[Rhodes22-list] lightning protection
Roger Pihlaja
cen09402@centurytel.net
Tue, 19 Nov 2002 18:39:06 -0500
Guys,
I've been involved in the design & construction of several fairly tall
structures over the course of my engineering career. Some of these
structures were distillation towers, which were 200 - 300 feet tall & were
handling large quantities of flammable, reactive chemicals. So, we paid
quite a lot of attention to lightning protection in these designs!
The American Society of Civil Engineers (ASCE) has done quite a bit of
research into the effects of the height of a structure & grounding vs. the
probability of it being struck by lightning. We always used the ASCE design
database.
Since I was downsized from my job at the Dow Chemical Co., I no longer have
access to this database. However, my recollection is that up to about 50
feet tall, the probability of a well grounded structure being stuck by
lightening was statistically no different than level ground nearby. Above
50 feet, the probablity of being struck increases exponentially to the point
where very tall structures, like the Empire State Building, are struck at
least once annually.
The ASCE data indicates that being poorly grounded tends to slightly
decrease the probablity of being struck.
My take on all of this data is that our R-22's with deck stepped masts are
poorly grounded. Since the probability of the 28 foot tall masthead being
struck is statistically no different than water level even if it were
grounded, my answer is, "Do nothing to ground the boat." The difference in
my likelyhood of being struck is probably statisically insignificant, but
intentionally not grounding the mast should tend to shift the probability
slightly in my favor. Plus, it's easier to do nothing.
One thing I do is disconnect the masthead VHF antenna at the base of the
mast & turn off the on-board electronics during a thunderstorm.
My other advice is, stay away from tall structures like bigger sailboats &
trees on hillsides. Many trees on the shoreline have extensive root systems
which extend quite a distance out into the water. The roots are a rather
poor conductor of electricity & lightening will tend to heat them instantly
to incredible temperatures, potentially causing a steam explosion right
underneath the boat! This could ruin your whole day. Likewise, you don't
want to be nearby a taller structure like a bigger sailboat. A lightning
strike puts out an incredible amount of EMP. A nearby strike could
inductively fry every piece of electronics on board your boat.
Trees, sailboats, the ground, & the water get struck everyday. Use common
sense & get off the water when lightning starts flashing if that's at all
feasible. But, if you're caught out on the water during a thunderstorm,
just realise it's an act of God & there is very little you or I can do to
prevent it. Quit worrying about it. It happens, but the odds of it
happening to you are slim.
Roger Pihlaja
S/V Dynamic Equilibrium
----- Original Message -----
From: "Rik Sandberg" <racerrik@rea-alp.com>
To: "The Rhodes 22 mail list" <rhodes22-list@rhodes22.org>
Sent: Tuesday, November 19, 2002 5:36 PM
Subject: Re: [Rhodes22-list] lightning protection
> Brad, Todd, Bill
>
> I have to agree with Brad on this point. He said this or she said that,
> really doesn't mean much unless it's backed up with some facts or proof of
> some kind.
>
> Lord knows, I mean no disrespect to the folks who have voiced an opinion
on
> this subject. But, in this case, we are dealing with such a powerful
> phenomenon, that testing and proving any theory is virtually impossible
with
> the technology available to us now. So, I don't believe that it is
possible
> to make a decision, based on the facts when talking about lightning. We
> don't have facts, just theories and statistical musings and I'd bet that
> we'll all be too darned old to sail by the time anyone figures it out for
> sure.
>
> While we do know that boats do get damaged by lightning fairly frequently,
> we can take comfort in the fact that very few of these incidents result in
> an injury, let alone a death.
>
> So, in the end, we can argue this back and forth just like we have in the
> past and when we're done, we'll still know just as much as we did the last
> time this came up; Nothing. There just isn't any conclusive proof that any
> particular way of taking care of this problem is any better than another
> way, or that either of those is much better than doing nothing at all.
>
> I think the best any of us can do with this is go with what feels like the
> best solution to us and know that there is probably as much chance that
> we're wrong as the next guy who does just the opposite.
>
> My personal choice is to not worry about it. :-)
>
> Rik
>
> ----- Original Message -----
> From: "brad haslett" <flybrad@yahoo.com>
> To: "The Rhodes 22 mail list" <rhodes22-list@rhodes22.org>
> Sent: Tuesday, November 19, 2002 3:11 PM
> Subject: Re: [Rhodes22-list] lightning protection
>
>
> > Ladies and Gentlemen:
> >
> > This discussion reminds me of being a fifteen year old
> > kid, apprentice aircraft mechanic, milling around the
> > hangar and listening to the "old guys" tell their "war
> > stories". The scientific facts say that lightning
> > strikes on sailboats is a real threat. How big a
> > threat it is to you is a function of where you sail,
> > how often you sail, and how much time you spend "on
> > the hook". We all have to live with the American
> > jurisprudence system based on precedent but just
> > because someone said something some time ago on the
> > Rhodes list doesn't make it gospel. Todd has a valid
> > point and I challenge anyone to "shoot it down" with
> > evidence, not emotion.
> >
> > Brad Haslett
> > "CoraShen"
> > --- Todd Tavares <sprocket80@mail.com> wrote:
> > >
> > > Thanks Bill,
> > >
> > > My only point was that lightning would
> > > find the shortest and
> > > probably straightest path to the water. My
> > > terminology wasn't
> > > correct. I thought you were implying that
> > > because of the construction
> > > of the R22 that we were less likely to be hit
> > > because of the distance
> > > between the mast and water, and materials in the
> > > boat.
> > >
> > > I have never been on a boat that has been
> > > hit by lightening, but
> > > have seen many ground strikes close up. My only
> > > hope would be to
> > > survive (including making it back or being
> > > rescued from a partially
> > > submerged boat). Boat and electronics can be
> > > replaced.
> > >
> > > Thanks for the posts.
> > >
> > > Todd
> > >
> > > ----- Original Message -----
> > > From: "Bill Effros"
> > > Date: Tue, 19 Nov 2002 12:52:34 -0500
> > > To: "The Rhodes 22 mail list"
> > > Subject: Re: [Rhodes22-list] lightning protection
> > > > Todd,
> > > >
> > > > The image of lightning "jumping gaps" is
> > > somewhat misleading. Rather
> > > than
> > > > rehash everything that has been said
> > > previously, let me post a few
> > > of the
> > > > responses that meant the most to me from
> > > previous discussions:
> > > >
> > > > Here is how Razz put it:
> > > >
> > > > Rummy,
> > > >
> > > > If your electrical thrills are the result of
> > > electrostatic charge
> > > > building up, grounding the rigging won't
> > > eliminate the problem as it
> > > > is already acting as ground. The charge most
> > > likely is building up
> > > on
> > > > your butt as you slide across the seats. Much
> > > like crossing a wool
> > > > carpet and getting shocked when you touch the
> > > door knob.
> > > >
> > > > Proper grounding for lightning protection
> > > involves installing a
> > > > lightning rod at the top of the mast and
> > > grounding that to a metal
> > > > surface of significant area below the water.
> > > Boats with exposed lead
> > > > or iron keels can use the keel as a grounding
> > > surface. Reuel Parker
> > > > uses a
> > > > 2'x2' copper plate laminated into the bottom of
> > > his centerboard boat
> > > > designs. The cable connecting the lightning rod
> > > to the ground plate
> > > > should be very heavy guage and insulated
> > > > from the mast and rigging. No other metal on
> > > the boat, nor any part
> > > of
> > > > your electrical system should be grounded to
> > > this lightning
> > > protection
> > > > system. The theory is to provide a
> > > > clear alternate path to ground for any
> > > potential charge buildup.
> > > This
> > > > does not attract lightning as it serves to
> > > dissipate most charges
> > > > before they build up to dangerous levels. If a
> > > strike occurs, the
> > > > lightning ground provides a cone of protection
> > > for the boat and its
> > > > occupants equal to the height of the mast. So
> > > as long as you aren't
> > > > touching metal you are not likely to be
> > > injured. One of Parker's 50'
> > > > centerboard schooners took a direct hit that
> > > blew a hole in the hull
> > > > where his ground plate was attached, but no one
> > > on board was injured
> > > > and the boat did not sink as his designs have
> > > water tight bulkheads
> > > > between compartments.
> > > >
> > > > Very few lightning strikes actually occur on
> > > inland lakes or on any
> > > > body of water where land is nearby. Despite the
> > > fact that the
> > > earth's
> > > > surface is 2/3rds water, 94% of all cloud to
> > > earth lightning strikes
> > > > occur on land. This is due to the fact that
> > > large bodies of water
> > > > usually carry a positive ionic charge which is
> > > the same polarity as
> > > > the positive charge build up in thunderstorms
> > > and therefore do not
> > > > represent a good ground potential compared to
> > > the negative charge of
> > > > land surfaces. Earth ground really does mean
> > > solid ground. A boat is
> > > > more likely to be struck sitting in the marina
> > > or anchored near land
> > > > than out sailing and then only if it is the
> > > tallest object around.
> > > >
> > > > RAZZ
> > > >
> > > >
> > > > Here is what Fred Lange had to say:
> > > >
> > > > West Marine will send you a free packet of
> > > articles about lightning.
> > > >
> > > > Included in the material are some interesting
> > > statistical studies
> > > about
> > > > how lightning damages boats. In summary,
> > > sailboats are more likely
> > > to
> > > > be hit than power boats. Boats near shore or
> > > docked are more likely
> > > to
> > > > be hit than boats on the open water. Boats on
> > > salt water are more
> > > > likely to be hit than boats on fresh water.
> > > >
> > > > The physics of it is that if your boat is in a
> > > lower voltage area
> > > > between sky and earth surface, the likelihood
> > > of getting struck is
> > > > higher.
> > > >
> > > > Boats with lightning protection on open salt
> > > water suffer only
> > > slightly
> > > > less damage from lightning strikes than boats
> > > without lightning
> > > > protection, but the people aboard fare better.
> > > Boats on open fresh
> > > > water pretty much always get sunk when they are
> > > hit, but here again
> > > > lightning protection helps the people.
> > > >
> > > > I had a sailboat boat hit at the dock in
> > > brackish water. I know
> > > because
> > > > I had an anchor chain shackled to a stay and
> > > the fiberglass next to
> > > the
> > > > chain was charred. No other damage.
> > > >
> > > > Since I'm mainly a fresh water sailor, I have
> > > extended my experience
> > > > with the chain to try and protect the people in
> > > the boat if I get
> > > hit.
> > > > I have 5 battery cables for clamping to the two
> > > back stays, the fore
> > > > stay and to the port and starboard sides of the
> > > mast. I hope the
> > > cables
> > > > with their frayed ends will ground most of the
> > > strike into the water
> > > > leaving the crew untouched. However, I expect
> > > there is till the
> > > > likelihood that much of the strike will jump
> > > from the base of the
> > > mast,
> > > > though the cabin and out of the bottom of the
> > > hull, filling the
> > > Rhodes
> > > > with water.
> > > >
> > > > Fred
> > > >
> > > > Here is Roger's 2 cents worth:
> > > >
> > > > Thena,
> > > >
> > > > Yes, the Beneteau 40 footer had one of the
> > > tallest masts in the
> > > marina that
> > > > night. But, it wasn't the biggest sailboat &
> > > there
> > === message truncated ===
> >
> >
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