Roger Long wrote:

IanM wrote:

Can you get a strap round the front of the mast bolted to the
copper bracket either side sufficiently far out that it doesn't have
sharp bends in it?


I can't get to the front of the bracket without major surgery that would
compromise the boat's structural integrity as well as appearance.

I'm beginning to realize that this subject is so complex that only tests
in a high voltage chamber (which would cost enough to simply buy a high
end boat with protection already built in) will really answer the
question but, do you think this is worth putting in?

http://home.maine.rr.com/rlma/Ground.jpg

This is the earlier drawing with a top view added. The horizontal
brackets would be top and bottom. I recognize that the long tail is
probably useless for the primary current flow but will assist in
attaching the copper outside the hull and give me a point to lead bonds
from the toe rail and other items to.

I may be cooked anyway. The mast post ends in a plate lagged into the
top of the fiberglass ballast encapsulation so four sharp pointed lag
screws lead right down close to the encapsulated lead. I'm can't
imagine now that there won't be enough current flow left over,
regardless of what I do, to prevent something gross happening down in
the keel area.

I see little benefit in the long diagonal strap. Current sharing with
the short strap wont be anywhere near equal. I know you have pipes and
wires the other side, but the big advantage of keeping the copper
bracket reasonably symmetrical (apart from one or more holes for the
wires etc.) port and starboard is the inductance from the change of
direction will partially cancel. If you try to take it out one side
only with too sharp a bend it *will* arc over to the other side and down
through the ballast. If its already got a heavy copper path out the
other side, it probably wont.

Several square feet of ground plate each side and you will have reached
the point of sharply diminishing returns. Just try not to leave the
boat in fresh water in storm country. If you need to do so and its
going to be on a shallow berth, take a strap down the side of the keel
to the bottom each side and pray.

As to the lag bolts, if there is any other way you could secure the
compression post foot like bonding it into place with Epoxy, do so.
Otherwise you are just going to have to gamble that you've provided a
good enough diversion path unless you want to bore through the ballast
and tap studs into it so its electrically bonded as well, then tap more
studs into it through the sides of the keel.

As long as nothings caught fire you couldn't put out, you have a means
of determining a course to make port (figuring your electronics is toast
and all compasses aboard have been magnetically damaged and are
untrustworthy) the underwater damage is less than you can cope with a
manual bilge pump, and you can still either make sail or get the engine
going you've succeeded in saving your boat, even though you may have to
stay on board pumping till you can be hauled out. Plenty of yachts have
been struck and survived. If your grounding system significantly exceeds
industry standards, with chain plates, toe rails etc. bonded, the odds
are in your favour. No guarantees though.

OTOH if you were designing a production series of yachts it would be
prudent to call in a specialist to do some heavyweight numerical
modelling and scale model testing to prove that it is effective enough
protection for 99.{as many 9's as you need)% of recorded lightning strikes.