Roger Long wrote:
How about this?

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

I hauled out 180 feet of chain and removed my bilge storm chain locker
for a better look and measurements. This is close to scale.

The ground is shown in red. This would be about two square feet of
1/16" copper glued to the hull and screwed in way of the ballast fill.
A 1/2" diameter silicon bronze bolt would go through the copper and
hull. This is the same cross section area as the stainless steel mast
stanchion.

The connection between the mast stanchion and the through bolt would be
a 1/8" copper bracket with bent flanges for resistance to the mechanical
forces of the charge. This would be machine screwed to the pipe
stanchion from the back.

A refinement would be to make the through bolt longer and fasten it to
the side of the bracket with through bolts for a more straight line
electrical path.

Doesn't look too bad, definately better than leaving it alone. As I
mentioned previously, this is much like a HF grounding problem (except a
DC path is required and the expected current level. That would lead me
to suspect that multiple bolts between the copper bracket and the
grounding plate would be in order. Take full width copper plates bolted
on fore and aft of the support strut out to ground plates port and
starboard. If you can persuade the current to split fairly evenly you
gain *much* more protection from explosive events round those 1/2"
through hull bolts as resistive heating will be proportional to I^2. If
you manage to split the current evenly between four bolts, two each
side, the energy dissipated in each bolt will be reduced by a factor of
16.

I wouldnt bother with the longer bolts bent and bolted to the bracket,
You'd be better off with triangular pieces brazed accross the corner of
the bracket to its flange either side of each bolt, leaving just enough
room to get the end of a spanner in. If everything is nicely faired in
and you round off all sharp edges to the largest radius possible you
should have minimal structural and underwater damage.

There is likely to be at least *some* moisture behind the exterior
grounding plates so I would expect a steam explosion especially at their
edges. Screwing them into the ballast keel is probably not a good idea.
Drill and countersink holes in the plates *ONLY* to provide a key and
epoxy them into place? They will probably come loose at the edges in a
strike but hopefully will remain connected at the through bolts.
If you can keep the encapsulated ballast from being involved, you've
basically won.

Have you considerd that the odds are that you *WONT* have a startable
engine unless you can either hand start it or have a spare starting
battery kept fully isolated and a spare starter motor. Also you will
probably have damage to control cables and possibly to any metal fuel
lines depending on their proximity to other items.

I've seen photos of lightning damage to a mast with multiple holes you
could stick several fingers through down it for about a quarter of its
length so rig failure is also a real possibility.