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:
- A
lightning protective mast will generally divert a direct
lightning strike within a cone-shaped radius two times the
height of the mast. Therefore, the mast must be of sufficient
height to place all parts of the boat within this cone-shaped
zone of protection (see Figure 6).
- The
path from the top of the mast to the "water" ground should
be essentially straight. Any bends in the conductor should
have a minimum radius of eight inches (see Figure 7).
- To
provide adequate protection, the entire circuit from the
top of the mast to the "water" ground should have a minimum
conductivity equivalent to a No. 8 AWG copper conductor.
If a copper cable is used, the individual strands should
be no less than No. 17 AWG. Copper metal or strips should
be a minimum of No. 20 AWG.
- Major
metal components aboard the boat, within six feet of the
lightning conductor, should be interconnected with the lightning
protective system with a conductor at least equal to No.
8 AWG copper. It is preferable to ground the engine directly
to the ground plate rather than to an intermediate point
in the lightning protection system.
- If
the boat's mast is not of a lightning protective design,
the associated lightning or grounding connector should be
essentially straight, securely fastened to the mast, extended
at least 6 inches above the mast and terminate in a sharp
receiving point.
- The
radio antenna may serve as a lightning protective mast,
provided it and all the grounding conductors have a conductivity
equivalent to No. 8 AWG copper and is equipped with lightning
arrestors, lightning protective gaps, or means for grounding
during electrical storms. Most antennas do not meet these
requirements. The height of the antenna must be sufficient
to provide the cone-shaped zone of protection.
- Antennas
with loading coils are considered to end at a point immediately
below the loading coil unless this coil is provided with
a protective device for by-passing the lightning current.
Nonconducting antenna masts with spirally wrapped conductors
are not suitable for lightning protection purposes. Never
tie down a whip-type antenna during a storm if it is a part
of the lightning protection system. However, antennas and
other protruding devices, not part of the lightning protection
system, should be tied down or removed during a storm.
- All
materials used in a lightning protective system should be
corrosion-resistant. Copper, either compact-stranded, concentric-lay-stranded
or ribbon, is resistant to corrosion.
- The
"water" ground connection may be any submerged metal surface
with an area of at least one square foot. Metallic propellers,
rudders or hull will be adequate.
- On
sailboats, all masts, shrouds, stays, preventors, sail tracks
and continuous metallic tracks on the mast or boom should
be interconnected (bonded) and grounded.
- Small
boats can be protected with a portable lightning protection
system. This would consist of a mast of sufficient height
to provide the cone of protection connected by a flexible
copper cable to a submerged ground plate of at least one
square foot. When lightning conditions are observed in the
distance, the mast is mounted near the bow and the ground
plate dropped overboard. The connecting copper cable should
be fully extended and as straight as possible. The boaters
should stay low in the middle or aft portion of the boat.
Thunderstorms in Florida and over its coastal waters are frequently
unpredictable. Even with the best weather reports, along with
constant and accurate observations of climatic conditions, boaters
can still be caught in open waters in a thunderstorm. Then,
with or without a lightning protective system, it is critical
to take additional safety precautions to protect the boat's
personnel. These precautions during a thunderstorm are:
- Stay
in the center of the cabin if the boat is so designed. If
no enclosure (cabin) is available, stay low in the boat.
Don't be a "stand-up human" lightning mast!
- Keep
arms and legs in the boat. Do not dangle them in the water.
- Discontinue
fishing, water skiing, scuba diving, swimming or other water
activities when there is lightning or even when weather
conditions look threatening. The first lightning strike
can be a mile or more in front of an approaching thunderstorm
cloud.
- Disconnect
and do not use or touch the major electronic equipment,
including the radio, throughout the duration of the storm.
- Lower,
remove or tie down the radio antenna and other protruding
devices if they are not part of the lightning protection
system.
- To
the degree possible, avoid making contact with any portion
of the boat connected to the lightning protection system.
Never be in contact with two components connected to the
system at the same time. Example: The gear levers and spotlight
handle are both connected to the system. Should you have
a hand on both when lightning strikes, the possibility of
electrical current passing through your body from hand to
hand is great. The path of the electrical current would
be directly through your heart--a very deadly path!
- It
would be desirable to have individuals aboard who are competent
in cardiopulmonary resuscitation (CPR) and first aid. Many
individuals struck by lightning or exposed to excessive
electrical current can be saved with prompt and proper artificial
respiration and/or CPR. There is no danger in touching persons
after they have been struck by lightning.
- If
a boat has been, or is suspected of having been, struck
by lightning, check out the electrical system and the compasses
to insure that no damage has occurred.
- Boating
in Florida's waters is an enjoyable activity for many people.
Keep it that way!
- Listen
to the weather reports! Learn to read the weather conditions.
Heed these reports and the conditions. Stay off or get off
the water when weather conditions are threatening.
- Install
and/or maintain an adequate lightning protection system.
Have it inspected regularly. Follow all safety precautions
should you ever be caught in a thunderstorm. By using good
judgment, it is less likely that first aid or CPR will
be needed while boating.
- National
Fire Codes. Lightning Protection Code--NFPA 78; Fire Protection
Standard for Motor Craft--NFPA 302, 14. National Fire Protection
Association, Batterymarch Park, Quincy, MA 02269.
- Standards
and Recommended Practices for Small Craft. Standard E-4,
Lightning Protection. American Boat and Yacht Council, P.O.
Box 806, Amityville, NY 11701.
- Sitarz,
Walter A. Boating Safety--Thunderstorms (MAP-5), Florida
Sea Grant College Program, University of Florida, Gainesville,
FL 32605.
This
publication, "Boating/Lightning Protection" (SGEB-7) replaces
"Boating Safety/Thunderstorms" (MAP-5), a Florida Sea Grant
bulletin published December 1978 and is acknowledged as a source
of information for this bulletin.
Disclaimer
and Reproduction Information: Information in NASD does not
represent NIOSH policy. Information included in NASD appears
by permission of the author and/or copyright holder. More
NASD Review: 04/2002
This
document,
SGEB-7
, was published May 1985 and last reviewed October 1992 by
the Florida Cooperative Extension Service. For
William J. Becker, Professor and Extension Safety Specialist,
Agricultural Engineering Department, Cooperative Extension
Service, Institute of Food
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