• TABLE OF CONTENTS
HIDE
 Introduction
 Thunderstorms
 Lightning
 Lightning interaction with...
 Technical aspects of the lightning...
 Personal safety
 Personal safety






Group Title: Florida Sea Grant Extension Bulletin SGEB-17
Title: Lightning and sailboats
CITATION PAGE IMAGE ZOOMABLE
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00073916/00001
 Material Information
Title: Lightning and sailboats
Alternate Title: Lightning & Sailboats
Physical Description: 24 p. : ill. ; 22 cm.
Language: English
Creator: Thomson, Ewen M
Florida Sea Grant College
Publisher: Florida Sea Grant College Program, University of Florida
Place of Publication: Gainesville Fla
Publication Date: [1992]
 Subjects
Subject: Lightning protection   ( lcsh )
Sailboats -- Safety measures   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: Ewen M. Thomson.
General Note: "SGEB-17"--Cover.
General Note: "A Florida Sea Grant College Program Publication"--Cover.
General Note: "July 1992"
General Note: "Sea Grant Project No. R/MI-10 ; Grant No. NA89AA-D-SG053"
Funding: This collection includes items related to Florida’s environments, ecosystems, and species. It includes the subcollections of Florida Cooperative Fish and Wildlife Research Unit project documents, the Sea Grant technical series, the Florida Geological Survey series, the Coastal Engineering Department series, the Howard T. Odum Center for Wetland technical reports, and other entities devoted to the study and preservation of Florida's natural resources.
 Record Information
Bibliographic ID: UF00073916
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: aleph - 001981603
oclc - 32207444
notis - AKF8534

Table of Contents
    Introduction
        Page 1
    Thunderstorms
        Page 2
    Lightning
        Page 2
    Lightning interaction with a sailboat
        Page 3
        Page 4
    Technical aspects of the lightning protection system
        Page 5
        Page 6
        Page 7
        Page 8
    Personal safety
        Page 9
    Personal safety
        Page 9
Full Text




SGEB-17
F UNIVERSITY of

UF FLORIDA
IFAS Extension



Lightning & Sailboats1


Ewen M. Thomson2


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.
A VHS copy is available for $15 from: Florida Sea
Grant, University of Florida, P.O.Box 110409,
Gainesville, FL32611

Make checks payable to the University of Florida.

E.M.Thomson, A Critical Assessment of the U.S.
Code for Lightning Protection ofBoats, Institute of
Electrical and Electronic Engineers Transactions on
Electromagnetic Compatibility, Volume 33, Number
2, pp. 132-138,1991. Available online:
http://www.thomson.ece.ufl.edu/lightning/IEEE.pdf
The copyright is owned by IEEE. It deals with some


1. This document is SGEB-17, one of a series of the Sea Grant Department, Florida Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida. Original publication date July 1992. Reviewed July 2006. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.
2. Associate Professor, Department of Electrical Engineering College of Engineering, University of Florida, Gainesville, Florida 32611

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and
other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex,
sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service,
University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry
Arrington, Dean







Lightning & Sailboats 2


problems in the code for lightning protection of boats
that existed at that time and suggests corrections.


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.


; ; ; 1 4 I ; MILE.


TIME


Figure 1. Thunderstorms ranging using time to thunder.

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 borne 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 electrified 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


10-. MILES







Lightning & Sailboats 2


problems in the code for lightning protection of boats
that existed at that time and suggests corrections.


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.


; ; ; 1 4 I ; MILE.


TIME


Figure 1. Thunderstorms ranging using time to thunder.

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 borne 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 electrified 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


10-. MILES






Lightning & Sailboats 3


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 protection
system 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.







Lightning & Sailboats 4


Figure 2. Lightning attachment to a sailboat.

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


100

00 ^^ .......... ......... ........................................

Boaxt
60- *---

40-
30

10-

0 1 2 3 4
Hul DOmage Index

Figure 3. Proportion of boats struck by lightning suffering
hull damage of varying degrees.

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.

Table 1. Severity of hull damage.

Hull Damage Type of hull damage
Index
0 No hull damage

1 Small non-leaking cracks
or burns


f Protrcon, Sah No Protetion, Sae
Protection Fresh No ProtecIon, Fresh







Lightning & Sailboats 5


Table 1. Severity of hull damage.


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


Figure 4. Possible effects of a lightning strike to an
ungrounded boat.

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


2 Small holes that did not
leak seriously
3 Holes larger than 1/4 inch
diameter above waterline
4 Holes larger than 1/4 inch
diameter below waterline






Lightning & Sailboats 6


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







Lightning & Sailboats 7


Figure 5. Effects of lightning strike to a grounded boat.

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 stem pulpits, antenna bases, the
ground wire for the electronics, etc.


Figure 6. Lightning effects on an unbonded boat.


Figure 7. Lightning effects on a bonded boat. Note: Being
in contact with a wheel or tiller during a lightning strike is
extremely hazardous, even in a grounded, bonded boat.

The illustrations in Figure 6 and 7 show what
happens on board a bonded (Figure 7) and unbonded
(Figure 6) 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 7 would not be smiling if he
had one hand on the tiller and the other on the engine







Lightning & Sailboats 8


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 8 that
shows the percentages of boats with electronics
damage of different magnitudes.


go ......... ............. ............................ .........
8. .......................................... ........................


0 ". ........................ .. .................
O. ...........................


2. ........................
10 ................. .

Noer Som AN
Elemn [Mmran u Index

Figure 8. Proportion of boats struck by lightning suffering
electronics damage of varying degrees.

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 modem 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


P-dubteeon SAIl No POdMd~Wn, Sat
PodUWsbOn. Fh No Pmdot~fio. F.h






Lightning & Sailboats 9


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 Project No. R/MI-10 Grant No.
NA89AA-D-SG053

Florida Sea Grant College Program Bldg. 803 -
University of Florida, Gainesville, Florida 32611


This publication was supported by the National
Sea Grant College Program of the US. Department of
Commerce s National Oceanic and Atmospheric
Administration (NOAA), Grant No.
NA89AA-D-SG053. The views expressed are those of
the authors and do not necessarily reflect the view of
these organizations. Additional copies are available
by contacting Florida Sea Grant, University of
Florida, PO Box 11040), Gainesville, FL,
32611-040), (352) 392.2801, www rllo,,,it ,,rg.






Lightning & Sailboats 9


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 Project No. R/MI-10 Grant No.
NA89AA-D-SG053

Florida Sea Grant College Program Bldg. 803 -
University of Florida, Gainesville, Florida 32611


This publication was supported by the National
Sea Grant College Program of the US. Department of
Commerce s National Oceanic and Atmospheric
Administration (NOAA), Grant No.
NA89AA-D-SG053. The views expressed are those of
the authors and do not necessarily reflect the view of
these organizations. Additional copies are available
by contacting Florida Sea Grant, University of
Florida, PO Box 11040), Gainesville, FL,
32611-040), (352) 392.2801, www rllo,,,it ,,rg.




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