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
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
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
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
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
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
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
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
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
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.
- 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
- 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
- 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
- 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
- Disconnect and do not use or touch the major electronic
equipment, including the radio, throughout the duration of the
- Lower, remove or tie down the radio antenna and other
protruding devices if they are not part of the lightning
- 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
- 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
- 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.
NASD Review: 04/2002
, 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