Introduction
The sight of a jagged lightning bolt licking the not-too-distant
horizon undoubtedly gives rise to concerned thoughts in the minds of many
sailors. Few actually act on their thoughts. And very few understand the
phenomenon well enough to act confidently.
Questions abound - What do I do if a lightning
storm is approaching or on me? What happens when lightning strikes a boat?
Does a lightning protection system help? But if I do install a lightning
protection system, won't it attract lightning? How do I install a
protection system anyway? Other questions relate to lightning itself-
Does lightning go up or down? Do the light and thunder originate at the
same time? What causes thunder? Why does lightning sometimes flicker? What
dictates whether lightning will strike an object on the ground or water?
In this Bulletin, and in the Sea Grant video
"Lightning and Sailboats" we attempt to answer these questions. We describe
the physics of lightning at a layman's level, discuss how a lightning protection
system is supposed to work, and explain some of the technical details
necessary for the correct installation of a protection system. A more technically
oriented paper is published in the technical literature.
Thunderstorms
From the sailor's point of view, thunderstorms are
best avoided. There are several techniques that can be employed to
recognize a growing storm and track one that is moving in your direction.
The thunderstorm, or cumulonimbus cloud, is best recognized in its
forming stages by its tightly packed "cotton wool" appearance. This
occurs because a tremendous amount of energy is being released to
produce powerful convection inside and around the cloud. Of course,
if the thunderstorm is forming directly overhead the cotton wool appearance
will not be visible, only a gray overcast that slowly darkens and
eventually produces torrential rain, lightning and strong winds.
The first few flashes of lightning in a thunderstorm typically do
not reach the ground and may be completely invisible during daytime.
One way to determine what is going on in the area is with a cheap
AM radio. (Note: FM radios do not work nearly as well for lightning
detection.) The characteristic crackle that we call 'static' on an
AM radio is caused by lightning. A common problem in summer
is that there are too many storms within radio range, which may be
hundreds of miles. In order to lower the sensitivity of your radio
to distant storms, tune it to a local radio station, or, if the
signal is too strong, slightly off tune. Any loud static can then
be interpreted as a warning that things are charging up.
Once a thunderstorm starts to produce lightning that hits the ground
or 'ground flashes', these can be used to locate a thunderstorm.
One method is to track a collision course using a hand bearing compass::
if the bearing to the lightning does not change, on average, the
storm is heading your way and it is time to adjust your course. Another
method that works once the thunder can be heard is to count the time between
the light and the thunder. Since the light arrives almost instantaneously
and the thunder travels at a speed of 1/5 mile/second, this time divided
by five gives the distance to the lightning. For example, if the thunder
starts 30 seconds after the lightning, the flash is 6 miles away. See Figure
1. 
Note that
the thunderstorm is about 10 miles across and that ground flashes originate
anywhere inside the storm at a height of about 5 miles. Further, lightning
channels usually slant away from vertical and can even emerge from the
side of the storm (the classical 'bolt from the blue'). The danger to the
boat is obvious. That boaters frequently underestimate this danger is bome
out by those whose boats have been struck by lightning, a typical comment
being that there were no thunderstorms in the area just before their boats
were struck. Others signs of imminent lightning are even more obvious.
St. Elmo's fire and buzzing sounds off radio antennas arise when a boat
is in the large electric field directly below an electrifled cloud. Although
lightning may not yet have begun, its occurrence in the immediate vicinity
is exceedingly probable when these electrical phenomena are observed. Act
as if your boat is about to be hit by lightning, as described below.
Lightning
The only type of lightning that need concern sailors
is the ground flash, since lightning that does not reach the ground does
not damage boats. Ground flashes can be expected to hit from 4-20% of
moored sailboats per year in Florida. Cruising sailboats typically get
hit at least once in their lifetimes. The standing records for the total
number of strikes to a single boat is five (in Sarasota, Florida) and the
highest strike repetition rate is twice within ten seconds (in the Indian
ocean). The typical ground flash starts at a height of about 5 miles above
water, inside a region of the thunderstorm that is charged negatively.
The path, or channel, that eventually connects this negative charge to
ground begins here. As the channel extends towards ground during the 'stepped
leader' phase, negative charge is funneled from the cloud into a spark
channel. When the tip of the stepped leader is about 30-100 yards above
ground level, another spark, this time positively charged, is launched
from the ground. A massive amount of power is generated when this positively
charged attachment spark and the negatively charged stepped leader connect.
At this time the peak lightning current is generated, during the 'return
stroke'. Although cresting at ten thousand to hundreds of thousands of
amps, it only lasts for about a millionth of a second. Longer lasting currents
of a few hundred to a few thousand amperes may persist for much longer
times (on the short time scale of the lightning) during a 'continuing current'.
These long-lasting continuing currents are responsible for large heating
effects and are thought to be responsible for forest fire ignition. After
a short pause, subsequent leaders may reenergize the channel, followed
by more return strokes and, on occasion, continuing currents. A typical
ground flash has about three leader/return stroke sequences. Lightning
frequently appears to flicker because each return stroke lights up the
channel, and the time between them is sufficiently long enough to be seen
by the human eye.The return stroke heats up the lightning channel to a
temperature about six times as hot as the sun. This causes the surrounding
air literally to explode. We hear this explosion as thunder that appears
to last for much longer than the lightning, which is all over in less than
a second, because the lightning channel network covers several miles. The
speed of sound is only about 600 knots and so thunder from more distant
parts of the cloud arrives later than thunder from closer parts. The important
thing is that the light and sound are generated at the same time since
they are both caused by the return stroke.
Lightning
Interaction with a Sailboat
Attachment
As the negatively-charged stepped leader moves downwards,
it induces a positive charge on the ground below. When the tip of the leader
is about 30-100 yards above ground level, the induced positive charge becomes
so concentrated that a new spark forms at the ground, as shown in Figure
2. 
This positively
charged spark is the crucial process as far as the attachment to a boat
is concerned. If it starts at the tip of a boat mast, then lightning strikes
the mast. Unfortunately, there is no scientifically accepted technique
to prevent this spark from forming. Even if a device were effective in
diverting the attachment spark, it would not be a good idea to mount it
on the masthead as the attachment spark may start elsewhere on the boat
or crew. The likelihood of lightning attaching to the masthead is a safety
feature as far as the crew is concerned.
Consequently, lightning protection means minimizing
the damage caused by lightning in the event of a strike, rather than preventing
a lightning strike. In general terms, a protected boat is one in which
there is a continuous conducting path from the water to the mast tip. The
current needed to feed the attachment spark is conducted through the protectionsystem
from the water. That is, the path that the lightning takes in the boat
is forced to be that of the conductors in the protection system. If this
conducting path is not continuous, for example, in a boat which is not
well grounded, there is little difference as far as the top of the mast
is concerned. The attachment spark still begins there as this is where
the positive charges have concentrated. The difference is what happens
where the conducting path, the mast, ends. Since current cannot flow from
the ground to feed the growing attachment spark, a negative charge accumulates
at the base of the mast and eventually arcs across in the general direction
of the water or a nearby conductor. (For this exercise, crew members are
conductors!) The result is an unharnessed electrical discharge between
the bottom of the mast and the water.
According to the above argument,
the likelihood that lightning will strike a boat does not depend on whether
the boat is well grounded or not. There is some support for this in the
experiences of marine surveyors. Nine marine surveyors in Florida, each
of whom had surveyed more than 200 sailboats in their career, reported
that between 2% and 67% (on average 34%) of the boats they surveyed for
any reason had a lightning protection system. Of the boats that they surveyed
because of a lightning strike, they reported that between 0% and 67% (on
average 29%) had a protection system. While the individual estimates varied
widely between surveyors, there is no support for the argument presented
by some sailors that they should not ground 'their sailboat since
it will increase the chances of it being struck by lightning.
Sideflashes
Data obtained from sailors whose boats have been
struck by lightning are consistent with the above scenario: boats that
do not have a protection system do indeed suffer more damage. The type
of water, whether salt or fresh, is also important. Damage is much more
extensive for boats struck by lightning in fresh water than for boats struck
in salt water because fresh water is a worse conductor. Consequently, it
is much more difficult to design an adequate protection system for boats
in fresh water than for boats in salt water. Figure 3 summarizes these
data for a sample of 71 boats that were struck by lightning. 
The
bars show the percentages of boats in each category that received various
magnitudes of hull damage. The four categories were boats with/without
protection systems in salt/fresh water. The damage indices indicate the
severity of hull damage as shown in Table 1.
In boats with a hull damage of 2 or higher
the lightning had formed its own path(s) through the boat hull. If a lightning
protection system was present it malfunctioned. As the statistics show,
malfunctioning protection systems are very common in fresh water:
40% of protected boats in fresh water experienced this effect. The most
likely way that this happened was through the formation of "sideflashes".
These are sparks that form between the lightning protection system and
ungrounded conductors or the water. Basically, in order to dissipate a
lightning current in fresh water a much more extensive underwater grounding
system is needed than that usually found in 'protected" boats. This is
described in more detail below.
Technical Aspects of
the Lightning Protection System
Overview
Although lightning protection needs to be designed
on a boat-by-boat basis and ideally installed during manufacture, there
are three major considerations in a good protection system: (a) grounding,
(b) bonding, and; (e) electronics protection. The grounding system is intended
to provide an adequate conducting path from the point of lightning attachment,
usually the masthead, to a system of conductors in the water, without producing
sideflashes. The bonding system protects the crew and consists of conductors
that short out large metal fittings so that large voltages cannot develop
between them. Electronics protection limits power supply and transducer
voltages through a combination of transient protection devices and careful
wiring techniques.
Grounding
The idea of the grounding system is to divert the
lightning current through a predetermined path so that it does not make
its own explosive path through fiberglass, teak, crew members, etc. Figure
4 shows what can happen when lightning strikes an ungrounded fiberglass
boat with an aluminum mast.
The
lightning charges all of the rigging but no conducting path exists to channel
the charge into the water. The result is destructive sparks between the
lower parts of the rigging, such as the mast base and chainplates, and
the water. Wherever these sparks travel through bad conductors (fiberglass
hull, teak bulkheads, through-hulls, porta-potties, etc.) sufficient heat
is generated to explode the impeding material into a nicely conducting
plasma that is hotter than the surface of the sun.
The components of the grounding system are: (i)
an air terminal at the top of the mast; (ii) downconductors, and; (iii)
grounding conductors that are immersed underwater ("ground strips" or "ground
plates"). The air terminal is the point where the lightning is supposed
to attach, the down-conductors conduct the current from the air terminal
to below the water, and the grounding conductors dissipate the current
into the water without forming any sideflashes. Usually the aluminum mast
is connected in as part of the down-conductor network.
On a sailboat with a VHF radio, the masthead VHF
antenna usually serves as a sacrificial air terminal. In fact, one of the
first signs that lightning has struck a boat is typically that shards of
antenna material are scattered around the deck. The presence of a VHF antenna
or other expensive masthead transducers makes a separate air terminal highly
desirable, although this will degrade the performance of the VBF. The top
of the air terminal should be sufficiently high that the angle from it
to any other masthead object is less than 45 degrees. That is, the air
terminal provides a 'cone of protection' that attracts lightning (or, more
accurately, launches an attachment spark) preferentially to any other object
that is below a conical surface whose apex is on the top of the air terminal
and that has a 90-degree apex angle.
An aluminum mast is the preferred down conductor,
being a much better conductor than stainless stays. If the mast base is
on top of the cabin, a downconductor is needed to connect the mast base
to the ground strips. Use at least #4 gauge copper with preferably bimetallic
copper/stainless connections to prevent galvanic corrosion. Alternatively,
make a strong mechanical connection and additionally braze or solder, to
improve the electrical contact and lessen the chance of contact corrosion,
then paint with an insulating coating. A keel-stepped mast similarly needs
to be connected to the keelbolts with at least #4 gauge copper.
The ground strips in contact with the water should
be connected to the down-conductors with care to avoid galvanic corrosion.
In salt water a single grounding conductor of a square foot or more in
area is usually enough. In this respect, a lead keel connected to the down-conductor
via the keel bolts is adequate. If the lead is either painted or encapsulated
in fiberglass, minor repairs may be needed after a lightning strike. However,
the paint or fiberglass does not seriously compromise the ballast lead
as a lightning ground. Note that this system does not work in river mouths
where there may be a less dense layer of fresh water riding on top of a
salt water "wedge". The situation in fresh water is much more complicated
as the voltages involved during a lightning strike are about a thousand
times larger than those that occur on a boat in salt water. A good start
is to lay a flat or 'D' cross section strip of 3/4" x 1/8 ' stainless or
brass along the outside of the stem of the boat. Connect this to the forestay,
mast base, and backstay with #4 gauge vertical copper down-conductors.
However, this is not usually enough. In addition, extra ground strips are
needed just outside the hull close to metal fittings such as gas tanks,
metal-cored plumbing pipes, wiring, etc. Connect these to the grounding
system using near vertical down-conductors. Under no event should these
down-conductors run close to the hull except where they penetrate the hull
to connect to the grounding strip: otherwise the conductor may cause a
sideflash through the hull. The engine, propeller shaft, and propeller
should be regarded as part of the grounding system and tied in appropriately.
The manner in which a correctly grounded.boat reacts
to a lightning strike is illustrated in Figure 5. 
The
lightning charge that flows onto the rigging does not accumulate to the
point where it forms destructive sparks, as was the case for an ungrounded
boat. Instead, it is discharged into the water over a wide region. The
more evenly the charge can be discharged into the water, the less likely
it is that a sideflash will occur through the boat hull.
Bonding
The difference between the grounding system
and the bonding system is only one of degree since both are interconnected
and both will conduct current during a lightning strike. Whereas the grounding
system is designed to handle the full lightning current, the bonding system
consists of mainly horizontal connections between metal fittings to short
out any voltages that might otherwise develop. Bonding is a measure that
is intended to protect the crew and enable them to work the boat without
getting shocks. This can occur from nearby lightning as well as from direct
strikes. Smaller gauge conductors than the grounding system are adequate
in the bonding system, down to #8 gauge copper. As with the grounding down-conductor
connections, all bonding connections should be made to minimize galvanic
corrosion. Metallic fittings that should be bonded to the grounding system,
using horizontal connections as much as possible and avoiding the hull,
are toe rails, chain plates, steering wheels, motor controls, bow and stern
pulpits, antenna bases, the ground wire for the electronics, etc.
The illustrations in Figure 6 show what happens
on board a bonded (bottom) and unbonded (top) boat during a lightning strike.
On the unbonded boat large voltages develop between the mast, chainplates,
forestay, backstay, wheel, rudder post, toe rails, electronics, wiring,
metal reinforcing in plumbing fixtures, engine, etc. These make working
the boat extremely hazardous, even if lightning is not striking the boat
directly. On the bonded boat these voltages are shorted out by bonding
conductors. Note, however, that the large magnetic fields associated with
a direct lightning strike make the concept of an electrical "short" a misnomer.
Appreciable voltages can develop between the ends of long conductors even
if the conductors are connected together at their other end. The helm is
a particularly dangerous place owing to its proximity to the engine controls,
boom, rudder post and backstay. The helmsman in Figure 6 (bottom) would
not be smiling if he had one hand on the tiller and the other on the engine
controls, for example. (Note that he is steering with one hand in his pocket
to minimize the risk of making a connection between two conductors at different
voltages. This is not as safe as throwing over the anchor and going below!)
For stations such as the helm that are usually manned, it is crucial that
the bonding conductors should be kept as short and straight as possible.
Electronics
Electronics-killing overvoltages may be introduced through
the DC power wires, antenna input, or any other external connection such
as a lead to a transducer. Electronics on a small sailboat that are struck
by lightning are particularly difficult to protect since it is impossible
to divert the lightning current any appreciable distance away from" the
electronics. This difficulty, and the pervasive nature of electronics damage,
is illustrated in Figure 7 that shows the percentages of boats with electronics
damage of different magnitudes.
In this case there is less of a distinction between
boats struck in fresh water versus salt water as there was for hull damage,
but the same trend is evident: boats with protection systems in salt water
fare best and boats with no protection systems in fresh water fare worst.
More notably, 96% of all boats sustained damage to at least some electronics
items. Apparently a lightning protection system, as installed on the boats
in the survey, does not necessarily save the electronics. Note that for
these boats "lightning protection" merely meant that the boat was grounded,
not necessarily bonded with transient protection devices, as explained
below.
In order to protect electronics, more is needed
than merely diverting the current to ground (water) without its blowing
a hole in the hull. Due to the low voltages typically used in modern marine
electronics, just a few extra volts is enough to cause extensive damage.
However, techniques that are used to protect computers, cable TV and radio
equipment on land can also be used in shipboard DC and AC equipment. Some
devices are readily available from electronics stores. Radio antennas can
be protected using lightning arrestor hardware designed for cable TV. Connect
the "ground' connection to the lightning grounding network. AC transient
protection outlets or plug-in metal oxide varistors (MOV) work also on
boats but need to have their ground connections connected to the shore
ground wire. Ideally this ground should also be connected to the lightning
protection ground but this circuit arrangement can cause ground current
problems in marinas. As for protection of DC electronics, which are probably
the most important, transient protection devices are available to clamp
voltages at the point where each piece of equipment is connected to the
DC supply. These are available from companies such as General Electric
or from mail order electronics distributors. They can be found under the
generic name "Transient Suppressors" and are of various types: metal oxide
varistor, silicon avalanche diode, and surge suppressor zener diode. It
is important to locate this protection device immediately next to the equipment
and each piece of equipment should have its own device. The overvoltages
that appear at DC inputs can be reduced by using twisted-pair wiring in
wiring harnesses, ideally with a conducting sheath that is connected to
the bonding system. The overall philosophy here is to minimize the spacing
between positive and negative DC lines. If a main control center exists,
surround it with a conducting enclosure that is connected to the bonding
system. Through-hull transducers are especially vulnerable. Due to the
typically vertical alignment of the cables connecting these to their main
electronics, they should be regarded as being part of the lightning grounding
system. Since the wires used in these cables are of an insufficient thickness
to withstand a lightning strike, a #4 gauge copper wire should be placed
parallel to any cable that leads to a through-hull transducer. The top
of this copper wire should be reconnected to the lightning grounding system
and the bottom to a ground strip close to the underwater transducer on
the outside of the hull.
As with all aspects of lightning protection, 100%
effectiveness cannot be guaranteed, even if all the above measures are
taken for electronics systems. Disconnecting equipment in advance of a
storm helps isolate it from voltages induced by lightning, and the larger
the lead separation the better. Use disconnects in preference to knife
switches, and these in preference to switch panels.
Personal Safety
Consider the worst case scenario for a lightning strike
to a sailboat - a small boat in fresh water. If the boat has been provided
with a well-built protection system it is still an exceedingly hazardous
situation. If lightning protection does not exist, the situation is life
threatening. In both cases, the areas to avoid are close to the waterline
and close to large metal fitting. In the unprotected boat, an additional
-danger zone is beneath the mast or boom. Even in the unprotected boat,
it is unwise to get in the water, as electrocution is highly probable if
lightning strikes nearby. In fact, there is no safe place on an unprotected
small sailboat, and in a protected boat only places of relative safety.
There is, however, one place that is more hazardous than a small unprotected
sailboat, that is a small unprotected boat without a mast. Every year there
are multiple deaths of boaters in open boats caused by lightning strikes,
but very few reports of sailors in sailboats killed by lightning.
The above general rules also apply to larger sailboats. These are generally
safer, if protected, since it is possible to get away from the waterline
and large metal objects, and yet still stay dry inside the cabin. As far
as unharnessed electricity is concerned, a dry human body is much less
attractive than a wet one.
Conclusions
Lightning protection on a sailboat means diverting the
lightning current into the water without its causing any hull damage, personal
injury, or electronics damage. This involves providing a continuous, mainly
vertical, conducting path from above any vulnerable masthead transducers
to grounding conductors immersed in the water (the grounding system) and
a network of mainly horizontal interconnected conductors attached to large
metal fittings, including the grounding system (the bonding system). Transient
suppressors are needed on each piece of electronics equipment, and wiring
should all be twisted pair for protection of electronics.
| Florida
Sea Grant College is supported by award of the Office of Sea Grant,
National Oceanic and Atmospheric Administration, U.S. Department of Commerce,
grant number NA 89 AA-D-SGO53, under provisions of the National Sea Grant
College and Programs Act of 1966. This information is published by the
Sea Grant Extension Program which functions as a component of the Florida
Cooperative Extension Service, John T. Woeste, Dean, in conducting Cooperative
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