[Rhodes22-list] Lighting Rod

[John Lock]

Hi folks,

Time for my biennial lightning re-post from the past.  This info is 
getting pretty old and links may not be valid any longer, but I think 
the overall message is still useful:

JULY 2006 -

I already did some research on the subject because it concerned me when
the sailing bug first bit (not long ago).  Here are some salient facts
and observations that I have found valuable - YMMV.

First, there are two schools of thought on adding lightning protection
to your boat:

1) Lightning is a random and poorly understood phenomenon.  Trying to
avoid or control it is probably futile and the results will be random
and poorly understood.

2) Doing something is better than doing nothing and maybe it will help.
Besides it's a cool project.

I suspect that both points of view have merit and which one you
subscribe to probably says more about your personality than your
technical skills

SOME INTERESTING FACTS _

* Boats in saltwater are more likely to be struck than boats in
freshwater, due to saltwater's higher conductivity.  However, boats
struck on freshwater are more likely to be severely damaged due to the
higher current loads in the strike itself.  (All this being relative to
the small likelihood of getting hit in the first place.)

* Powerboats are potentially more dangerous in a storm than sailboats,
because their lower profile means a greater amount of current is needed
to make a strike.  So, if you're out in a typical fiberglass runabout
and get struck, poooof.

* Being caught out in a sailboat during a lightning storm does not mean
you are going to be struck.  There are many accounts of people
witnessing water strikes very near their boats.  Many other factors are
involved in setting up a lightning strike.

* Boats with lightning protection systems "may" be more likely to be
struck, but experience less damage.  There only seems to be anecdotal
evidence of this, but the theory seems sound.  That is - if you give
lightning somewhere to go, it may hit you first, but be dissipated more
readily (see more on this further down).

SOME MYTHS TO BE DEBUNKED -

"Mooring your boat among boats with taller masts will protect you"

Lightning is seeking it's best path to ground.  Height (or the distance
of the "air gap") is only one factor.  Other factors - such as mast/keel
composition, deck or keel stepped masts, presence of other grounding
objects near the waterline, etc - will ultimately decide the lightning
path.  For example, a lead-keeled, keel-stepped boat may be more likely
to be struck than a deck-stepped, centerboard boat with a taller mast.
And you can't survey all those boats you've parked amongst, so it's
false security.

"Clamping jumper cables on a shroud and dangling the other end in the
water is good enough"

While that sounds good on the surface, it is in fact a very bad idea.
The problem is that you are depending on relatively small surface areas
to conduct a helluva lot of current.  The connection points between the
shroud and the mast and the jumper cable clamp and the shroud are not
sufficient to conduct the amount of current a strike produces.  However,
you have increased the likelihood of a strike by providing a grounding
path.  I would strongly discourage this practice. (There is also a
similar method, which involves wrapping the anchor chain around the
mast.  Same problem.)

"Adding a good lightning protection system will protect me and my boat"

Well, maybe...  There is at least one documented case of a
well-protected boat being sunk by a strike.  The mast and all the
shrouds were grounded via heavy copper cable to a copper plate epoxied
onto the bottom of the hull.  However, there was some moisture behind
the plate.  When the strike occurred, that moisture was instantly
vaporized into steam and exploded the plate off the hull (with obvious
results).

SOME GOOD IDEAS IF YOU PLAN TO ADD A LIGHTNING SYSTEM -

* Use nothing but heavy-gauge (#4 or larger) copper conductors.  All
other materials will corrode or provide inferior conductivity.

* Keep all leads as straight as possible.  Any sharp bends or kinks will
defeat the purpose.

* Provide lots of contact surface.  Snaps, hooks, turnbuckles, etc. will
not conduct the current loads you get in a typical strike.  Use large
connecting plates, bolts, and flat washers, clean connecting surfaces
and seal from weather.

* If you have a system installed, don't do anything to defeat it if
you're caught in a storm.  For example - don't hold onto the backstay
while you pull up the swim ladder or fiddle with the outboard.  You may
involuntarily become an integral part of the lightning system (as Bill
E. so eloquently described )

VARIOUS LIGHTNING PROTECTION SYSTEMS -

There are basically three commercially available systems in use at
varying costs and perceived effectiveness.  Again, this assumes that you
subscribe to the "something is better than nothing" school of thought -

1) Complete grounding systems - the mast, shrouds, motor, electronics
and any other conductive materials are wired into one or more grounding
leads, which go thru the hull to a flat copper plate affixed to the
exterior.  Yep, that means you have to drill one or more holes to bring
the conductor thru and (as shown in the example above) must be mounted
with great care to eliminate all possibility of moisture behind the
plate.  These systems are usually professionally installed, custom
designed for each boat, and cost mucho bucks.  See
http://www.marinelightning.com/Information/GroundingGuide.htm for some
details on this.

2) Static dissipators - These are like inverted stainless steel "whisk
brooms" attached to the top of your mast.  The theory is that the many
small metallic points offered by the strands of the device will
dissipate charges gradually as they build up, rather than allowing
potentials to increase to the level of a full strike. There seems to be
little evidence that this actually works, since it's supposed to prevent
a strike.  So... you could say if you don't get hit, it must be
working!  They are cheap and have the added benefit of keeping birds off
your masthead.  See example at http://www.yachtgard.com/lightning.html 
<http://www.yachtgard.com/lightning.html>

3) Mast grounding systems - These work on the same principal as #1
above, except the focus is entirely on the mast, rather than the whole
boat.  The idea being that if lighting strikes the mast (most likely
point), we should give it somewhere to go before it can cause any
damage.  In concept, this is similar to the "jumper cable" method
mentioned earlier, but approaches the problem in a more realistic
manner.  A large copper conductor is bolted to the mast and attached to
heavy copper cable, which can be removed and attached when needed,
leading into the water.  The water-end usually has some kind of device
attached to increase its surface area in contact with the water.  See
http://www.strikeshield.com/ for a commercial example.

There are many online resources on lightning and boats, protection
systems, theories, rumors, innuendo... hey, after all it IS the Internet
    Try a search on "lightning protection for sailboats" and you'll get
plenty to confuse you further.

-- 
Cheers!
John Lock
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s/v Pandion - '79 Rhodes 22
Lake Sinclair, GA
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