[Rhodes22-list] sailing and lightning (long reply)

[John Lock]

At 03:31 PM 7/28/2006 -0700, Tootle wrote:
>http://www.cdc.gov/nasd/docs/d000001-d000100/d000007/d000007.html
>
>And since John Lock would rather read than sail, maybe he should reasearch
>this one.

That is an incorrect statement.  I would rather be sailing!  But 
since I am nowhere near water and don't have a boat, well...

I already did some research on the subject because it concerned me 
when the sailing bug first bit (not long ago).  Here are some salient 
facts and observations that I have found valuable - YMMV.

First, there are two schools of thought on adding lightning 
protection to your boat:

1) Lightning is a random and poorly understood phenomenon.  Trying to 
avoid or control it is probably futile and the results will be random 
and poorly understood.

2) Doing something is better than doing nothing and maybe it will 
help.  Besides it's a cool project.

I suspect that both points of view have merit and which one you 
subscribe to probably says more about your personality than your 
technical skills ;-)

SOME INTERESTING FACTS _

* Boats in saltwater are more likely to be struck than boats in 
freshwater, due to saltwater's higher conductivity.  However, boats 
struck on freshwater are more likely to be severely damaged due to 
the higher current loads in the strike itself.  (All this being 
relative to the small likelihood of getting hit in the first place.)

* Powerboats are potentially more dangerous in a storm than 
sailboats, because their lower profile means a greater amount of 
current is needed to make a strike.  So, if you're out in a typical 
fiberglass runabout and get struck, poooof.

* Being caught out in a sailboat during a lightning storm does not 
mean you are going to be struck.  There are many accounts of people 
witnessing water strikes very near their boats.  Many other factors 
are involved in setting up a lightning strike.

* Boats with lightning protection systems "may" be more likely to be 
struck, but experience less damage.  There only seems to be anecdotal 
evidence of this, but the theory seems sound.  That is - if you give 
lightning somewhere to go, it may hit you first, but be dissipated 
more readily (see more on this further down).

SOME MYTHS TO BE DEBUNKED -

"Mooring your boat among boats with taller masts will protect you"

Lightning is seeking it's best path to ground.  Height (or the 
distance of the "air gap") is only one factor.  Other factors - such 
as mast/keel composition, deck or keel stepped masts, presence of 
other grounding objects near the waterline, etc - will ultimately 
decide the lightning path.  For example, a lead-keeled, keel-stepped 
boat may be more likely to be struck than a deck-stepped, centerboard 
boat with a taller mast.  And you can't survey all those boats you've 
parked amongst, so it's false security.

"Clamping jumper cables on a shroud and dangling the other end in the 
water is good enough"

While that sounds good on the surface, it is in fact a very bad 
idea.  The problem is that you are depending on relatively small 
surface areas to conduct a helluva lot of current.  The connection 
points between the shroud and the mast and the jumper cable clamp and 
the shroud are not sufficient to conduct the amount of current a 
strike produces.  However, you have increased the likelihood of a 
strike by providing a grounding path.  I would strongly discourage 
this practice. (There is also a similar method, which involves 
wrapping the anchor chain around the mast.  Same problem.)

"Adding a good lightning protection system will protect me and my boat"

Well, maybe...  There is at least one documented case of a 
well-protected boat being sunk by a strike.  The mast and all the 
shrouds were grounded via heavy copper cable to a copper plate 
epoxied onto the bottom of the hull.  However, there was some 
moisture behind the plate.  When the strike occurred, that moisture 
was instantly vaporized into steam and exploded the plate off the 
hull (with obvious results).

SOME GOOD IDEAS IF YOU PLAN TO ADD A LIGHTNING SYSTEM -

* Use nothing but heavy-gauge (#4 or larger) copper conductors.  All 
other materials will corrode or provide inferior conductivity.

* Keep all leads as straight as possible.  Any sharp bends or kinks 
will defeat the purpose.

* Provide lots of contact surface.  Snaps, hooks, turnbuckles, etc. 
will not conduct the current loads you get in a typical strike.  Use 
large connecting plates, bolts, and flat washers, clean connecting 
surfaces and seal from weather.

* If you have a system installed, don't do anything to defeat it if 
you're caught in a storm.  For example - don't hold onto the backstay 
while you pull up the swim ladder or fiddle with the outboard.  You 
may involuntarily become an integral part of the lightning system (as 
Bill E. so eloquently described :-) )

VARIOUS LIGHTNING PROTECTION SYSTEMS -

There are basically three commercially available systems in use at 
varying costs and perceived effectiveness.  Again, this assumes that 
you subscribe to the "something is better than nothing" school of thought -

1) Complete grounding systems - the mast, shrouds, motor, electronics 
and any other conductive materials are wired into one or more 
grounding leads, which go thru the hull to a flat copper plate 
affixed to the exterior.  Yep, that means you have to drill one or 
more holes to bring the conductor thru and (as shown in the example 
above) must be mounted with great care to eliminate all possibility 
of moisture behind the plate.  These systems are usually 
professionally installed, custom designed for each boat, and cost 
mucho bucks.  See 
http://www.marinelightning.com/Information/GroundingGuide.htm for 
some details on this.

2) Static dissipators - These are like inverted stainless steel 
"whisk brooms" attached to the top of your mast.  The theory is that 
the many small metallic points offered by the strands of the device 
will dissipate charges gradually as they build up, rather than 
allowing potentials to increase to the level of a full strike.  There 
seems to be little evidence that this actually works, since it's 
supposed to prevent a strike.  So... you could say if you don't get 
hit, it must be working!  They are cheap and have the added benefit 
of keeping birds off your masthead.  See example at 
http://www.yachtgard.com/lightning.html

3) Mast grounding systems - These work on the same principal as #1 
above, except the focus is entirely on the mast, rather than the 
whole boat.  The idea being that if lighting strikes the mast (most 
likely point), we should give it somewhere to go before it can cause 
any damage.  In concept, this is similar to the "jumper cable" method 
mentioned earlier, but approaches the problem in a more realistic 
manner.  A large copper conductor is bolted to the mast and attached 
to heavy copper cable, which can be removed and attached when needed, 
leading into the water.  The water-end usually has some kind of 
device attached to increase its surface area in contact with the 
water.  See http://www.strikeshield.com/ for a commercial example.

There are many online resources on lightning and boats, protection 
systems, theories, rumors, innuendo... hey, after all it IS the 
Internet ;-)  Try a search on "lightning protection for sailboats" 
and you'll get plenty to confuse you further.

And finally, to Mike W: there are two problems with your system - an 
aluminum plate (1) with a right-angle bend in it (2).  You'd be much 
better off with a flat copper plate attached to the conductor without 
any bends.  I don't know what the physical constraints inside the 
trunk are, but there you have it.

Cheers!

John

"Ever wonder what the speed of lightning would be if it didn't zigzag?"