Jere Lull wrote:


No, no, No, NO, *NO*! Find something to occupy those idle hands that
will likely add positive survival probability.

The thing I've learned in this very interesting thread on via Google is that
lightning will follow every path; not just the path of least resistance.
There is so much current that even a small fraction can do enormous damage.
When it gets to the end of an ungrounded conductor, it's going to go
somewhere. The approach of leading it down the stays might work with a
non-conductive mast that didn't have any wiring in it but, as Ian points
out, when the largest conductor on the boat just ends either right above the
heads of people huddled inside (in the case of a non-conductive support
pillar) or at a non-grounded keel, bad things are going to happen.

There seems to be an inconsistency in the historical fear level of marine
lightning and current statistics. I suspect this is due to two main
reasons. First, up until about 40 years ago, the typical vessel had a
wooden mast with outside chainplates that lead near the waterline. This is
far from effective protection but may actually be as good as can be obtained
with secondary grounding of a metal mast. Second, boating in Florida and
other high strike probability areas has become vastly more common in the
same time period.

Most of the strikes I have heard of anecdotally in this part of the world
have only resulted in electronics wipe out. I've never heard of a sinking or
fatality in a sailboat in New England. Strikes clearly vary in intensity.
Some would probably sink a boat with a 4" diameter solid copper conductor
running to 50 square feet of ground plate. There is a huge probability
factor at work here.

The Sea Grant study

http://www.thomson.ece.ufl.edu/lightning/

showed that 75% of Florida boats struck in salt water suffered no hull
damage, and less than 10% had watertight integrity breaches, a large
proportion of which were survivable. These translate into pretty good odds
for a boat operating in northeast waters or only in Florida during the dry
season.

In view of the difficulties doing anything clearly effective on my boat, I'm
now tending towards your quoted statement. There is a big element of "fun
for it's own sake" in these projects. I enjoy watching weather and
thunderstorms and that enjoyment would be increased by a lower anxiety level
about a strike. However, similar money and effort spent on similarly
interesting projects would probably increase the overall safety of my boat
more than grounding the mast.

If lighting wants to go in a straight line to large masses of metal, my mast
is probably somewhat grounded anyway. There is a lot of lead down there and
the keel is quite wide. Side flash would probably go down into that large
mass and spread out below the top of the encapsulation. There would still
be major flashes around inside the boat but it sounds like there would be
with any expensive and complex grounding plates I added as well. My
grounding scheme might well just attract the charge towards the thin part of
the hull. Damage down in the encapsulation would probably be major but
there would be so many paths that it probably wouldn't result in
catastrophic leaks. Leaking would take care of any fire that resulted in
the ballast area.

A small blood clot in a heart artery is probably an order of magnitude
greater in probability than a boat sinking strike in this part of the world
so perhaps I should just have two asprin and call back in the morning.

--
Roger Long

On Tue, 11 Nov 2008 07:46:38 -0500, "Roger Long"
wrote:

Jere Lull wrote:


No, no, No, NO, *NO*! Find something to occupy those idle hands that
will likely add positive survival probability.

The thing I've learned in this very interesting thread on via Google is that
lightning will follow every path; not just the path of least resistance.
There is so much current that even a small fraction can do enormous damage.
When it gets to the end of an ungrounded conductor, it's going to go
somewhere. The approach of leading it down the stays might work with a
non-conductive mast that didn't have any wiring in it but, as Ian points
out, when the largest conductor on the boat just ends either right above the
heads of people huddled inside (in the case of a non-conductive support
pillar) or at a non-grounded keel, bad things are going to happen.

There seems to be an inconsistency in the historical fear level of marine
lightning and current statistics. I suspect this is due to two main
reasons. First, up until about 40 years ago, the typical vessel had a
wooden mast with outside chainplates that lead near the waterline. This is
far from effective protection but may actually be as good as can be obtained
with secondary grounding of a metal mast. Second, boating in Florida and
other high strike probability areas has become vastly more common in the
same time period.

Most of the strikes I have heard of anecdotally in this part of the world
have only resulted in electronics wipe out. I've never heard of a sinking or
fatality in a sailboat in New England. Strikes clearly vary in intensity.
Some would probably sink a boat with a 4" diameter solid copper conductor
running to 50 square feet of ground plate. There is a huge probability
factor at work here.

The Sea Grant study

http://www.thomson.ece.ufl.edu/lightning/

showed that 75% of Florida boats struck in salt water suffered no hull
damage, and less than 10% had watertight integrity breaches, a large
proportion of which were survivable. These translate into pretty good odds
for a boat operating in northeast waters or only in Florida during the dry
season.

In view of the difficulties doing anything clearly effective on my boat, I'm
now tending towards your quoted statement. There is a big element of "fun
for it's own sake" in these projects. I enjoy watching weather and
thunderstorms and that enjoyment would be increased by a lower anxiety level
about a strike. However, similar money and effort spent on similarly
interesting projects would probably increase the overall safety of my boat
more than grounding the mast.

If lighting wants to go in a straight line to large masses of metal, my mast
is probably somewhat grounded anyway. There is a lot of lead down there and
the keel is quite wide. Side flash would probably go down into that large
mass and spread out below the top of the encapsulation. There would still
be major flashes around inside the boat but it sounds like there would be
with any expensive and complex grounding plates I added as well. My
grounding scheme might well just attract the charge towards the thin part of
the hull. Damage down in the encapsulation would probably be major but
there would be so many paths that it probably wouldn't result in
catastrophic leaks. Leaking would take care of any fire that resulted in
the ballast area.

A small blood clot in a heart artery is probably an order of magnitude
greater in probability than a boat sinking strike in this part of the world
so perhaps I should just have two asprin and call back in the morning.

Lightning can just as easily strike the fiberglass hull at the same
time as it hits the mast, as it heads towards the water below it.
Lightning, as I have mentioned, has no brains. Who says it is more
likely to score a bulls eye on the top of the mast, just because it's
the highest point? It doesn't have that kind of accuracy, and it's not
a thin "arrow" of energy.

I've also heard of boats being struck and left with a myriad of
pinholes, rather than any large openings.

On Nov 11, 7:46 am, "Roger Long" wrote:
If lighting wants to go in a straight line to large masses of metal, my mast
is probably somewhat grounded anyway. There is a lot of lead down there and
the keel is quite wide. Side flash would probably go down into that large
mass and spread out below the top of the encapsulation.

It's not that lightning wants to go straight. It's also not about
resistance. In an earlier question, you asked if a heavier gauge wire
would help. No. The concept is called wire impedance. Increasing
that 8 AWG wire to a heavier gauge does little to decrease impedance.
Shorter wire length - not wire diameter - makes better wire
conductivity.

Bending a wire increases impedance. A quarter round bent wire is an
inductor. Basically zero inductance to electricity such as 60 Hertz
AC. But a massive inductance to lightning.

How much lightning current can an 18 AWG lamp cord wire carry?
Something less than 60,000 amps. Lightning typically is only 20,000
amps. So we run larger 6 or 8 AWG wire to make it sufficient for even
largest lightning.

Routine is to have lightning strikes with no damage and no knowledge
that the lightning even struck. But that means some simple grounding
concepts as discussed in that article. If electronics are damaged,
well, electronics made a lower impedance connection to water; the
damage is how a weakness in that grounding is located and corrected.

Somewhere earlier, you worried about a 6" radius verses 8". Well,
that bend is an inductor trying to stop lightning currents. If
lighting does not travel through that bend, then what wire closer to
the cloud will arc to water (due to a sharper bend closer to water)?
IOW you are worrying about a minor thing. If that eight inch bend is
only feet from the grounding plate, then lightning will still go to
the grounding plate; not through the hull.

I did not see all posts. However there should have been a caution
somewhere about keeping those 8 AWG ground wires well separated from
all other wires. Even factory installers often don't understand this
concept which is why electronics damage occurs. If a ground wire is
bundled with other wires, then lightning induced surges is now on
those other wires (just another in a long list of reasons why plug-in
protectors also don't protection in the home).

Not having metal items bonded to that plate is the worst thing you
can do. Even simple lamp cord can conduct lightning because lightning
does not contain the high energy content so often assumed in myths.
How lightning gets to water is equivalent to "a battle is lost for the
want of a nail". It may not be the best, but it still may conduct
that current non-destructively into water.

One final point. In shallow water, lightning is seeking earth
beneath that water. Water is actually a less conductive material.
Lightning may even pass through the hull rather than use that ground
plate if bottom is closer to some other part of the hull. Just
another reason why we prefer that ground plate to be deeper; closer to
the bottom when in the shallows.

If is quite routine to have a direct lightning strike without even
any appreciable indication that the strike occurred. Lightning
strikes more often without any damage than you might imagine. Do
make metallic items (mast, rails) bonded to that ground plate. Then,
where possible, improve that connection by eliminating sharp bends and
separation from other wires.

On 2008-11-11 07:46:38 -0500, "Roger Long" said:

In view of the difficulties doing anything clearly effective on my
boat, I'm now tending towards your quoted statement. There is a big
element of "fun for it's own sake" in these projects. I enjoy watching
weather and thunderstorms and that enjoyment would be increased by a
lower anxiety level about a strike. However, similar money and effort
spent on similarly interesting projects would probably increase the
overall safety of my boat more than grounding the mast.

Oh, thank you! I nearly had a heart attack before I got to that part.

Lightning's scary. I lived in Clearwater, would spend hours on the
causeway watching the light shows over the lightning capital of the
world (Tampa).

But there are funner things to do since it seems the commercial
products seem to attract strikes.

--
Jere Lull
Xan-à-Deux -- Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/

Jere Lull wrote:

But there are funner things to do since it seems the commercial products
seem to attract strikes.

I don't see a shred of evidence to support this. I think it more likely
that people who are on the water enough in frequent strike zones to be at
high risk install protection and therefore get struck more often simply
because they are at higher risk.

--
Roger Long

On 2008-11-12 05:25:05 -0500, "Roger Long" said:

Jere Lull wrote:

But there are funner things to do since it seems the commercial
products seem to attract strikes.

I don't see a shred of evidence to support this. I think it more
likely that people who are on the water enough in frequent strike zones
to be at high risk install protection and therefore get struck more
often simply because they are at higher risk.

My evidence is anecdotal only, primary one was one boat getting a
bottle brush installed by the factory team. Though the boat's mast was
relatively short compared to dozens of boats around it, it was the only
one hit -- a couple of weeks later. The device's insurance ensured they
paid nothing to get everything fixed, but they weren't able to get
enough of the systems up to use the boat that season.

Even land-based lightning rods have to be very carefully installed or
they attract strikes. (that's something I read in school, perhaps
connected to Ben Franklin.)

--
Jere Lull
Xan-à-Deux -- Tanzer 28 #4 out of Tolchester, MD
Xan's pages: http://web.mac.com/jerelull/iWeb/Xan/
Our BVI trips & tips: http://homepage.mac.com/jerelull/BVI/

Jeeezeus H Christ............what a thread!

This makes my worrying about a little rust seem like nuthin.

The more I learn about plastic boats the more I like steel.