[Alberg30] Lightning protection

George Dinwiddie gdinwiddie at min.net
Fri Mar 15 07:59:54 PST 2002

I pretty much agree with Michael.  I did a bunch of research on the
subject some years back, and there's a lot of conflicting info and
misinformed opinions available.

Let's consider the different risks.

Direct strike:  A direct strike contains enough energy to lift
the QE-II out of the water.  If that strike conducts through
your mast to ground, it will surely melt the mast.  In reality,
you can't put a heavy enough conductor on the boat to carry 
this much current.  If you could, you couldn't put a large
enough conductive surface on the boat to transfer the current
to the water.

Side discharge from a nearby strike:  This could theoretically
be handled if the size of the current is low enough, but I doubt
that it can in practice.  The amount of energy is still huge,
and it acts very much like radio frequencies because of the 
sharp rise time of the voltage.  This means that the current
tends to travel on the surface of conductors and it doesn't
like to make turns.  If the current is low enough, the
situation is similar to the next category.

Induced current from a nearby strike:  The flow of current 
produces an electromagnetic field around it.  This field
will build quickly as the bolt passes and dissipate quickly
after it.  The change in magnetic field will generate
electricity in any conductor within the field.  This is 
the principle that makes your alternator and transformers
work.  If that electricity has a path to flow, it will do
so.  The current can be quite large.  For the wiring in
your boat, the path will likely be through your electronics
and lights, to the negative connection on the engine block
and then out the shaft to the water.  Your insurance will
likely pay for new electronics.  There is no scheme that 
will protect it yet still have it available for use.  (You
could put it inside a metal box with no wires coming out.
I've known people to store a backup GPS that way for use
after a lightning event offshore.)  Depending on the current
and your luck, the engine may still work.  This is less
likely if you've converted to electronic ignition, I believe.
If you're unlucky, the current will be enough to damage or
destroy the stuffing box and sink the boat.

In high-current cases, if the through-hulls are connected
to ground either for bonding against galvanic corrosion or
in an attempt to provide a lightning ground, they could be
blown out of the hull.  If they are not connected, it's
possible for the electricity to take a myriad of paths
that you would not normally consider conductive.  This 
has been known to leave a hull full of pinholes and 
subsequently full of water.  You pays your money and you
takes your chances.

If you are trying to direct the current flow to ground
by providing a designated path, you'll have to ensure
a large conductor, a straight path, and a large surface
to discharge the current into the water.  From memory,
I believe that a #6 copper wire (in good shape) and a
2-square-foot copper plate are considered the minimum
useful size.  I also believe that they're the maximum
useful size, as if this is not enough, you're not likely
to succeed anyway.  In spite of this, the current may
spurn the designated path for a reason that you cannot

Static electricity:  There is a school of thought that
says that the buildup of static electricity at the top
of the mast tends to attract lightning strikes.  This
implies that you should bleed off this charge, so the
top of the mast is nearly the same potential as the 
water around you.  This leads people to mount air terminals
(either a single sharpened point or one of those "bottle
brush" commercial jobs) with a conductor to the water.
Here the current capacity is less important, but the
conductance (inverse of resistance) is.  A corroded
connection can render the whole thing useless.

As Michael says, there is another school of thought that
says that this air terminal is likely to attract a
lightning strike, as it provides a similar potential
closer to the other end of the lightning bolt than is
the surrounding water.  I believe that both of these
cases are true, in different circumstances.  It's quite
difficult to do repeatable experiments in lightning 
protection.  There's a lot of literature pro and con.
Much of the studies in favor of the air terminal has
been paid by those with a financial interest in them.

The company that makes the "bottle brush" offers to
pay the deductible of your insurance if you are struck
by lightning and sustain damage.  This really says
nothing about the effectiveness of the unit.  It's
a sucker bet.  If you pay me the same amount, I can
give you the same terms without requiring any hardware.
The odds of lightning strike are low enough that this
will always pay off if you bet enough people.

I've never made up my mind on the advisability of mounting
an air terminal.  Therefore I've taken the path of least
resistance and done nothing.  If you live outside of
southern Florida (where lightning strikes are 100 times
more prevalent than most of the US and Canada), then
the odds are very much against having your choice be

 - George

  George Dinwiddie                             gdinwiddie at alberg30.org
  The gods do not deduct from man's allotted span those hours spent in
  sailing.                                    http://www.Alberg30.org/

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