On Thu, 02 Jul 2009 12:42:43 -0400, Gene
wrote:

On Thu, 2 Jul 2009 09:38:49 -0700 (PDT), Loogypicker
wrote:

On Jul 2, 11:50*am, Frogwatch wrote:
On Jul 2, 11:42*am, Gene wrote:





On Thu, 2 Jul 2009 08:37:03 -0700 (PDT), Frogwatch

wrote:
On Jul 2, 11:23*am, Gene wrote:
On Thu, 2 Jul 2009 08:19:45 -0700 (PDT), Frogwatch

wrote:
I once did a calculation of lightning strike probability.......
What I do remember is that your probability of being struck went up
with the SQUARE of mast height so a mast or tuna tower twice as high
as another quadrupled your probability of being struck.
This number said nothing about the probability of a sailboat with
others being in a marina being struck.
So, after this calculation, I am very careful about sailing in
thunderstormy weather.

Uh.... is this some sort of justification of a low transom.....
--

Forté Agent 5.00 Build 1171

"Twenty years from now you will be more disappointed by
the things you didn't do than by the ones you did do.
So, throw off the bowlines. Sail away from the safe harbor.
Catch the trade winds in your sails.
Explore. Dream. Discover." * - Unknown

Grady-White Gulfstream, out of Oak Island, NC.

Homepagehttp://pamandgene.tranquilrefuge.net/boating/the_boat/my_boat.htm

No, its my way of avoiding work.
OK, I cannot help myself. *I looked up the strike density.year for
where I am, roughly 10 km2/yr multiplied by protected cone area of my
mast gives a roughly 1/1000 chance of being hit IF THE BOAT WAS ALL
ALONE. *This is NOT the same as the probabiluity of being hit if you
sail under a storm. That probability is much higher.

Man, are we feeling lazy today or what? If things suck that bad, I'm
guessing you could get a couple more distant places of PI before
lunch!
--

Forté Agent 5.00 Build 1171

"Twenty years from now you will be more disappointed by
the things you didn't do than by the ones you did do.
So, throw off the bowlines. Sail away from the safe harbor.
Catch the trade winds in your sails.
Explore. Dream. Discover." * - Unknown

Grady-White Gulfstream, out of Oak Island, NC.

Homepagehttp://pamandgene.tranquilrefuge.net/boating/the_boat/my_boat.htm

Assume a storm is 10 km X 10 km which is 10^8 m2. *If you sail under
said storm and stay, your (my) sailboat is then 10^-5 of the storm
area. *We have to guess the number of cloud to ground strikes for a
storm and multiply by it to get our probability for getting struck if
you insist on sailing into a thunderstorm. *I think this number will
come out to anywhere from .001 to .01.

CLOSE ENOUGH, now I can go back to work.- Hide quoted text -

- Show quoted text -

That's all assuming some things, of course. Lightning will take the
path of least resistance (most of the time!), and that path may alter
the odds.

That's certainly true, but I think it is a fair assumption that
sticking that 50' aluminum conductor into the air just made you the
number one candidate for "... path of least resistance...."

Eisboch had the real answer - it's all about differing potentials for
current flow. For that, you need to analyze more than just the number
of strikes per sq/ft. like ground conductivity, saline content, heavy
metals, etc. Additionally, in particular water, there's surface
temperature which is a part of the equation also. You also have to
account for ionization of the air particles, partial pressures of the
various components of gases in the atmosphere - yada, yada, yada.

Admittedly, sticking a 50 foot piece of aluminum into the air helps
the probability a little, but if it were the only variable, why do
sail boats in marinas with short masts (by comparison) get hit as
often as larger boats with taller masts?

I know when I had my towers, the number of strikes went down
dramatically when the heavy duty ground system was installed - a
copper wire grid that used bare #10 stranded welding wire - about a
mile of it I think.

A good grounding system is a must to allow for equalization of
potential.