider a few possibilities. Lightning strikes the ungrounded
radio antenna on your boat. The metal antenna carries the electrical
charge to the radio, which does not have a good conductor to the
water. Your hand is on the radio, or on metal connected to the
radio. Your feet are on a wet surface, which is in contact with
metal which extends through the hull of the boat to the water. Your
body may then become the best conductor for the electrical charge.
A second example is a sailboat. Lightning strikes the mast. The
electrical current follows the mast or wire rope to your hands,
through your body to the wet surface, and then through the hull to
the water.
Or, while operating a motor boat, the lightning strikes you,
passes through your body to the motor, and then to the water.
Or, sitting in your aluminum or fiberglass rowboat, you are
holding a graphite (a good electrical conductor) fishing rod. The
rod is struck by lightning. The electrical charge passes through the
rod, your body, then to the boat to the water.
In all four examples you could be seriously injured. You could be
dead.
You need not even be in contact with the components of the boat
struck by lightning. Unless the components of the boat which could
conduct electricity are bonded together and are adequately grounded,
there could be side flashes. A side flash occurs when the electrical
charge jumps from one component to another seeking a better path to
ground. You might be that "better path."
Do not become a lightning target. Preferably stay off, and
definitely get off, the water whenever weather conditions are
threatening. Check the weather. The National Weather Service (NWS)
provides a continuously updated weather forecast for Florida and its
coastline via the VHF/FM channels WX1 (162.550 MHz), WX2 (162.400
MHz), WX3 (162.475 MHz). Never go boating without listening to this
service. Their short-term forecasts are quite accurate, but small
localized storms might not be reported. Therefore, it is important
that boaters learn to read the weather.
Watch for the development of large well-defined rising cumulus
clouds. Once they reach 30,000 feet the thunderstorm is generally
developing. Now is the time to head for shore. As the clouds become
darker and more anvil-shaped, the thunderstorm is already in
progress.
Watch for distant lighting. Listen for distant thunder. You may
hear the thunder before you can see the lightning on a bright day.
Seldom will you hear thunder more than five miles from its source.
That thunder was caused by lightning 25 seconds earlier. The sound
of thunder travels at one mile per five seconds (see Figure 4).
You are two miles from shore. The thunderstorm which is now five
miles away is traveling in your direction at 20 miles per hour,
which means it could be overhead within 15 minutes. Can you reach
shore--two miles away--and seek shelter within that time? You better
move!
There is no such thing as lightning-proof boats, only
lightning-protected boats. All-metal ships are rarely damaged, and
injuries or deaths are uncommon. These ships are frequently struck,
but the high conductivity of the large quantities of metal, with
hundreds of square yards of hull in direct contact with the water,
causes rapid dissipation of the electrical charge.
But small boats are seldom made of metal. Their wood and
fiberglass construction do not provide the automatic grounding
protection offered by metal-hulled craft. Therefore, when lightning
strikes a small boat, the electrical current is searching any route
to ground and the human body is an excellent conductor of
electricity!
Today's fiberglass-constructed small boats, especially sailboats,
are particularly vulnerable to lightning strikes since any
projection above the flat surface of the water acts as a potential
lightning rod. In many cases, the small boat operator or casual
weekend sailor is not aware of this vulnerability to the hazards of
lightning. These boats can be protected from lightning strikes by
properly designed and connected systems of lightning protection.
However, the majority of these boats are not so equipped.
Lightning protection systems do not prevent lightning strikes.
They may, in fact, increase the possibilities of the boat being
struck. The purpose of lightning protection is to reduce the damage
to the boat and the possibility of injuries or death to the
passengers from a lightning strike.
If you are considering the purchase of a new or used boat,
determine if it is equipped with a properly designed and installed
lightning protection system. Such a system is generally more
effective and less costly than a system installed on a boat after it
has been constructed.
The major components of a lightning protection system for a boat are
an air terminal, main conductor, and a ground plate. Secondary
components are secondary conductors, lightning arrestors, lightning
protective gaps, and connectors (see Figure 5).
The mast, if constructed of conductive material, a conductor
securely fastened to the mast and extending six inches above the
mast and terminating in a receiving point, or a radio antenna can
serve as the air terminal.
The main conductor carries the electrical current to the ground.
Flexible, insulated compact-stranded, concentric-lay-stranded or
solid copper ribbon (20- gauge minimum) should be used as the main
conductor.
The ground plate, and that portion of the conductor in contact
with the water, should be copper, monel or navel bronze. Other
metals are too corrosive. The secondary conductors ground major
metal components of the boat to the main conductor. However, the
engine should be grounded directly to the ground plate.
Lightning arrestors and lightning protective gaps are used to
protect radios and other electronic equipment which are subject to
electrical surges.
The connectors must be able to carry as much electrical current
as other components of the system. Further, the connections must be
secure and noncorrosive.
On a large power boat or sailboat, a properly designed and
grounded antenna could provide a cone of protection. Presently,
however, the vast majority of the radio antenna is totally
unsuitable for lightning protection. This is also true of the wires
feeding the antenna. If the antenna is not properly grounded, it may
result in injury or death and cause considerable property damage.
Sailboats with portable masts, or those with the mast mounted on
the cabin roof, are particularly vulnerable as they are usually the
least protected as far as grounding or bonding is concerned.
Ideally, an effective ground plate should be installed on the
outside of all boats when the hulls are constructed. Unfortunately,
this is not often done. Such a ground plate would help manufacturers
design safer lightning protection systems for the boats.
The National Fire Protection Association, Lightning Protection Code,
suggests a number of ways in which the boater can protect his boat
and minimize damage if the boat is struck or is in the vicinity of a
lightning strike. These suggestions are summarized below:
