[Rhodes22-list] Lighting Rod

[James Nichols]

Just a little math to show what I am referring to.

Air is about 3000V/mm or about 3,000,000V/M.  Assuming 10 meters of aluminum
mast is perfect conduction just for ease, but in reality aluminum is only a
reasonable conductor used mostly for its durability rather than its ability
to conduct.  That means the lightning can bypass 30,000,000 volts worth of
air passage.  However, once it reaches the fiberglass hull, 1 meter of
fiberglass is requires 20,000,000 V to transverse, and with the hull's
circumference to the waterline being about 9 feet (or 3 meters) that means
there is 60,000,000 V increased requirements to pass through the hull.

If the direct line distance from the tip of the mast to the water line is 12
meters, we're talking about 36,000,000 V to traverse that leap.
The same distance through the mast and then hull requires the lightning to
use 60,000,000 V.  

So it just makes sense that the lightning won't pass through the hull unless
there is something wrong with the integrity of the hull.

A more likely scenario for a boat in well maintained condition would be that
the lightning might use the mast to bypass 10 meters of air, then make a
leap into the air again at the base of the mast around the side of the boat
to the water, saving a net of 7 meters of air passage.  I could see the mast
being seriously damaged in that situation and probably the connection to the
hull at the foot of the mast as well.

-----Original Message-----
From: rhodes22-list-bounces at rhodes22.org
[mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of James Nichols
Sent: Monday, March 10, 2014 10:55 PM
To: 'The Rhodes 22 Email List'
Subject: Re: [Rhodes22-list] Lighting Rod

There is something else that is shorting out the route between the mast and
the water, not the length of the mast.  High power lines run at 120kV and
only have a meter or two of glass insulating them from the 50 meter steel
tower that the lines are attached to.  The lines and the tower are vastly
larger than the glass insulators so there is no possibility that the 3+
meters (circumference because the lightning will try and travel on the skin
of the material first) of fiberglass on the boat is more than enough to
insulate the mast from the water.

The problem is most likely related to the metal fittings that pierce the
skin of the hull from the mast hull and allow the lightning to get to the
core.  Which if it is like my boat, my core is saturated with water from a
poor repair job on the forward hatch.  That linked with other imperfections
in the hull could lead to lightning finding a path through the boat, however
a well maintained boat that has not had water penetrate into the core would
most likely not have any problems insulating the mast from the water
sufficiently.

Which on a side note, the '71 Rhodes has a balsa core and not a plywood core
like I had originally thought.  I've got most of inside fiberglass skin
peeled off the forward deck on my boat and I'm getting ready to go back with
some nice waterproof ply and oak strips to frame and reinforce the forward
deck.  I'll upload pictures on my previous thread when I get a little
further along.

James

-----Original Message-----
From: rhodes22-list-bounces at rhodes22.org
[mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of Ron Lipton
Sent: Monday, March 10, 2014 10:33 PM
To: The Rhodes 22 Email List
Subject: Re: [Rhodes22-list] Lighting Rod

I got interested in this because I know that lightning will punch through
fiberglass hulls. The dielectric strength (breakdown voltage) of dry air is
about 3000 V/mm. The dielectric strength that I could find for fiberglass is
about 20000 V/mm.  So fiberglass (at least the stuff used in PC boards) is
only about a factor six or seven better than air.  Humidity actually appears
to increase the breakdown voltage a bit. As a rough estimate I would guess
that a 10 meter mast "shorts out" the equivalent of 1 meter of fiberglass,
much thicker than the hull (the number seems too large to me-I will check
some more). So if given the choice the lightning will proceed through the
mast and fiberglass rather than the air. That explains why lightning will
often punch a hole in the fiberglass on the bottom of a boat, sinking it,
rather than find a path outside. 

I am pretty sure fields generated by the motor have no effect. 

Ron

> On Mar 10, 2014, at 21:42, "James Nichols" <jfn302 at yahoo.com> wrote:
> 
> Typically Lightning will not strike anything that does not give it a 
> clear path to ground, so running a wire from the mast to the water is 
> only inviting lightning to strike the boat more often.  Lightning, 
> like water, always looks for the easiest path to ground.  Fiberglass 
> is an awesome insulator,  so the mast doesn't typically attract 
> lightning.  Also, while lake and sea water conduct electricity (sea 
> water is a much better conductor because of the salt) Lightning on a 
> lake setting will more likely hit a tree on the shoreline because the 
> path through the tree into the earth is much easier than the path 
> through the boat, through the water, then into the ground.  Also, 
> while most lightning happens during a storm, so the fiberglass on the 
> boat will be wet, the amount of water clinging to the deck doesn't 
> usually offer enough of an electrical pathway from the mast to the 
> main body of water  that the lightning would choose to strike the boat
over striking the water directly.
> 
> All these statements aside though, as Ron said, stay away from large 
> metal objects on the boat during a storm, and you should be fine.
> 
> As a side note about the quote about deaths of boaters but not sailboats.
> The boats that have deaths are because they are running motorboats 
> that have large engines that create large electro-magnetic fields to 
> drive the motor, and they tend to run them all out, so the 
> electro-magnetic field is as large as it can get, and as they are 
> screaming across the water, what they don't realize is they are 
> screaming,
"STRIKE ME!!"  And nature says, "Ok."
> 


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