Lee Huddleston wrote:
Any thoughts?


How about a steel hulled and decked boat? Mine has a deck-stepped
mast (actually two) with a steel compression post under them. Is the
lightening likely to hit the steel deck and go around the outside like
a Faraday's (sp?) cage, or is it more likely to make the jump to the
compression post and keep going straight to the steel keel?

Lee Huddleston
s/v Truelove

If all the bits are well connected (ie. no salty rust layers
between parts , etc.) I would expect the lighning to go via the
shortest path, given a choise of paths with similar geometry. Is
there a rubber pad or something under the mast heel?

This means I would expect your king posts(s) to sink most of the
bolt's current.

Something I realised isn't thought much by many is that all that
power has to be dissipated, or absorbed between the cloud and the
earth. That means that somewhere, the place with the biggest
resitivity, will get zorched while the other parts of the discharge
path may carry mucho current but will not dissapate the major whack
like what would happen at the point where a rusty old bolt is
expected to complete the circuit. That old bolt between the
overhead and the king post, or between the king post and the keel
could become the focus for a billion watts or so. That bolt might
vaporise, forming a metallic vapor arc lamp for a split second
while the rest of the path remains relatively unaffected. It is all
a question, like so many others, of relative resistivity.

If the path struck is pretty well connected, with no salient
resistance point, the surfaces in contact with bolt and earth will
bear the brunt. A large low resistivity contact area with the sea
will mean that the point where the bolt strikes, usually the mast,
will take a kicking. My mast had 25 half inch holes burned in it,
all along the port side upper foreward half, while there was little
else to show for damage except at the point where the keel bolt
passed through the hull, which aquired a slow drip leak.

I believe this was because the aluminium oxide coating the mast,
being a relatively poor conductor, is the point at which the most
resistance was developed in the current path, boiling the mast
material at that point and caused each mini stroke to extinguish
completely and requiring the next mini stroke to re-establish an arc
somewhere else on the mast with the same (near minimum) surface
curvature. It narrowly avoided the upper(ungrounded at the deck)
shroud. The holes were spaced about 6 inches apart. It is a textbook
example, I would expect, of typical and classic lightning behavoir,
consistant within it's parameters, not a freak. It repeated this
behavoir 24 more times. Perhaps the keel bolt got hot enough to
swell, squashing the bedding material, at the least.

Insurance replaced the mast. There was no other damage, surprising,
really, because the mast was in the return path for the spreader
light, using the VHF antenna coax ground. The spreader light was
wired this way when I got the boat, and frankly, given my
experiences, I am loathe to wire it "properly." The lightning ground
is a 3-0 wire from tabernacle to a keel, using the 4 mounting bolts
for the tabernacle passing through the now epoxy, then cored
fiberglass overhead, connected to an aluminium backing plate at the
top of the wooden king post. Some of the balsa there was rotton.

I believe I watched the top end of the bolt as I sat on my back
porch watching the storm from 10 miles away. The bolt appeared to
be quite heavy and of long duration, but that impression may be only
a result of the dynamics involved in the discharge, ie. 25 seperate
strokes, spread out a little in time, seemingly longer in duration
and enhancing my perception. I had no suspicion at the time that my
boat had been struck and it wasn't until a week later that I noticed
marks on the topmast.

It was, or seemed, by far the strongest bolt in that storm.

People inside your steel hull and cabin would be well protected as
long as all the connections between parts inside do not contain
relatively high resistance connections, though, being relative, one
point will always be "the highest" resistance and no matter how
low, that is where the sparks will fly as they did on my mast, where
the aluminium oxide presented the point to develop most of the power
dissipated above the sea, discounting the lightning bolt's display
itself.

Terry K